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Collection and Determination of Metal Contaminants in Gases A Review URSULA TELGHEDER*a AND VLADIMIR A. KHVOSTIKOVb aUniversity of Duisburg, Department of Instrumental Analytical Chemistry, L otharstrasse, 1, D-47057 Duisburg, Germany bInstitute ofMicroelectronics T echnology and High Purity Materials RAS, Chernogolovka, Moscow distr., 142432 Russia SUMMARY OF CONTENTS In some cases, particle counters are used for the determination of particulates in gases.7,20 Introduction The most suitable methods for collection are filtration, Determination of Solid Contaminants in Gases impaction and impingers.Further possibilities are electrostatic Filtration precipitation and nuclear condensation.21 Dissolution of the Particles on Filters Impactors Filtration Impingers Analytical Methods Filtration is the most frequently used method because of the References ease of operation, low cost, and the availability of pure filter materials. Different filter materials have been investigated.Keywords: Metal contamination; gases; analytical methods; Usually, membrane filters with a pore size ranging from review 0.0511,22 to 5 mm23–31 and cellulose filters32 have been used. Other materials used are PTFE and polycarbonate. Because of their high mechanical strength, and despite their high chemical blanks, glass-fibre filters have been widely INTRODUCTION applied.33–38 Polystyrene-fibre filters,26 quartz fibres34,39 and In dealing with the problem of the determination of metal silver membranes27 have also been investigated.contaminants in gases it is important to consider the following The advantages of membrane filters are the good separation points: Which gases will be analysed? What type of trace of particles of different sizes, a high pore density and a rapid metals are the contaminants? How can the different types of filtration. On the other hand, the disadvantages are the low metal contaminants be collected, enriched and separated? loading capacity and the rapid clogging.The element contents Which analytical methods will be used? of the blanks are lower, but trace element determinations by Most publications deal with the determination of contami- ETAAS andNAA have shown that the purity of the membranes nants in air. Additionally, other gaseous components have can vary from box to box, even if the filters are from the same been analysed such as HCl,1–5 Cl2,6,7 high-purity volatile manufacturer.29 inorganic hydrides (SiH4, GeH4), halogenides (SiCl4, GeCl4, The glass-fibre filters have a high loading capacity and are POCl3, BBr3 , BCl3), organometallic compounds (Et2Zn, Et2Te, inexpensive.However, the pore size is not defined and the Me2Cd), and organochlorosilanes (Me2SiCl2, MeSiHCl2),8–13 filters have a low strength. pure gas dust from coal power stations14 and incinerators, or All filters for the measurement of particles in air have to waste gas.15–17 Nevertheless, the determination of particles in fulfil particular requirements. Apart from the degree of depos- air is the most important and will be systematically and ition, the flow resistance of the uncovered filter and its change extensively pursued because of the guidelines18 in Germany.with increasing coverage must also be taken into consideration. Generally, in gaseous compounds metals could be solid Furthermore, attention must be paid to the factors leading to particles, liquids and gaseous contaminants.The following changes in the filter material caused by the measuring object, expressions have to be defined: (i) Particulate compounds: e.g., the humidity, the amount of air, or particles of aggressive among these all dusty compounds passing the filter system compounds. Finally, the filters should be selected for a good will be subsumed. (ii) Compounds that are able to pass the prior interpretation. filter system: by this definition, all compounds passing the filter system are included as well as gaseous and liquid Dissolution of the Particles on Filters compounds. For chemical analysis the particulate matter collected on the filter has to be dissolved, except for some physical methods DETERMINATION OF SOLID CONTAMINANTS such as X-ray fluorescence28,40 and NAA.24,29,36 Membrane IN GASES filters have to be decomposed by different mixtures of acids in closed and open systems. The most commonly used mixtures A large number of measuring methods exist for the registration of the suspended-particulate concentration in gases due to the are those of hot HNO3 and HCl,22,23 or HNO3 , HClO4 and HF (with a temperature programme from 80 to 230 °C),41 or VDI-Guideline 246318 in Germany. The procedure mainly used in practice is the high-volume sampler principle.19 This is a HNO3 and HF.After digestion, the acids are evaporated and the residue is dissolved in diluted HNO325 and concentrated sampling method with a very high rate of air flow for the determination of particulates in ambient air.In this process HNO3 only.29 If glass-fibre filters are used, the sample may be digested by the air is sucked from below by a diffuser working as an aerodynamic rectifier. Thus, a regular, distributed, laminar a mixture of HNO3 and HClO4, followed by heating, evaporation, and dissolution of the residue in HCl33 or in a mixture current of air impinges on the filter element. The amount of sample is sufficient to carry out a gravimetric of HNO3, HF, HCl and H2O2 .34,38 The digestion can also be carried out using potassium chlorate and HNO3 with gentle determination of mass as well as specific analysis.Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 (1–6) 1boiling on a hot-plate followed by the addition of HCl,35 In addition, a special Venturi separator for small amounts of dust has been investigated.66 The TU� V Rheinland sampling dissolution in HNO3,39 and the extraction of the metals using a Soxhlet apparatus.42 Polystyrene filters have been dissolved system67,68 was developed with parts of well known measuring techniques.Field tests with frit wash bottles as absorption in xylene.26 devices showed a good and in some cases a better efficiency than more complicated absorption apparatus such as the jet Impactors absorber or Venturi scrubber. The passing compounds are sucked through a heated tube and divided into several by-pass Size-fractionated aerosol samples were obtained by using a Battelle cascade impactor.43 streams.The compounds are then supplied to parallel absorption systems. A mixture of 5% HNO3 with 3% H2O2, and Wagner and Georgii44 developed a five-stage impactor which separates the particulate matter into five fractions (diameter aqua regia [HCl–HNO3 (3+1)] have been used as absorption solutions for metallic compounds. For the determination of 6.3–1.0 mm). Each stage is made of a PVC funnel connected to a brass tube.Nozzles with different diameters are located at mercury, a 3% KMnO4 solution in 10% H2SO4 has been found to be more efficient. the bottom of the tube. Sneddon and co-workers45–52 have described an impactor, directly connected to a commercial Bruckmann et al.69 used two impingers cooled to -12 °C and filled with 25% HNO3 as well as a further impinger for electrothermal atomizer. The sampling process is often performed with cascade impactors of various designs.15,53–55 The condensation.The impingers were connected by glass pipes to sampling equipment following VDI 2463 Sheet 7.70 most widely used cascade impactor is the Anderson sampler, which has the advantage of being simple and inexpensive.53,54 In order to precipitate suspended particles, Scharf71 suggested a procedure and described an instrument to determine The collecting discs, for example, are made of Plexiglas; the back-up filters are deep bed filters of glass fibre.The back-up the content of the suspended particles quantitatively. The amount of precipitated suspended particles was increased by filters of each fraction are extracted with HNO3 followed by acid filtration through XAD-4 amberlite resin. The solutions the addition of lightly condensing solvents to the gas being analysed followed by cooling the mixture. The particles were are then analysed by ETAAS. Weiswler and Gund55 described the chemical analysis of separated by a cooled impinger made of high-purity quartz.single aerosol particles collected on a nine-stage cascade impactor. The separate parts (sulfate, nitrate, chloride and ammonium ions) were characterized by a combination of ANALYTICAL METHODS microchemical precipitation and subsequent interpretation by scanning electron microscopy. Several methods for the analysis of particles for metals have been described, including AAS,4,15,22,23,25,29,30,33–35,41,42,44–53,70 AFS,47,72 XRF,28,40,43,55 NAA,22,24,29,36 ICP- Impingers AES,15,26,27,34,37,38,41 including sealed ICP-AES5–7,9–11,73–76 as well as ICP-MS,77 MIP,78 anodic stripping voltammetry,32,79 Filters have been mainly used for the precipitation of suspended particles in gases.Unfortunately, the amounts of elements in ion chromatography (IC),31 differential-pulse polarography, energy dispersive X-ray spectrometry (EDX) and laser micro- the blank solution of the filters were sometimes found to be high and varied from filter to filter.29 Therefore, the detection mass analysis (LAMMA).14 The most frequently used technique for the determination limit of the procedure involving gas filtration is limited by the quality of the filters.Impingers are more suitable for an of trace metals is atomic absorption, especially ETAAS. This method possesses the extreme sensitivity that is required extensive precipitation of suspended particles without contamination, especially for a low flow rate gas stream.In this section, without the need for time-consuming preconcentration methods. It is reasonably rapid and does not require excessively different sampling systems are exemplary characterized. (i) The single-line sampling system, based on the US expensive instrumentation. Pickford and Rossi25 constructed an automatic laboratory sampling system for the determination Environmental ProtectionAgency (EPA) method.56–59 Initially, the sample, which is sucked through, passes a cyclone lying of Pb in atmospheric particulates by ETAAS after acid dissolution. This system permits a large number of samples to be outside the exhaust channel and then a plane filter for the precipitation of particles, after which the compounds are analysed in the minimum time without operator attendance.A passive sampler designed to simulate pulmonary retention caught in several wash bottles. A similar system is the WEP (Wet-Electrostatic-Precipitator)-Sampling-Train. Here, a Wet- of sub-micrometre particles was described by Tennant and Rees.30 The device can accumulate lead from ambient air over Electrostatic-Precipitator is used instead of the cyclone followed by a plane filter.60 a period of 4 weeks.The analysis is carried out by ETAAS. A major advantage of the electrothermal atomizeris the feasibility (ii) The condensation nucleus collector developed by the Technical University of Lyngby (Denmark).This collector uses of the direct analysis of metal compounds collected by impaction. 45–52 There are several applications using ETAAS connec- a filter holder (following the guidelines in the German VDI 2066 Sheet 261 and a separator for compounds that are able ted with cascade impactors.44,80 Liang et al.47 reported the use of an impactor–ETAAS system for the direct determination of to pass the filter system (condensation nucleus collector57). The measuring arrangements described by Gutberlet62 and Cu, Fe, Mg, Pb, Sn and Tl in air at ng m-3 levels as well as a laser atomic fluorescence system for determination at pg m-3 Mu�ller63 consist of a filter holder, also following VDI 2066 Sheet 2.61 The outlet pipe is connected to many wash bottles.levels. The potential of atomic fluorescence with a tantalum coil For the precipitation of metals which may pass the filter system, a beam absorber has been developed at the University atomizer for direct analysis of gases was demonstrated for Fe determination in Ar.72 The detection limit obtained was of Hamburg.64 The sample gas is absorbed in a liquid using a pump and a Venturi tube which allows an extensive mass 4 ng l-1.Baaske and co-workers4,81 described the introduction of transfer between the gaseous and liquid phase. Two other prototype separators65 for such compounds are also based on gaseous samples into an electrothermal atomizer using a modified by-pass–back-flush–balancing injection system and this model: the first is the condensation origin absorber, which consists of a wash bottle with a frit, and the second is the later using an automated sampling system.The detection limit of Fe in gaseous hydrogen chloride was found to be electro-absorber, which is based on the origin absorber but also contains two platinum electrodes. 0.7 ng ml-1. 2 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12Table 1 Survey of measurements of metals in air and gases Element Matrix Sample collection Sample treatment Technique Ref.Air Impaction–ETAAS 49 Cd Fe Air High-volume sampler, low-volume Dissolution of the filter in xylene ICP-AES 26 sampler, polystyrene-fibre filter Fe HCl Direct introduction of the HCl into ICP-AES 3 the ICP-AES system Fe HCl ETAAS 4 Fe Ar AFS with Ta coil 72 Fe Ar MIP-AES 78 Fe Air Impaction–ETAAS 51 Hg Air Adsorption on PbS AES 80 Hg Air Impaction–ETAAS 50 P Ar Sealed ICP-AES 76 Pb Air Polycarbonate filters coated with a Atomization and excitation in the ICP-AES 27 thin film of high-purity Ag high-temperature plasma produced by the electrical vaporization of the Ag film Pb Air Impaction–ETAAS 52 Pb Suspended Membrane filters Acid digestion in 50% HNO3 , ETAAS 25 particles heating at 100 °C for 30 min Pb Air Membrane filters with a pore size of Membrane filters wet-ashed in HNO3 ETAAS 30 1.45 mm Pb Air Nuclepore filters with a pore size of Digestion in crucibles with a mixture AAS 23 5 mm of 8 mol l-1 HNO3 and concentrated (12 mol l-1 ) HCl (5+1) at 90 °C As, P Ar Sealed ICP-AES 75 Te, Se Atmospheric High-volume sampler, collection on Removal of interfering elements by ETAAS 33 aerosol glass-fibre filters cation exchange, digestion in samples concentrated HNO3 and 60% HClO4 , dissolution of the residue in 0.05 mol l-1 HCl As, Hg, Se Waste gas Nuclepore filters with a pore NAA 84 diameter of 0.4 mm Cd, Cu, Mn Aerosols Impaction–ETAAS 48 Cd, Cu, Mn Air Impaction on an electrothermal ETAAS 45 atomizer Cr, Mn, Pb Suspended High-volume sampler, glass-fibre ETV–ICP 37 particles filters As, Cd, Co, Suspended Glass-fibre and quartz filters Dissolution in a mixture of HNO3 , ETAAS 34 Tl particles HF, HClO4 and H2O2 Al, Fe, Pb, Air Impaction on glass-fibre filters XRF 21 Si Cd, Cu, Pb, Waste gas The sampler is based on The sampler is led to a vaporizer and Differential-pulse 39 Zn vaporization/condensation mixed with a strong oxidizing acid polarography principles (65% HNO3 ) Cd, Cu, Pb, Air Celotate cellulose filters Ashing in a high vacuum at low Anodic stripping 32 Zn temperature, adding concentrated voltammetry HNO3 and HF, heating for 1 h at 120 °C Cr, Fe, Mn, HCl ETAAS 81 Ni Cu, Mn, Ni, Fog; Fan collector; Zn Fog droplets Impact on PFTE strings (diameter HPLC 31 0.3 mm) Cd, Fe, Mn, Air Five-stage impactor ETAAS 44 Pb Al, C, Ca, Cl2 Sealed ICP-AES 7 Cu, Sn Al, Fe, Mg, Semiconductor- Adsorption of the trace metals on an Elution with a mixture of HCl, ICP-AES 85 Si, Zn grade gases adsorbing material HNO3, H2O As, Cd, Cu, Suspended High-volume sampler, various types Digestion with KClO3, 20% HNO3 , Flame AAS 35 Pb, Zn particles of glass-fibre filters simmering for 15 min on a hot-plate Ca, Mg, Na, Fog Ion chromatography 31 NH4+, K Cd, Cu, Fe, Airborne High-volume sampler, glass-fibre Soxhlet extraction; acid digestion Flame AAS 42 Mn, Pb particulates filters with a mixture of HNO3, H2SO4 , HClO4 , at 120°C for 2 h; autoclave digestion, HNO3 at 50°C for 1 h Cr, Cu, Fe, HCl Polycarbonate filters Digestion with HCl–HNO3 ETAAS 22 Mn, Ni As, Cd, Cr, Air Impinger, filling solution: 25% Digestion in a mixture of HNO3, ETAAS 69 Cu, Ni, HNO3 ; membrane filter, 1.2 mm HClO4 , HF with increasing Pb pore size tempetures, 80–200°C Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 3Table 1 (continued) Element Matrix Sample collection Sample treatment Technique Ref.Air Precipitation on glass-fibre and Cr, Cu, Fe, XRF 28 Mn, Ni, Zn membrane filters Cu, Fe, Mn, Air Impaction–ETAAS 47 Pb, Sn, Tl or LAFS As, Cd, Cr, Air Impinger, different procedures 84 Hg, Ni, Pb, Se Al, C, Ca, Cr, HCl Sealed ICP-AES 5 Fe, Ni, Sn Cd, Co, Cr, Airborne Cascade impactor, collection on Extraction with 1 mol l-1 HNO3 at ETAAS 53 Cu, Ni, Pb, particulates Plexiglas 80 °C for 1 h, filtration over Zn Amberlite XAD-4 C, Fe, Ge, AsCl4 Sealed ICP-AES 10 Mg, Mo, Ni, Sn, V Al, Ca, Cr, Cl2 Sealed ICP-AES 6 Cu, Fe, Mg, Mo, Ni, Sn Al, Ca, Cr, Suspended Glass-fibre and quartz filters ICP-AES 34 Cu, Fe, Mg, particles Ni, Pb, Zn Al, Cd, Cr, Filter Millipore filters Dissolution in concentrated HNO3 in ETAAS 29 Cu, Fe, Mn, a PTFE screw-cap dissolution bomb Ni, Pb, Zn Br, Mg, Mn, HCl Polycarbonate filters NAA 22 Na, Sb, Sn, Te, Ti, Zn Co, Cr, Fe, Filter Millipore filters NAA 29 Mn, Na, Sb, Sc, Se, Zn Be, Cd, Cr, Airborne High-volume sampler; low-volume HNO3–H2O2 digestion; aqua regia ICP-AES 38 Cu, Fe, Mn, particulates sampler; glass-fibre filters and quartz- digestion; HNO3–HClO4 digestion Ni, Pb, V, fibre filters Zn Ag, Co, Mn, Pure gas dust Cascade impactor LAMMA 14 Mo, Nb, Ni, Pb, Sn, Ti, V, Zn As, Cd, Cr, Waste gas Impinger, filling solution: quartz Digestion: HNO3–HF, heating ETAAS 15 Cu, Hg, Ni, cotton, two-steps: acid washing ICP-AES Pb, Sb, Se, solutions, absorption solution: 5% V, Zn HNO3+3% H2O2 , aqua regia Al, Au, Co, NAA 24 Cr, Cs, Cu, La, Mn, Sc, Se, V, Zn As, Al, Bi, Suspended Filter made of air filter cardboard XRF 40 Ca, Cd, Cu, particles Cr, Fe, K, Mn, Ni, Sb, Sn, Si, Ti, Pb, Zn Al, Ca, Cd, Air High-volume sampler; membrane Digestion with a mixture of HNO3 , ICP-AES 41 Co, Cr, Cu, filters with a pore size of 1.2 mm HClO4 ; temperature programme ETAAS Fe, Mg, Mn, 80–230 °C Ni, Pb, Sb, Sr, Ti, V, Zn As, Ba, Ca, Air High-volume sampler; glass-fibre NAA 36 Cd, Ce, Co, filters Cr, Cs, Fe, La, Lu, Mo, Ni, Rb, Sb, Sc, Se, Sm, Sn, Ta, Yb, Zn Al, As, Ca, Air Battelle cascade impactor in six EDXRF PIXE 43 Cu, Fe, K, particle-size fractions Mn, Si, Ti, Pb, Rb, Sr, Zn Ag, Al, Bi, Gases Concentration of the metals on a Dc-arc 8 Ca, Cd, Co, carbon collector by vacuum spectrographic Cr, Cu, Fe, distillation method Mg, Mn, Mo, Na, V, Zn, Ni, Pb, Sb, Ti 4 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12Table 1 (continued) Element Matrix Sample collection Sample treatment Technique Ref.Air Nine-stage cascade impactor AAS Metals 55 IC Many Semiconductor- The gas is carried by an argon stream ICP-MS 2 elements grade gases into the ICP-MS system Chiou and Manuel33 determined, for example, Te and Se comparison of the impulse sequence of the spectral lines with calibrating filters. after removing interfering elements by cation exchange. Flame AAS has also been used.23,35,42 A disadvantage of PIXE can also be used for the determination of trace contaminants43 because of the detection sensitivity obtained flame AAS is that direct analysis of solids is difficult.On account of its multi-element capability, AES has been widely by the higher ‘signal-to-bremsstrahlung’ relationship; X-rayinduced photoelectron spectroscopy has been used for the used to determine a number of different elements in a sample. Because of high sensitivity, fewer interferences and multi- chemical analysis of single aerosol particles.The main advantage of NAA is the simple sample prep- element capability, ICP-AES has become an ideal technique5 –7,9–11,15,26,27,34,37,38,41,73–76 for trace element aration. Faix et al.22 have determined particle-bound trace metals in highly pure hydrogen chloride by NAA. Trace determination. Standardized techniques have been mostly used. Sugimae elements in ambient air24,36 and the concentrations of several elements in the vapour-phase in the stack of a coal-fired power and Mizoguchi26 described the direct determination of Fe in airborne particulate matter by direct nebulization of suspen- plant83 have been determined in the same way.Occasionally, electrochemical procedures, such as differen- sions into the ICP. A similar procedure for the direct determination of Pb in urban particulate material has been carried tial-pulse polarography and, particularly, anodic stripping voltammetry, have been used for the determination of metals.39 out using thin silver films electrically vaporized from membrane filters.27 A number of workers have used simultaneous multi- Larjava and Kauppinen39 developed a sampler to collect gaseous metals and to fractionate the particles into three element analysis by conventional ICP-AES.15,34,38,41 Schram3 described a method for direct introduction of reactive gases classes.The sampler is based on vaporization/condensation principles. into an ICP-AES system. The gaseous sample is led into a mixing chamber, positioned under the aerosol tube of the ICP- Table 1 gives a general survey of the field of analysis of air and gases for metals and their compounds including AES torch.Regulation of the gas flow is effected by the by-pass–back-flush method or the peristaltic pump. applications. The method of sealed ICP, developed by Jacksier, Barnes and co-workers5–7,9–11,73–76 seems very promising for the direct analysis of reactive gases. The main advantage of this method REFERENCES is that the plasma is excited inside a closed container so that the direct contact of aggressive gases with the parts of the 1 Yin, I.H., Denyszyn, R. B., and Bandy, T., paper presented at Microcontamination, West Conference, Anaheim, 1989. analytical system is excluded. These workers demonstrated the 2 Hutton, R. C., Bridenne, M., Coffre, E., Marot, Y., and possibilities of the method for the analysis of arsine, silane, Simondet, F., J. Anal. At. 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L ett., 1985, 18(A10), 1261. 75 Jahl, M. J., and Barnes, R. M., J. Anal. At. Spectrom., 1992, 46 Sneddon, J., Smith, M. V., Indurthy, S., and Lee, Y., Spectroscopy, 7 (6), 825. 1995, 10(1), 26. 76 Jahl, M. J., and Barnes, R. M., Spectrochim. Acta, Part B, 1992, 47 Liang, Z., Wie, G. T., Irwin, R. L., Walton, A. P., Michel, R. G., 47B(7), 923. and Sneddon, J., Anal. Chem., 1990, 62(14), 1452. 77 Rosamilia, J. M., presented at the 1994 Pittsburg Conference, 48 Sneddon, J., Anal. L ett., 1990, 23(6), 1107. Chicago, IL, March, 1994. 49 Sneddon, J., Appl. Spectrosc., 1989, 43(6), 1100. 78 Kirschner, S., Golloch, A., and Telgheder, U., J. Anal. At. 50 Sneddon, J., Spectrosc. L ett., 1987, 20 (6–7), 527. Spectrom., 1994, 9(9), 971. 51 Sneddon, J., Int. L ab., 1986, 16 (4), 18. 79 Pfeffer, H. U., and Buck, M., J. Aerosol Sci., 1986, 17, 290. 52 Sneddon, J., Anal. Chem., 1984, 56(11), 1982. 80 Zehringer, M., Hohl, C., Schneider, A., and Schu�pbach, M. R., 53 Dzubay, T. G., Hines, L. E., and Stevens, R. K., Atmos. Environ., Staub-Reinhalt. L uft, 1989, 49, 439. 1976, 10, 229. 81 Baaske, B., and Telgheder, U., J. Anal. At. Spectrom., 1995, 54 Laskus, L., and Bake, D., Staub-Reinhalt. L uft, 1976, 36, 102. 10, 1077. 55 Weisweiler, W., and Gund, G., Staub-Reinhalt. L uf t, 1990, 50, 53. 82 Alexandrov, S., Fresenius’ Z. Anal. Chem., 1985, 321, 578. 56 Rom, J. J., Maintenance, Calibration and Operation of Isokinetic 83 Meier, H., and Unger, E., Staub-Reinhalt. L uft, 1975, 35, 321. Source-sampling Equipment, EPA: APTD-0576/PB 209022, EPA 84 Germani, M. S., and Zoller, W. H., Environ. Sci. T echnol., 1988, Research, Triangle Park, NC, USA, 1972. 22, 1079. 57 Peters, E. T., Valentine, D. R., and Adams, J. W., Metal Particulate 85 Miyazaki, K., and Nakagawa, K., Koatsu Gasu, 1992, 29(4), 281. Emissions from Stationary Sources, Vol. 1: Standard Sampling and Analysis Method, EPA-600/2-80-202, PD 81-120024, EPA Paper 6/02984A Research, Triangle Park, NC, USA, 1980. Received April 29, 1996 58 Schwitzgebel, K., Coleman, R. T., Collins, R. V., Mann, R. M., and Thompson, C. M., T race Element Study of a Primary Copper Accepted September 17, 1996 6 Journal of Analytical Atom

ISSN:0267-9477    

DOI:10.1039/a602984a

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Novel Laser Sampling Technique for Inductively Coupled Plasma Atomic Emission Spectrometry KENNETH K. K. LAM AND W. T. CHAN* Department of Chemistry, T he University of Hong Kong, Pokfulam Road, Hong Kong A novel laser sampling technique, back-surface ablation, has through the substrate. As the sample at the sample–substrate interface is vaporized by the laser beam, enormous pressure is been developed for ICP-AES. Samples are coated onto a transparent substrate and a laser beam is irradiated onto the developed between the sample and the substrate at the laser spot.21 The sample at the laser spot is removed explosively by sample through the substrate instead of sampling directly from the sample surface.As part of the sample is vaporized by the a single laser pulse. A clean plug of sample is easily removed with moderate laser power density (#107 Wcm-2); therefore, laser beam, a high pressure develops at the sample–substrate interface. The sample at the laser spot is explosively and preferential vaporization is minimized.Furthermore, sampling efficiency is enhanced. The amount of material removed is up completely removed by the expanding vapour. Sampling efficiency is up to ten times higher than conventional ‘front- to 100 times larger than direct ablation of a sample (frontsurface ablation).21 The actual enhancement of laser sampling surface’ laser sampling. Also, preferential vaporization is minimized because of complete removal of the sample at the efficiency depends on the thickness of the sample film.A 10–20-fold enhancement was obtained in this work. Sensitivity laser spot. The risk of inaccurate chemical analysis associated with non-stoichiometric thermal vaporization in front surface enhancement (in terms of ICP emission intensity), however, is only about 3-fold, probably because of poor transport efficiency laser sampling is reduced. Two methods of calibration, viz., standard additions and calibration with standards in a of the laser-sampled material.The laser-sampled materials travel at high speed on taking off from the substrate, collide poly(vinyl alcohol ) matrix, were used for quantitative elemental analysis of household paints using front- and back- with the ablation chamber and break into small particles. Some of these particles are probably still too large for efficient surface ablation–ICP-AES. Internal standards were used to compensate for pulse-to-pulse laser energy fluctuation and transport to the ICP by the carrier gas.22 Household emulsion paints were analysed using both front- sample thickness variation across a sample.Ten elements with different thermal properties and at concentrations ranging and back-surface ablation to compare the sensitivity, precision, and laser-sampled material stoichiometry of these sampling from 10 to 1000 ppm were determined and the elemental concentrations were compared with those of microwave techniques.Quantitative elemental analysis using standard additions and calibration with standards in a poly(vinyl digestion/solution nebulization–ICP-AES. Back-surface ablation appears to be more accurate than conventional front- alcohol) (PVA) matrix is demonstrated. The results were compared with those of microwave acid digestion. surface ablation. Keywords: L aser sampling; back-surface laser ablation; inductively coupled plasma; atomic emission spectrometry EXPERIMENTAL Instrumentation Laser ablation is a versatile sampling technique for analytical A schematic diagram of the laser ablation set-up is shown excitation sources.It can be applied directly to a wide range in Fig. 1. The ICP spectrometer [Carl Zeiss (Jena, Germany) of sample types with little or no sample preparation.1–4 Sample Plasmaquant 110] uses a 1 m high-resolution echelle preparation, especially dissolution of solids, is time consuming and often a source of contamination and analyte loss.Laser sampling has been coupled with ICP-AES and ICP-MS as a tandem technique that allows independent optimization of the sampling and excitation processes.5–13 ICP is a mature high power density atomic source that can vaporize and atomize the laser-sampled materials efficiently. During laser sampling–ICP operation, the ICP parameters usually do not need much adjustment. However, laser–material interactions at the sample surface are complicated, the amount and stoichiometry of the laser-sampled materials vary with laser power density and pulse energy, gas atmosphere in the ablation chamber, as well as the properties of the sample.14–19 Preferential vaporization and laser-induced plasma shielding of the laser beam20 are especially troublesome for chemical analysis.Routine analyses using laser sampling have yet to be realized. A laser sampling technique without these problems is desirable. This paper describes a novel laser sampling technique, backsurface ablation, that makes use of the high pressure generated at the sample surface during laser sample vaporization to Fig. 1 Experimental set-upfor laser ablation–ICP-AES. During back- remove the sample at the laser spot. A film of a sample, a few surface ablation, the sample is placed at position 1 with the quartz tens of micrometres thick, is coated onto a transparent sub- substrate facing the laser beam. In conventional front-surface ablation, the sample is placed at position 2, facing the laser beam directly.strate. A laser beam is irradiated onto the sample surface Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 (7–12) 7spectrometer for wavelength selection. A fibre block with 132 optical fibres was positioned at the exit of the spectrometer; each fibre corresponds to a specific wavelength. Up to 12 emission lines can be monitored simultaneously. Optical fibres for the selected lines were connected to an array of 12 photomultiplier tubes (PMTs) for measurement.The operating parameters of the ICP-AES system and the selected spectral lines are given in Tables 1 and 2, respectively. Since the lasersampled materials take about 5 s to be completely swept into the ICP, ICP emission intensity was integrated for 6 s for each laser shot during quantitative analysis. A shorter integration time (0.03 s) was used for temporal study of the ICP emission. A KrF excimer laser [Lumonics 510 (Lumonics, Kanata, Fig. 2 Details of the glass laser ablation chamber. Canada) with stable optics, wavelength=248 nm, pulse duration= 12 ns FWHM] was used for laser ablation. Pulse ing is discussed below.) A sample-coated quartz plate was energy is approximately 60 mJ. A single plano-convex lens of attached to the front of the chamber with a plastic ring that focal length 200 mm at 248 nm was used to focus the laser fitted snugly to the laser ablation chamber so that the chamber beam onto the sample.The samples were always placed before was air-tight. The laser-sampled materials were carried to the the focal point to avoid laser breakdown of the atmosphere at ICP by an Ar stream via a Teflon tube (0.80 m×5 mm id). A the laser focus. Because of the stable cavity configuration, the three-way valve was placed between the chamber and the laser beam divergence is relatively large. A rectangular aperture torch. The bottom of the ICP torch was sealed from (30×10 mm) was placed between the excimer laser and the the atmosphere during sample changing.The Ar flow into the focusing lens to limit the laser beam size and select the central central tube of the ICP torch was resumed after the sample homogeneous portion of the laser beam. The laser energy after had been attached and the chamber was flushed with Ar the aperture is 20–30 mJ. Typical laser power density is in the for 1 min. range 107–108 Wcm-2, depending on the lens-to-sample dis- Conventional front-surface ablation using the same laser tance.Laser power density was determined from laser pulse and ICP operating parameters was performed for comparison. energy, pulse duration and measured spot size. Spot size was In this set-up, the chamber is rotated 180° so that the quartz also calculated from the lens-to-sample distance using geoplate at the end of the chamber now faces the laser beam and metric optics principles, which agreed reasonably well with the becomes the chamber window.A quartz plate coated with measured spot size. As all material at the laser spot is compaint sample was placed at the other end of the ablation pletely removed by a single laser shot during back-surface chamber, with the sample facing the laser. ablation, single laser pulse ablation was used. The layout of the laser ablation chamber for back-surface ablation is shown in Fig. 2. The glass ablation chamber is Preparation of Samples and Standards 30 mm in diameter and 30 mm long.One end of the chamber was sealed with a quartz plate. The chamber was mounted on Local latex emulsion paints (The China Paint MFG. Co., Hong Kong) of different colours were used. The water-based a xyz-translation stage to facilitate raster laser sampling of the sample and fine adjustment of the laser beam focus. Quartz paints can be coated on a substrate readily and reproducibly. Furthermore, aqueous standard solutions mix readily with the plates, 30 mm in diameter and 1 mm thick, were used as transparent substrates to hold the sample films.(Sample coat- samples for quantitative analysis using the standard additions method. The wet paint was diluted with an equal volume of distilled water to reduce its viscosity. A thin film of the diluted Table 1 ICP operating parameters paint was then coated onto a quartz plate by spreading a few ICP forward power 1.0 kW drops of the paint evenly on the quartz plate surface with a Observation height 10 mm above load coil glass rod.The paint was air-dried at room temperature. Coolant argon flow rate 12 l min-1 Different sample thicknesses were obtained by varying the Auxiliary argon flow rate 1.0 l min-1 number of drops of sample added to the substrate. A sample Carrier gas argon flow rate 1.0 l min-1 thickness of 35 mm was used during quantitative analysis. The Integration time 0.03–1.0 s thickness of the film, and thus the mass per unit area of the paint sample, varies slightly across the surface and from sample Table 2 ICP emission spectral lines used to sample.Variation in ICP emission intensity due to the variation in the amount of laser-ablated material was compen- Element Wavelength/nm sated using the internal standard method. For quantitative Al I 396.152 analysis using the standard additions method, titanium was Ba II 455.403 used as an internal standard because of its abundance in the Ca II 317.933 paint samples (about 5–10% TiO2 as pigment).Co II 228.616 Two calibration methods were used during quantitative Cr II 205.559 Cu I 324.754 elemental analysis: standard additions to the paint samples Fe II 238.207 and standards in a PVA matrix. In the standard additions Mg I 285.213 method, aqueous standard solutions in 1% nitric acid were Mn II 257.610 added directly to the wet paint samples and the resulting Ni I 341.477 mixture was coated onto the quartz plate for ablation. Pb II 220.351 Standards in a PVA matrix were prepared by dissolving 4 g Si I 212.412 Sn I 235.484 of PVA powder [Aldrich (Milwaukee, WI, USA), 88% hydro- Sr II 407.771 lysed, average molecular mass 80000–100 000] in 100 ml of Ti II 368.520 water.After dissolution, 4 g of titanium dioxide powder were V II 292.402 added and the mixture was stirred for 1 h with a magnetic Zn II 202.551 stirrer. Titanium dioxide absorbs strongly at the UV laser 8 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12wavelength. It is added to enhance laser energy absorption carrier gas flow rate.17 Since the operating parameters are the same except for the laser sampling method, ICP emission for and thus the ablation efficiency. In addition, PVA films become more brittle with titanium dioxide and the laser-sampled both back- and front-surface ablation has similar rise and fall times. materials from back-surface ablation can be fragmented into fine particles more efficiently.For quantitative analysis using The amount of laser-sampled material, however, is larger for back-surface ablation. With a single laser pulse of moderate standards in a PVA matrix, titanium was not used as an internal standard as its concentration varies in different paint power density (#107 W cm-2), the sample at the laser spot is completely removed using back-surface ablation. A small frac- samples and is not known. Ni (200 ppm) was used instead because it is absent from the paint samples and does not give tion of the sample is ablated using front-surface ablation at the same laser power density.The amount of material removed rise to spectral interference with other elements. A series of standards in a PVA matrix was prepared by adding aqueous by back-surface ablation can be 100 times larger than by frontsurface ablation.21 In this work, a 35 mm thick sample required standards (in 1% nitric acid) to the PVA solution. The standards were then coated onto quartz plates in a similar manner about ten laser pulses for complete removal of the sample at the laser spot using front-surface ablation (Fig. 4). Therefore, to the paint sample preparation. a 10-fold improvement in sampling efficiency is obtained using back-surface ablation at this sample thickness. Microwave Digestion It appears that enhancement of sampling efficiency using back-surface ablation can be further improved by increasing Quantitative elemental analysis of the paints using laser samthe sample thickness.However, there is an optimum sample pling–ICP-AES was compared with that of microwave digesthickness for maximum ICP emission intensity (Fig. 5). ICP tion and solution nebulization into the ICP. A modification of the method of Paudyn and Smith23 was used to digest the paints. Since the large amount of organic matter in the sample results in excessive pressure in the bomb and may cause damage to both the oven and the bomb, the sample was ashed before microwave digestion.A portion (3–4 g) of wet paint was weighed in an ashless filter-paper (Whatman No. 540) and ashed in a crucible with a Bunsen burner. About 1 g of ash remained after ashing. A portion (20–30 mg) of the ash was weighed and placed in a Parr Microwave Digestion Bomb [Parr Instrument (Moline, IL, USA) Model 4782], followed by 5 ml of concentrated HNO3 [analytical-reagent grade, Merck (Darmstadt, Germany)] and 2 ml of concentrated HF (analytical-reagent grade, Merck).The bomb was placed in the oven and irradiated for 2 min at about 600 W. After cooling Fig. 4 ICP emission intensity of Fe II 238.2 nm versus laser pulse for about 2 h, 20 ml of water were added to the vessel and the number using front-surface ablation. Laser power density, approxi- solution was transferred into a Teflon beaker and evaporated mately 5×107 W cm-2. to 1 ml on a hot-plate at 120°C. The resulting solution was diluted and filtered and made up to 100 ml with 1% nitric acid for spectrochemical analysis.The method of standard additions was used for quantitative elemental analysis of the digested samples. RESULTS AND DISCUSSION Sensitivity and Sample Thickness Typical ICP emission intensity for back- and front-surface laser ablation of a paint sample is shown in Fig. 3. The emission is transient for single pulse ablation. The peak shape and peak width are related to the volume of the ablation chamber, the length and diameter of the transfer tube, and the Fig. 5 ICP emission intensities of (a) Fe II 238.2 nm and (b) Mg I Fig. 3 Temporal ICP emission intensity of Fe II 238.2 nm for back- 285.2 nm versus paint sample thickness using back-surface ablation. Laser power density, approximately 5×107 Wcm-2. and front-surface ablation of a paint sample. Integration time, 0.3 s. Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 9Table 4 Oxide melting-points of Al, Ca, Cu, Fe, Mg, Mn and Sr25 emission intensity first increases and then reduces with sample thickness.Samples of all thicknesses used in Fig. 5 were Oxide melting-point/ completely removed by a single laser pulse during back-surface Element Oxide °C ablation. The initial increase in ICP intensity is probably Cu CuO 1326 related to the amount of sample ablated. Since back-surface Fe Fe2O3 1565 ablation removes all sample material at the laser spot, a thicker Mn Mn3O4 1564 sample means a larger sample mass and thus an increase in Al Al2O3 2072 ICP intensity.However, as the sample thickness increases, the Sr SrO 2430 Ca CaO 2614 efficiency of sample fragmentation into fine particles reduces. Mg MgO 2852 Larger amounts of paint fragments were found at the bottom of the laser sampling chamber as the sample thickness increased. Thin samples are probably more efficiently fragmented into fine particles than thick samples. Since large particles are not transported to the ICP efficiently,22 ICP strated.Stoichiometric sampling is possible only if critical or emission intensity decreases even when the mass ablated supercritical points are attained.26 However, if melting occurs increases with sample thickness. A sample thickness of 35 mm at the laser spot, the vapour pressure of low-melting oxides was used throughout this work. should be higher because of a larger difference between the melting-point and the molten oxide temperature.Differential vaporization occurs during front-surface ablation, leading to Preferential Vaporization enrichment of lower melting/boiling copper oxide (mp 1326 °C) With the optimum sample thickness of 35 mm, the ICP emission and depletion of high melting/boiling magnesium oxide (mp ratios of back-surface ablation to front-surface ablation range 2852°C) in the gas phase. The ICP intensity ratio therefore from 1 to 6 (Table 3), i.e., there is an enhancement of ICP reduces as the oxide melting-point increases (Fig. 6). In con- emission of up to 6-fold. The enhancement is smaller than that trast, preferential vaporization is minimal during back-surface of sampling efficiency (#10-fold, Fig. 4), probably because of laser ablation as all sample materials at the laser spot are the transport efficiency of the particles. However, the enhance- removed by a single laser shot. ment of ICP emission is also element-dependent, which may be related to preferential (thermal) vaporization of volatile elements during laser sampling.Preferential vaporization Precision during laser sampling leads to enrichment of volatile elements The typical RSD for single-pulse laser sampling reported in and depletion of refractory elements in the gas phase.14–19 the literature is 30–70%,1–4 which is mainly due to sample The extent of preferential vaporization during conventional heterogeneity and pulse-to-pulse variation in laser energy. front-surface laser sampling can be shown by the ICP-AES Laser power density is a major factor influencing the amount intensity ratios of front-surface ablation to back-surface of laser-sampled materials.27,28 In this work, the RSD for ablation for elements with a large difference in the melting- single-pulse front-surface ablation is 10–23%; it is reduced to point of their oxides (Fig. 6). Oxides are considered here 2–14% when Ti is used as an internal standard (Table 5). because the elements do not exist as metals in paint but mainly The RSD of the ICP-AES intensity for back-surface ablation as oxides.24 Oxides with melting-points25 ranging from 1326 is 9–16% (Table 5).The fluctuation represents the variation to 2852°C (Table 4) were studied. With such a wide range of in sample film thickness and laser pulse energy. Consistent melting-points, the effects of thermal vaporization of the molten and uniform thickness of the film from sample to sample is oxides at the laser spot during laser sampling can be demon- difficult to achieve using our sample-coating method.However, the variation in sample thickness can be compensated using Table 3 Enhancement of ICP emission intensity using back-surface an internal standard. The RSD of the ICP intensity is 2–10% ablation versus front-surface ablation with Ti as the internal standard (Table 5). Using an internal standard, the thickness of the film need not be strictly con- Intensity ratio, trolled and the preparation of sample films becomes simple Element back-surface ablation: front-surface ablation and straightforward. Cu I 1.0 Fe II 1.6 Si I 2.0 Al I 1.6 Sr II 2.8 Table 5 Precision (RSD %) of back- and front-surface ablation Ca II 5.7 Mg I 4.7 RSD (%) Back-surface Front-surface Back-surface Front-surface ablation ablation ablation ablation (without (without (with (with internal internal internal internal Element standard) standard) standard) standard) Ba 12 10 4 5 Co 13 14 3 3 Cr 11 11 4 5 Cu 13 11 5 3 Fe 11 14 2 3 Mg 9 23 8 5 Pb 15 16 9 10 Si 12 14 2 6 Sn 10 13 5 5 Sr 10 19 3 2 Fig. 6 ICP intensity ratios of front-surface to back-surface ablation Zn 16 21 10 12 versus oxide melting-points. 10 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12Fig. 7 Typical standard additions calibration graphs for (a) backsurface ablation and (b) front-surface ablation of a blue paint sample. Ti was used as an internal standard. Semi-quantitative Analysis of Paint Samples Using Standard Additions Method Fig. 8 Correlation of the elemental concentrations of Ba, Co, Cr, Cu, Paint samples were analysed using both back-surface and Fe, Mg, Mn, Pb, Sn, Sr, V and Zn in blue and red paints determined by the standard additions method using (a) back-surface ablation and front-surface laser sampling–ICP-AES. For comparison, the (b) front-surface ablation with those of the microwave digestion/ same samples were analysed using ICP-AES with microwave solution nebulization method.digestion/solution nebulization. Standard additions was used to compensate for matrix effects for all analyses. Ti was used as the internal standard to compensate for laser pulse energy dards are needed to ensure accurate analysis.29 However, fluctuation and sample thickness variation. Typical calibration matrix-matched solid standards are usually not available and graphs for laser sampling–ICP-AES are shown in Fig. 7. preparation of the standards can be tedious and time consum- Elemental concentrations of the paint samples determined ing.The standard additions method is applicable only for by back-surface and front-surface ablation–ICP-AES are com- certain samples and is also tedious. A laser sampling technique pared with those of microwave digestion/solution nebulization that does not require matrix-matched standards for calibration in Fig. 8. Good correlation between back-surface ablation and would, overall, simplify the analysis. microwave digestion at concentrations above 20 ppm for up Since sample matrix effects such as preferential vaporization to two orders of magnitude is observed [Fig. 8(a)]. The large are not significant in back-surface ablation, the use of non- deviation for elements of low concentration (<10 ppm) is matrix-matched standards for calibration seems feasible. probably due to the small amount of the elements present in Standards in a PVA matrix were prepared for the analysis of the sample.The mass of the paint material removed per laser paints using both front- and back-surface laser sampling. TiO2 pulse using back-surface ablation can be estimated from the was added to the PVA solution to enhance UV laser pulse laser spot size and the amount of sample added to the substrate. absorption by the PVA film and thus the laser sampling About 5 mg are removed by each laser pulse. If the elemental efficiency. Ni was used as an internal standard to compensate concentration is 10 ppm, the mass of metal sampled is #50 ng.for any film thickness variation. The sampling error for such a small amount of material may Calibration graphs using standards in a PVA matrix are be a major limiting factor for quantitative analysis of trace shown in Fig. 9. Again, the data of laser ablation using PVA elements. calibration were compared with those obtained by a microwave The difference in elemental concentration determined by digestion method (Fig. 10). There is a good correlation between front-surface ablation and microwave digestion is larger back-surface ablation and microwave digestion for major [Fig. 8(b)], probably due to preferential vaporization during elements [Fig. 10(a)]. On the other hand, the values obtained laser sampling. There is also a larger scattering of the data by front-surface ablation using PVA calibration and microwave points because of the smaller emission intensity and thus digestion do not agree so well [Fig. 10(b)]. Matrix-matched smaller S/N ratios for front-surface laser sampling. standards are needed for front-surface ablation calibration. Semi-quantitative Analysis of Paint Samples Using Standards in CONCLUSIONS PVA Matrix Back-surface ablation is potentially an effective laser sampling In laser sampling analysis, matrix effects strongly influence the amount of the laser-sampled material. Matrix-matched stan- method for coating materials, polymers and biological samples. Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 11The sensitivity and accuracy are improved compared with the conventional front-surface laser ablation method. Preferential vaporization is minimized as the sample at the laser spot is completely removed. Semi-quantitative analyses using the standard additions method and calibration using non-matrixmatched standards in PVA have been demonstrated. Elements of concentration from 1000 to 10 ppm were determined and the data correlate well with those of the microwave digestion/ solution nebulization method.Successful calibration using standards in a PVA matrix suggests that universal standards for a wide variety of sample matrices may be possible when back-surface ablation is used. Financial support from the Department of Chemistry and CRGC research grants of the University of Hong Kong is gratefully acknowledged. REFERENCES 1 Dittrich, K., and Wennrich, R., Prog.Anal. At. Spectrosc., 1984, 7, 139. 2 Darke, S. A., and Tyson, J. F., J. Anal. At. Spectrom., 1993, 8, 145. 3 Monenke-Blankenburg, L., Spectrochim. Acta Rev., 1993, 15, 1. 4 Russo, R. E., Appl. Spectrosc., 1995, 49, 14A. Fig. 9 Typical calibration graphs of standards in a PVA matrix (1% 5 Thompson, M., Goulter, J. E., and Sieper, F., Analyst, 1981, PVA+1% TiO2+standards) using (a) back-surface ablation and 106, 32. (b) front-surface ablation. Ni was used as an internal standard. 6 Carr, J. W., and Horlick, G., Spectrochim. Acta, Part B, 1982, 37, 1. 7 Ishizuka, T., and Uwamino, Y., Spectrochim. Acta, Part B, 1983, 38, 519. 8 Gray, A. L., Analyst, 1985, 110, 551. 9 Richner, P., Borer, M. W., Brushwyler, K. R., and Hieftje, G. M., Appl. Spectrosc., 1990, 44, 1290. 10 Pang, H. P., Wiederin, D. R., Houk, R. S., and Yeung, E. S., Anal. Chem., 1991, 63, 390. 11 Denoyer, E. R., Fredeen, K. J., and Hager, J. W., Anal. Chem., 1991, 63, 445A. 12 D’Silva, A. P., Baldwin, D.P., and Zamzow, D. S., Anal. Chem., 1994, 66, 1911. 13 Russo, R. E., Mao, X. L., Chan, W. T., Bryant, M. F., and Kinard, W. F., J. Anal. At. Spectrom., 1995, 10, 295. 14 Arrowsmith, P., Anal. Chem., 1987, 59, 1437. 15 Hager, J., Anal. Chem., 1989, 61, 1243. 16 Thompson, M., Chenery, S., and Brett, L., J. Anal. At. Spectrom., 1989, 4, 11. 17 Chan, W. T., and Russo, R. E., Spectrochim. Acta, Part B, 1991, 46, 1471. 18 Chan, W. T., and Russo, R. E., Appl. Spectrosc., 1992, 46, 1025. 19 Cromwell, E. F., and Arrowsmith, P., Anal. Chem., 1995, 67, 131. 20 Mao, X. L., Chan, W. T., Shannon, M. A., and Russo, R. E., J. Appl. Phys., 1993, 74, 4915. 21 Fletcher, T. R., J. Appl. Phys., 1993, 73, 5292. 22 Arrowsmith, P., and Hughes, S. K., Appl. Spectrosc., 1988, 42, 1231. 23 Paudyn, A. M., and Smith, R. G., Fresenius’ J. Anal. Chem., 1993, 345, 695. 24 Parsons, P., Surface Coatings, Chapman and Hall, London, 1993, vol. 1. 25 CRC Handbook of Chemistry and Physics, eds. Weast, C., Astle, M. J., and Beyer, W. H., CRC Press, Boca Raton, FL, 64th edn., 1984. 26 Ready, J. F., Effects of High-Power L aser Radiation, Academic Press, New York, 1971. 27 Imai, N., Anal. Chim. Acta, 1990, 235, 381. 28 McLeod, C. W., and Booth P. K., Mikrochim. Acta, 1990, 3, 283. 29 Darke, S. A., Long, S. E., Pickford, C. J., and Tyson, J. F., J. Anal. At. Spectrom., 1989, 4, 715. Paper 6/02822E Fig. 10 Correlation of the elemental concentrations of Ba, Co, Cr, Received April 23, 1996 Cu, Fe, Mg, Mn, Pb, Sn, Sr, V and Zn in blue and red paints Accepted August 27, 1996 determined by PVA calibration using (a) back-surface ablation and (b) front-surface ablation with those of the microwave digestion/ solution nebulization method. Standards in a PVA matrix were used for calibration. 12 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12

ISSN:0267-9477    

DOI:10.1039/a602822e

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Comparison of Two Objective Functions for Optimization of Simultaneous Multi-element Determinations in Inductively Coupled Plasma Spectrometry CHRISTINE SARTOROS AND ERIC D. SALIN* Department of Chemistry, McGill University,Montreal, Quebec, Canada H3A 2K6 Two objective functions for multi-element optimization in ICP- Leary et al.6 developed and tested several objective functions AES were compared using signal-to-background ratios as a based on SBRs, the best one being represented by the sum of figure of merit.Complete three-dimensional response surfaces the reciprocals of the SBRs for the elements studied: were generated for a number of elements (Ca, Cu, Al, Na, Ni, Mn and Ba) and two artificial ‘elements’ to evaluate the F= n .n i=1 (S/B)i-1 performance of both objective functions in locating the optimum compromise instrumental operating conditions in multi-element determinations. In the determination of the best where n is the number of elements studied for the optimization compromise instrument operating conditions for most and (S/B)i are the SBRs of each of the ith elements. This combinations of the elements used, both objective functions equation generates a value for each set of instrumental performed equally well; however, one occasionally performed operating conditions using the SBRs of all the elements.Ebdon significantly better than the other. and Carpenter8 used a modified version of Leary’s objective function in their study.Kalivas9 also used Leary’s objective Keywords: Multi-element optimization; inductively coupled function in the optimization of operating conditions for mini- plasma atomic emission spectrometry ; objective functions mal interferences. Instead of SBRs, Kalivas used selectivity, sensitivity and accuracy, as derived by Lorber,13 as response Optimization of instrumental operating conditions may improve functions. Moore et al.10 also used Leary’s objective function analytical accuracy and precision.The instrumental operating with SBRs in addition to ionization interference as the figure conditions of an ICP that may be optimized are rf power, flow of merits. Belchamber et al.11 developed an objective function rates of gases (the outer or coolant, intermediate or plasma and based on a measure of the magnitude of the matrix effects injector gas flow rates), observation region in the plasma and such as to minimize or remove these matrix effects. solution pump rate to the nebulizer.1 The primary optimization We tested another approach towards satisfying the two techniques that have been used with ICPs are simplex2–11 and the conditions required for obtaining optimum compromise Davidon–Fletcher–Powell12 algorithm.Traditionally the response operating conditions: (1) obtaining the maximum compromise functions used have been signal-to-background ratios (SBRs), SBRs for all elements and (2) emphasizing the maximization signal-to-noise ratios (SNRs), precision and accuracy.Thomas of the SBR of the elements close to their detection limits. This and Collins12 also used detection limits as the response function. objective function is called the combined ratio method (CRM) Any of these response functions maybe directlyusedfor determinand is given by: ing optimum instrument operating conditions for single-element analysis.2–5 Signal-to-background ratios are easily obtained and require the fewest measurements of the response functions listed above.The SBR is also a good figure of merit since it can be CRM= .n i=1 (S/B)i .k j=1 Rj correlated to detection limits. Therefore, SBRs are used throughout this work as calculated using the following equation: where n is the number of elements, k is (n-1)+(n-2)+...+1, S/B=total signal-background background (S/B)i are the SBRs of each of the ith elements and Rj is the ratio of the SBRs of two given elements ( jth combination) where where S/B is the SBR.the maximum SBR of the two is in the numerator such that The difficulty arises in simultaneous multi-element analysis Rj1. For example, given the following SBRs of three elements because optimization techniques generally require a single Zn, Na and K, the values for R1, R2 and R3, would be: value representing each set of operating conditions but, using (S/B)Zn=1; (S/B)Na=2; (S/B)K=10 any of the response functions mentioned above, multiple values (one for each element) are obtained for each set of conditions.R1=(S/B)K/(S/B)Zn=10/1=10 Galley et al.,5 in their automated simplex optimization of R2=(S/B)K/(S/B)Na=10/2=5 multi-element solutions, used the following choices for optimization criteria: (1) the maximization of the net signal or signal- R3=(S/B)Na/(S/B)Zn=2/1=2 to-background noise ratio, (2) the minimization of the relative and the CRM would be calculated as: standard deviation of the background and (3) the maximization of the ratio of atomic or ionic lines.An objective function, CRM=(S/B)Zn+(S/B)Na+(S/B)K R1+R2+R3 which by definition would result in a single value for a set of operating conditions, is needed. It should comprise the SBRs of all the elements studied with emphasis on the elements =1+2+10 10+5+2=13 17=0.76 closer to the detection limits. Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 (13–19) 13The CRM performs a weighted average on the sum of the SBRs and maximizes the individual SBRs while minimizing the difference among these ratios (i.e., minimizing .Rj ). In this work, we performed a comparison of Leary’s objective function and the CRM by examining the response surfaces generated by each function and evaluating the performance of each function in determining the optimum instrument operating conditions. EXPERIMENTAL The instrument used was a Thermo Jarrell Ash (Franklin, MA, USA) Model 25 sequential scanning spectrometer.The instrument functionsare all automated and controlled by an independent computer via an RS-232 port. The solution pump rate to the nebulizer was 0.9 ml min-1. A signal integration time of Fig. 1 Surface of SBRs of Al. 1.0 s was used. The background for each line was selected at 0.05 nm on both sides of each spectral line peak. An average of the background was used for all readings. All operating conditions were held constant except for observation height and rf power.The observation height was varied from 3 to 24 mm above the top of the load coil (ATOLC) in steps of 3.0 mm and the rf power was varied from 750 to 1550 W in steps of 200 W. All these combinations generated a response surface which characterized the parameter space. While any combination of rf power, observation region in the plasma, flow rates of gases and solution pump rate to the nebulizer could be optimized, we chose to vary only two of these parameters for simplicity of the graphical representation of the response surfaces.A stock standard solution was prepared from Fisher (Pittsburgh, PA) certified sodium, calcium, copper, aluminum, nickel, manganese and barium 1000 ppm standard solutions Fig. 2 Surface of SBRs of Ca. and the concentrations and spectral lines of these seven elements studied are listed in Table 1 along with their ionization potentials. These elements have both hard and soft lines. The SBRs were determined for all seven elements at each set of instrumental operating conditions.Based on these SBRs, a response surface was generated for each element. Using these seven response surfaces for the seven elements, two other surfaces were generated, one using Leary’s objective function and the other using the CRM. The optimum operating conditions were determined from these latter surfaces and used in the comparison of the two objective functions. In addition, theoretical models were used to compare the two objective functions further.RESULTS AND DISCUSSION Three-dimensional response surfaces were obtained for each Fig. 3 Surface of SBRs of Cu. element by plotting the SBRs of each element against the observation heights and rf powers (Figs. 2–8). The SBRs of each element at each set of operating conditions are listed in Tables 2–8. The optimum instrumental operating conditions for each element are listed in Table 9. Low power seems to be the best choice for these elements at their present concentrations.This is expected since an increase in power increases Table 1 Elements used Concentration Wavelength/ Ionization potential/ Element (ppm) nm eV14 Al I 10 309.28 5.99 Ca II 10 317.93 11.87 Cu I 10 324.75 7.73 Ba II 2 455.40 10.00 Ni I 10 232.00 7.64 Mn II 2 257.61 15.64 Na I 10 589.59 5.14 Fig. 4 Surface of SBRs of Ba. 14 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12Table 2 SBRs of aluminum Rf power/W Observation height/mm 750 950 1150 1350 1550 3 0.66 0.75 0.57 0.38 0.34 6 1.68 1.03 0.81 0.54 0.40 9 4.10 2.01 1.48 0.78 0.56 12 8.18 4.14 2.77 1.57 1.07 15 13.02 7.09 4.67 2.69 1.97 18 11.96 7.99 7.42 3.86 2.84 21 9.83 10.57 9.89 6.00 3.26 24 10.09 10.50 10.30 8.47 7.05 Table 3 SBRs of calcium Fig. 5 Surface of SBRs of Ni. Rf power/W Observation height/mm 750 950 1150 1350 1550 3 0.59 0.75 0.75 0.75 0.61 6 1.55 1.76 1.68 1.36 1.12 9 4.01 3.59 2.99 2.15 1.70 12 7.66 5.89 4.92 3.40 2.66 15 7.81 7.44 6.78 4.80 3.93 18 5.30 6.92 6.55 6.31 5.21 21 2.54 4.61 5.73 5.62 5.65 24 1.19 1.81 3.08 3.85 4.78 Table 4 SBRs of copper Rf power/W Observation height/mm 750 950 1150 1350 1550 3 4.05 2.46 1.78 1.18 0.92 6 6.92 3.87 2.99 1.79 1.32 Fig. 6 Surface of SBRs of Mn. 9 14.02 7.65 4.93 2.73 1.76 12 25.38 13.73 9.41 5.13 3.32 15 33.97 21.41 16.19 8.72 6.06 18 35.04 26.98 22.53 14.28 9.80 21 33.95 32.95 30.77 23.27 16.28 24 32.83 35.57 38.44 31.99 28.40 Table 5 SBRs of barium Rf power/W Observation height/mm 750 950 1150 1350 1550 3 0.39 0.29 0.27 0.16 0.13 6 0.85 0.55 0.42 0.26 0.19 9 1.50 0.73 0.52 0.24 0.15 12 3.94 1.61 1.09 0.49 0.32 15 7.65 3.75 2.30 1.05 0.77 18 10.32 5.87 4.20 2.18 1.38 Fig. 7 Surface of SBRs of Na. 21 9.16 8.64 6.64 3.72 2.57 24 6.35 8.23 7.70 5.68 4.73 Table 6 SBRs of nickel Rf power/W Observation height/mm 750 950 1150 1350 1550 3 1.28 1.15 0.73 0.67 0.47 6 2.38 2.23 1.55 1.20 0.93 9 6.64 5.51 4.08 2.11 1.54 12 12.61 9.55 6.77 3.84 2.22 15 14.48 12.05 9.03 6.56 4.12 18 8.45 10.55 10.05 6.14 3.83 21 3.23 5.65 5.83 4.99 2.21 24 2.50 2.25 2.35 2.99 3.78 Fig. 8 Surface obtained using the CRM method. Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 15Table 7 SBRs of manganese Table 10 Best compromise conditions for all the combinations of the elements Rf power/W Observation CRM Leary height/mm 750 950 1150 1350 1550 Power/ Height/ Power/ Height/ 3 5.47 6.17 5.60 4.60 3.26 Line Combination W mm W mm 6 13.38 12.19 9.95 8.34 6.56 9 26.31 24.57 19.61 12.66 8.70 1 Al–Ca 750 12 750 15 2 Al–Cu 750 15 750 15 12 41.05 32.97 28.80 19.58 14.98 15 36.87 39.03 36.83 26.53 20.32 3 Al–Ba 750 18 750 18 4 Al–Ni 750 15 750 15 18 27.46 35.66 36.65 32.85 26.49 21 14.08 22.28 22.34 22.73 18.74 5 Al–Mn 1150 24 750 15 6 Al–Na 750 15 750 18 24 4.44 6.63 10.57 9.51 11.21 7 Ca–Cu 950 15 750 15 8 Ca–Ba 750 15 750 15 9 Ca–Ni 750 12 750 15 Table 8 SBRs of sodium 10 Ca–Mn 750 15 750 12 11 Ca–Na 950 15 750 15 Rf power/W 12 Cu–Ba 750 18 750 18 Observation 13 Cu–Ni 750 15 750 15 height/mm 750 950 1150 1350 1550 14 Cu–Mn 750 15 750 15 3 3.20 1.69 1.27 0.75 0.58 15 Cu–Na 950 24 950 24 6 3.90 2.05 1.37 0.81 0.64 16 Ba–Ni 750 18 750 15 9 5.10 2.45 1.60 0.94 0.63 17 Ba–Mn 750 21 750 18 12 9.50 4.54 2.94 1.60 1.21 18 Ba–Na 750 18 750 18 15 14.24 7.59 5.31 3.08 2.59 19 Ni–Mn 750 15 750 15 18 15.98 12.37 8.65 5.32 4.09 20 Ni–Na 750 15 750 15 21 14.87 14.26 11.97 8.82 7.25 21 Mn–Na 750 21 750 15 24 16.17 17.73 15.81 11.75 10.84 22 Al–Ca–Cu 750 15 750 15 23 Al–Ca–Ba 750 15 750 15 24 Al–Ca–Ni 750 15 750 15 25 Al–Ca–Mn 750 15 750 15 Table 9 Optimum operating conditions for each element 26 Al–Ca–Na 750 15 750 15 27 Al–Cu–Ba 750 18 750 15 Observation 28 Al–Cu–Ni 750 15 750 15 Element Power/W height/mm 29 Al–Cu–Mn 750 15 750 15 Al 750 15 30 Al–Cu–Na 750 15 750 18 Ca 750 15 31 Al–Ba–Ni 750 18 750 15 Cu 1150 24 32 Al–Ba–Mn 750 21 750 18 Ba 750 18 33 Al–Ba–Na 750 18 750 18 Ni 750 15 34 Al–Ni–Mn 750 15 750 15 Mn 750 12 35 Al–Ni–Na 750 15 750 15 Na 950 24 36 Al–Mn–Na 750 18 750 15 37 Ca–Cu–Ba 750 15 750 15 38 Ca–Cu–Ni 750 15 750 15 39 Ca–Cu–Mn 750 15 750 15 the background more than the signal with a subsequent 40 Ca–Cu–Na 750 15 750 15 decrease in SBR.1 For example, looking at the combination of 41 Ca–Ba–Ni 750 15 750 15 manganese and sodium, the optimum operating conditions for 42 Ca–Ba–Mn 750 15 750 15 manganese are 750W rf power and 12 mm ATOLC, whereas 43 Ca–Ba–Na 750 15 750 15 those for sodium are 950 W rf power and 21 mm ATOLC. 44 Ca–Ni–Mn 750 15 750 15 45 Ca–Ni–Na 750 15 750 15 However, using either of these operating conditions in the 46 Ca–Mn–Na 750 15 750 15 simultaneous determination of these two elements would pro- 47 Cu–Ba–Ni 750 15 750 15 duce poor results for one of them. 48 Cu–Ba–Mn 750 18 750 18 Using the data in Tables 2–8, the optimum compromise 49 Cu–Ba–Na 750 18 750 18 instrumental operating conditions were determined for all 50 Cu–Ni–Mn 750 15 750 15 combinations (Table 10) of the seven elements studied by 51 Cu–Ni–Na 750 15 750 15 52 Cu–Mn–Na 750 18 750 15 applying Leary’s objective function and the CRM.The appli- 53 Ba–Ni–Mn 750 18 750 15 cation of these two objective functions to any combination of 54 Ba–Ni–Na 750 15 750 15 the elements for all sets of operating conditions produces two 55 Ba–Mn–Na 750 18 750 18 response surfaces (one for Leary’s objective function and the 56 Ni–Mn–Na 750 15 750 15 other for the CRM).For example, in the optimization of 57 Al–Ca–Cu–Ba 750 15 750 15 operating conditions over all seven elements, the resulting 58 Al–Ca–Cu–Ni 750 15 750 15 59 Al–Ca–Cu–Mn 750 15 750 15 surfaces are depicted in Figs. 8 and 9. The maximum point on 60 Al–Ca–Cu–Na 750 15 750 15 a surface indicates the best compromise operating conditions 61 Al–Ca–Ba–Ni 750 15 750 15 given by each method. Considering the combination of these 62 Al–Ca–Ba–Mn 750 15 750 15 seven elements, the resulting surfaces are similar to each other 63 Al–Ca–Ba–Na 750 15 750 15 and give the same set of operating conditions as the optimum 64 Al–Ca–Ni–Mn 750 15 750 15 compromise. 65 Al–Ca–Ni–Na 750 15 750 15 66 Al–Ca–Mn–Na 750 15 750 15 The optimum compromise settings were determined for each 67 Al–Cu–Ba–Ni 750 15 750 15 combination of the elements using both objective functions 68 Al–Cu–Ba–Mn 750 18 750 18 and are listed in Table 10.In many cases, both objective 69 Al–Cu–Ba–Na 750 18 750 18 functions give the same set of instrumental operating conditions 70 Al–Cu–Ni–Mn 750 15 750 15 as the best compromise.In the cases where they give different 71 Al–Cu–Ni–Na 750 15 750 15 operating conditions (Table 11), the CRM puts a greater 72 Al–Cu–Mn–Na 750 15 750 15 73 Al–Ba–Ni–Mn 750 18 750 15 emphasis on decreasing the difference between the SBRs (i.e., 16 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12Table 10 (continued) Table 11 SBR of elements for the best compromise conditions obtained using the two methods CRM Leary Line Method SBR of elements Power/ Height/ Power/ Height/ 1 Al Ca Line Combination W mm W mm CRM 8.18 7.66 Leary 13.02 7.81 74 Al–Ba–Ni–Na 750 15 750 15 75 Al–Ba–Mn–Na 750 18 750 18 5 Al Mn CRM 10.30 10.57 76 Al–Ni–Mn–Na 750 15 750 15 77 Ca–Cu–Ba–Ni 750 15 750 15 Leary 13.02 36.87 6 Al Na 78 Ca–Cu–Ba–Mn 750 15 750 15 79 Ca–Cu–Ba–Na 750 15 750 15 CRM 13.02 14.24 Leary 11.96 15.98 80 Ca–Cu–Ni–Mn 750 15 750 15 81 Ca–Cu–Ni–Na 750 15 750 15 7 Ca Cu CRM 7.44 21.41 82 Ca–Cu–Mn–Na 750 15 750 15 83 Ca–Ba–Ni–Mn 750 15 750 15 Leary 7.81 33.97 9 Ca Ni 84 Ca–Ba–Ni–Na 750 15 750 15 85 Ca–Ba–Mn–Na 750 15 750 15 CRM 7.66 12.61 Leary 7.81 14.48 86 Ca–Ni–Mn–Na 750 15 750 15 87 Cu–Ba–Ni–Mn 750 15 750 15 10 Ca Mn CRM 7.81 36.87 88 Cu–Ba–Ni–Na 750 15 750 15 89 Cu–Ba–Mn–Na 750 18 750 18 Leary 7.66 41.05 11 Ca Na 90 Cu–Ni–Mn–Na 750 15 750 15 91 Ba–Ni–Mn–Na 750 18 750 15 CRM 7.44 7.59 Leary 7.81 14.24 92 Al–Ca–Cu–Ba–Ni 750 15 750 15 93 Al–Ca–Cu–Ba–Mn 750 15 750 15 16 Ba Ni CRM 10.32 8.45 94 Al–Ca–Cu–Ba–Na 750 15 750 15 95 Al–Ca–Cu–Ni–Mn 750 15 750 15 Leary 7.65 14.48 17 Ba Mn 96 Al–Ca–Cu–Ni–Na 750 15 750 15 97 Al–Ca–Cu–Mn–Na 750 15 750 15 CRM 9.16 14.08 Leary 10.32 27.46 98 Al–Ca–Ba–Ni–Mn 750 15 750 15 99 Al–Ca–Ba–Ni–Na 750 15 750 15 21 Mn Na CRM 14.08 14.87 100 Al–Ca–Ba–Mn–Na 750 15 750 15 101 Al–Ca–Ni–Mn–Na 750 15 750 15 Leary 36.87 14.24 27 Al Cu Ba 102 Al–Cu–Ba–Ni–Mn 750 15 750 15 103 Al–Cu–Ba–Ni–Na 750 15 750 15 CRM 11.96 35.04 10.32 Leary 13.02 33.97 7.65 104 Al–Cu–Ba–Mn–Na 750 18 750 18 105 Al–Cu–Ni–Mn–Na 750 15 750 15 30 Al Cu Na CRM 13.02 33.97 14.24 106 Al–Ba–Ni–Mn–Na 750 15 750 15 107 Ca–Cu–Ba–Ni–Mn 750 15 750 15 Leary 11.96 35.04 15.98 31 Al Ba Ni 108 Ca–Cu–Ba–Ni–Na 750 15 750 15 109 Ca–Cu–Ba–Mn–Na 750 15 750 15 CRM 11.96 10.32 8.45 Leary 13.02 7.65 14.48 110 Ca–Cu–Ni–Mn–Na 750 15 750 15 111 Ca–Ba–Ni–Mn–Na 750 15 750 15 32 Al Ba Mn CRM 9.83 9.16 14.08 112 Cu–Ba–Ni–Mn–Na 750 15 750 15 113 Al–Ca–Cu–Ba–Ni–Mn 750 15 750 15 Leary 11.96 10.32 27.46 36 Al Mn Na 114 Al–Ca–Cu–Ba–Ni–Na 750 15 750 15 115 Al–Ca–Cu–Ba–Mn–Na 750 15 750 15 CRM 11.96 27.46 15.98 Leary 13.02 36.87 14.24 116 Al–Ca–Cu–Ni–Mn–Na 750 15 750 15 117 Al–Ca–Ba–Ni–Mn–Na 750 15 750 15 52 Cu Mn Na CRM 35.04 27.46 15.98 118 Al–Cu–Ba–Ni–Mn–Na 750 15 750 15 119 Ca–Cu–Ba–Ni–Mn–Na 750 15 750 15 Leary 33.97 36.87 14.24 53 Ba Ni Mn 120 All 750 15 750 15 CRM 10.32 8.45 27.46 Leary 7.65 14.48 36.87 73 Al Ba Ni Mn CRM 11.96 10.32 8.45 27.46 Leary 13.02 7.65 14.48 36.87 better compromise operating conditions over the other for all seven elements.When fewer element responses are combined, such as for the combination of Al, Ba and Mn (line 32 in Tables 10 and 11), Leary’s objective function provides better compromise operating conditions than the CRM since the CRM puts more emphasis on minimizing the difference between the SBRs of these three elements.However, for other combinations, such as that of Al, Ba and Ni (line 31 in Tables 10 and 11), it is a question of which is of greater importance, Fig. 9 Surface obtained using Leary’s objective function. maximizing the smallest SBR obtained (i.e., minimizing the difference between the SBRs of the elements) (CRM) or maximizing the total of the SBRs (Leary’s objective function). When decreasing Rj ). The similarities between the surface produced with Leary’s objective function (Fig. 8) and that produced with considering SBRs which can vary widely, these data suggest that the Leary approach is better, as significant improvements the CRM (Fig. 9) indicate that neither function would provide a better surface over the other for use with simplex optimization are sometimes found with relatively small losses compared with the CRM approach. techniques or the Davidon–Fletcher–Powell algorithm. It is also difficult to tell whether one of the two functions provides Given that the response surfaces were relatively similar for Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 17the various elements, several artificial ‘elemental’ surfaces were model (Fig. 11) is similar to the first except that the SBRs of the two elements at the maximum peak height are completely generated to compare the two approaches. Two of these models depicting extreme situations are presented (Figs. 10 and 11). different. Again, the surfaces obtained using the CRM and Leary’s objective function are very similar. All models gener- The first model (Fig. 10) illustrates SBR surfaces of two elements that peak under completely different instrumental ated gave the same operating conditions or produced the same situation described earlier where Leary’s objective function operating conditions but with approximately the same SBR at the top of the peak. The surfaces obtained using both the performed better since the CRM decreased the difference between the SBRs.CRM approach and Leary’s objective function are very similar, peaking under the same operating conditions. The second Signal-to-background ratios are convenient to use for Fig. 10 Theoretical model of two elements with similar SBRs. Fig. 11 Theoretical model of two elements with dissimilar SBRs. 18 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12optimization since fewer measurements are required compared acknowledges financial support from the Fonds pour la Formation de Chercheurs et l’Aide a` la Recherche.with SNRs and they often are easily related to detection limits. While one expects SBRs to vary widely, SNRs should be relative similar given concentrations well above the detection REFERENCES limit. In this case the CRM may be advantageous. With many 1 Inductively Coupled Plasma Emission Spectroscopy. Part I: ICP-MS instruments one tends to adjust a variety of operating Methodology, Instrumentation, and Performance, ed.Boumans, parameters (e.g., lens settings) to obtain a roughly uniform P. W. J. M., Wiley-Interscience, New York, 1987, pt. 1, ch. 4. pp. 100–257. sensitivity for all elements. Because of its tendency to promote 2 Ebdon, L., Cave, M. R., and Mowthorpe, D. J., Anal. Chim. Acta, uniformity of performance, the CRM may be more advanta- 1980, 115, 179. geous when used with a technique such as ICP-MS. 3 Norman, P., and Ebdon, L., Anal.Proc., 1986, 23, 420. Both objective functions, Leary’s and the CRM, could easily 4 Werner, P., and Friege, H., Appl. Spectrosc., 1987, 41, 32. use SNRs, accuracy or any of the other figure of merits 5 Galley, P. J., Horner, J. A., and Hieftje, G. M., Spectrochim. Acta, Part B, 1995, 50, 87. mentioned previously instead of SBRs in the computation of 6 Leary, J. J., Brokes, A. E., Dorrzapf, A. F., Jr., and Golightly, the objective function values. They could also be applied to D. W., Appl. Spectrosc., 1982, 36, 37. optimization of the other instrument parameters (e.g., gas flow 7 Terblanche, S. P., Visser, K., and Zeeman, P. B., Spectrochim. rates, solution pump rate to the nebulizer) using any group of Acta, Part B, 1981, 36, 293. 8 Ebdon, L., and Carpenter, R. C., Anal. Chim. Acta, 1987, 200, 551. analyte elements. With the evolution of instruments, most 9 Kalivas, J. H., Appl. Spectrosc., 1987, 41, 1338. instruments perform simultaneous multi-element analysis rap- 10 Moore, G. L., Humphries-Cuff, P. J., and Watson, A. E., idly and are completely computer controlled. The use of these Spectrochim. Acta, Part B, 1984, 39, 915. objective functions would be ideal in the optimization of these 11 Belchamber, R. M., Betteridge, D., Wade, A. P., Cruickshank, instruments since the information for the optimization is A. J., and Davison, P., Spectrochim. Acta, Part B, 1986, 41, 503. 12 Thomas, R. J., and Collins, J. B., Spectroscopy, 1990, 5, 38. readily available and the computations involved are trivial 13 Lorber, A., Anal. Chem., 1986, 54, 989. relative to the computational power of modern computers. 14 CRC Handbook of Chemistry and Physics, ed. Lide, D. R., CRC Press, New York, 76th edn., 1995–96, pp. 10–207 and 10–208. The authors gratefully acknowledge financial support from the Paper 6/06319E Received September 13, 1996 National Sciences and Engineering Council of Canada. C.S. Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 19

ISSN:0267-9477    

DOI:10.1039/a606319e

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Application of Inductively Coupled Plasma Atomic Emission Spectrometry in Forensic Science MARKO LALCHEVa, IONTCHO IONOVa AND NONKA DASKALOVAb aResearch Institute of Forensic Sciences and Criminology–Ministry of Interior, BG-1000 Sofia, P.O.Box 934, Bulgaria bInstitute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, BG-1113 Sofia, Bulgaria Table 1 Specifications of the spectrometer, ICP source and ICP-AES was applied to the determination of trace amounts operating conditions of toxic elements in animal tissues, and also a large number of trace elements in bullet lead and silver.The analysis of these JY 38 (Jobin-Yvon) Monochromator— materials is important in forensic science applications. Mounting Czerny–Turner, focal length 1 m Grating Holographic, 2400 grooves mm-1 Keywords: Inductively coupled plasma atomic emission Wavelength range 170–700 nm (1st order) spectrometry; forensic science; toxic elements; animal tissues; Dispersion 0.38 nm mm-1 bullet lead; silver Entrance slit 0.02 mm Exit slit 0.04 mm Resultant spectral slit 15.2 pm The Research Institute of Forensic Science and Criminology Practical spectral band-width 15.6 pm at the Ministry of Interior of Bulgaria maintains a great Photomultiplier Hamamatsu TV, R 446 HA interest in techniques for the multi-element analysis of different Rf generator— PlasmaTherm, Model HFP 1500 D materials according to the requirements of modern forensic Frequency 27.12 MHz (±0.05%) science examination. Recently, a project was initiated for the Oscillator Crystal-controlled at 13.56 MHz determination of a number of elements in animal tissues, bullet Power output 0.5–1.5 kW lead and silver for the following reasons: the determination of toxic trace elements in animal tissues is interrelated with cases Nebulizer— Meinhard, concentric glass of acute toxic poisoning of people and animals; the elemental Pump— Peristaltic, ten-roller, analysis of bullet lead is important in connection with the Gilson Minipuls II (Gilson investigation of cases of murder or physical injury of people Medical Electronics, France) and animals with firearms; the wide application of silver and its increasing price on the world market has led to illegal Operating conditions— operations involving this metal.Incident power 1.0 kW Reflected power 10 W There are several papers in the literature describing the Outer argon flow rate 15 l min-1 application of various techniques for trace element determi- Carrier flow rate 0.5 l min -1 nations in materials of concern to forensic scientists.1–6 For Liquid uptake rate 1.3 l min-1 the solution of the above-mentioned problems, techniques are Transport efficiency of ICP 3% needed which combine the following features: (i) trace analyt- system ical methodology with multi-element capability; and (ii) the possibility of element determinations in a broad concentration range including detection limit levels.ICP-AES is a powerful Sample Digestion Procedure analytical technique combining all these features.7 The merits of ICP-AES for the analysis of biological and/or clinical Animal tissues materials have been discussed8 on the basis of real practical A 10 g sample was treated with HNO3–H2SO4 (30 ml) by a analytical tasks, whereby ICP-AES has been selected as the wet decomposition procedure. The wet digestion was carried method of choice. out in a closed system by using an autoclave with Teflon The purpose of this work was to show the possibilities of vessels [Perkin-Elmer (Norwalk, CT, USA), N3; working ICP-AES in the determination of trace elements in tissues, volume=0.12 l, pressure=50 bar and maximum temperature= bullet lead and pure silver. 160°C].A notable advantage of digestions carried out in closed systems is that volatilization losses can be minimized.8 EXPERIMENTAL The final volume of the sample solution was 100 ml.The solvent blank had a concentration of 183 mg ml-1 H2SO4. Instrumentation The matrix blank was a 10 g wet sample of the tissue in 100 ml The experiments were performed with the Jobin-Yvon of solution. The tissues contained the ‘normal values’ of element (Longjumeau, France) equipment specified in Table 1. concentrations. Bullet lead Reagents and Reference Solutions All reagents were of analytical-reagent grade (Merck, A 0.250 g sample was dissolved under heating with 1.5 ml of HNO3 (1+3) in the presence of tartaric acid (0.250 g).The Darmstadt, Germany) and doubly distilled water was used throughout. The stock solutions of the elements (1 mg ml-1) final volume of the sample solution was 25 ml. The solvent blank had a final concentration of 10 mg ml-1 tartaric acid were prepared from Merck Titrisol solutions. The reference solutions for the determination of the analytes were prepared and 27 mg ml-1 HNO3 . The matrix blank contained 10 mg ml-1 lead and the solvent blank.by precise matching of sample acidity and matrix content. Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 (21–24) 21Silver understanding the ambivalentnature (i.e., toxic versus essential) of many elements by developing methods that determine A 0.250 g sample was dissolved under heating with 2.0 ml of minimum absolute amounts of trace elements in these materials HNO3 (1+1). The final volume of the sample was 25 ml. The in the most precise and accurate way.Detection limits and solvent blank was a 60 mg ml-1 solution of HNO3. The matrix accuracy of ICP-AES depend on both multiplicative and blank contained 10 mg ml-1 silver and the solvent blank. spectral interferences.8 The multiplicative and spectral interferences on the prominent lines19 were investigated. The net analyte signals in the RESULTS AND DISCUSSION presence of 183 mg ml-1 H2SO4 decreased by 30% in compari- ICP-AES is not free from matrix effects9–14 including acid son with the corresponding values in pure aqueous solution.matrix interferences.15–17 The latter was of significance since These results were in accordance with those reported by the samples were prepared for analysis by acid dissolution (see Chudinov et al.15 A 10 g biological sample digest at the same Sample Digestion Procedure). The multiplicative (non-spectral) acidity did not change the slope of the calibration graphs. The and additive (spectral) interference effects in the presence of influence of H2SO4 and the biological matrix was studied at a the above-mentioned matrices were studied.concentration level equal to that in the final sample solution. Multiplicative interferences are related to sensitivity changes No spectral interferences in the trace element determination in the analyte signals so that the signal-to-background ratio is were registered. Hence, the detection limits in solution were modified, i.e., the slope of the calibration graph is affected and calculated from the equation for pure aqueous solutions10 the accuracy may be reduced.This effect is relatively small, i.e., it hardly affects the detection limits. In trace analysis, CL=2Ó2×0.01×RSDBL×BEC (1) spectral interferences may deteriorate detection limits considerably and endanger accuracy. The signal-to-background ratio where CL is the detection limit in solution, RSDBL is the decreases as a result of enhanced background caused by stray relative standard deviation of the blank in % and BEC is the light, line wing or direct line overlap.If the presence of an background equivalent concentration. interfering line is not recognized, the result of the analysis will Table 2 compares the detection limits with respect to the be inaccurate. Spectral interferences affect the intercept of a dissolved sample of wet tissue and the range of ‘normal values’ calibration graph plotted on a linear scale.7 The type and of trace element concentrations in human liver, compiled from magnitude of both multiplicative and spectral interferences are various sources.20–22 The detection limits were measured by specific for the sample type.The effect of the sample matrix using eqn. (1), RSD=2%. It should be noted that the ‘normal on the accuracy and detection limits cannot be predicted in levels’ of trace element concentrations in tissues vary for the general terms. Only detailed experimental data can reveal the different countries.The difference in these levels might be due situation for each analytical line of the analyte in the presence to eating habits, types of food, geographical conditions or of a given matrix and acidity. The influence of matrix type anthropogenic sources of pollution. and acid concentration on the detection limits and accuracy In our toxicological work, the analytical task required the was investigated. determination of lethal concentration levels of the abovementioned toxic elements.These levels are higher than the ‘normal values’ by a factor of 10–100 and are therefore readily Animal Tissues measurable by direct ICP-AES analysis. The mean values were determined from the analysis of three separate dissolutions of Recently,18 the developments in atomic spectrometric techniques for the analysis of clinical and biological materials were each sample. We have no tissues with ‘certified values’ available and hence reviewed.It was noted that the analytical chemist can help in Table 2 Comparison between the detection limits with respect to the dissolved sample of wet tissue (in ng g-1) obtained in this work by using ICP-AES and ranges of ‘normal values’ or mean values of trace element concentrations in human liver of clinical interest, compiled from various sources This work (ICP-AES) Concentration range of ‘normal values’ in accordance with reference data/ng g-1 Selected analytical Detection limit/ line/pm ng g-1 ref. 20 ref. 21 ref. 22 Ag I 328 068 75 —* — — As I 234 984 1600 5–15 33–70 10 Ba II 455 403 5.2 — — 1300 Be I 313 042 5.0 — — — Bi I 223061 430 — 12–56 5 Cd II 214 438 200 300 1100–2300 1000 Cr II 205 559 110 8 12–230 50 Cu I 324 754 26 500–800 2100–2300 6000 Hg I 253650 560 30–150 160–1300 100 Mn II 257610 15 1000–2000 450–2100 1000 Pb II 220 353 1700 350–550 160–1000 1000 Sb I 206 833 330 — <10–70 5–10 Se I 203985 220 250–400 — 300 Sn I 233 484 880 — — <400 Tl I 276787 940 — — — Zn I 213 856 30 40000–60 000 21000–82 000 — Co II 238 892 120 6 13–62 — Fe II 239562 94 150000–250 000 — 150000 Mo II 202030 140 3600 200–1200 — Ni II 221 647 200 5–13 28–220 30 V II 309 311 120 — — 2–40 * No data available. 22 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12cannot confirm the accuracy of the procedure in this way. The tration, CA) in the presence of lead or lead–antimony alloy. In these cases, QI(la)=0; QW(Dla)=SW(Dla)/SA, where SW(Dla) absence of losses was established as follows: three separate dissolutions of a sample of human liver (10 g) with ‘normal is the sensitivity associated with wing background (signal per unit matrix concentration, CI).The use of the symbol Dla values’ were made in the presence of As, Bi, Cd, Hg, Pb, Sb, Se and Sn (volatile elements). The analytes were added at instead of la expresses that SW(Dla) refers to the overall background level in the spectral window (Dla) viewed and not concentration levels equal to five times the detection limits (Table 2).The reference solutions contained the matrix blank specifically to la as happens with the partial line sensitivity. The background level is defined as the minimum background and the above-mentioned analytes at concentrations ranging from the detection limits to 10 times the detection limits. No in the ‘smoothed’ scan in a given spectral window (Dla). No line and wing background interferences were registered volatilization losses were observed in the decomposition procedure.in the presence of 10 mg ml-1 silver as a matrix. In this case, QI (la)=0 and QW(Dla)=0. Detection limits were calculated In cases of chronic poisoning with toxic elements, their concentrations are several times higher than the ‘normal using eqn. (1). The multiplicative interferences in the presence of the above- values’. In this work, the ‘normal levels’ of the toxic elements were used as reference values, i.e., in all cases tissues from a mentioned matrices were investigated.The net analyte signals decreased by 15% (bullet lead) and 20% (silver). The effect is normal body and corpse were analysed for evidence of poisoning. However, for As, Bi, Cr, Hg, Sb, Sn and V, the detection negative in both cases. A sensitivity change of this magnitude is not negligible in analysis because the results can be inaccur- limits obtained in this work by direct introduction of the sample solutions into the ICP with conventional pneumatic ate.For example, the content of silver in a certified reference material (E3–4, NIPKI, Plovdiv, Bulgaria) was obtained from nebulization were higher than the corresponding ‘normal levels’ (Table 2). Hence, analysis at the level of ‘normal values’ calibration graphs constructed from standards prepared in the solvent blank and in the solvent blank+10 mg ml-1 lead in requires analyte preconcentration.These possibilities will be discussed in a separate paper. solution (Table 3). Using Student’s criterion, a statistical difference between the two sets of data was found. Reviewing the data presented in Table 3, it can be concluded that accurate Bullet Lead and Silver results for the analyte content can be obtained if the calibration standards contain the same matrix concentration as the sample. The spectral interferences were studied in the presence of The sensitivity change depends on the type and concentration 10 mg ml-1 lead, 10 mg ml-1 lead–antimony alloy (9+1) and of the matrix. 10 mg ml-1 silver, respectively. Information on the interfering matrix lines was derived from the wavelength scans centred around the analytical lines. Details of how the results were Table 3 Content of silver in the lead certified reference material E3–4, obtained and quantified are given elsewhere.12 Fig. 1 shows an obtained by using calibration procedures in the solvent blank and in example of wing background interference by Pb 257 726 pm the solvent blank+10 mg ml-1 lead in solution.The mean values were on Mn II 257 610 pm. XW is the net wing background signal. determined from the analysis of three replicates of E3–4 The detection limits in the presence of 10 mg ml-1 lead or Readings from 10 mg ml-1 lead–antimony alloy (9+1) were calculated from calibration graph the following equation:23,24 Readings from in prepared calibration graph in solvent blank CL,conv=2Ó2×0.01×RSDBL×[BEC+QI(la)CI+QW(Dla)] prepared solvent +10 mg ml-1 (2) blank (%) lead (%) Certified where CL,conv is the conventional detection limit; QI(la)=SI /SA, No.value (%) Xi X� Xi X� where SI is the partial sensitivity of an interfering line, defined as the signal per unit matrix concentration (CI), produced by 1 0.00140 0.00118 0.00119 0.00138 0.00139 2 0.00140 0.00120 0.00142 the line at the peak wavelength of an analytical line and SA is 3 0.00140 0.00119 0.00139 the sensitivity of the analytical line (signal per analyte concen- Table 4 Detection limits with respect to the dissolved solid sample in the solution (in %) for solid concentrations of 10 mg ml-1 Pb (column 1) and 10 mg ml-1 Pb–Sb alloy (Pb5Sb=951) (column 2) Detection limit (%) Analytical line/pm 1 2 Al I 396152 2.9×10-4 3.4×10-4 Ag I 328068 7.0×10-5 7.0×10-5 As I 234984* 1.8×10-3 1.0×10-3 Bi I 223 060 4.6×10-4 4.6×10-4 Cd I 214 438 4.0×10-5 4.0×10-5 Cu I 324 754 4.7×10-5 4.7×10-5 Fe II 238204 6.0×10-5 6.0×10-5 Mg II 279 553 1.2×10-5 1.2×10-5 Mn II 257610 2.0×10-5 2.0×10-5 Sb I 206838 4.5×10-4 —† Sn I 235484 1.2×10-3 1.2×10-3 Tl I 276 781 9.8×10-4 9.8×10-4 Z213856 3.2×10-5 3.2×10-5 * The most sensitive arsenic line, As I 193696,20 could not be used Fig. 1 Example of a spectral scan over a spectral region ±200 pm since it lies outside the wavelength range of the ICP equipment (see Table 1). around the analytical line, Mn II 257 610 pm.Interferent: 10 mg ml-1 lead. † Matrix element. Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 23Table 5 Detection limits with respect to the dissolved solid sample The financial support from the National Fund for scientific in the solution (in %) for solid concentrations of 10 mg ml-1 Ag research of the Ministry of Science, Education and Technology obtained in this work (column 1), ref. 25 (column 2) and ref. 26 of Bulgaria under registration No.X-499A is gratefully (column 3) acknowledged. Detection limit (%) Analytical line/pm 1 2* 3† REFERENCES Cu I 324 754 1.0×10-5 —‡ 1.0×10-5 1 Krasnobaeva, N., Nedyalkova, N., and Lalchev, M., T heses of the Zn I 213 856 5.0×10-5 — 5.0×10-5 Institute of Forensic Science and Criminology–Ministry of Interior, Mn II 257610 1.0×10-5 5.0×10-5 2.0×10-5 1980, 9, 111. Mg II 279553 2.6×10-6 — 2.0×10-5 2 Locke, J., Anal. Chim. Acta, 1980, 113, 3. Na I 588995 5.5×10-5 — — 3 Lalchev, M., Ionov, I., and Daskalova, N., Anal.L ab., 1992, 1, 77. Fe II 359940 5.0×10-5 5.0×10-5 5.0×10-5 4 Lalchev, M., Ionov, I., and Daskalova, N., Presented at the 29th Se I 196096 2.3×10-3 — 5.5×10-4 Colloquium Spectroscopicum Internationale, Leipzig, 1995. Ni II 231 604 5.0×10-5 5.0×10-5 ×10-4 5 Peters, A., and Koons, R., ICP Inf. Newsl., 1988, 13, 751. Cd I 214 438 2.0×10-4 — 1.0×10-4 6 Powell, G. L., Robinson, R. R., Cocks, B., and Wright, M., Te II 214281 1.0×10-3 — 8.0×10-4 J.Forensic Sci., 1978, 23, 712. Pb II 283 307 5.0×10-5 5.0×10-5 ×10-4 7 Boumans, P. W. J. M., in Inductively Coupled Plasma Emission Ca II 393 366 1.6×10-5 — 2.0×10-5 Spectroscopy, Part 1, Methodology, Instrumentation and Perfor- Pd II 340 458 2.2×10-4 — — mance, ed. Boumans, P. W. J. M., Wiley, New York, 1987, p.100. Sb I 206 833 3.3×10-4 — 4.5×10-4 8 Maessen, F. J. M. J., in Inductively Coupled Plasma Emission Bi I 223061 4.0×10-4 — 5.0×10-4 Spectroscopy, Part 2, Applications and Fundamentals, ed.Boumans, Hg I 253652 1.0×10-4 — 1.5×10-4 P.W. J. M., Wiley, New York, 1987, p. 100. 9 Boumans, P. W. J. M., Fresenius’ Z. Anal. Chem., 1986, 324, 397. * 56 MHz ICP was used. 10 Boumans, P. W. J. M., and Vrakking, J. J. A. M., J. Anal. At. † 40.68 MHz ICP was used. Spectrom., 1987, 2, 513. ‡ No data available. 11 Boumans, P. W. J. M., in Inductively Coupled Plasma Emission Spectroscopy, Part 1, Methodology, Instrumentation and Performance, ed. Boumans, P.W. J. M., Wiley, New York, Tables 4 and 5 summarize the detection limits with respect 1987, p. 358. to the dissolved solid sample in the solutions: 10 mg ml-1 12 Daskalova, N., Velichkov, S., Krasnobaeva, N., and Slavova, P., lead (Table 4, column 1), 10 mg ml-1 lead–antimony alloy Spectrochim. Acta, Part B, 1992, 47, E1595. (Pb5Sb=951) (Table 4, column 2) and 10 mg ml-1 silver 13 Velichkov, S., Daskalova, N., and Slavova, P., Spectrochim. Acta, (Table 5). The RSD was 2%.Table 5 shows for comparison Part B, 1993, 48, E1743. the literature data.25,26 It should be noted that under the 14 Daskalova, N., Velichkov, S., and Slavova, P., Spectrochim. Acta, Part B, 1996, 51, 733. influence of the strong irradiation by the ICP, partial reduction 15 Chudinov, E. G., Ostroukhova, I. I., and Varvanina, G. V., of silver to the metallic state was observed. The deposition of Fresenius’ Z. Anal. Chem., 1989, 335, 25. the silver metal on the nozzle and capillary of the concentric 16 Brenner, I.B., Mermet, J. M., Segal, I., and Long, G. L., glass nebulizer leads to changes in the nebulization efficiency Spectrochim. Acta, Part B, 1995, 50, 323. and sample uptake rate. This effect was eliminated by using a 17 Brenner, I. B., Segal, I., Mermet, M., and Mermet, J. M., concentric metallic nebulizer with a Pt-capillary (Jobin-Yvon). Spectrochim. Acta, Part B, 1995, 50, 333. 18 Taylor, A., Branch, S., Crews, H. M., Halls, D. J., and White, M., The good analytical characteristics of the proposed methods J.Anal. At. Spectrom., 1994, 9, 87R. satisfy the requirements for identification and classification of 19 Boumans, P. W. J. M., L ine Coincidence T ables for Inductively materials in forensic examinations. Bullet lead or silver was Coupled Plasma Atomic Emission Spectrometry, Pergamon Press, identified on the basis of their trace element content. This Oxford, 1980, p. 1984. information and other evidence can be used to answer the 20 Iyengar, G.V., Concentrations of 15 T race Elements in some question: what is the likely origin of these materials? Selected Adult Human T issues and Body Fluids of Clinical Interest from Several Countries: Results from a Pilot Study for the Establishment of Reference Values, Report No. 1974 of the Institute of Medicine, Ju�lich Nuclear Research Center, 1985. CONCLUSIONS 21 Sumino, K,, Hayakawa, K., Shibata, T., and Kitamura, S., Arch. Environ. Health, 1975, 30 487. ICP-AES as a multi-element plasma source was applied to the 22 Popov, T. A., Zaprianov, Z. Z., Bentchev, I. B., and Georgiev, elemental analysis of materials relevant to forensic science G. K., Atlas of T oxicokinetics, ‘Medicina i fizkultura’, Sofia, 1984. 23 Boumans, P. W. J. M., and Vrakking, J. J. A. M., Spectrochim. applications (tissues, bullet lead and pure silver). The loss of Acta, Part B, 1988, 43, 69. accuracy caused by systematic errors in the forensic examin- 24 Boumans, P. W. J. M., Tielrooy, J. A., and Maessen, F. J. M. J., ation is a serious problem for material identification and Spectrochim. Acta, Part B, 1988, 43, 173. classification. In order to achieve the required accuracy, cali- 25 Vankatasubramanian, R., Biswas, S. S., and Murty, P. S., Indian bration by precise matching of matrix content and sample J. T echnol., 1991, 29, 605. acidity, as well as essential information on the type of spectral 26 Jobin-Yvon Division d’Instruments, Applications P009, Longjumeau, 1985. interferences, is required. In this work, only a few wing background interferences were registered in the determination Paper 6/03869G of trace elements in bullet lead. These results indicate that Received June 4, 1996 ICP-AES is a powerful tool for the analysis of materials of Accepted October 2, 1996 interest to forensic scientists. 24 Journal of Analytical Atomic Spectrometry, January 1997, Vol.

ISSN:0267-9477    

DOI:10.1039/a603869g

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Determination of Toxic Elements in Liquid Hazardous Waste Using High-resolution Energy-dispersive X-ray Fluorescence Spectrometry P. A. RUSSELL*a AND R. JAMESb aOxford Instruments, Industrial Analysis Group, Abingdon, Oxfordshire, UK OX14 1TX bRechem International, Gwent, UK NP45DQ The analysis of liquid hazardous waste (LHW) prior to Process Control/Acid-generating Elements ( Br, Cl, I, P, S) disposal using high-temperature incineration or as an These elements are required analytes for the effective control alternative fuel is required for process and regulatory control.of the composition of combustion off-gases. The feed composi- The typical requirements of the industry sector are rapid tion must be controlled within the capacity of gas treatment screening to support decisions on the most appropriate facilities to ensure that emission limits are not breached. Where treatment of the waste. This paper reports the use of high- waste-derived fuels are manufactured to a specification, the resolution EDXRF spectrometry for the determination of level of confidence in the measured value must be high to halides and toxic heavy elements using a rapid technique ensure satisfactory performance.combined with a unique sample preparation methodology. Calibrations were developed using traceable certified 1000 mg l-1 aqueous standards and pure organic solvents. Heavy Metals (Cr, Cu, Ni, V, Zn) Samples were stabilized in an alumina matrix and measured against a suitable calibration.The benefits of the proposed These elements are required analytes to ensure environmental technique are as follows: (i) ease of obtaining calibration performance meets regulatory requirements for releases to all standards; (ii ) removal of sample history, i.e., control of media. These elements are commonly occurring (e.g., as wear matrix effects; (iii) minimization of analyte loss during sample metals/trace elements in fuel oils) and are often regulated in preparation; (iv) wide range of matrix types measurable using discharges to air and water.a single calibration, i.e., clean solvents to turbid sewage sludge; (v) accuracy of measurement, typically within 10% relative in the concentration range 10–100 mg kg-1 with a precision of Toxic/Volatile Metals (As, Cd, Hg, Pb, Sb, Se, Sn, Tl ) better than 5% relative; and (vi) speed of analysis, for >20 elements typically <15 min from receipt of sample. The These elements are required analytes owing to their potential results presented show high-resolution EDXRF to be ideally presence in trace amounts sufficiently high to result in unacsuited to the analysis of LHW owing to good heavy element ceptable atmospheric emission.The elements are sufficiently detection in the atomic number range 30–82 (Zn–Pb). volatile to evade many of the treatment stages that are effective Detection limits are in the range 3–17 mg kg-1 for heavy for heavy metals and, in addition, are potentially more hazardelements and below the working calibration range for Cl, P ous in their own right.The volatility of these metals has and S. These limits satisfy typical requirements for process historically resulted in techniques involving ashing, heating or and regulatory control, which are of importance in the range digestion being found inadequate for determinations in waste >0.1% m/m for P, S and Cl and >50 mg kg-1 for the matrices owing to volatile loss.heavier elements. Keywords: T oxic elements; liquid hazardous waste; high- Precision, Accuracy and Appropriate Analysis resolution energy-dispersive X-ray fluorescence spectrometry; matrix modification The typical level of accuracy and precision achieved in the analysis of liquefied waste is generally considered to be acceptable if better than 20% relative. The contribution from interelement effects to the accuracy of an analysis can be significant The analysis of waste for the purposes of disposal risk assess- in LHWs.The use of matrix modification methods reduces ment features some unique difficulties resulting from the these inter-element or matrix effects and their accuracy needs extremely wide range of matrices encountered. The types of typically to be <5% for the matrix modification technique to elemental analytes and relevant ranges of measured values be of value. The alumina method proposed in this paper relies selected for waste assessment usually reflect requirements of on the principle of little or no inter-element effects being environmental regulation and waste disposal licensing.1 The present in the final specimen, which is demonstrated by the key purposes of these measurements are (i) to ensure com- linearity of the calibrations for all elements studied.Dilution pliance with regulatory requirements (e.g., for waste-derived into the linear calibration ranges is used to maintain the fuels), (ii) to allow appropriate selection of disposal strategy, minimization of matrix effects while still achieving appropriate (iii) to confirm the waste composition and (iv) to ensure detection limits for the elements of interest for both process appropriate process control where a treatment or incineration and regulatory control.The reduction of matrix effects enables disposal option is selected. The target analytes can be con- calibrations to be developed from pure element standards sidered to fall into three groups, (i) process control/acid- prepared in aqueous or organic solvent.generating elements, (ii) heavy metals and (iii) toxic/volatile Table 1 identifies the main areas of potential error encountered in any analytical technique used for the analysis of LHWs. metals, as follows. Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 (25–32) 25Table 1 Key sources of error in the analysis of liquid hazardous waste Estimated error Systematic Means of error control proposed Source of error magnitude (%) (Y/N) in this work Phase distribution of sample 0–typically 50 N Addressed by mixing with alumina in one stage Analyte losses 0–100 N No preparation/digestion stages ensuring no analyte loss Chemical interferences 0–100 N No preparation/digestion stages preventing uncontrolled chemical reactions Instrumental interferences 1–15 Y XRF spectra simple.Alumina normalizes matrix effects and reduces inter-element effects to <5% Linearity over measured range 0–25 Y Simple sample dilution after s/w out of range warnings Stability of calibrations 0–15 Y Very stable technique and measures a static sample identical to calibration standards REVIEW OF EXISTING METHODOLOGIES FOR explosives and, although this may be known, they cannot be easily and/or safely exposed to thermal digestion sample LHW ANALYSIS preparation techniques.Bomb digestion is used, followed by A brief review of the literature reveals a surprising lack of either potentiometric titration for determination of halogens work being carried out to solve some of the difficult sample or ion chromatography13 for determination of halogens, PO42- preparation problems encountered in the analysis of LHWs.and SO42-. The bomb technique relies on complete conversion The most recent review of environmental analysis by Cresser of organic compounds into inorganic forms, i.e., halogenated et al.2 reported only five references relating to LHWs, all of solvents to X- (X=Br, Cl, F, I) and subsequent quantitative which were on the determination of a few elements in waste transfer of these components to the final measurement tech- water.In general, dissolution techniques used for AAS or AES nique. Typical analysis times for these methods are 30–45 min. analysis of soils and waters are also used for LHWs. Various Common interferences occur in the measurement of I in the reviews2–6 of atomic spectrometry show an emphasis on extrac- presence of sulfate using ion chromatography and the forma- tion, preconcentration and digestion methods of sample prep- tion of stable metal halides during digestion yields low recover- aration.A number of key problems occur when using extensive ies. In the analysis of LHWs a flame test should be carried dissolution and/or extraction procedures for LHW analysis. out before attempting bomb digestion.If the flame test indicates (i) The effects of organic solvents on AAS/AES element a very high calorific value, i.e., potentially explosive, then the sensitivities7–9 and the variable and unpredictable nature of bomb technique would not be appropriate and an alternative LHWs preclude even direct introduction of clean solvents, i.e., technique must be used. Frequently there is no alternative containing no undissolved material. This drives the need for digestion technique available. sample preparation methods based on digestion or preconcentration to produce a common matrix form.XRF is capable of measuring samples in their ‘as-received’ state and therefore should be an ideal technique for the analysis (ii) Potential loss of analytes such as Cd, Hg, Pb, Tl and Se during digestion10 and/or atomization11 will always affect the ofLHW in all its forms, i.e., sludges to clean solvents. Decreases in analysis time and cost, in addition to lowering potential confidence of measurements made when digestion preparation techniques are employed.analyte losses through eliminating extensive sample preparation, demonstrate the obvious advantages to this approach. (iii) Sample preparation time is an issue for industrial incineration plants. Methods that employ lengthy clean-up West et al.14 reported on various methods for the analysis of electroplating bath sludges. They first dried the samples, then preparation procedures which, although ultimately producing high-quality analyses, are not cost-effective.Peraniemi et al.12 prepared lithium metaborate fusions or pressed pellets for WDXRF analysis. This approach relies on a preconcentration reported on a preconcentration techniquefor the determination of P in waste water using EDXRF. The sample preparation drying stage followed by digestion (fusion) or pelletization to allow a traditional XRF analysis to be used. As the sludge time was quoted as between 3 and 6 h plus overnight drying.The spectrometer available to the researchers was unable to matrix is aqueous and of a predictable nature, this approach, although time consuming, gave good results. However, it would determine P directly from a filtrate (which would have reduced the preparation time) owing to spectral overlap from neigh- be inappropriate for organic-based sludges and suffers from an inability to determine halogens or S in organic form. Lucke bouring elements. (iv) Spectral interferences, especially from Cl and other et al.15 dried sewage sludge prior to analysis and results were reported on the basis of spikes made to this dried matrix.halogens, are encountered in AAS,3 ETAAS,11 ICP and ICP-MS.9 Vanhoe et al.9 described a number of mass spectral Results from determinations of elements such as Cd, Pb and Hg made using this approach may be low owing to losses of interferences encountered in the ICP-MS analysis of biological samples due to Cl, Ar and O species combinations.Biological these volatile elements during the drying stage. A few researchers have studied the use of simple matrix samples can be considered to be similar in nature to LHW sample types because of their high concentrations of C, O and modification methods for XRF analysis which avoid heating and/or digestion. Seiber,16 as reported in a review by Marshall often Cl. Species combinations of Cl–Ar–O were reported as potential interferents with many environmentally sensitive et al.,5 used a binding powder obtained from Chemplex Industries to stabilize grease and oil samples prior to XRF elements, e.g., As, Cr, Se and V.Suppression of these interferences again relied upon complicated extraction or digestion analysis. In a Chinese paper by Liu et al.,8 reported in a review by Bacon et al.,17 MgO powder was used to stabilize oil techniques. (v) The use of bomb digestion can be highly dangerous samples followed by low-temperature heating (270 °C) prior to the determination of Cr, Cu, Fe, Mn, Ni, V and Zn by EDXRF when unknown organic solvents are mixed with acids or oxygen.Many LHWs are explosive or may be precursors to spectrometry. Carbon is also used as a simple matrix modifier 26 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12in a newly released ASTM method18 for the determination of such as Cd are linear at trace concentrations, i.e., the mass of alumina+sample used to fill a sample cup does not affect the toxic metals in liquefied waste prior to its use as an alternative fuel for cement kilns.The carbon matrix method also uses sensitivity of the heavy elements, e.g., the infinite thickness of Cd in alumina is approximately 2.3 cm compared with 26.4 cm low-temperature heating similar to that of Liu et al.8 The heating of the sample in both of the above techniques precludes in a carbon matrix. (iv) Compared with a calibration based on liquids of varying the determination of halogens, S and P, and as these are critical elements the techniques have limited use in process density, there is no need to carry out a background correction, i.e., ratio to Compton scatter.For elements determined from control analysis for a high-temperature incinerator. The use of a light matrix, i.e., carbon or MgO, is an their X-ray lines at energies above the Compton peak, background correction is not possible using the Compton scatter important aspect in the development of simple matrix modifi- cation techniques for use in the analysis of LHW by XRF.peak. (v) Mixing a sample with alumina ensures that liquid phases The use of low-power X-ray tubes in many EDXRF spectrometers reduces potential losses of analyte from samples cannot separate prior to or during analysis and that undissolved solids are correctly incorporated into the analysis, i.e., through heating. Excellent detection of elements with X-ray lines in the energy range 8–40 keV makes EDXRF the preferred do not settle on the support film, thus introducing a bias.This is especially important in the analysis of sewage sludge. choice of XRF for the analysis of LHW. Existing methods for inorganic elemental analysis of LHW (vi) The sample used for analysis is safer to handle in the spectrometer, i.e., leakages cannot occur in the sample support and their limitations are summarized in Table 2. film owing to small punctures or chemical attack during analysis.FUNDAMENTALS OF THE ACTIVATED (vii) The sample mass used for analysis is relatively large ALUMINA METHOD FOR XRF (5 g) compared with alternative techniques. All techniques used for the analysis of liquefied waste suffer from a common problem, i.e., matrix variation and its effect EXPERIMENTAL on analyses based on multiple standards calibrations. Using Sample Preparation high-resolution EDXRF the use of activated alumina mixed with the sample minimizes this effect.Materials A key feature of this technique is the use of a well known Calcined alumina (1500°C) (Merck/BDH, Poole, Dorset, UK), principle for the analysis of complex matrices by XRF. For 50–60 ml wide-mouthed HDPE sample bottles, a minimum of many years the technique of preparing glass fusion beads using two 1 cm diameter stainless-steel ball-bearings or similar heavy lithium borate mixes for the analysis of geological and other stable material, Chemplex Industries (Tuckahoe, NY, USA) or materials has been used to great effect.This principle relies on Spex Industries (Edison, NJ, USA) 31.5 mm X-ray sample reducing, by dilution, all samples to a common matrix form cups, 4 mm Prolene film (Chemplex Industries) and 4 mm and thus largely eliminating the sample history. The alumina Hostaphan high-purity polyester film (Oxford Instruments, method uses this principle of diluting a sample to a common Abingdon, Oxfordshire, UK) were used.matrix form with the following additional benefits in respect of the analysis of LHW: (i) The alumina is activated. This will stabilizehighly volatile Standard solutions solvents, reducing losses that may occur due to thermal and Standard solutions used were for AAS 1000 mg l-1 pure ana- X-ray heating in the spectrometer during analysis [a common lyte aqueous standard solutions of As, Cd, Cr, Cu, Fe, Hg, Ni, problem with pure solvent(s) samples especially in high wattage Pb, Sb, Se, Sn, Tl, V and Zn, for P, triethyl phosphate, for S, spectrometers], thus avoiding an obvious area of bias.dithioglycol, for Cl, trichlorobenzene, for Br, (ii) The diluting matrix, i.e., alumina, is significantly heavier 1-bromonaphthalene and for I, iodobenzoic acid, and low in atomic number than its fusion bead equivalent. This gives molecular mass polyethylene glycol (PEG 400). the benefit of reducing background in the XRF spectrum compared with that produced by a low-Z organic or aqueous solution in addition to substantially reducing inter-element Preparation Procedure for Standards and Samples and matrix effects.(iii) The density of an alumina mixed sample compared Weigh 15±0.01 g of alumina into a 60 ml wide-mouthed polyethylene bottle, add 5±0.01 g of sample to the bottle, with the original liquid form of the sample helps to ensure that the alumina sample is infinitely thick with respect to the place two 1 cm diameter stainless-steel ball-bearings or similar mixing device in the bottle, seal the bottle with the screw-cap K-series fluorescence X-rays emitted from heavy elements such as Cd or Sb.This ensures that calibrations for heavy elements and shake vigorously until the sample and alumina are com- Table 2 Summary of existing methods and limitations Technique Reference Elements Sample preparation Key limitations Classical 13 Metals Presence of unknown metals; colour; difficult to Digestion, pH and buffering control, titrations control pH Bomb calorimetry 10, 13 Halides, P, S Addition of reactive agents Potentially determines only organic halides not total owing to complexing of inorganic halides with metal species; oxidation prevents determination of I ICP-AES, ICP-MS 3–5, 7, 9, 10 Halides, P, S, Digestion Halide interference; heating during preparation trace metals causes loss of important analytes, e.g., Pb, Tl, Hg; particulates; large conc.of alkali metals can cause significant interference; organic solvents AAS 3–5, 10, 11 Trace metals Digestion Halides interference; particulates; organic solvents XRF 8, 12, 14–16, Halides, P, S, As received, fusion, digestion Homogeneity; loss of analytes in fusion or 18 trace metals digestion process Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 27pletely mixed (approximately 30 s). Tapping the bottle on a another. Tables 3 and 4 list the parameters used in this work for the two calibrations studied.The instrument used was an hard surface will aid the mixing process. Transfer sufficient sample to fill a standard 31.5 mm diameter vented Chemplex Oxford Instruments ED2000 EDXRF spectrometer fitted with a silver target X-ray tube. The benefit of the Ag target tube sample cup fitted with a suitable X-ray transmission film for the elements to be measured. Tap the cup gently on a flat compared with the more commonly used Rh target tube are twofold: improved excitation of Cl by Ag L lines; Rh L lines surface (analysis face down) to compact the sample and remove any air gaps.The sample is now ready for analysis. The do not excite Cl and produce serious spectral interference in energy-dispersive spectrometers at low Cl concentrations; and analysis is carried out using a calibration based on the same alumina to sample ratio. a heavier Z number target, i.e., Ag, gives more high-energy excitation for improved heavy element sensitivity. Intensities were obtained using the method of least-squares Calibration peak fitting to library spectra19 using XpertEase software (an Oxford Instruments proprietary EDXRF software package for Analytical standards should be prepared gravimetrically by Microsoft Windows operating systems).blending the pure element standards into a suitable calibration suite as determined by the final analytical requirements. A requirement for two calibrations was identified based on the process and regulatory controls needed.These are identified Routine Analysis as: ‘light elements and halogens’ and ‘toxic elements.’ The concentration ranges for each calibration are as follows: light An important advantage of this technique is the ability to dilute a sample into the concentration range of the calibration. elements and halogens, P, S and Cl 0.05–5%, Br 0–1% and I 0–0.2%; and toxic elements, all elements 0–600 mg kg-1. If the result reported from an analysis is outside the calibration range, then a dilution of the sample can be made by changing Standards can be single elements or mixtures. Standard solutions can then be mixed at a 15 g alumina to 5 g standard the mass of sample placed on the alumina but maintaining the alumina to sample ratio. This is carried out either by adding ratio to produce the final calibrants. Note: more than one standard can be added to a single 15 g the required balance up to 5 g as PEG or similar material to a reduced mass of sample, or by taking a sub-sample of the alumina measure so long as the total mass of standard equals 5 g.This will maintain the alumina to sample ratio whilst waste–alumina mix and mixing it with a PEG–alumina blank at a suitable diluting ratio, e.g., 1 g of waste–alumina sample allowing mixtures of incompatible standards/elements to be manufactured. made up to 10 g using the blank gives a 10-fold dilution. A simple post-analytical calculation will give the true concen- Empirical calibrations using a suite of standards as described above were used to develop the two methods required, i.e., tration of the analyte in the original sample. Bulk sampling will depend on two main factors, i.e., viscosity light elements and halogens and toxic elements.Standard concentrations were limited to a maximum of 600 mg kg-1 for and homogeneity. Larger samples can be taken to ensure a suitable sample size if homogeneity is poor.The alumina to the heavy trace elements as this was the maximum concentration which could be obtained from 1000 mg kg-1 stock sample ratio is maintained by increasing the mass of alumina used. Liquids that are highly viscous can be transferred by AAS aqueous standard solutions whilst minimizing the number of individual standards required, i.e., a maximum of three thoroughly mixing and then pouring or spooning an accurately weighed amount of approximately 5 g. Again the alumina to elements in the highest standards were used.Serial dilutions of pure analyte standards were used to produce each element sample ratio is maintained by adding three times as much alumina as sample used. Both of the above approaches will calibration. Mixed analyte standards were used to assess the spectral interferences of the spectrometer and check for cali- allow direct reporting of the final results from the calibration. bration linearity. Note: samples are diluted into the linear range using a suitable diluent, i.e., distilled water or PEG, e.g., 5000 mg kg-1 Method Validation Pb solution is diluted into the 0–600 mg kg-1 calibration range by using 15 g of alumina to 0.5 g of sample+4.5 g of The use of reference standards for validating this method is diluent.This gives a 10-fold dilution and maintains the alumina not possible. The term liquid waste implies that the material to sample ratio. for analysis cannot be classified in terms of a specific matrix and as such no standard reference material is available. Method validation is therefore based entirely upon spiking real waste Recommended Spectrometer Conditions samples with known concentrations of analytes (similar to the technique of standards additions).Experiments were carried Specific parameters for obtaining the optimum performance for ranges of elements will vary from one spectrometer to out to determine the following: Table 3 Instrumental conditions for toxic elements Condition Path Voltage/kV Current/mA Primary beam filter Elements measured Live time/s 1 Air 15 1000 Thick Al V–Fe 100 2 Air 35 65 Thin Ag Ni–Br 100 3 Air 45 100 Thick Ag Hg–Pb 150 4 Air 50 500 Thick Cu Cd–I 150 Table 4 Instrumental conditions for light elements and halogens Condition Path Voltage/kV Current/mA Primary beam filter Elements measured Live time/s 1 He 5 1000 None P–K 150 2 He 35 65 Thin Ag Br 50 3 He 50 500 Thick Cu I 50 28 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12(1) Blank measurement of the alumina sample preparation.Accuracy A sample containing only PEG was used to check for any Table 7 gives the results of the measurement of calibration bias in the calibrations at the zero concentration level. standards. The accuracy figure shows the match of given versus (2) Accuracy of the calibrations. Standards were run against calculated concentration for each element. the calibrations to assess accuracy and to check for bias in the calibrations due to either matrix or spectral effects.Matrix/matrix spike recoveries (3) Analyte spiked recoveries using real waste samples. Three types of actual waste solutions were selected from routine Tables 8–10 for the toxic elements and Tables 11–13 for the test samples taken at an incineration plant: clear solutions, light elements and halogens give results for the measurement turbid solutions, i.e., containing significant solids not in of the spiked samples separated into the three matrix types.suspension, and biphasal solutions, i.e., containing two Using eqn. (2), a recovery figure for each analyte in each distinctly immiscible liquid phases. matrix type was determined. The results, referred to as a matrix spike/matrix spike duplicate (MS/MSD), are shown below. For each matrix type a sub-sample was spiked with a known concentration of analyte. The spiked sample was prepared MS/MSD recovery (%)=[(C2-D1C1 )/C3]×100 (2) using the alumina technique and measured.where D1=dilution factor due to matrix spike addition= 1-(mass of spike)/(total mass of sample+spike), C1=calcu- RESULTS lated concentration of matrix without spike, C2=calculated concentration of matrix+spike and C3=given concentration Calibration Details and Standard Errors of matrix spike. Table 5 shows the 3s lower limit of detection and regression details for all elements included in this study. The lower limits Method Reproducibility of detection (LLD) for each analyte in Table 5 are based on the following equation: A number of replicate analyses were made on a waste sample to demonstrate the reproducibility of the sample preparation on the two calibrations, i.e., toxic elements and light elements LLD= 3Óbg net peak × 1 ÓT ×conc.(1) and halogens. (1) A single measurement from each of 10 repeat sample preparations was made for Cl content. A repeat of this where bg=background intensity under analyte peak (cps), net process was made on newly prepared samples 48 h later.(2) A peak=fitted peak intensity of analyte (cps), T=count time (s) single measurement from each of 10 repeat sample preparations and conc.=concentration of analyte. was made for a waste sample spiked with 54.3 mg kg-1 Cd. A single Cl analysis of the waste sample measured on an Blank, Accuracy and Matrix Spike Results Oxford XR400 EDXRF spectrometer, at a separate site and by a second operaror, using the alumina sample preparation Errors shown in the following tables were taken from the technique, is shown in the last column of Table 14.results output of the instrument and are nominally ±2s.19 These represent the total error attributed to spectrum processing and counting statistics. Details of the calculations used DISCUSSION in the error calculations were given by Statham.19 The recovery results show that all but three, i.e., 93%, of the results from the toxic elements calibrations lie within ±15% Blank relative of the spiked value; 81% are within 10% relative and after correcting for possible error based on the 2s errors Table 6 gives the results of the measurement of the PEG shown, a further 12% lie within the ±15% range.Three blank sample. measurements lie outside this range. These anomalous values are explained as follows: first, the determination of Tl in the turbid waste gave 43% recovery; this anomaly was due to spectral interference from a high Br content in this sample of Table 5 Calibration results (measurement times according to Tables 3 and 4) 0.58%; and second, the absence of Pb in the single-phase solvent and low Cu recovery in the biphasal solution are Element Units LLD/mg kg-1 Std.error conc. attributed to errors in spike preparation. Repeat analysis of P % n/a* 0.132 these samples was not possible owing to a lack of sample. S % n/a 0.056 All of the results from the light elements and halogens Cl % n/a 0.066 calibration lie within ±15% relative of the spiked values and Br mg kg-1 5 147 most are within ±10% relative.I mgkg-1 7 35 During the running of these experiments a number of V mgkg-1 8 6.1 possible areas of bias were observed. These effects are Cr mg kg-1 8 9.6 Fe mg kg-1 14 7.9 summarized as follows: Ni mg kg-1 16 8.5 (i) Under certain conditions the Prolene film used in the Cu mg kg-1 17 7.7 determination of the light elements and halogens was seen to Zn mg kg-1 11 5.6 relax and become crinkled.This was found to be associated As mg kg-1 5 8.2 with samples containing high concentrations of certain chlori- Se mg kg-1 5 5.3 nated compounds. The exact nature and type of compound Cd mg kg-1 3 1.9 Sn mg kg-1 6 2.5 was not known and was outside the requirements of this study. Sb mg kg-1 5 4.3 This film relaxation was associated with halogens migrating Hg mg kg-1 7 7.5 through the film, often resulting in spurious results if the Tl mg kg-1 4 13.1 sample was not measured immediately. For this reason, it was Pb mg kg-1 4 5.4 recommended that samples are measured within 30 min of preparation.This was typically the way the technique is used * n/a: These elements were calibrated at concentrations significantly higher than their respective detection limits. at the incinerator sites where the alumina method has been Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 29Table 6 Blank measurement results V Cr Fe Ni Cu Zn As Se Cd Sn Sb I Hg Tl Pb mg kg-1 1.2 0.0 17.3 0.0 0.0 1.6 1.9 2.6 0.0 1.3 0.0 3.9 1.9 2.2 0.0 mg kg-1 error 0.5 0.7 0.2 3.6 0.04 0.4 7.6 2.5 2.5 0.4 0.9 10 2.9 3.9 1.0 Table 7 Accuracy measurement results P S Cl Se/ As/ Br/ Cd/ Sn/ Sb/ I/ (% m/m) (% m/m) (% m/m) mg kg-1 mg kg-1 mg kg-1 mg kg-1 mg kg-1 mg kg-1 mg kg-1 Calc.conc. 0.98 2.09 0.53 93 393 4779 197 47 201 587 Error 0.02 0.02 0.005 4 10 15 4 3 7 13 Given conc. 1.024 2.095 0.598 100 400 4838 200 50 200 600 Accuracy (%) 96 100 89 93 98 99 98 94 100 98 Hg/ Tl/ Pb/ V/ Cr/ Fe/ Ni/ Cu/ Zn/ mg kg-1 mg kg-1 mg kg-1 mg kg-1 mg kg-1 mg kg-1 mg kg-1 mg kg-1 mg kg-1 Calc. conc. 99 96 94 109 6 589 53 55 95 Error 9 13 7 7 3 0.2 8 3 5 Given conc. 100 100 100 100 10 600 50 50 100 Accuracy (%) 99 96 94 109 60 98 106 110 95 Table 8 Toxic elements spiking results: biphasal waste V Cr Fe Ni Cu Zn Se MS/MSD recovery (%) 96 111 84 86 73 105 109 Error/mg kg-1 6 5 1 9 4 5 4 Spike conc./mg kg-1 97.4 98.3 97.1 98.2 99.3 102.0 104.6 As Cd Sn Sb Hg Tl Pb MS/MSD recovery (%) 109 108 103 84 103 98 99 Error/mg kg-1 9 3 3 6 9 10 7 Spike conc./mg kg-1 109.7 47.1 53.5 53.3 101.8 102.5 183.7 Table 9 Toxic elements spiking results: single-phase waste V Cr Fe Ni Cu Zn Se MS/MSD recovery (%) 110 114 109 104 105 90 108 Error/mg kg-1 6 6 2 10 5 5 4 Spike conc./mg kg-1 116.8 109.5 109 105.7 109.9 105.3 103.8 As Cd Sn Sb Hg Tl Pb MS/MSD recovery (%) 108 100 95 112 98 77 Error Error/mg kg-1 9 3 3 6 9 10 Spike conc./mg kg-1 105 45.5 55.5 54.6 97.0 96 Table 10 Toxic elements spiking results: turbid waste V Cr Fe Ni Cu Zn Se MS/MSD recovery (%) 102 110 104 118 94 105 90 Error/mg kg-1 6 6 2 11 5 5 5 Spike conc./mg kg-1 103.4 102 106.4 104.5 103.5 106.6 93.6 As Cd Sn Sb Hg Tl Pb MS/MSD recovery (%) 121 92 103 108 114 43 108 Error/mg kg-1 10 3 3 6 11 13 10 Spike conc./mg kg-1 94.8 44.8 53.2 53.2 96.8 97.7 177 Table 11 Light elements and halogens spiking results: biphasal waste hence the effects of any matrix interaction at the Prolene film are minimal.The toxic elements calibration uses Hostaphan Cl film and is not affected in the same way. MS/MSD recovery (%) 98 (ii) High concentrations of Br can affect the performance of Error (% m/m) 0.008 Tl owing to the fitting of the Tl peak in the spectrum. The Spike conc. (% m/m) 1.654 performance for Tl will depend on detector resolution and accuracy of peak-fitting routines used to determine the countrate of individual elements in a spectrum.The type of sample, introduced. Heavy elements are not affected by this problem i.e., the major heavy elements present, will govern to a large as the bias appears to be due to loss and/or matrix interactions extent the accuracy of any determination of trace heavy toxic of light element analytes at the Prolene/alumina interface. The elements. This is not considered a limitation as the requirement of the method is to provide data upon which risk assessment determination of Br and I in the alumina is from the bulk, 30 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12Table 12 Light elements and halogens spiking results: single-phase waste P S Cl Br I MS/MSD recovery (%) 111.3 120 110 95 100 Error (% m/m) 0.02 0.01 0.007 0.0007 0.0032 Spike conc. (% m/m) 0.9156 0.926 1.279 0.0599 0.856 Table 13 Light elements and halogens spiking results: turbid waste P S Cl Br I MS/MSD recovery (%) 98 91 89 104 100 Error (% m/m) 0.01 0.009 0.005 0.0006 0.0034 Spike conc.(% m/m) 0.849 0.805 0.858 0.042 0.845 Table 14 Sample preparation reproducibility results 10 repeats: day 1 10 repeats: day 3 Second laboratory Mean s* RSD (%) Mean s* RSD (%) (single result) Cl (% m/m) 1.45 0.027 1.9 1.43 0.037 2.6 1.38 Cd/mg kg-1 56 1.3 2.2 * s=1s standard deviation. in terms of process control and environmental regulations can CONCLUSIONS be monitored. High concentrations and/or element(s) close to, The alumina matrix modification technique provides a reliable, or exceeding, decision threshold concentrations with respect fast and robust method of sample preparation prior to analysis to their control or regulated limits will govern the disposal by high-resolution EDXRF spectrometry. The typical levels of route of any waste.In the presence of elements with high accuracy achieved for both major acid-producing elements and concentrations, in terms of decision thresholds, the inability environmentally sensitive toxic trace elements were within 15% to determine accurately trace elements such as Tl, i.e., relative using spiked real waste samples.The precision of total <100 mg kg-1, will not affect, in most instances, the disposal measurement was better than 2.5%. route or whether to accept or reject a consignment. The wide elemental range, low power and non-destructive Perhaps the most useful spectroscopic feature of the alumina nature of EDXRF spectrometry strongly support its being the matrix technique is the control of spectrum background.Fig. 1 ideal instrument for LHW screening. The technique outlined shows two spectra, one a blank and the other containing in this paper shows that it is ‘fit-for-purpose,’ by exhibiting 1.25% Br. The backgrounds of these two spectra are almost traceable analysis, relative accuracy better than 20% and rapid identical. The effect of the variations in sample matrix can be analysis (<15 min) for the determination of elements in all considered to be negligible in the alumina matrix.In order to LHW. maintain and control the background component of the sample The following matrix types are suitable for analysis using matrix, each element is restricted to the linear response range, high resolution EDXRF spectrometry with the activated alumi. e., <1000 mg kg-1 for trace elements. This is usually of the ina method of sample preparation: industrial waste solvents, order of 1–2% m/m but as suitable standards are often not sewage sludge, liquefied waste fuels, paints and inorganic available at these concentrations the range is restricted to pigments in liquid form, electroplating solutions and waste oils. 0–1000 mg kg-1 where pure aqueous solutions are readily A single calibration for the determination of light elements obtainable. A wider concentration range is permitted for low and halogens and a second for the determination of heavy atomic number elements as these do not significantly effect the trace elements can be used for most types of LHW samples.spectral background. The mechanism that allows any hazard- The simple alumina sample preparation technique allows for ous liquid waste to be determined using the alumina technique rapid analysis, with typically a total time, including sample is to dilute into the calibration range. preparation, of <15 min for halogens and light elements determination and an additional 15 min for the heavy trace elements.Based on sample preparation consumables and instrument overheads (helium, liquid nitrogen and power) the cost was estimated at £1.00 per analysis for a minimum of five halogens and light elements, £0.80 for a minimum of 16 heavy element traces and £1.35 for a minimum of 21 element analyses using both calibrations. The proposed alumina matrix modification technique is tailor-made for the determination of most elements encountered in LHW, unlike alternative instrumental techniques which must undergo some kind of aggressive (digestion) pretreatment.This absence of pre-treatment minimizes unpredictable analyte losses, producing a high degree of analytical confidence. The implementation of the alumina matrix modification technique can improve the reliability and accuracy of results Fig. 1 Spectra for a sample of real waste containing Br at 12.45% such that regulators and regulatory bodies could increase the (diluted 10-fold to bring the concentration into the calibration range) superimposed on a PEG blank sample.number of elements determined beyond the current minimum. Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 316 Cresser, M. S., Armstrong, J., Cook, J. M., Dean, J. R., Watkins, The acceptance of the alumina technique coupled with high- P., and Cave, M., J. Anal. At. Spectrom., 1995, 10, 9R. resolution EDXRF spectrometry as an industry standard 7 McCrindle, R.I., and Rademeyer, C. J., J. Anal. At. Spectrom., would ensure that a near total elemental analysis of all LHW 1995, 10, 399. would become possible. Currently, a lack of appropriate avail- 8 Lui, Y. W., Li, D. L., Fan, Q. M., and Wei, C. L., Guangpuxue Yu able techniques and matrix difficulties restrict the type and Guangpu Fenxi, 1992, 12, 83. 9 Vanhoe, H., Goossens, J., Moens, L., and Dams, R., J. Anal. At. number of analyses carried out on any particular waste. The Spectrom., 1994, 9, 177. method has been extensively used at one UK incineration site 10 Applied Zeeman Graphite Furnace Atomic Absorption after the UK Environmental Agency (EA) approved its use for Spectrometry: Chemical L aboratory T oxicology, eds., Minoia, C., the determination of Br. and Caroli, S., Pergamon Press, Oxford, 1992, p. 79. 11 Tserovsky, E., Arpadjan, S., and Karadjava, I., J. Anal. At. Spectrom., 1993, 8, 85. Thanks are due to Jeanette Gravell and David James for the 12 Peraniemi, S., Vepsalainen, J., Mustalahti, H., and Ahlgren, M., preparation of samples and standards and to Andy Ellis for Fresenius’ J. Anal. Chem., 1992, 344, 118. his advice. 13 Meltsh, B., Muenzberg, I., and Janssen, A., L aborPraxis, 1995, 19 (4), 64 and 67. 14 West, H., Cawley, J., and Wills, R., Analyst, 1995, 120, 1267. 15 Lucke, N., Wehner, B., Thi Hong Lan, T., and Kalla, E., Acta REFERENCES Hydrochim. Hydrobiol., 1991, 19, 275. 16 Seiber, J. R., Adv. X-Ray Anal., 1993, 36, 155. 1 James, R., paper presented at Chemspec Europe 95 BACS 17 Bacon, J. R., Ellis, A. T., McMahon, A. W., Potts, P. J., and Symposium, 1995. Williams, J. G., J. Anal. At. Spectrom., 1994, 9, 267R. 2 Cresser, M. S., Garden, L. M., Armstrong, J., Dean, J. R.,Watkins, 18 ASTM Method D5839, 1997 Annual Book of ASTM Standards, P., and Cave, M., J. Anal. At. Spectrom., 1996, 11, 19R. ASTM, Philadelphia, PA, USA, 1997, vol. 11.04. 3 Marshall, J., Carroll, J., Crighton, J. S., and Barnard, C. L. R., 19 Statham, P., Anal. Chem., 1977, 49, 2149. J. Anal. At. Spectrom., 1993, 8, 337R. 4 Marshall, J., Carroll, J., Crighton, J. S., and Barnard, C. L. R., Paper 6/03605H J. Anal. At. Spectrom., 1994, 9, 319R. ReceivedMay 23, 1996 5 Marshall, J., Carroll, J., and Crighton, J. S., J. Anal. At. Spectrom., 1995, 10, 359R. Accepted October 9, 1996 32 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12

ISSN:0267-9477    

DOI:10.1039/a603605h

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Effects of Limiting Orifice (Anode) Geometries on Charged Particle Characteristics in an Analytical Radiofrequency Glow Discharge as Determined by Langmuir, Current and Voltage Probes† YUANCAI YE AND R. KENNETH MARCUS* Department of Chemistry, Howard L . Hunter Chemical L aboratories, Clemson University, Clemson, SC 29634-1905, USA Fundamental studies were performed to assess the role of detailed analytical performance characterization, but also limiting orifice (anode) diameter on the charged particle requires an amount of fundamental knowledge of the physical characteristics of a radiofrequency glow discharge (rf-GD) processes taking place within the devices.The Langmuir probe atomic emission source. Measurements of the electron and ion is a powerful tool for investigating the charged particle paramnumber densities, electron temperature, average electron eters in low temperature, reduced pressure rf plasmas.10 Several energy and electron energy distribution function were made by papers have reported the measurement of charged particle using an impedance-tuned Langmuir probe. The electrical parameters including electron number density (ne), ion number features of the rf-GD were studied by the simultaneous use of density (ni), electron temperature (Te), average electron energy voltage and current probes. Studies were focused on the effects (e ) and electron energy distribution function (EEDF) in of the limiting orifice geometry on the rf-GD operation under rf-GD sources using Langmuir probes.10–13 A computervarious conditions of operating pressure and probe sampling controlled, impedance-tuned Langmuir probe system has been position.The results show that limiting orifice diameters and designed and implemented for the diagnostic study of analytdischarge pressures have an important role in the excitation ical, rf-GD sources in this laboratory.11 conditions of rf-GD sources. In addition, rf-power generator One important set of discharge parameters in the determisystems can greatly affect electron and ion densities, and to a nation of cathodic sputtering and gas-phase energetics is the lesser extent the electron temperature, average electron energy combination of dc-bias voltage and current passing through and electron energy distribution functions.Some phenomena the discharge. The dc-bias voltages on the cathode surface are previously reported, including conditions that produce self- strongly related to the discharge processes in the sheath region absorption in atomic emission applications and the high degree very near to the cathode.Therefore, measuring the dc-bias of spatial specificity in MS applications, are reasonably voltages is a good way to investigate the fundamental discharge explained. processes within the sheath as they dictate the kinetic energies of the bombarding ions as well as the resulting secondary Keywords: Radiofrequency glow discharge; atomic emission electrons leaving the surface.A number of previous rf-GD spectrometry ; L angmuir probe ; plasma characteristics studies have reported dc-bias voltages under different discharge conditions and geometries.3,4,14–18 The measurement of the current at the cathode is of additional interest as it should Over the past few years, the radiofrequency (rf ) powered glow assist in the understanding of the flow of electrons and ions discharge (GD) has been developed as a viable analytical tool.within the plasma. Because rf-GDs typically run in a constant The primary advantages of the rf-GD over traditional analyt- power mode, most systems are monitored by in-line measure- ical techniques for analysing solid samples such as arc and ment of the cumulative forward and reflected powers. While spark spectrometries lie in the fact that the rf-GD sources can one hopes that these values accurately reflect the discharge be used in the direct bulk and surface analyses of metals and processes, these values cannot be taken literally.As such, it alloys and electrically non-conducting materials such as cer- would be instructive to measure separately the actual input amics, glasses and geological materials, without any prior power (as voltage and current components) as close as possible matrix modification. Additionally, these sources have demon- to the sample surface. The voltage and current, measured with strated high sensitivity in optical emission and mass spec- appropriate probes by an oscilloscope, can be used to calculate trometry studies, with detection limits of less than 0.1 mg g-1 the actual power delivered to the GD and allow evaluation of and excellent stability.1 In addition, the rf-GD has a wide the validity of the power values measured by the commercial dynamic range and lower degrees of matrix effects than arc systems.The theoretical prediction and experimental measure- and spark emission spectrometries, which should lead to more ment of electrical currents have been detailed by Paranjpe convenient use of the source.et al.19 and Hargis et al.,15 respectively. In this laboratory, research efforts have been focused on the The plasma characteristics of rf-GDs are affected by the development of various rf-GD sources.2,3 The devices have configuration of the electrodes and the chamber, materials of been successfully applied to the bulk and surface analyses of construction, external power circuitry, means of coupling different materials, through the use of atomic absorption, between the rf generator and electrodes, and the discharge emission and mass spectrometries.4 –9 The application and gases and working pressure.Initial results of studies, employing evolution of any spectrochemical device necessitates not only various spectrometric sampling modes, have shown that the limiting orifice (anode) geometry is one of the most important † Presented at the 1996 Winter Conference on Plasma Spectrochemistry, Fort Lauderdale, FL, USA, January 7–13, 1996.factors.4,5,9,16 For example, Lazik and Marcus9 found that Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 (33–41) 33copper sample sputtering rates were inversely related to orifice Pearson Electronics, Palo Alto, CA, USA) is employed here diameter, while also exhibiting lesser amounts of self- to measure the effective sputtering current.This probe has absorption in AES applications. They concluded that atom been shown to have exceptional band-width and linearity, diffusion paths away from the more confined negative glow providing excellent frequency response and current inforregion were probably responsible for the lesser extent of self- mation.15 The rf power cable is passed through a doughnutabsorption. No conclusions could be made with regard to shaped disc (6.3 mm id, 10 mm thick) wherein the magnetic changes in excitation conditions that might be occurring by fields associated with the rf current induce an ac current which changing orifice diameter; which would also be a viable is sampled by the probe.Because the thickness of the RG-238 contributor to the changes in the degree of self-absorption. In power cable is greater than the aperture in the probe, a splice a seemingly contradictory set of experiments, Shick and was placed in the power line such that a metal connector box Marcus20 found that analyte ion signals in rf-GDMS increased would allow mounting of the probe about the power cable.with increases in the limiting orifice diameter for a comparable The bare Cu connector is encased in a glass sleeve to protect source geometry. Those data suggest that while sputtering the current probe from touching the hot lead. The cubic metal rates are the lowest for large orifice diameters (10 mm), the box serves to shield the Langmuir probe measurement circuitry ionization efficiencies are greatest.This paper focuses on the from interference due to the exposed connections at the current investigation of the effects of limiting orifice geometries on the probe. The output of the current probe is connected to one charged particle populations in an analytical rf-GD source by input channel of the above oscilloscope by a coaxial cable. the use of Langmuir, current and voltage probes. Direct Representative temporal waveforms of the applied voltages measurement of electron and ion number densities, as well as and induced currents are plotted with an x–y recorder electron temperatures, distribution functions, and average ener- (Yokagawa, Model 3203, Baxter Scientific Products, McGaw gies are intended to give insights into the cited analytical Park, IL, USA) for storage and subsequent waveform analysis. responses.The rf-GD source has also been described in detail previously. 9 The Ar discharge gas (purity>99.999%) passes through a flow rate meter and a needle valve, with the flow INSTRUMENTATION AND THEORIES rate fixed at a relatively low value of 30 ml min-1 throughout Since previous papers reported in detail the choice of Langmuir this work.The discharge pressure is set by varying the pumping probe theories and the automated probe acquisition system rate of the source through a bellows valve located between the employed in this study,11,12 the specifics need not be reviewed pump and the source.The rf power was supplied by an RF-5S comprehensively here. The operation of the tuned Langmuir generator (13.56 MHz, RF Plasma Products, Marlton, NJ, probe is straightforward. Insertion of a metal wire into the USA) equipped with an automatic matching network (AM-5). rf-GD and applying a voltage (±60 V) produces a response The forward and reflected powers were displayed on the front current drained from the plasma which is recorded at different panel of the supply, with the reflected power consistently bias voltages, generating an i–V curve.Based on the shape of reading less than 1 W throughout these studies. Two vacuum the curve, the charged particle characteristics may be deduced. gauges are used in tandem to monitor the base vacuum quality The simplest and most classical ‘collisionless’ theories,21–23 (usually <4 mTorr) and the operating source pressure originally put forward by Langmuir24 and then further devel- (2–10 Torr) (1 Torr#133 Pa).oped by Laframboise,25 are chosen to process the obtained Prior to each experiment, the sample (oxygen-free, hard data. A C-language program has been written for the evalu- copper) surface is polished to a mirror finish with an alumina ation of data, with the calculation methods employed being slurry, rinsed with anhydrous methanol and dried in air. The very similar to those described by Fang and Marcus.26 probe is cleaned, to remove sputtered material which deposits The computer-controlled data acquisition system is based during the course of each sputtering cycle, after each data set on an Apple Macintosh IIsi computer with an NB-MIO-16XL with dilute (10%) nitric acid, rinsed with dry methanol and interface board (National Instruments, Austin, TX, USA).A dried in air. Finally, in order to ensure proper removal of high voltage operational amplifier (PA08A, Apex residual vapors from the source and/or sample surface, a Microtechnology, Tucson, AZ, USA) supplies a voltage range 30 min plasma stabilization time is adopted to remove residual from -100 to +100 V to the probe, based on a factor of ten vapors from the system.11 amplification of the interface board DAC output.The As will be seen in subsequent sections, one of the aims of impedance tuning portion of the probe circuit consists of a this study is to measure the difference in phase angle between variable capacitor (2–25 pF) and an inductor (2.5 mH) which the voltage and the current components of the delivered power.are connected in parallel and located between the plasma and Phase differences provide a qualitative, and possibly quantitat- the voltage source. The probe is impedance-tuned by the ive, measure of changes in the plasma impedance. To confirm adjustment of the variable capacitor to affect the lowest voltage that the voltage, the current and the difference in phase are produced on the electrically floating probe.The applied voltage measured accurately, the probes were tested by simultaneously and the voltage drop (i.e., current) across a high precision applying 13.56 MHz voltage signals from a waveform function 3.34 kV resistor are measured by and stored in the computer, generator (Wavetek, Model 166, San Diego, CA, USA) through representing the necessary i–V data. An applied LabView 50 and 100 V resistors. The responses were synchronous and program (National Instruments) supports the hardware used in agreement with the sensitivities presented by the commercial to acquire the data from the probe.The time required to manufactures, indicating that phase differences in the applied acquire a complete i–V curve is less than 1 min, where 200 voltage and current could be measured without bias.15 pairs of data are typically obtained, with excellent precision Once the waveforms of the voltage and the current have (less than 7% RSD) of measurement.11 been accurately measured, the instantaneous power input to A 15100 commercial voltage probe (PM 9100, capacitive, the cathode from the RF-5S generator can be accurately 3 pF, John Fluke Mfg., Everett, WA, USA) is employed to calculated by the following equation: measure the peak-to-peak (Vp–p) and dc-bias (Vdc) voltages present on conductive samples.16 The voltage probe is connecp( t)=V(t)×i(t) (1) ted to a 100 MHz digital storage oscilloscope (Model PM3375; Philips, Eindhoven, The Netherlands), and the voltage readings where p(t), V (t) and i(t) are instantaneous power, voltage and are taken from the CRT screen.A commercial, magnetically inductive, current probe (Pearson 2878, sensitivity=0.1 V A-1, current, respectively, at time t. The actual power (P) dissipated 34 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12in the plasma can be calculated by: P=APT 0 p(t)dtB/T (2) where T is the period (T=7.375×10-8 s in this case) of the waveforms of voltage and current.The electrical characteristics of the discharge can be examined as a function of the electrode potentials and the various regions of the plasma as shown in Fig. 1. These processes can be simplified as shown in the ‘classical’ equivalent circuit depicted in the center of Fig. 1.27 As depicted here, the electrical behavior in GD sources is determined by the resistances and capacitances of the target/ cathode (Rt and Ct) and wall/anode (Rw and Cw) sheaths and the resistance of the bulk plasma (Rp).It is well known that the sheath adjacent to the cathode is primarily capacitive in Fig. 2 Configuration of the anode orifice disc and the positioning of nature.15,19,27,28 Because of the high degree of asymmetry of the Langmuir probe. the current source design (i.e., the cathode is much smaller than the anode), the capacitance of the anode sheath is much larger than that of the cathode.27 The classical equivalent It should be pointed out that the above equations [(3)–(8)] circuit model can be further simplified for the plasma under are only strictly valid under the supposition that no inelastic study here into the form of the right-hand one in Fig. 1 (see collision reactions occur among the electrons, ions, atoms and Electrical Characteristics of the Rf-GD). The instantaneous molecules within the plasma. This obviously contradicts the voltage, current and the dissipated power can be expressed as observed spectroscopic features of rf-GDs, and a small fraction a function of time through the following equations:15,19,27,28 of the dissipated energy is consumed by the above collisions.28 Therefore, the equations are only able to describe electrical V (t)=Vdc+Vrf sin(2pt/T) (3) characteristics of rf-GD sources approximately. Nevertheless, i(t)=irf sin(2pt/T-w) (4) these mathematical predictions of electrical characteristics are useful for our understanding.If the cathode sheath and the P=0.5×Vrfirf cos(w) (5) bulk plasma are perfectly capacitive and resistive, respectively, where Vdc, Vrf, irf and w are the dc-bias voltage, the peak and no inelastic collisional reactions take place in the glow, voltage, the peak current and the difference in phase between the waveforms of V(t) and i (t) should be perfectly sinusoidal the waveforms of current and voltage, respectively. Here the in nature. The differences between ideal (sinusoidal) and actual w-value, which is always less than 90°, is dependent on the shapes of V (t) and i(t) qualitatively indicate how strongly the resistance of the bulk plasma and the capacitance of the above various inelastic collisional reactions take place within cathode sheath by: the GD source.The focus of this study is to evaluate the influence of anode tan(w)=T /2pRpCT (6) geometry on the characteristics of an analytical rf-GD source. where Rp and CT are the resistance of the bulk plasma and In order to facilitate this, six removable orifice discs, as shown the total capacitance of the system, respectively.Both quantities in Fig. 2, are employed.9 Each disc was 3 mm thick, with orifice depend on many factors such as the thickness of the sheath, diameters ranging from 2.5 to 12 mm. In an effort to minimize the area of the cathode, the discharge gas and operating the complexity of this study, a constant power of 20 W was pressure, the geometry of the cathode and the gas flow rate.employed in all experiments. Previous studies have indicated The cumulative plasma impedance, Z, can be obtained by two that while input power influences the charged particle densities, methods, either: it does not appreciably affect electron temperatures, average electron energies and electron energy distribution func- Z=Vrf/irf (7) tions.10–13 As such, it is believed that the influences of the or input power on the charged particle properties would be easy to infer based on the results of those studies.Z2=(Rp)2+(T /2pCT)2 (8) In this work, Vrf, irf and w are measurable. Therefore, Z, Rp RESULTS AND DISCUSSION and CT can be estimated by the above equations. Electron Number Density Previous studies have shown that sputtering rates are strongly dependent on the limiting orifice diameter, presumably due to higher power densities and consequently higher dc-bias voltages. 9 Fig. 3 clearly shows the influence of limiting orifice diameter and probe sampling distance (from the cathode surface) on electron densities at a working pressure of 5.2 Torr.As has been seen in previous rf-GD studies,12,13 electron densities are spatially homogeneous at distances of 5–15 mm from the surface, regardless of the limiting orifice diameter. This phenomenon is not surprising,11–13 and can be ascribed to the high electron diffusion rate in the GD environment. As will be seen in the following section, and has been observed in MS studies,20 positively charged ions are not so evenly distributed.AES and MS studies have shown that limiting orifice Fig. 1 Equivalent circuit of the rf-GD plasma. diameter and discharge gas pressure can play counteractive Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 35Ion Number Density Ion number density is an important factor as it may influence the cathodic sputtering rate and also serves as an indication of how and where ionization reactions take place in the plasma.This latter information dictates the fact that ion sampling positions must be optimized in analytical GDMS.2,4 Fig. 5 depicts the spatial distribution of positive ions for three different orifice disc sizes at a source pressure of 5.2 Torr (the same conditions as those employed in Fig. 3). Ion densities show steady decreases with increasing sampling distance, having an approximately inverse dependence on distance.As with electron densities, this response is analogous to results of previous ion density maps in rf-GDs.10–13 In comparison with electrons, ions have larger masses, and thus their diffusion Fig. 3 Electron number densities as a function of probe sampling rates are much lower. The negative dc-bias voltage on the position for different limiting orifice diameters: A, 2.5 mm; B, 6 mm; cathode may also hinder ionic diffusion from the region of the and C, 12 mm (source pressure=5.2 Torr, rf power=20 W).cathode dark space/negative glow interface.12 Given the strong dependence of the determined ion number roles in analytical response, where the observations in those density on the sampling position, the effects of orifice diameter studies suggest variations in excitation/ionization con- and discharge pressure were evaluated at a distance of 5 mm ditions.9–11 Fig. 4 graphically demonstrates the dependence of from the cathode surface, rather than the 7.5 mm distance in electron density on working pressure and limiting orifice the electron measurements.As can be seen in Fig. 6, both diameter at a sampling position 7.5 mm away from the cathode. limiting orifice diameter and working pressure have strong The results show that orifice diameters strongly affect the way influences on the determined ion densities. The maximum ion in which the electron densities respond to changes in working densities are seen to occur at lower discharge pressures as the pressure.Electron number densitiesare seen to increase steadily orifice diameters are increased. This trend of increasing densiwith increases in working pressure for the smallest orifice ties with increased orifice diameter reflects the same obserdiameter, 2.5 mm. This response is very similar to that observed vations seen in MS studies by Shick and Marcus.20 The for analyte emission intensities for that orifice size, where influences seen for the 5 mm sampling position are similar to higher pressures produce greater analytical response even those seen for each sampling position, albeit with differences though sputtering rates decrease.A nearly opposite trend is seen for the larger orifice diameters. Each of the orifice discs with diameters between 4 and 10 mm exhibit a maximum in electron number density at a defined pressure range. This optimum pressure shifts from 8 Torr for the 4 mm diameter to 3 Torr for the largest (12 mm) orifice size.The shift to lower pressures suggests a dependence on the discharge voltage, with higher voltages tending to produce higher number densities.12 The highest number densities (7×1010 cm-3) are seen for an 8 mm diameter orifice and a pressure of 4 Torr. Owing to the homogeneity of the electron distributions in the discharge (negative glow) volume, the electron densities obtained for this single sampling position are probably a fair representation of the negative glow region. Fig. 5 Ion number densities as a function of probe sampling position for different limiting orifice diameters: A, 2.5 mm; B, 6 mm; and C, 12 mm (source pressure=5.2 Torr, rf power=20 W). Fig. 4 Electron number densities as a function of limiting orifice Fig. 6 Ion number densities as a function of limiting orifice diameter and discharge pressure (probe sampling position=5 mm, rf power= diameter and discharge pressure (probe sampling position=7.5 mm, rf power=20 W). 20W). 36 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12in absolute values. Comparison of Figs. 6 and 4 reveals similar immediately from Fig. 7. First, the ratio shows a distinct inverse relationship with sampling distance. These variations trends as both densities show their highest values when the pressures are on the lower end of the range and the orifice in ni/ne reflect more the changes in spatial distribution of ions, as the electron densities are fairly constant as a function of sizes are large.The slight differences in the trends are attributed in part to the effect of orifice diameter on ionic spatial sampling distance. As shown previously, ion densities decrease with increasing distance, with the ion densities near the cathode distribution. It must be stressed that these density maps are not expected to be identical as the negative glow of the plasma sheath being much higher than those at the bulk negative GD for each limiting orifice.This distribution was illustrated is not charge-neutral over any spatial region sampled by the probe in a given experiment. empirically in the early rf-GDMS studies where optimum analyte signals were obtained by sampling the region of the It is necessary to point out here that both the electron and ion densities in this study are as much as 30 times higher than negative glow/dark space interface, albeit at much lower pressures than those employed here.4 The second obvious result is those presented in the two previous studies performed with the same discharge source under the same conditions of power that while the absolute values of ni and ne vary with orifice diameter, the ni/ne ratio is constant.This indicates that changes and working pressure.11,12 This discrepancy results from the different power supply systems employed in these studies. In in orifice diameter do not affect appreciably the spatial location where ionizing events take place within the plasma.This is an the present study, the complete RF-5S power supply and matching system replaces the earlier system, which consisted important point in the analytical application of the devices in GDMS. While changes in orifice diameters are often dictated of independent components including a function generator (Wavetek, Model 166), an rf amplifier (Amplifier Research, by sample sizes, no changes in ion source geometry (i.e., ion sampling distance) need be made.20 Model 50A220, 10 kHz–200 MHz frequency range), a tuner (MFJ Versa Tuner 5, Model MFJ-989C), and an in-line rf An interesting situation exists at the sampling distance of 7.5 mm, where for this set of discharge conditions the ni /ne power meter (Thruline Model 43, Bird Electronics).11,12 Parker and Marcus16 similarly observed that the component system values are approximately unity.Studies of the roles of source pressure and orifice diameter on the ni/ne characteristics were resulted in much lower atomic and ionic absorbances (i.e., sputtering rates), as well as dc-bias voltages, than the present initiated at this position as it was expected that any effects would be the most profoundhere. Fig. 8 depicts the dependence system. A comprehensive study of the roles of power supplies, coupling and plasma design for semiconductor fabrication of the ni/ne ratio on orifice diameter and working pressure. As observed in previous studies in this laboratory,12 these results applications suggests that such differences as seen here are not surprising.15 Intuitively, the larger dc-bias potentials seen here demonstrate that increases in working pressure dramatically compress the ion populations towards the sample surface, indicate better power coupling to the plasma in this case, in comparison with the former studies.It is believed that the which is probably due to the shortening of the cathode dark space as electron mean free paths are decreased.28 It is generally system employed here allows a more accurate measurement of the power delivered to the plasma.The results in this study observed that high operating pressures compress the size of GD plasmas for all geometries. The extent of the compression also indicate that the power supply system affects electron temperatures, average electron energies and electron energy here seems to be greatest for those orifices with smaller diameters, suggesting that the grounded anode orifices confine distribution functions, although to a lesser extent.the plasmas not only in terms of the surface sputtering area but also in terms of confining the negative glow. Ratio of Ion Number Density to Electron Number Density As mentioned previously, the negative glow region of a GD Electron Energy Distribution Function plasma cannot be assumed to be charge-neutral or homogeneous. These qualities not only necessitate careful spatial While the effects of orifice diameter on the charged particle densities give insights into the structure of the plasma negative optimization in GDMS, but also provide clues as to basic plasma processes.Fig. 7 depicts the dependence of the ratios glow and to spatial considerations required for analytical rf-GDMS,4 studies by Ye and Marcus12 have shown that of ion number density to electron number density (ni/ne) on the sampling distance and orifice diameter at a working factors affecting electron energetics seem to dictate analytical pressure of 6 Torr.Two definite sets of relationships emerge Fig. 8 Ratio of ion number densities to electron number densities as Fig. 7 Ratio of ion number densities to electron number densities as a function of probe sampling position and limiting orifice diameter a function of source pressure and limiting orifice diameter (probe sampling position=7.5 mm, rf power=20 W). (source pressure=6 Torr, rf power=20 W). Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 37responses to a greater extent, i.e., fewer electrons with higher Average Electron Energy energies produce greater analytical emission than more elec- While visual inspection of EEDFs gives insights into plasma trons of lesser energy. For this reason, analytical responses are excitation efficiencies, they are only qualitative in nature and not as much suppressed for non-conductive sample analyses difficult to put into perspective in parametric studies such as as would have been predicted based on the far lower sputtering these.A better measure of the changes in the energies of the rates in comparison with metals and alloys. The dependence overall electron populations is the weighted average electron of electron energy distribution functions (EEDFs) on working energy e . As shown in Fig. 10, e values obtained at a pressures, orifice diameters and sampling distances is pro- sampling position 5 mm from the cathode surface dramatically nounced, particularly for orifice diameters.As is depicted in increase with decreases in limiting orifice diameter. For Fig. 9, the location of EEDF maxima dramatically shifts to instance, at a pressure of 2 Torr the average electron energies higher electron energies with decreases in limiting orifice for the 10 mm orifice diameter are approximately 0.6 eV, while diameters. For example, maxima are observed at energy values a value of #5 eV is found for the 2.5 mm case.As reported of 0.6 and 5 eV for the 10 and 2.5 mm orifice sizes, respectively, previously,11 average electron energies are very sensitive to at a pressure of 2 Torr. This disparity is not as severe for the working pressure.The data depicted here more clearly illustrate 10 Torr discharge pressure, where for the large orifice disc the the (now expected) enhancement in optical emission intensities maximum has shifted to 4 eV, while the first maximum for at high operating pressures and small orifice diameters.Plots the 2.5 mm orifice is #8 eV at the higher pressure. (The of e values as a function of sampling distance show a very negative-going portion of the latter is due to the bi-modal slight increase with increasing distance, as would be expected electron populations and is an artifact of the second-derivative from the EEDF data of Fig. 9. calculation procedure.26) The trend of increasedEEDF maxima with increasing pressure was observed in previous studies.12 The effect of sampling distance on the measured EEDFs is not as pronounced as the effects of orifice diameter and pressure, but there is a definite shift of the maxima to higher energy values at increasing distance.12 This shift probably reflects the lower densities of sputtered atoms at greater distances, removing effective electron energy loss mechanisms.The observed dependences of the EEDFs on limiting orifice diameters sheds new light on early observations in rf-GDAES.Lazik and Marcus9 found that the intensity of the Cu I 324.7 nm resonance emission strongly depended on orifice diameter. It was concluded in those studies that small orifice diameters (2 mm) suffered less from self-absorption effects than orifices of larger diameter, yielding resonant state emission intensities which were more than an order of magnitude greater in some instances. In contrast, emission from high-lying Cu I levels (402.3 nm) improved with increasing orifice diameter while sputtering rates decreased.The data depicted in Fig. 9 suggest that the higher resonant state emission intensities are the result of the combination of higher sputtering rates with Fig. 10 Average electron energy as a function of source pressure and the smaller orifices and the increased excitation efficiencies limiting orifice diameter (probe sampling position=5 mm, rf power=20W). (thus lower self-absorption) resulting from the higher EEDFs.12 Fig. 9 Effects of orifice disc, diameter, source pressure [(a) 10 mm, 2 Torr; (b) 2.5 mm, 2 Torr; (c) 10 mm, 10 Torr; and (d) 2.5 mm, 10 Torr], and probe sampling position (A, 15 mm; and B, 5 mm) on the determined EEDFs. 38 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12Electron Temperature Electrical Characteristics of the Rf-GD Langmuir probe measurements have been effective in studying The electron temperature derived from Langmuir probe plots should be the most relevant of these data in terms of compari- the gas-phase results of the application of rf potential to the GD sample and its effects on charged particle densities and sons of different spectroscopic excitation sources.The results shown in Fig. 11, obtained at a 5 mm sampling distance from energies. Previous studies of dc-bias potentials have also provided supporting evidence for observed differences in sput- the cathode surface, show the dependence of electron temperatures on orifice disc diameter and working pressure. Basically, tering rates for different sample types and discharge conditions. 9,16 Questions still remain in terms of the partitioning the effects of these parameters are very similar to those seen in the e data (Fig. 10), albeit with lesser degrees of depen- of the applied rf power between the instantaneous voltage and current components, their phase relationships and the dence. It is obvious that both increasing the working pressure and decreasing the limiting orifice diameter increase the elec- impedance of the cathode dark space.It is these parameters that dictate initial energy input to the plasma and the observed tron temperature. The temperatures obtained here are generally in agreement with those obtained from other low pressure steady-state atomization/excitation/ionization characteristics. Fig. 12depicts the waveforms(captured on the digital oscillo- discharge systems,11–13,29–32 although the experimental conditions here differ from those in the antecedent studies dealing scope) of the voltage on the cathode and of the current passing through the cathode at pressures of 2 and 10 Torr for discs with low pressure inductively coupled and sputter deposition plasmas.More pronounced than for average electron energies, with limiting diameters of 2.5 and 10 mm. The first qualitative feature apparent from the comparison of Fig. 12(a) and (b) electron temperatures definitively increase with increasing sampling distances, particularly at higher pressures. with Fig. 12(c) and (d ) indicates that high working pressures not only decrease Vrf values, dc-bias voltages and currents, but A comparison of the derived average energy and temperature values across the range of discharge conditions studied here they also dramatically distort the waveforms of both the voltage and the current. This observation suggests that the provides further insights into rf-GD source operation.It should be pointed out that temperatures do not smoothly change with rf-GD becomes less capacitive in nature at high working pressure as the waveforms deviate far from their original the changes in orifice disc diameters because the actual electron energy distributions are probably comprised of multiple sinusoidal shape. This reflects the fact that the inelastic collisions/ reactions are more prominent in the cathode sheath at Maxwellian functions as shown in Fig. 9(b) and (d ). Non- Maxwellian behavior (i.e., lack of a single Maxwellian function) high working pressure than at low working pressure. In particular,symmetric charge exchange reactions between argon leads to errors in the calculation of electron temperature. Nevertheless, in most cases, the ratios of the e values to Te ions and atoms are more favorable in terms of shorter mean free paths (high Ar atom densities) and lower ion velocities are between 1.2 and 2.0, close to the value of 1.5 which is indicative of an electron energy distribution that is (low dc-bias potentials).The second observation to be made from these waveforms Maxwellian.11,12,33 If this situation is a correct representation, any of the electron energy parameters (EEDF, e or Te) can is that the discs with small diameters do not exhibit as much distortion of the current and voltage waveforms as for the be used to derive the others. As was the case for the charged particle densities, comparison large orifice diameters.This basic observation reveals that the effects of inelastic collisions are more pronounced in affecting of the EEDF, e and Te values obtained with this power supply system with those obtained with component system10,11 the current (ion) flow with the large diameter discs. This relationship follows directly that of the previous paragraph in indicates that the power supply system can affect electron energetics (each value slightly lower in this case).The influences that the large orifice discs operate at lower dc-bias potentials than the smaller orifice discs for the same applied power. Thus, are not as strong as those seen for the electron and ion densities demonstrated above (see Electron Number Density we would expect more extensive charge exchange reactions to take place under these conditions. and Ion Number Density), as was the case in studies of sputter deposition systems.16 This suggests that power supply systems The total impedance (Z) of the plasma can be directly calculated through eqn.(7). The Z-values depend only slightly will influence the optical and mass spectral characteristics as they are dependent on EEDF, e and Te values in a plasma. Fig. 12 Waveforms obtained by voltage and current probes at pressures of 2 [(a) and (b)] and 10 [(c) and (d)] Torr and limiting orifice diameters of 2.5 [(a) and (c)] and 10 [(b) and (d)] mm.(a) dc bias= Fig. 11 Electron temperature as a function of source pressure and 435 V, Vrf=476 V, irf=3.8 A, Z=125 V; (b) dc bias=461 V, Vrf=436 V, irf=4.3 A, Z=101 V; (c) dc bias=284 V, Vrf=288 V, irf=3.0 A, Z= limiting orifice diameter (probe sampling position=5 mm, rf power=20 W). 96 V; and (d) dc bias=206 V, Vrf=232 V, irf=2.8 A, Z=84 V. Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 39on the working pressure and disc diameters, having values of An interesting aspect of these measurements is that the discharge current is very large (>2 A), comparable to those #100 V.These values follow the general trends of the Vrf values, increasing with applied rf power. Based on the directly reported for sputter deposition systems.15 In addition, the electron flow velocity is very fast in this rf-GD source (peak measured parameters, eqn. (8) can be solved to obtain the total capacitance (CT) by first rearranging eqn. (6) to solve for current#2.5 A for ne=4×1010 cm-3) in comparison with dc-GD sources (current#0.02 A for ne=15×1010 cm-3).26 Rp [eqn.(9)] and then substitution into eqn. (8). By virtue of the fact that the observed This is reflected in the fact that electrons in the rf-GD have higher energies than those in the dc-GD. This is probably the Rp=T /2p tan wCT (9) result of a larger fraction of the input energy being carried by electrons in the negative glow and less of the energy being phase angles are very close to 90° (86–90°), the tan w values (>14) make the resistive component of the total impedance consumed in the cathode sheath for rf-GDs than for dc-GDs, thus the observations that rf-GD-AES sources are more effec- very small (<1%) in comparison with the capacitive component and thus can be ignored to yield total capacitance values tive in their excitation while sputtering less material than their dc counterparts.1,2,6 This makes intuitive sense because elec- which vary from 139 to 94 pF. These values are consistent with those obtained for sputter deposition systems.15 The trons in the rf plasma are continuously accelerated and decelerated by the varying applied potential while the electrons in resistance of the plasma (Rp) is interesting as it is a measure of the input energy transferred into the bulk negative glow; the dc plasma move by diffusion and the slight potential gradient within the negative glow.28 Finally, since the current the higher the resistance to electron flow the higher the power dissipation.This determination is a major goal in this work; (normally 2500 mA) is large in rf-GD sources in comparison with the current drawn by the Langmuir probe (generally unfortunately, the difference in phase angle between the current and voltage waveforms cannot be measured with sufficient 15 mA), the operation of the probe does not appreciably perturb the bulk plasma properties so that the electron precision (#5% RSD here), making these values very prone to large errors.While small variations in the phase angle have densities, etc., should be correctly determined. little effect on the contribution of the resistive component to the total impedance, the resulting large changes in the tangent CONCLUSIONS values lead to very large differences in the calculation of the resistance. Nevertheless, Rp-values can be roughly calculated The measurements of the charged particle parameters including ne , ni, Te, EEDF and e have been made in an analytical to be in the range 1–7 V, with values of #3 V being the norm.Again, such values are in general agreement with those pub- rf-GD atomic emission source by use of an impedance-tuned, computer-controlled Langmuir probe. In addition, voltage and lished for deposition plasmas.15 The above electrical parameters are useful in the successful design and optimization of current probes have been simultaneously employed to yield new insights into the electrical features of this rf-GD.Studies rf-GD sources and power supply systems for different process plasma applications.28 Improvement in the ability to assess the here have been focused on the effects of limiting orifice geometry on the rf-GD under various conditions of working phase differences between the waveforms in this system will hopefully also add new insights in terms of accurately describ- pressure and probe sampling position.Based on the results obtained here, a number of conclusions can be reached and ing the distribution of input power between the sputtering and gas phase processes. summarized as follows: (1) The limiting orifice diameter plays important roles in The electrical parameters obtained here strongly support a simplification of the ‘classical’ equivalent circuit for rf-GD the charged particle densities in rf-GD sources. At lower pressures, both ion and electron number densities increase plasmas to the model depicted on the right-hand side of Fig. 1. First, the Ct-impedance must be much less than the Rt-value incrementally with orifice diameter. At high pressures, electron densities are observed to decrease while ion densities are not in Fig. 1 because each of the observed phase differences (as shown in Fig. 12) are close to 90°. Second, because the present affected by increasing diameters. Interestingly, the conditions that lead to optimum AES and MS signals correlate well with source design has a high degree of asymmetry, with the cathode surface area being #1/100 that of the anode, the capacitance maxima of electron and ion number densities on the pressure– orifice diameter coordinates.While there is a spatial depen- of the anode sheath should be much larger than that of the cathode.27 Quantitatively, the relative capacitances can be dence to the ratio of ion-to-electron number densities (decreasing with distance), these values are only affected at very low calculated from data of the sort depicted in Fig. 12 through the relationship:27 pressures. (2) As reported previously,11 working pressures greatly Vdc=Vrf[(Ct-Cw)/(Ct+Cw)] (10) influence electron temperature, average electron energy and EEDFs. Increasing pressures tend to increase electron tempera- On average, the capacitance of the cathode sheath is #50 times less than that of the walls over the range of discharge tures and average electron energies, also shifting electron energy distribution to higher energies. Small orifice diameters conditions applied here.This value is higher than those found for sputter deposition systems, but not inconsistent given the lead to the shifting of EEDFs towards higher energies (particularly at low pressures) with average electron energies following disparity in discharge pressures. By analogy, the anode sheath resistance, Rw, can be assumed to be infinite in comparison suit. Electron temperatures are less sensitive to changes in orifice diameter, although small orifice sizes yield higher values with the capacitive impedance of the anode sheath.27 Therefore, it can be assumed that most of the discharge current (electrons) at low pressures.(3) The data reported here provide fundamental support for passes through the Ct and Cw components of the equivalent circuit rather than Rt and Rw, and the total impedance of this the observed differences in the analytical responsivity of rf-GD spectrometries. Specifically, the preferable operation of the source is mainly determined by Ct.Therefore, the discharge processes in this source can be simplified to the form of the sources in an atomic emission mode under conditions of high pressure and small orifice diameters9 reflects a combination of right-hand equivalent circuit in Fig. 1. That is, the electrical behavior in this rf-GD source is determined mainly by the higher sputtering rates for small orifice diameters coupled with the trends of higher average electron energy and temperatures. capacitance of the cathode sheath and the resistance of the bulk plasma, without a strong need to consider the resistances This combination of effects would be expected to produce optimum rf-GD-AES response.The observed dependences of of either the cathode and anode sheaths or the capacitance of the anode sheath. MS signal intensities on limiting orifice diameter and source 40 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 1212 Ye, Y., and Marcus, R. K., Spectrochim. Acta, Part B, 1996, 51, 509. pressure are also supported in these studies.20 In fact, both ion 13 Heintz, M. J., and Hieftje, G. M., unpublished work. and electron number densities tend to reach a maximum under 14 Lazik, C., and Marcus, R. K., Spectrochim. Acta, Part B, 1994, conditions of low pressure and large orifice diameter, as 49, 649. observed in rf-GDMS operation.Interestingly, these conditions 15 Hargis, P. J., Jr., Greenberg, K. E., Miller, P. A., Gerardo, J. B., are the opposite to those that produce high energy electrons. Torczynski, J. R., Riley, M. E., Hebner, G. A., Roberts, J. R., Olthoff, J. K., Whetstone, J. R., Brunt, R. J. V., Sobolewski, This dichotomy would support a mechanism by which most M. A., Anderson, H. M., Splichal, M. P., Mock, J. L., Bletzinger, P., electrons present in the negative glow are secondary electrons Garscadden, A., Gottscho, R.A., Selwyn, G., Dalvie, M., resulting from the sputtering process rather than due to gas Heidenreich, J. E., Butterbaugh, J. W., Brake, M. L., Passow, phase ionization events. M. L., Pender, J., Lujan, A., Elta, M. E., Graves, D. B., Sawin, (4) Simultaneous measurement of the applied rf voltage and H. H., Kushner, M. J., Verdeyen, J. T., Horwath, R., and Turner, current waveforms gives extended insights into the collisional T. R., Rev.Sci. Instrum., 1994, 65, 140. 16 Parker, M., and Marcus, R. K., Spectrochim. Acta, Part B, 1995, processes within the plasma. Strong inelastic collision reactions 50, 617. are seen to take place in the cathode sheath, particularly at 17 Heintz, M. J., Galley, P. J., and Hieftje, G. M., Spectrochim. Acta, high pressures. These collisions, observed as distortions of the Part B, 1994, 49, 745. respective waveforms, also seem to increase with increasing 18 Woo, J., Cho, K. H., Tanaka, T., and Kawaguchi, H., Spectrochim. orifice diameters, even for low pressure operation.These trends Acta, Part B, 1994, 49, 915. correspond to those conditions wherein low dc-bias potentials, 19 Paranjpe, A. P., McVittie, J. P., and Self, S. A., J. Vac. Sci. and thus low ion/electron velocities, exist. T echnol. A, 1990, 8, 1654. 20 Shick, C. R., Jr., and Marcus, R. K., Appl. Spectrosc., 1996, 50, 454. (5) Finally, the results of these studies demonstrate the effect 21 Chen, F. F., in Plasma Diagnostic T echniques, ed. Huddlestone, that a particular rf-power supply/matching network system R. H., and Leonard, S. L., Academic Press, New York, 1965, ch. 4. has on electron and ion densities, as well as electron energies. 22 Swift, J. D., and Schwar, M. J. R., Electrical Probes for Plasma These differences, along with differences in sputtering rates, Diagnostics, Elsevier, New York, 1971. can greatly affect the analytical performance of rf-GD sources. 23 Clements, R. M., J. Vac. Sci. T echnol., 1978, 15, 193. 24 Langmuir, I., in Collected Works of Irving L angmuir, ed. Suits, G., Pergamon, Long Island City, NY, 1961, vols. 4 and 5. Financial support of the National Science Foundation under 25 Laframboise, J. G., University of Toronto, Institute for Aerospace grant No. CHE-9420751 is gratefully acknowledged. Studies Report No. 100, 1966. 26 Fang, D., and Marcus, R. K., Spectrochim. Acta, Part B, 1990, 45, 1053. REFERENCES 27 Kohler, K., Coburn, J. W., Horne, D. E., Kay, E., and Keller, 1 Marcus, R. K., Harville, T. R., Mei, Y., and Shick, C. R., Jr., J. H., J. Appl. Phys., 1985, 57, 59. Anal. Chem., 1994, 66, 902A. 28 Chapman, B., Glow Discharge Processes, Wiley, New York, 1980. 2 Marcus, R. K., J. Anal. At. Spectrom., 1994, 9, 1029. 29 Ruzic, D. N., and Wilson, J. L., J. Vac. Sci. T echnol. A, 1990, 3 Lazik, C., and Marcus, R. K., Spectrochim. Acta, Part B, 1993, 8, 3746. 48, 863. 30 Busch, K. W., and Vickers, T. J., Spectrochim. Acta, Part B, 1973, 4 Duckworth, D. C., and Marcus, R. K., Anal. Chem., 1989, 61, 1879. 28, 85. 5 Parker, M., and Marcus, R. K., Appl. Spectrosc., 1994, 48, 623. 31 Wilson, J. L., Caughman II, J. B. O., Nguyen, P. L., and Ruzic, 6 Harville, T. R., and Marcus, R. K., Anal. Chem., 1993, 65, 3636. D. N., J. Vac. Sci. T echnol. A, 1989, 7, 972. 7 Bordel-Garcý�a, N., Pereiro-Garcý�a, R., Ferna�ndez-Garcý�a, M., 32 Walters, P. E., Chester, T. L., and Winefordner, J. D., Appl. Sanz-Medel, A., Harville, T. R., and Marcus, R. K., J. Anal. At. Spectrosc., 1977, 31, 1. Spectrom., 1995, 10, 671. 33 Lai, C., Breun, R. A., Sandstrom, P. W., Wendt, A. E., 8 Shick, C. R., Jr., Raith, A., and Marcus, R. K., J. Anal. At. Hershkowitz, N., and Woods, R. C., J. Vac. Sci. T echnol. A, 1993, Spectrom., 1994, 9, 1045. 11, 1199. 9 Lazik, C., and Marcus, R. K., Spectrochim. Acta, Part B, 1992, 47, 1309. Paper 6/06346B 10 Paranjpe, A. P., McVittie, J. P., and Self, S. A., J. Appl. Phys., Received September 16, 1996 1990, 67, 6718. 11 Ye, Y., and Marcus, R. K., Spectrochim. Acta, Part B, 1995, 50, 997. Accepted September 25, 1996 Journal of Analytical Atomic Spectrometry, January 1997, Vo

ISSN:0267-9477    

DOI:10.1039/a606346b

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Improved Signal-to-noise Ratio in Glow Discharge Ion Trap Mass Spectrometry via Pulsed Discharge Operation†‡ DOUGLAS C. DUCKWORTH, DAVID H. SMITH AND SCOTT A. MCLUCKEY Chemical and Analytical Sciences Division, Oak Ridge National L aboratory, Oak Ridge, TN 37831-6375, USA An improvement in the S/N ratio is reported for the analysis ence of intense matrix and discharge gas (Ar) ion beams.9 This of trace elements in brass by glow discharge ion trap mass observation was important because it allowed the dynamic spectrometry.This was achieved by synchronizing the pulsed range of the trap, which has an ion volume limited to 104–105 discharge voltage with the ion injection and acquisition events; ions, to be extended through a variety of selective ion accumu- ‘on’ during ion injection and ‘off’ during data acquisition. Two lation methodologies. Several tactics were shown to eliminate modes of operation were evaluated: (1) a high duty cycle matrix ions, which would normally fill the trapping volume pulse, which allowed a continuous injection over the duration and limit the dynamic range to #102 (due to space-charge of the pulse; and (2) a low duty cycle pulse with multiple data effects), allowing unimpeded accumulation of ions of minor gates, which allowed gated injections of ions at selected regions constituents. In effect, the dynamic range was extended to 105 of the pulse profile.The latter afforded a means of selective ion (i.e., 10 ppm–100%).injection since discharge and residual gas species are formed at Having realizeda substantial improvement in signal intensity different times in the pulse event than analyte ions. for trace species, attention was given to the reduction of noise Improvements in S/N ratios greater than 40-fold were for further improvement in the S/N ratio. This was driven by observed, primarily due to a reduction in background and the following observations: (1) the primary source of noise is background noise after the discharge was extinguished.charged species extracted from the glow discharge; (2) most of Evidence is presented which suggests that electrons emanating the noise arises from outside of the ion trap volume (charged from the ion source are the precursors of most of the noise. particles from the ion gauge contribute substantial noise to Detection limits for various elements were 0.2–0.5 ppm. the detector); and (3) background and background noise increase in the presence of the He buffer gas (1–4 mTorr).Keywords: Glow discharge ion trap mass spectrometry ; (Whether it is detector dark current or non-mass resolved elemental analysis; direct solids analysis ; noise; pulsed signal, background refers to the average signal from the discharge; gated ion injection detector; noise refers to the deviations from that average value.) Shielding the detector resulted in only modest improvement. Quadrupole ion traps have realized wide acceptance as mass In its simplest form, glow discharge analysis using an ion analyzers in organic applications, primarily as a mass analyzer trap is a two-step process, comprised of an ion injection period and tandem mass spectrometer in GC–MS.1 More recently and a data acquisition period, in which ions are ejected from they have been employed as tandem mass spectrometers the trap and detected.Ion injection periods typically occur coupled with liquid chromatography and capillary electrophor- over a time frame of 1 ms–1 s, and mass analysis requires of esis.2 Ion traps are rapidly being extended into the inorganic the order of 20 ms.To provide acceptable ion statistics for mass spectrometry domain with application in the elemental accurate abundance measurements and to improve S/N ratios, analysis of solids and solutions. Such applications include laser a series of scans is typically acquired and averaged for display. ablation,3–7 glow discharge,8,9 secondary ion10 and inductively Because most of the noise originates from the glow discharge, coupled plasma11–13 mass spectrometries.Reported here is the it was reasonable to expect the noise to be largely eliminated use of pulsed glow discharges for substantial improvements in by turning the discharge off during the acquisition period. The S/N ratios in glow discharge ion trap mass spectrometry relatively long time frames associated with ion injection and (GD-ITMS).mass analysis are easily compatible with pulsed discharge In a prior report the interface and operational characteristics operation. of a glow discharge ion trap mass spectrometer for bulk solid Pulsed glow discharges have been used in conjunction with elemental analysis were described.8 Several promising charac- linear quadrupole14–18 and time-of-flight19 mass spectrometers. teristics of the ion trap were identified, including MS–MS Typically, the pulsed discharge operates at 50 Hz with 25% capabilities, ‘spontaneous’ dissociation of some polyatomic duty cycle. There are advantages to operation in the pulsed ions, and the reduction of argon-related species through charge mode.Atomization and ionization are increased relative to exchange processes with adventitious water in the vacuum continuous discharges because higher instantaneous power is system. These observations held promise for the generation of required to maintain the same average power.14 Furthermore, purely atomic spectra of most elements.with gated data collection, one can preferentially sample Also noted in early experiments was the ability to trap ionized sputtered species over discharge gas species by sam- selectively and analyze minor sample constituents in the pres- pling ions in the so-called pre-peak and after-peak portions of the ion signal profile.14,15,18 Descriptions of the mechanisms † Paper presented at the 44th ASMS Conference on Mass giving the observed temporal characteristics have been Spectrometry and Allied Topics, May 12–16, 1996, Portland, OR, USA.‡ The submitted manuscript has been authored by a contractor of given.15–18 the US Government under contract No. DE-AC05-96OR22464. Two modes of operation are reported and characterized Accordingly, the US Government retains a paid-up, nonexclusive, here. The first employed a single continuous injection over the irrevocable, worldwide license to publish or reproduce the published discharge ‘on’ period.The injection time employed is inversely form of this contribution, prepare derivative works, distribute copies proportional to the analyte concentration, resulting in high to the public, and perform publicly and display publicly, or allow others to do so, for US Government purposes. duty cycle, low frequency pulses for trace species. (Duty cycle, Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 (43–48) 43as used here, refers to the fraction of the period the discharge the entire elemental mass range.This mode of operation was effectively a continuous discharge (single continuous injection pulse is on. It does not refer to the sample consumption, which only takes place during the discharge. As long as the discharge period) which was turned off after the injection period. A trigger pulse, generated by the 3DQ electronics at the beginning pulse is fully encompassed by the time allotted for ion accumulation, the duty cycle, in terms of sample consumption, is 100%.of each scan, signals a square-wave pulse generator (Model 8010, Berkeley Nucleonics Corporation, Richmond, CA, USA). This is, of course, a significant advantage to pulsed discharge operation.) Alternatively, multiple injections were made using The pulse generator controlled the pulse delay, width and amplitude. The pulse signal drove an operational power supply several pulses at a higher frequency.Used in combination with appropriately timed injection periods, the multiple injection (Model OPS 3500, Kepco, Flushing, NY, USA). Experiments were monitored and traces (glow discharge and rf trapping mode can be used to generate a temporally resolved spectrum for the selective accumulation of analyte ions. potentials) were stored on a digital oscilloscope (Phillips, Model PM3382). Because the ion source was turned off after the injection period, the gate voltage was not necessary; in EXPERIMENTAL some experiments, both half-plates (L2) were operated at a common and optimal potential (#-325 V).The GD-ITMS system employed in this study is schematically The second operational mode used multiple injections in a presented in Fig. 1. The instrument is a Teledyne 3DQ ion single scan function. A discharge pulse of fixed frequency and trap-based mass spectrometer (Teledyne Electronic duty cycle was used in association with a data gate, positioned Technologies, Mountain View, CA, USA), modified for injecover the pulse region of interest.This is analogous to the tion of externally generated glow discharge ions. The glow typical mode of pulse discharge operation.14–19 Data gates discharge was operated in the pulsed-direct current mode. were 1.5 ms long. The discharge was pulsed at 36 Hz and had Samples were introduced through a vacuum interlock on a a 33% duty cycle. To accumulate a sufficient number of ions, direct insertion probe.20 Argon was used as a support gas, and multiple gated injections (n=4) per scan (one per pulse) were pressures ranging from 200–500 mTorr were optimized for made before the acquisition period.This was accomplished by analyte signal intensity and discharge stability. Discharge including multiple triggers in the scan function. Pulses and ion voltage was maintained at 1.5 kV for all modes of operation. gates were synchronized by the use of appropriate delays in Ion focussing and injection were effected using a simple the scan functions and pulse generator.The scan function is three element lens system described previously (see Fig. 1).21 described in more detail later. Ions were vacuum-extracted from the discharge through a Two brass standard reference materials were used: NIST SRM 250 mm orifice. Lens 2, comprised of two half-plates, was used 1101 Cartridge Brass B and NIST SRM 1102 Cartridge Brass to gate ions into the trap.Positive ions transmitted through C. The trace element concentrations of interest are presented in lens 1 were either deflected from, or passed to, lens 3 by the the relevant parts of Results and Discussion. application of +200 or -200 V, respectively. The 200 V potential was supplied by the modified (i.e., voltage inverted) electron gate voltage of the 3DQ electronics. RESULTS AND DISCUSSION Ions were injected through an aperture in the center of an Noise Characterization endcap electrode, which was operated at 0 Vdc during injection.Trapping efficiency was improved by the addition of helium A series of mass spectra, acquired under differing analyzer as a buffer gas (pressure, 2–4 mTorr).3,21 Ions were measured conditions and shown in Fig. 2 (ion gauge off ), illustrates the directly on a Channeltron electron multiplier (Model 4773G, origin of background and background noise in the GD-ITMS. Galileo Electro-optics, Sturbridge, MA, USA), mounted off No scan averaging was made and a steady-state discharge was axis to reduce photon-induced noise.Mass spectra were used. Fig. 2(a) shows a mass scan of brass with the glow acquired and displayed using Teledyne Electronic discharge on, multiplier voltage on (1900 V), but with no He Technologies’ 3DQ Discovery and Sequel Data System buffer gas added to the analyzer. Argon pressure in the analyzer software, respectively. was 3×10-5 Torr (base pressure was 5×10-7 Torr). In the Two operational modes were employed: single continuous absence of buffer gas, no analyte signal is noted with a 5 ms injection, whose duration fully encompassed the glow discharge injection.Noise arising with the initiation of the glow discharge pulse, and multiple injections. The single continuous injection is the first noise of any consequence above the baseline of 83 mode used a high duty cycle, low frequency discharge voltage counts (arbitrary units). Baseline counts are attributed to the pulse.The ranges of duty cycles and frequencies were 5–90% detector dark current; approximately 5 counts are attributed and 4.5–45 Hz, respectively. Both frequency and duty cycle to rf pick-up from the trap electrodes. Noise, 83–658 counts, were dependent on the required injection period, with trace is eliminated when the discharge is turned off. This indicates species sometimes requiring injection times in excess of 150 that the glow discharge is the source of the noise: charged ms; acquisition time (pulse ‘off ’ period) was about 20 ms for species, photons or fast neutrals.With the addition of He buffer gas (5 mTorr) to the ion trap vacuum manifold [Fig. 2(b)], an increase in ion trapping efficiency, background and background noise is observed. Improved efficiency in the injection of externally generated ions through the use of a buffer gas is well known3,20 and is now common practice in ITMS. The background increased from 83 counts to 1600 counts with the addition of He.Background noise increased from 3 to 33% of background. Charged species are responsible for most of the background and background noise. A laboratory magnet, positioned above the exit orifice and outside the vacuum housing, was used to reduce noise on the detector [Fig. 2(c)]. Peak noise, with the magnet in place, ranges from 90–700 counts per channel; this count level approaches the noise characteristics in Fig. 2(a). Since a small laboratory magnet, positioned outside the Fig. 1 Pulsed glow discharge ion trap-based mass spectrometer system. vacuum chamber, is capable of reducing the noise by more 44 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12Fig. 2 S/N ratio comparison of steady-state glow discharge mass spectra of brass (1.5 kV, 1.5 mA, 330 mTorr Ar, single scan, ion gauge off) with (a) no He buffer, no magnetic deflection of electrons; (b) He buffer (5 mTorr), no magnetic deflection of electrons; and (c) He buffer (5 mTorr), with magnetic deflection of electrons. than 80%, electrons are believed to be the precursor of most voltage is pulsed at 5.5 Hz, has an 80% duty cycle and an of the noise (see below).amplitude of -1.5 kV. The rf amplitude trace consists of an Most of the noise arising from charged species is believed injection period of 158 ms and a subsequent acquisition ramp. to come from outside of the trapping volume. This is because The rf levels correspond to a low mass cutoff of m/z 130 (qz= lens and ion trap potentials have little effect on background 0.57 for 208Pb) during the injection period and an acquisition and background noise.It is possible that electrons promote ramp of m/z 40–290. The single injection period corresponds electron impact ionization of Ar and He, which can lead to to the duration of the pulse (146 ms) as the ion gate (L2) is the presence of ions in the detector volume via charge exchange ‘open’ over 158 ms injection period.In this manner a single processes. This is supported by the observation that the ion injection is made per scan which corresponds to the pulse gauge, when on during operation (data not shown), contributes period in length (146 ms here). significant amounts of background and noise. Charge mobility A comparison of steady-state and pulsed glow discharge via symmetric charge exchange processes is known to be very operation is presented in Fig. 4, which shows the mass spectra high for both Ar (cross section #60×10-16 cm2)22 and He of Pb (500 ppm) in NIST SRM 1101 Cartridge Brass B. The (cross section #30×10-16 cm2).23 steady-state discharge [Fig. 4(a)] was generated by gating ions with L2 during a 183 ms injection period. The pulsed-discharge spectrum [Fig. 4(b)] was obtained as described above. Each Pulsed Operation spectrum is the average of 25 scans. The improvement in S/N Single injection is most evident in the detection of 204Pb (7 ppm), which is not discernible above the noise from the steady-state discharge.The voltage traces in Fig. 3 show the synchronized glow Background is reduced by about 100 counts (from #180 to discharge voltage pulse and the rf signal on the ring electrode during the injection and acquisition periods. The discharge 83 counts). Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 45Fig. 3 Voltage traces showing the synchronization of the rf trapping Fig. 5 Voltage traces showing the rf trapping voltage and the glow voltage with the glow discharge voltage.discharge voltage, which is extinguished after the acquisition ramp is initiated to illustrate noise reduction. Fig. 4 Glow discharge ion trap mass spectra of Pb (500 ppm) in NIST SRM 1101 Cartridge Brass B (150 ms injection, 25 scans averaged). (a) Steady-state discharge; and (b) pulsed discharge. Fig. 6 Pulsed glow discharge ion trap mass spectra of Pb (500 ppm) in NIST SRM 1101 Cartridge Brass B (250 ms injection, single scan).The low m/z values illustrate background and noise contributions from the discharge. The improvement in S/N is more clearly illustrated by the results of the experiment shown in the voltage traces in Fig. 5 and the resulting spectrum of Fig. 6. The discharge pulse remains on during a portion of the acquisition ramp (Fig. 5); The 20-fold improvement in the noise characteristics of the the discharge voltage is turned off at an rf level (time) during GD-ITMS spectra should result in proportional improvements the ramp corresponding to m/z 130.In this case, L2 deflects in limits of detection. Examples illustrating the detection limits (gates) ions from the trap so that noise is not from ions being are shown in Figs. 7 and 8. Fig. 7 is the GD-ITMS spectrum injected into the trap during acquisition; magnetic deflection of Pb (200 ppm) and Bi (5 ppm) in NIST SRM 1102 Cartridge of electrons was employed.During the discharge ‘on’ portion Brass C. The spectrum is the average of 32 scans acquired of the mass scan (low m/z, Fig. 6), 163 background counts are using a 950 ms pulse (950 ms single injection/scan), 0.95 Hz measured (dark current count subtracted), while only 5 counts pulse, 1.5 kV, #3mA current, and 0.4 Torr Ar. A factor of two are observed during the discharge ‘off’ period (a more than in sensitivity was gained by optimizing both L2 half-plates at 30-fold improvement).With 11 scans averaged, there is a -325 V. Detection limits (3s) for Pb and Bi are 0.2 ppm and 20-fold improvement in the average peak-to-peak back- 0.5 ppm, respectively. Scale expansion [Fig. 7(b)] shows back- ground noise. ground and background noise levels consistent with the dark Even though the discharge is turned off at a time during the current in Fig. 2(a). Fig. 8 shows a number of other elements acquisition which corresponds to m/z 130, noise is observed in the low ppm concentration level in NIST SRM 1102 up to mass m/z 160.This results from the decay period required Cartridge Brass C5Cd (45 ppm), Ag (30 ppm), Sn (60 ppm), for charged species to be lost from the sampled volume of the Sb (50 ppm). Detection limits for each of these is <1 ppm. discharge after it is extinguished. From the ramp rate, this These detection limits are better than the #10–50 ppm limits time is calculated to be#1.5 ms, consistent with that reported in the literature for pulsed glow discharges.15 of detection reported previously,8,9 and are consistent with the 46 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12sampling) without the detector ‘seeing’ the discharge during acquisition (see below). This scan function is generated in the software’s scan function editor. The overlap of the pulse period and the injection period determines the total injection time per pulse. In the experiment described in Fig. 9, a 36 Hz pulse with a 33% duty cycle is used. Only ions formed during the last 1.5 ms of the pulse period and the after-peak ions, formed through ionization by Ar metastables,15–18 are injected. Using the pulse set-up described, four pulses were sampled (n=4) for a total injection time of 6 ms to generate the spectrum shown in Fig. 10(a). The primary peaks in the spectrum are from the isotopes of Cu (72.85%) and Zn (27.10%) in NIST SRM 1102 Cartridge Brass C. Argon and ArH, usually ten times more intense than the matrix, are present at only about 5% of the base peak.Only small peaks of water, argon dimer and other residual Fig. 7 Pulsed glow discharge ion trap mass spectrum of Pb (200 ppm) gases are observed. This demonstrates the analyte ion selec- and Bi (5 ppm) in NIST SRM 1102 Cartridge Brass C (950 ms injection, 32 scans averaged). Limits of detection (3s) for Pb and Bi tivity that is possible by selective sampling of the pulse profile.are 0.2 and 0.5 ppm, respectively. (a) Scale: 1×; (b) scale: 16×. If ions are sampled over the initial 1.5 ms of the pulse, primarily discharge and residual gas species are observed [Fig. 10(b)]. The pulse-injection gate overlap was adjusted by 20-fold reduction in noise and two-fold improvement in increasing the pulse delay after the trigger pulse. The observed signal intensity. preferential sampling of gases in the discharge volume is consistent with that observed on both linear quadrupole14,15,18 Multiple injections and time-of-flight mass analyzers.19 In sampling the early pulse ions, Cu+ intensity is reduced by a factor of three, while argon An alternative method for operation of the pulsed discharge is and related species increase nearly two orders of magnitude to make multiple (n) injections over n pulse periods prior to [note the difference in relative counts shown in Figs. 10(a) and data acquisition. An ion gate is used to sample a portion of (b)].Also noted is continuum noise between H2O+ and Ar+ each pulse. In this manner, constant frequency and duty cycle arising from charge exchange processes during the period of can be maintained; ion intensity increases with n. This should acquisition from low to high m/z values. have several advantages: (1) reduction in detector noise; (2) higher instantaneous powers;14 and (3) temporal selectivity over the ion population.14,15,18 The voltage traces in Fig. 9 indicate the sequence of events associated with the experiment.Two voltage traces are shown: the glow discharge voltage and the rf trapping voltage. The trapping voltage trace consists of a repetitive set of scan periods: a delay period, an injection period, and a cool period. Each region is shown with a different rf amplitude for clarity (delay, rf: m/z 25; inject, rf: m/z 20; and cool, m/z 15). This set is repeated n times for n injections and is followed by an acquisition ramp of m/z 15–150.The delay period and glow discharge pulse are initiated by a software-initiated trigger Fig. 9 Voltage traces showing the synchronization of the rf trapping pulse, which originates in the Teledyne emission control card voltage with the glow discharge voltage during a multiple injection (Fig. 1). The injection period determines when the ion gate is experiment (36 Hz, 33% duty cycle). Overlap of the pulse profile and ‘open’. The cool period allows the pulse to be positioned over the injection period result in selective sampling of ions formed in the post-peak period.the final milliseconds of the injection period (i.e., post-pulse Fig. 8 Pulsed glow discharge ion trap mass spectrum of Cd (45 ppm), Ag (30 ppm), Sn (60 ppm), and Sb (50 ppm) in NIST SRM 1102 Cartridge Brass C. Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 47minor and trace species relative to the matrix element(s), consistent with traditional quantification methodology used in GDMS.These issues are the subject of ongoing investigations. Research sponsored by the Office of Basic Energy Sciences, US Department of Energy, under contract number DE-AC05-96OR22464 with Oak Ridge National Laboratory, managed by Lockheed Martin Energy Research Corporation. REFERENCES 1 Yates, N. A., Booth, M. M., Stephenson, J. L., Jr., and Yost, R. A., in Practical Aspects of Ion T rap Mass Spectrometry, Vol. 3, Chemical, Environmental, and Biomedical Applications, ed.March, R. E., and Todd, J. F. J., CRC Press, New York, 1995, ch. 4. 2 Kleintop, B. L., Eades, D. M., Jones, J. A., and Yost, R. A., in Practical Aspects of Ion T rapMass Spectrometry, Vol. 3, Chemical, Environmental, and Biomedical Applications, ed. March, R. E., and Todd, J. F. J., CRC Press, New York, 1995, ch. 5. 3 Louris, J. N., Amy, J. W., Ridley, T. Y., and Cooks, R. G., Int. J. Mass Spectrom. Ion Processes, 1989, 88, 97. 4 Gill, C. G., Daigle, B., and Blades, M. W., Spectrochim.Acta, Part B, 1991, 46, 1227. 5 Gill, C. G., and Blades, M. W., J. Anal. At. Spectrom., 1993, 8, 261. 6 Alexander, M. L., Hemberger, P. H., Cisper, M. E., and Nogar, N. S., Anal. Chem., 1993, 65, 1609. 7 Garrett, A. W., Hemberger, P. H., and Nogar, N. S., Spectrochim. Acta, Part B, 1995, 50, 1889. 8 McLuckey, S. A., Glish, G. L., Duckworth, D. C., and Marcus, R. K., Anal. Chem., 1992, 64, 1606. 9 Duckworth, D. C, Barshick, C. M., Smith, D. H., and McLuckey, S.A., Anal. Chem., 1994, 66, 92. 10 Appelhans, A. D., Groenewold, G. S., Ingram, J. C., Delmore, J. E., and Dahl, D. A, Secondary Ion Mass Spectrometry, SIMS X, Wiley, New York, 1996. 11 Barinaga, C. J., and Koppenaal, D. W., Rapid Commun. Mass Fig. 10 Time-resolved glow discharge ion trap mass spectrum of Spectrom., 1994, 8, 71. NIST SRM 1102 Cartridge Brass C; (a) sampled as pulse was extingu- 12 Koppenaal, D. W., Barinaga, C. J., and Smith, M. R., J. Anal.At. ished (1.5 ms injection gate), and (b) sampled during pulse initiation Spectrom., 1994, 9, 1053. (1.5 ms injection gate). 13 Frum, C. I., Presented at the 44th ASMS Conference on Mass Spectrometry and Allied Topics, Portland, OR, May 12–16, 1996. 14 Klingler, J. A., Savickas, P. J., and Harrison, W. W., J. Am. Soc. CONCLUSION Mass Spectrom., 1990, 1, 138. 15 Klingler, J. A., Barshick, C. M., and Harrison, W. W., Anal. The primary benefit from the use of pulsed discharges is the Chem., 1991, 63, 2571.>40-fold improvement in detection limits brought about pri- 16 King, F. L., and Pan, C., Anal. Chem., 1993, 65, 735. marily from a 20-fold reduction in noise. Additional benefits 17 Pan, C., and King, F. L., J. Am. Soc.Mass Spectrom., 1993, 4, 727. hold promise. Future studies will show whether there is an 18 Pan, C., and King, F. L., Anal. Chem., 1993, 65, 3187. advantage in preferentially injecting analyte ions over discharge 19 Steiner, R. E., Lewis, C.L., and King, F. L., Proceedings of the gas ions as a means of selective ion accumulation. Present 43rd ASMS Conference on Mass Spectrometry and Allied Topics, Atlanta, GA, May 21–26, 1995, p. 42. methodologies use resonance24–26 and chemically8,27 selective 20 Duckworth, D. C., and Marcus, R. K., J. Anal. At. Spectrom., means of mass discrimination of ions in the trap. There could 1992, 7, 711. be some benefit in minimizing interaction between analyte ions 21 McLuckey, S. A., Glish, G. L., and Asano, K. G., Anal. Chim. with a high abundance of gas ions or with the resonance Acta, 1989, 225, 25. frequencies employed to remove these abundant species. 22 Hergerberg, R., Elford, M. T., and Skullerud, H. R., J. Phys. B: Using multiple injections holds promise for quantification At. Mol. Phys., 1982, 15, 797. 23 Helm, H., J. Phys. B: At. Mol. Phys., 1977, 10, 3683. methodologies in GD-ITMS. It is reasonable to assume that 24 McLuckey, S. A., Goeringer, D. E., and Glish, G. L., J. Am. Soc. the use of a constant frequency and constant duty cycle pulse Mass Spectrom., 1991, 2, 11. will improve the quantitative characteristics of the trap over 25 Julian, R. K., Cox, K. A., and Cooks, R. G., Anal. Chem., 1993, the use of single injections of variable pulse length (i.e., pulse 65, 1827. time is inversely proportional to concentration). Major, minor 26 Goeringer, D. E., Asano, K. G., McLuckey, S. A., Hoekman, D., and trace constituents sampled in this manner are extracted and Stiller, S. W., Anal. Chem., 1994, 66, 313. 27 Eiden, G. C., Barinaga, C. J., and Koppenaal, D. W., J. Anal. At. from a discharge with identical current and voltage character- Spectrom., 1996, 11, 317. istics. Additionally, the use of multiple (n) pulses should allow n data gates to be used to accumulate minor constituents, Paper 6/05312B followed by a single gate (e.g., a 10 ms pulse) over one pulse Received July 29, 1996 for sampling matrix ions. This may allow quantification of Accpeted October 16, 1996 48 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12

ISSN:0267-9477    

DOI:10.1039/a605312b

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Quantitative Analysis of Zirconium Oxide by Direct Current Glow Discharge Mass Spectrometry Using a Secondary Cathode WIM SCHELLES AND RENE� VAN GRIEKEN* Department of Chemistry, University of Antwerp (UIA), Universiteitsplein 1, B-2610 Antwerpen, Belgium A dc GD mass spectrometer has been used for the analysis of of the research work in this area has been restricted to qualitative analysis of only a few matrices. In the present work, ZrO2 samples with known amounts of artificial impurities.For this purpose the secondary cathode technique has been used. three samples of compacted ZrO2 with added amounts of artificial impurities are measured, mainly aiming at a method- The research work focused both on the methodological aspects of the atomization of the ZrO2 and on some quantitative ological investigation. This results in sputter–atomization data, specifically for this type of matrix, and an evaluation of some aspects, with direct analytical interest.The reproducibility was found to be better than 10% RSD in most cases; the accuracy quantitative aspects. Moreover, from the application point of view, dc GDMS in of the ‘standardless’, raw results was within a factor of 2–3 of the known concentration. combination with the secondary cathode technique can be considered as a novel approach for trace analysis of refractory Key words: Glow discharge mass spectrometry; nonconductor insulators such as ZrO2, which have always been a challenge analysis; secondary cathode; quantitative results to the analytical chemist.22–24 These sample types are commonly measured with typical solid analysis techniques (X-ray In the last decade GDMS has become a powerful and accepted methods,25 laser-based methods)26 or wet-chemical techniques tool for trace analysis of solid samples.Its main advantages such as ICP-OES and ICP-MS (using fusion and subsequent are the ability to measure all elements, even with isotopic dissolution for sample preparation, slurry nebulization, matrix information, with minimum sample preparation, over a wide removal)22–24,27 or (ET) AAS.28 Because all these techniques dynamic range (sub-ppb to 100%) and with a relatively uniform have intrinsic advantages and disadvantages, the present study response over the mass range.1–4 The concept of the GD, in can add dc GDMS to this list, without necessarily being which the sample acts as a cathode, seems to restrict the superior in all aspects to the alternative techniques.applications to conducting materials. However, considerable efforts have been undertaken to extend the inherent advantages of GDMS to insulating materials as well. A well known method to overcome the conductivity requirement is to substitute the EXPERIMENTAL classical dc GD source by an rf powered source.5–8 Here it GDMS becomes possible to sputter–atomize directly nonconducting samples such as glasses and ceramic materials. The develop- The work reported in this study was performed with a VG9000 ments over the last years concerning this type of instrumen- double-focusing dc GD mass spectrometer (VG Elemental, tation are promising, and rf sources have even been coupled Fisons Instruments, Winsford, UK), described in detail else- with commercial mass spectrometers.9–11 It is, on the other where.29 A working resolution of 3500–4000 was routinely hand, also possible to apply dc GD sources to the analysis of used for these studies.The detection system consists of a nonconducting materials. A common approach is to mix the combination of a Faraday cup and a Daly detector, providing powdered sample with a conducting binder material.12–15 a dynamic range of more than nine orders of magnitude (i.e. However, when the sample under investigation is a solid <1×10-18–1×10-9 A). The flat cell of the second generation, material, grinding can be troublesome and can cause severe often referred to as ‘the new flat cell’, was used. It has already contamination.To handle solid sample types with a dc GDMS been described elsewhere.30 The opening of the anode body instrument (commercially available, unlike rf GDMS), a con- was 7.5 mm, as determined by the opening in the front plate ducting diaphragm should be placed in front of the sample, of the sample holder. The cell was cryogenically cooled to the so-called secondary cathode technique.16–21 This method, reduce the background due to residual gases.The glow dis- introduced by Milton and Hutton in 1993,16 has not yet been charge was supported with high-purity argon (Air Liquide, widely applied. The concept is based on continuously coating Lie`ge, Belgium, 99.9997%) that was not further purified. the nonconducting sample with a thin conducting film, sputterdeposited from the secondary cathode. The major advantages are the ease and low cost of the technique; the major limitations are the restricted operating conditions and, for some elements, Materials the possible blank contribution due to the sputtering of the secondary cathode material.In general, one can state that Three samples of 5 g each, based on Tosoh-Zirconia TZ-3Y (93.96% ZrO2, and an uncertified amount of Y2O3), produced the secondary cathode technique can, in certain cases, be a viable alternative for rf GDMS, as previously proven in the by Ceraten (Madrid, Spain) and supplied by VG Elemental, were measured.These samples contained small amounts of restricted number of publications concerning this method.16–21 The analysis of a new type of matrix with the secondary Mg, Al, Si, Ca and Fe, resulting in samples with 10, 100 and 1000 ppm of these elements. The diaphragms used as secondary cathode method is not yet straightforward, mainly because of limited operating conditions to create stable atomization of cathodes were made of 0.25 mm thick tantalum (Goodfellow, Cambridge, UK, 99.9%).the nonconductor under investigation.17–19 Therefore most Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 (49–52) 49RESULTS AND DISCUSSION larities between the sputtering of SiO2 and ZrO2 are striking. For Si and Zr, the MO+5M ratio is 3.0 and 4.1%, respectively; Atomization of ZrO2 for the MO2+5M ratio (the molecular species) these values are 0.011 and 0.015%, respectively. This resemblance can be When using a secondary cathode, it is important to know whether the matrix under investigation is ‘poorly conducting’ understood as both sample types were measured under the same operating conditions and as the M–O bond strength is or ‘nonconducting’. It has previously been shown that there is, for GDMS measurements, a certain ‘edge of conductivity’ comparable for both (for Si and Zr, it is 798.7±8.4 and 759.8±8.4 kJ mol-1, respectively).31 The fact that cluster (about 1010–1011 V cm).17 Below this edge, samples can be directly used as a cathode resulting in a stable discharge.The species are only present in the discharge at relatively low concentrations is favourable from the analytical point of view. sputter yield can, however, be extremely low, for example for samples with an electrical resistivity close to the edge (e.g. Clusters can interfere with analytical peaks; moreover, the absence of clusters facilitates quantitative procedures, because about 109 V cm). This makes it useless to perform the analysis of these ‘poorly conducting’ samples directly, i.e., without a the matrix peak(s) can reliablybe used, as afirst approximation, as internal standard (see below). The other values in Table 1 secondary cathode.If samples with an electrical resistivity greater than about 1010 V cm are used directly, no discharge have more analytical significance, and will therefore be discussed in the next section. can be obtained. In these cases, the secondary cathode is absolutely necessary to create stable atomization of the nonconducting sample.Poorly conducting and nonconducting Quantitative Results and Analytical Figures of Merit samples require other optimum operating conditions and Three ZrO2 based samples with known amounts of artificial poorly conducting terials reveal better analytical character- impurities were measured under the operating conditions istics when measured with the secondary cathode technique. described in the previous section.The matrix element Zr Therefore, information concerning the electrical resistivity is (74% m/m of ZrO2) was used as an internal standard. This is significant. The most useful way to distinguish between ‘non- the well known ion beam ratio approach, a predecessor of the conducting’ and ‘poorly conducting’ materials is to run the relative sensitivity factors (RSFs) approach commonly used sample directly as a cathode in the dc GD, without exposing for the analysis of high purity conducting samples; in that case, any auxiliary conductor to the plasma.17 If no discharge at all the matrix is considered to have a 100% concentration.The can be obtained, this means that the sample can be classified raw, uncorrected concentrations were calculated according to: as ‘nonconducting’. With this method, the compacted ZrO2 samples were found to be ‘nonconducting’, i.e., the use of the Concentration X (ppm)= Signal intensity X Signal intensity Zr ×0.74×106 secondary cathode was absolutely necessary to create a stable discharge.When a nonconductor is measured with a secondary cath- For each sample (containing 10, 100 or 1000 ppm of each artificial impurity), at least five data were acquired over a ode, the discharge conditions (pressure, voltage, current) cannot be chosen arbitrarily. Therefore, initially, two different range of more than half an hour of sputtering. The mean uncorrected concentrations and their RSDs are listed in sets of discharge conditions, which had previouslybeen successfully applied for ‘nonconductor’ analysis (i.e. for the analysis Table 2.The rather poor reproducibility for Ca in comparison with the other elements is caused by the low abundance of the of glass and of Macor19) were used to sputter–atomize these samples in a stable way. A 3 mA/0.7 kV discharge, used for isotope measured (44Ca, 2.13% abundance). Generally, it is clear that the atomization and ionization of ZrO2 in a dc GD the atomization of glass, always caused instabilities and/or a tantalum coated sample, thereby preventing the underlying using a tantalum secondary cathode can be considered as reproducible.ZrO2 from being sputtered. On the other hand, the (slightly modified) conditions used for Macor, allowed reproducible Table 2 also reveals that acceptable semi-quantitative results can be obtained (within a factor of 2–3), based only on an and successful analysis of ZrO2 .The conditions were a 0.8 mA/1.15 kV discharge, and, as for the glass analysis, the internal and no external standards. The exception is the 10 ppm level, where deviations obtained were rather high. This is either use of a tantalum secondary cathode with a 4 mm hole, an anode body opening of 7.5 mm diameter (as determined by the due to blank values in the ZrO2 sample or to blank values in the secondary cathode. To evaluate this and to have a general opening in the sample holder front plate), and a 0.5 mm thick Teflon spacer between the cathode and anode. The reason for overview of the influence of the blank contribution of the tantalum mask, three panoramic analyses of the tantalum were the different optimum discharge conditions is not yet completely understood; the sample surface itself and the adhesion performed.The data obtained for the impurities are listed in Table 3 in decreasing order of average measured concentration. of the redeposited tantalum atoms seem to play a major role.19 In the first part of this study, the signal intensities of the These ‘real’ blank values should be multiplied with a weight factor of the secondary cathode sputtering to obtain apparent matrix species and the tantalum secondary cathode were evaluated.To make comparisons possible, the raw, isotopic blank values, i.e. the blank contribution of the mask in the spectrum of the sample. The weight of the blank values in the intensities were converted into elemental signal intensities by means of the isotopic abundances.These intensities and their mask is given by the Ta5Zr signal intensity ratio (5.0, see Table 1) and the concentration of Zr in the sample (74%). ratio to the elemental Zr signal intensity are listed in Table 1. When one compares the data with previous results obtained This results in a weight factor of less than 4 (namely 5.0×0.74). This factor is significantly lower than the weight factor 10, with the secondary cathode technique on Macor,19 the simireported for the analysis of Macor ceramic.19 In practice, this means that, for the selected elements, the impurities in the Table 1 Mean maximum elemental signal intensities (A) for the matrix constituents and their ratio to the Zr signal ZrO2 rather than the impurities in the tantalum secondary cathode can be considered as the cause of the significant Mean value Ratio to Zr deviations at the 10 ppm level.O+ 4.9×10-13 1.0×10-2 Based on the known concentrations and the measured, Zr+ 5.2×10-11 1 uncorrected concentrations calibration graphs were drawn.ZrO+ 2.2×10-12 4.1×10-2 The correlation coefficients for the different elements are listed ZrO2+ 8.2×10-16 1.5×10-5 in Table 2. The concept of a calibration curve is useful, Ta+ 2.5×10-10 5.0 especially if a blank contribution is involved. In GDMS, 50 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12Table 2 Measured, uncorrected concentrations (ppm), based on Zr (74%) as internal standard.Values between brackets are relative standard deviations over at least five measurements on the same sample. Correlation coefficients and calculated RSFsZrOx are based on the calibration lines Measure concentration (ppm) Correlation 10 100 1000 coefficient RSFZrOx RSFstand RSFoxid RSFMacor Mg 6 46 556 0.99985 1.9 1.5 1.0 0.61 (5.3%) (2.4%) (2.9%) Al 28 89 797 0.99994 1.4 1.4 0.57 0.30 (1.4%) (5.5%) (3.3%) Si 18 109 562 0.99723 2.0 1.8 1.3 (5.2%) (7.3%) (5.6%) Ca 18 128 1200 0.99999 0.91 0.55 (25%) (16%) (2.6%) Fe 37 152 1114 0.99989 1.0 1.0 1.0 1.0 (3.8%) (3.7%) (1.6%) Table 3 Mean measured concentrations (ppm) of impurities in the tantalum secondary cathode Concentration Concentration Concentration Impurity (ppm) Impurity (ppm) Impurity (ppm) Nb 185 Ni 0.039 Ti 0.0056 O 39 Ga 0.037 Sm 0.0047 C 20 Li 0.030 U 0.0047 W 14 Cr 0.025 Dy 0.0047 Sn 1.1 Hf 0.025 Pd 0.0037 Mo 0.73 Th 0.023 Er 0.0037 K 0.54 Cd 0.022 Tl 0.0033 Fe 0.45 P 0.022 Mn 0.0026 Na 0.43 Sb 0.016 La 0.0021 In 0.27 Pb 0.015 Eu 0.0020 Cu 0.23 B 0.015 Rb 0.0019 S 0.19 Ba 0.013 Be 0.0019 Ca 0.18 Sr 0.012 Ce 0.0018 Zn 0.092 Zr 0.0097 Y 0.0017 Mg 0.072 Ag 0.0087 Co 0.0016 Pt 0.055 Nd 0.0067 Rh 0.0013 Au 0.052 Yb 0.0063 Pr 0.0010 Al 0.049 Bi 0.0057 V 0.0006 Si 0.045 Gd 0.0057 Sc 0.0006 quantification is, however, commonly performed by means of view (i.e.if one wants to generalize towards other matrices), only blank values due to the sputtering of the tantalum should RSFs.This is defined as the reciprocal value of the slope of the calibration curve, and is calculated relative to Fe (RSF= be taken into account (and not those due to the ZrO2 impurities). As mentioned earlier, these values are rather low 1). Actually, the RSF is a measure of the ‘in-sensitivity’ of an element. The obtained RSFs are also listed in Table 2. For for most elements (see Table 3).In practice, blank contributions restrict the LODs for several elements to a level of about 100 reasons of comparison, ‘standard RSFs’ (obtained for metallic samples),32 RSFs obtained for powdered oxide-based samples14 ppb or even higher. On the other hand, even sub-ppb LODs have been reported for U and Th, using the secondary cathode and limited data obtained for Macor19 (also an oxide-based ceramic, but measured as a solid, unlike the compacted ZrO2 technique.21 In this case, the low resolution mode (and the inherent signal enhancement) and extremely long integration powder samples used in this study) are also listed in Table 2.Although there is no absolute match, the same trend can be times were applied. Generally, one can state that about 85% of the elements can routinely be measured at sub-ppm levels. seen in the different sets: RSFCa<RSFFe<RSFAl etc. Nevertheless, it is clear that RSFs acquired for varying materials, even for ‘matrix-matched’ materials, are not exchangeable, W.S. acknowledges financial support by the Vlaams Instituut if an accurate result is required. voor de bevordering van het Wetenschappelijk-technologisch For the LODs, two factors have to be taken into account Onderzoek in de Industrie (IWT). The authors thank VG for each element: the sensitivity and the blank contribution. Elemental for the supply of samples and K. De Cauwsemaecker The sensitivity for the selected elements, expressed as the and R. Saelens for technical support.maximum signal intensity/ppm is between 2×10-18 A/ppm (44Ca, 2.13% abundant) and 1×10-16 A/ppm (27Al, 100% REFERENCES abundant). Depending on the integration time used, it is possible, for elements not subject to interference, to distinguish 1 King, F. L., Teng, J., and Steiner, R. E., J. Mass Spectrom., 1995, peak heights of 2×10-17 A (20 ms per channel) to 1×10-18 A 30, 1061. (8 s per channel) from the background. In this context, it may 2 King, F.L., and Harrison, W. W., in Glow Discharge Spectroscopies, ed. Marcus, R. K., Plenum Press, New York, 1993, be useful to know that, commonly, 60–100 channels are pp. 175–214. measured for each isotope. Thus, only taking into account the 3 Gijbels, R., van Straaten, M., and Bogaerts, A., in Advances in sensitivity, LODs for most elements are in the range between Mass Spectrometry, ed. Cornides, I., Horva�th, G., and Ve� key, K., 10 ppb (high elemental sensitivity, long integration time) and Wiley, New York, 1995, vol. 13, pp. 241–256. 10 ppm (poor elemental sensitivity, short integration time). 4 Harrison, W. W., J. Anal. At. Spectrom., 1992, 7, 75. However, in addition, the restrictions due to blank values have 5 Coburn, J. W., and Kay, E., Appl. Phys. L ett., 1971, 19, 350. 6 Duckworth, D. C., and Marcus, R. K., Anal. Chem., 1989, 61, 1879. to be taken into account. From the methodological point of Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 517 Marcus, R. K., Harville, T. R., Mei, Y., and Shick, C. R., Jr., Anal 22 Broekaert, J. A. C., Graule, T., Jenett, H., To� lg, G., and Tscho�pel, Chem., 1994, 66, 902A. P., Fresenius’ Z. Anal Chem., 1989, 332, 825. 8 Harville, T. R., and Marcus, R. K., Anal. Chem., 1995, 67, 1271. 23 Broekaert, J. A. C., and To� lg, G., Mikrochim. Acta, 1990, II, 9 Duckworth, D. C., Donohue, D. L., Smith, D. H., Lewis, T. A., 173. and Marcus, R. K., Anal. Chem., 1993, 65, 2478. 24 Broekaert, J. A. C., Lathen, C., Brandt, R., Pilger, C., Pollmann, 10 Shick, C. R., Jr., Raith, A., and Marcus, R. K., J. Anal. At. D., Tscho�pel, P., and To� lg, G., Fresenius’ J. Anal. Chem., 1994, Spectrom., 1993, 8, 1043. 349, 20. 11 Saprykin, A. I., Becker, J. S., and Dietze, H.-J., J. Anal. At. 25 Radha Krishna, G., Ravindra, H. R., Gopalan, B., and Spectrom., 1995, 10, 897. Syamsunder, S., Anal. Chim. Acta, 1995, 309, 333. 12 Tong, S. L., and Harrison, W.W., Spectrochim. Acta, Part B, 26 Becker, J. S., and Dietze, H.-J., Fresenius’ J. Anal. Chem., 1993, 1993, 48, 1237. 346, 134. 13 Woo, J. C., Jakubowski, N., and Stuewer, D., J. Anal. At. 27 Lobinsky, R., Van Borm, W., Broekaert, J. A. C., Tscho�pel, P., Spectrom., 1993, 8, 881. and To� lg, G., Fresenius’ J. Anal. Chem., 1992, 342, 563. 14 De Gendt, S., Schelles, W., Van Grieken, R. E., and Mu�ller, V., 28 Hauptkorn, S., Schneider, G., and Krivan, V., J. Anal. At. J. Anal. At. Spectrom., 1995, 10, 681. Spectrom., 1994, 9, 463. 15 Duckworth, D. C., Barshick, C. M., and Smith, D. H., J. Anal. 29 Robinson, K., and Nayler, R., Eur. Spectrosc. News, 1986, 68, 18. At. Spectrom., 1993, 8, 875. 30 van Straaten, M., Gijbels, R., and Vertes, A., Anal. Chem., 1992, 16 Milton, D. M. P., and Hutton, R. C., Spectrochim. Acta, Part B, 64, 1855. 1993, 48, 39. 31 Handbook of Chemistry and Physics, West, R. C., Ed., 55th edn., 17 Schelles, W., De Gendt, S., Muller, V., and Van Grieken, R. E., Appl. Spectrosc., 1995, 49, 939. CRC Press, Cleveland, 1982–1983, pF185. 18 Schelles, W., De Gendt, S., Maes, K., and Van Grieken, R. E., 32 VG9000, software version 5.3, Fisons Instruments, VG Elemental, Fresenius’ J. Anal. Chem., 1996, 355, 858. Winsford, Cheshire, UK, 1990. 19 Schelles, W., and Van Grieken, R. E., Anal. Chem., 1996, 68, 3570. 20 Schelles, W., De Gendt, S., and Van Grieken, R. E., J. Anal. At. Paper 6/05397A Spectrom., 1996, 11, 937. Received August 1, 1996 21 Betti, M., Giannarelli, S., Hiernaut, T., Rasmussen, G., and Koch, L., Fresenius’ J. Anal. Chem., 1996, 355, 642. Accepted October 10, 1996 52 Journal of Analytical Atomic Spectrometry, J

ISSN:0267-9477    

DOI:10.1039/a605397a

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Determination of Total Mercury in Sediments by Microwave-assisted Digestion–Flow Injection–Inductively Coupled Plasma Mass Spectrometry A� GNES WOLLER*†a, HERVE� GARRAUDa‡, FABIENNE MARTINa‡, OLIVIER F. X. DONARDa‡ AND PE� TER FODORb aL aboratoire de Photophysique et PhotochimieMole�culaire, Universite� de Bordeaux I, CNRS URA 348, 351, Cours de la L ibe� ration, 33405 T alence Cedex, France bDepartment of Chemistry and Biochemistry, University of Horticulture and Food Industry, V illa�nyi u. 29–34, Budapest, 1114, Hungary A method for the determination of total mercury in sediment cury by ICP-MS, on the other hand, is not very easy. The major problem is the extensive memory effect, which requires samples was developed. Extraction of mercury from a sample long washout times. It seems that mercury, even at relatively matrix was carried out in an open vessel microwave digestion low concentrations, can adhere to the walls of the spray system while maintaining mild conditions during digestion in chamber and the transfer tubing of the introduction system, order to avoid any loss of mercury due to volatilization.A causing contamination of subsequent samples and a continuous complexing agent (EDTA) and a surfactant (Triton X-100) decrease in sensitivity in aqueous calibration. A possible expla- were added to the samples in order to eliminate memory nation of this phenomenon could be that mercury vapour effects associated with mercury determinations and to obtain builds up slowly in the spray chamber and this is caused by reproducible linear calibration curves.Standard additions and the increased volatility of mercury due to the large pressure internal standardization were used for calibration and drop during high-pressure pneumatic nebulization.6,13 This correction in an FI–ICP-MS detection system. The method situation makes accurate and precise determinations of mercury was validated using the certified reference material PACS-1 in environmental samples impossible.Possible solutions to this and the reference materials IAEA-356 and S19 and gave problem include alternative sample introduction systems, e.g., results in good agreement with the certified and reference a direct injection nebulizer (DIN), which has been proposed values. Sediment samples from Arcachon Bay were also by several workers.14,15 The technique is promising, although analysed. Detection limits of 10 ng g-1 for solution and in sediment analysis the very small internal diameter of the 1 ng g-1 for dry sediment samples were obtained.DIN capillary would possibly cause problems due to clogging. Keywords: Mercury; microwave-assisted digestion; flow Flow injection (FI) techniques have been used with various injection ; inductively coupled plasma mass spectrometry; atomic spectroscopic measurements to facilitate calibration sediment samples; reference materials and dilution, to reduce sample consumption and to lower the risk of user-introduced contamination.16 With ICP-MS, a continuous aspiration solution analysis with large amounts of Lakes, rivers and coastal waters are important indicators of total dissolved solids may cause matrix deposition on the environmental pollution.The determination of contaminants cones. In consequence, reduced sensitivity, instrument drift, in aquatic systems helps in evaluating the present state of the degradation of measurement precision and cone orifice clog- environment.Mercury is of considerable interest as it is widely ging may be expected. When FI sample introduction is used, used in industry in the production of chemicals, pesticides, the attack of harsh matrices on the sampler and skimmer electrical apparatus, paints, dental mateials, etc. Concerning cones and instrument drift when analysing difficult samples the determination of mercury in aquatic systems, because this are reduced and there is essentially no degradation of the element tends to accumulate in bottom sediments,1 the estab- ICP-MS detection power.17 Combined with standard additions lishment of its concentration in sediments can play a key role and internal standardization, FI is an excellent tool to decrease in detecting sources of pollution.matrix effects with complex samples. Various methods for the determination of total mercury in Sample preparation is a critical factor in the analysis of natural samples have been reported, including AFS,2 AAS,3 environmental samples.With mercury, special care must be ICP-AES4 and ICP-MS,5–9 although the most commonly taken to avoid losses by volatilization and/or adsorption. employed and most sensitive method for the determination of Microwave digestion seems to be a method of choice over mercury in environmental samples is still cold vapour atomic conventional procedures for the digestion of mercury in sedi- spectrometric techniques (CVAAS, CVAFS).10–12 ment and soil samples.Amongst the reasons are the shorter ICP-MS has become a widely accepted technique applied to extraction time, the higher extraction efficiency, the easier environmental samples because of its unique capabilities. control of digestion parameters and the ease of ICP-MS, when compared with AFS, FAAS and ICP-AES, automation.3,6,10,12 offers exceptional sensitivity and excellent accuracy along with In this paper, we present a method for the determination of multi-element and isotope ratio measurement capabilities. The mercury in sediments by FI sample introduction followed by accuracy of isotope dilution techniques was shown to be ICP-MS detection after a microwave-assisted sample digestion.superior to other techniques when the optimum isotopic ratio The method was validated using two reference materials and for analysis was maintained in the case of mercury determi- a certified reference material. Real samples collected from nation in environmental samples.9 The determination of mer- Arcachon Bay (France) were also analysed.† Present address: The Department of Chemistry and Biochemistry, EXPERIMENTAL University of Horticulture and Food Industry, Villa�nyi u. 29–34, Instrumentation Budapest, 1114, Hungary. An ELAN 5000 ICP-MS instrument (Perkin-Elmer SCIEX, ‡ Present address: CNRS EP 132, He�lioparc, 2 Avenue du Pre�sident Angot, 64000 Pau, France. Thornhill, Ontario, Canada) equipped with a Scott-type Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 (53–56) 53double-pass spray chamber was used throughout the analyses. 2 ml of H2O2 (Merck, Darmstadt, Germany) and digestion was continued at a power setting of 20W for a further 5 min. The standard ICP-MS operating conditions used in this study are presented in Table 1. Optimization was carried out daily After cooling, the samples were diluted with Milli-Q water according to the concentration range of the samples.Analysis with a normal verification solution (10 ng g-1, Rh, Mg, Pb, Ce, Ba). Raw data were collected by the ELAN software was carried out either on the same day as, or the day after, sample digestion. Samples were kept cool (4°C) in the dark through a personal computer (IBM PS/2 Model 70). A FIAS-200 FI system (Bodenseewerk Perkin-Elmer, overnight in case of delayed analysis. Stability tests for 3 d storage carried out with standard reference materials showed U� berlingen, Germany) was used, fully controlled by the ELAN software.The system includes two peristaltic pumps and a that the aforementioned conditions preserved the total amount of extracted mercury. flow injection valve fitted with a 0.2 ml injection loop. Peak areas were used to characterize the analytical response and Analytical Procedure concentrations were calculated after normalization of the data to the internal standard signal followed by appropriate blank The final solutions (5 ml) contained 0.1% Triton X-100 (Sigma, subtraction.St. Louis, MO, USA), 0.1% EDTA (added as 1 g l-1 solution) Sample digestion was carried out in a single-mode reflux (Aldrich–Chemie, Steinheim, Germany), 0.1% v/v ammonia focused microwave system (Microdigest A-301, Prolabo, Paris, solution (Merck) and an internal standard consisting of a Frawer setting of 200 W. The power 20 ng ml-1 solution of thallium (Merck). It was necessary to system used with a focused microwave apparatus provides a add Triton X-100 as a surfactant and EDTA as a complexing continuous microwave emission at each power level.During agent in order to obtain a linear calibration curve (see Results digestion of sediments, in all cases 10% (20W) of the maximum and Discussion). Mercury working standard solutions were power output was used. The construction of the system allows prepared from a 1000 mg ml-1 Hg stock standard solution a change of time setting in 1 min steps.Specially designed glass (Spex Industries, Edison, NJ, USA) by successive dilutions vessels and the construction of the system allow digestion with Milli-Q water. Mercury standard spikes were added to under atmospheric pressure without any microwave leakage. the sample prior to determination by FI–ICP-MS to give As the system is an open-vessel microwave system, an Aspivap additions to the final concentrations of 0, 10, 20 and fume treatment system (Prolabo) is used, which effectively 40 ng ml-1.Once the solutions had been prepared they were neutralizes the acid fumes generated. analysed within 2 h. The stability of these solutions does not seem to exceed 24 h, which could be attributed to the effect of Sample Digestion the surfactant. The rinsing solution for the FI system was 5% v/v hydro- High-purity de-ionized water purified with a Milli-Q analytical- chloric acid (Romil). reagent grade water-purification system (Millipore, Chester, Cheshire, UK) was used throughout.Acids used were all super- Reference Materials and Real Samples purity acids and other reagents were of analytical-reagent grade. Samples, standards and final volumes were measured A certified reference material (PACS-1, National Research by mass in all cases. Vessels were rigorously cleaned, soaked Council of Canada) and two reference materials (IAEA-356, for 24 h in 10% nitric acid and thoroughly rinsed with Milli-Q S19), the latter produced for European intercomparison studies water before use.(BCR, EC),18 were used for validation of the method. Digestion parameters used throughout this work are given Sediment samples from Arcachon Bay were collected at in Table 2. In the procedure, in all cases 0.25 g of dry sediment several points and were placed in clean polyethylene bottles. sample was digested with 8 ml of HNO3 (Romil, Cambridge, Surface sediments were refrigerated at -20 °C immediately. UK) at a power setting of 20W (10%) for 5 min.The sample As the extraction procedure has been developed for dry was left to cool for about 5 min, followed by the addition of sediments, it was necessary to freeze-dry the wet sediments before analysis. The freeze-dryer used was an RP 2V (Department CIRP Lyophilisation, Argenteuil, France). Table 1 Operating conditions of the ELAN 5000 ICP-MS instrument Amounts of 10–30 g of wet sediment were introduced into the used for mercury determination stainless steel vessels provided by the manufacturer.In order ICP-MS conditions— to prevent losses of mercury due to volatilization, mild con- Forward rf power 1100W ditions were maintained throughout lyophilization. Samples Plasma gas flow rate 15 l min-1 were left to dry for 52 h before being transferred into polyethyl- Auxiliary gas flow rate 0.8 l min-1 ene bottles and kept in a refrigerator at 4°C. Similarly to the Nebulizer gas flow rate 0.98 l min-1 reference materials, samples were homogenized by shaking for Sampler and skimmer cones Nickel about 5 min before addition of acids and microwave digestion.Data acquisition— Scan mode Peak hop transient Measurements Dwell time 100 ms The performance of the FI–ICP-MS system was verified regu- Sweeps per reading 5 larly according to the daily procedure. Two isotopes of mercury Readings per replicate 60 No. of replicates 1 (masses 200 and 202) were always measured and calculations Signal processing Spectral peak integrated were normally based on the mercury-202 isotope.Mercury-200 Isotope measured 202Hg, 200Hg was used to check and confirm results in case of doubt. Internal standard 205Tl Analyses were carried out in duplicate and appropriate blank subtraction was used. To check for contamination of the digestion procedure and sample manipulation, a blank solution Table 2 Digestion parameters in an open-vessel (Prolabo) microwave was prepared and carried through each set of analyses.system for determination of total mercury in sediment samples by FI–ICP-MS. Amount digested: 0.25 g of sediment sample RESULTS AND DISCUSSION Step Reagent added Power setting/W Time/min Minimizing the Memory Effect of Mercury 1 8 ml conc. HNO3 20 5 Mercury is a difficult element to determine in real samples by 2 2 ml 30% H2O2 20 5 ICP-MS. Problems arising from its strong memory effect are 54 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12Table 3 Summary of results for the determination of mercury in mostly manifested through unacceptable correlation of cali- reference sediment samples by microwave digestion and FI–ICP-MS bration curves or a continuous decrease in sensitivity.Our with standard additions. Analytical figures of merit of the system are first aim before performing analyses of sediment samples was also shown therefore to find a procedure for eliminating these effects.We first optimized the analytical conditions with standards Reference material Reference value/mg g-1* Measured value/mg g-1† n‡ in simple solutions before carrying out analyses of real samples. To ensure the stability of mercury during transport and S19 91.07 95±3 6 nebulization, the pH of the solutions was kept alkaline by the IAEA-356 7.62 7.3±0.1 4 PACS-1§ 4.57±0.16 4.7±0.3 4 addition of ammonia solution, and the mercury was complexed with 0.1% EDTA. Also, 0.1% Triton X-100 was added to Correlation coefficient of calibration 0.995–1.000 improve the sample transport efficiency through its action as Limits of detection 10 ng g-1 (for solution) a surfactant.The method proposed by Campbell et al.6 resulted 1.0 ng g-1 (for dry sediment in linear calibration curves with a correlation coefficient samples) between 0.995–1.000. The fact that the sensitivity did not Recovery 95–105% change when repeated calibrations were executed indicates * Concentration values expressed on dry-mass basis.that the memory effect was completely eliminated in this way. † Results given with 95% confidence interval. It is likely that complexation transforms mercury into a more ‡ Number of replicates used for calculation. uniform and inert form, so no losses from volatilization and § Certified reference material. no sensitivity changes due to vapour build-up in the nebulization spray chamber occur. Triton as a surfactant might also additions separately also did not gave acceptable results, help to ensure uniform conditions and nebulization seems to whereas the combination of the two techniques proved to be be more stable, because losses during sample transport are excellent when determining mercury in sediment matrices. minimized.The behaviour of mercury becomes more predict- It has been shown that using an internal standard with a able and controllable. With these conditions, we were able to mass number close to that of the analyte element improves obtain a calibration curve with an excellent correlation the precision.19 Thallium, having a mass number of 205, is coefficient and without any memory effect. In Fig. 1, a transient close enough to the mercury-202 isotope, although its calcu- signal of the FI–ICP-MS system can be seen. Comparing the lated extent of ionization in argon ICP is 100% whereas for shape of peaks obtained for thallium (used as internal standard, mercury it is only about 38%.18 The internal standard chosen, see later) with those obtained for the two mercury isotopes, combined with standard additions, corrected the instrument we can see that minimum tailing and asymmetry resulted in instabilities and the interferences to a reasonable extent.This the case of mercury. This indicates that problems emerging seems to reinforce the observations of Vanhaecke et al.19 which from the memory effect of mercury have been overcome. indicated that the ionization energy of the internal standard has no or only secondary importance with regard to signal Determination of Mercury in Sediments precision and accuracy. Results for the total mercury content obtained for the analysis of reference materials are given in Table 3.Analytical figures Digestion Procedure of merit of the system are also indicated. For the determination of mercury in solid environmental The sample matrix can significantly change the instrument samples by FI–ICP-MS, a pre-treatment is necessary.This was sensitivity, preventing the use of external calibration methods. achieved by a microwave-assisted digestion procedure that has The technique of standard additions and the use of internal basically two objectives with regard to mercury determination: standards have traditionally been used to overcome the matrix first, to oxidize the organic matter in the sample and thus to effects of an analyte, since these quantification methods take liberate the mercury species from the sample matrix; and into account the instrument sensitivity for each sample matrix.second, to oxidize fully the liberated mercury to HgII in order In the given system, the analysed sediment samples contributed to prevent losses. These two objectives must also be kept in a complex matrix effect to the determination of mercury such mind when the digestion reagents are chosen. A microwave- that external calibration was found to be inadequate.Even assisted digestion procedure developed by Lamble and Hill10 though linear calibration curves with excellent correlation was used as a starting point, after having completed some coefficients were obtained when standard solutions (with added preliminary experiments on the digestion system. A modifi- EDTA and Triton X-100) were analysed, the results were not cation was made, however, since the procedure needed to be in agreement with the reference values when the S19 material adapted to ICP-MS detection. The sulfuric acid used as a was analysed.The use of internal standard or standard reagent in the first step of the digestion procedure was replaced with nitric acid in order to protect the sampler and skimmer cones from the strong corrosive effect of sulfuric acid. Conditions of the digestion procedure are summarized in Table 2. Elemental mercury (Hg0), being a highly volatile element, can easily be lost during microwave digestion in an open system.Care has to be taken to avoid the formation of Hg0 by overheating (too high power settings or too long a heating time) or by aspirating the generated fumes too strongly. This is the reason why the lowest possible power setting (20W, 10%) was used throughout the heating and why care was taken when adding reagents to avoid venting off acid fumes which may have resulted in losses of mercury. The fume aspiration head was set to the highest position in order to reduce aspiration of the fumes.Another important factor when Fig. 1 Transient signal of spiked sediment sample from Arcachon performing a microwave-assisted digestion procedure is the Bay. ICP-MS parameters are given in Table 1. A, 202Hg; B, 200Hg; and C, 205Tl. amount of the digested sample. Preliminary experiments Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 55Table 4 Quantitative analysis of sediment samples from Arcachon showed that recoveries were low (30–60%) and not repetitive Bay.The total mercury content was determined by microwave diges- when 0.5 g of sediment sample was digested with 10 ml of acid tion followed by FI–ICP-MS reagent. However, 0.25 g of dry sediment proved to be sufficient to obtain exact and reproducible results when performing the Sample Total Hg/ng g-1 Water content of wet sample (%) digestion procedure using the optimum conditions. All these A 190 66.5 precautions together provided conditions mild enough to retain B 260 62.7 all of the mercury.On the other hand, the digestion procedure C 180 63.7 was rigorous and long enough to recover 100% of the mercury. D 240 64.6 The digestion procedure is simple, and addition of the second Mean 218 acid reagent causes no problems since the system is open. The complete digestion procedure for one sample takes only 15 min, which can be further shortened when analysing samples in series. There is a considerable risk of contamination with open- wave-assisted digestion and FI–ICP-MS. A simple and fast vessel microwave systems, but throughout our experiments no digestion procedure was used and, with FI-ICP-MS detection, contamination was observed.resulted in an average 100% recovery for total mercury in the mercury concentration range 4–100 mg g-1. The application Reference Materials and Real Samples of FI with standard additions, internal standardization and The certified reference material PACS-1 and the reference complexation of mercury eliminated the classical problems materials IAEA-356 and S19 were used to validate this method.experienced when using direct calibration with conventional Results obtained for the determination of total mercury in ICP-MS methods. Disadvantages of this method are the small these three sediment reference materials are presented in sample throughput, which is a direct consequence of the Table 3. The values obtained for the reference materials (which standard additions method.The possibility of on-line standard have different concentration ranges and different matrix com- additions is offered by the FI system. The complete automation positions) are in good agreement with the given values. The of the detection system retaining the same limits of detection calibration curves for standard additions gave correlation may yet prove to be a successful method for routine analysis. coefficients in the range between 0.995–1.000.Recoveries between 95 and 105% were obtained, which represents an The authors thank the French Government for support (Bourse average maximum error of 10%. Limits of detection (LODs) du Gouvernement Franc�ais). were determined as three times the standard deviation of the blank divided by the slope of the calibration curve. For REFERENCES solutions an LOD of 10 ng g-1 and for dry sediment samples 1 Fo� rstner, U., and Wittmann, G. T. W., Metal Pollution in the about 1.0 ng g-1 (depending on the sample matrix) were Aquatic Environment, Springer, Berlin, 1983.obtained. The average precision of the whole procedure was 2 Godden, R. G., and Stockwell, P. B., J. Anal. At. Spectrom., 1989, 4, 301. better than 90% for the mercury concentration range 3 Bulska, E., Kandler, W., Paslawski, P., and Hulanicki, A., 4–100 mg g-1. Mikrochim. Acta., 1995, 119, 137. When the optimization and validation of both the extraction 4 Can�ada Rudner, P., Garcý�a de Torres, A., and Cano Pavo�n, J.M., procedure and the analytical determination with sediment J. Anal. At. Spectrom., 1993, 8, 705. reference materials had been completed, we determined the 5 Stroh, A., and Vo� llkopf, U., J. Anal. At. Spectrom., 1993, 8, 35. mercury content in real samples. The results obtained for 6 Campbell, M. J., Vermeir, G., Dams, R., and Quevauviller, P., J. Anal. At. Spectrom., 1992, 7, 617. sediment samples from Arcachon Bay are given in Table 4. 7 McLaren, J.W., Beauchemin, D., and Berman, S. S., Spectrochim. The mercury content was determined for samples from four Acta, Part B, 1988, 43, 413. different sampling areas. The water contents of these samples 8 Kalamegham, R., and Owen Ash, K., J. Clin. L ab. Anal., 1992, were not identical. The second column of Table 4 gives data 6, 190. concerning the water content of wet samples. Standard 9 Smith, G. R., Anal. Chem., 1993, 65, 2485. additions gave excellent calibration and the results seem to 10 Lamble, K.J., and Hill, S. J., J. Anal. At. Spectrom., 1996, 11, 1099. indicate an equal distribution of mercury of around 218 ng 11 Hall, A., Duarte, A., Caldeira, M. T. M., and Lucas, M. F. B., Sci T otal Environ., 1987, 64, 75. g-1 in the sediment f Since these samples have 12 Scifres, J., Cheema, V., Wasko, M., and McDaniel, W., Am. lower mercury concentrations than the reference standards, Environ. L ab., 1995, 6, 6. another analytical technique was used to confirm that the 13 Bushee, D.S., Analyst, 1988, 113, 1167. FI–ICP-MS results were accurate. Determination of total and 14 Wiederin, D. R., Smyczek, R. E., and Houk, R. S., Anal. Chem., methylmercury in this case was carried out by aqueous phase 1991, 63, 1626. ethylation followed by cryogenic GC–quartz furnace AAS after 15 Powell, M. J., Quan, E. S. K., and Boomer, D. W., Anal. Chem., 1992, 64, 2253, a microwave-assisted acid leaching.21 A two-sample paired 16 Tyson, J., Fresenius’ Z. Anal. Chem., 1988, 329, 663. Student’s t-test was used to calculate the significance of differ- 17 Jarvis, K. E., Gray, A. L., and Houk, R. S., Handbook of ences between mean values of the two different methods. At Inductively Coupled Plasma Mass Spectrometry, Blackie, New the 95% confidence level the results showed no significant York, 1992, pp. 119–124. difference. 18 Quevauviller, Ph., Fortunati, G. U., Filippelli, M., Baldi, F., According to the environmental regulations for soil in Bianchi, M., and Muntau, H., Appl. Organomet. Chem., 1996, 10 in the press. Europe, the sediment from Arcachon Bay is not polluted and 19 Vanhaecke, F., Vanhoe, H., and Dams, R., T alanta, 1992, 39, 737. further action is not required (limit in Germany 250 ng g-1 20 Houk, R. S., Anal. Chem., 1986, 58, 97A. and in the UK 1000 ng g-1). However, considering the assess- 21 de Diego, A., Tseng, C. M., Stoichev, T., Martin, F., and Donard, ment criteria of the Interim Canadian Environmental Quality O. F. X., unpublished data. Criteria for Contaminated Sites, where the limit is 100 ng g-1, 22 Visser, W. J. F., Contaminated L and Policies in Some Industrialized action should be considered.22 Countries, TCB, The Hague, Netherlands, 1994. CONCLUSION Paper 6/06044G Received September 2, 1996 This work has demonstrated the ability to determine total mercury with good precision in sediment samples by micro- Accepted October 29, 1996 56 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12

ISSN:0267-9477    

DOI:10.1039/a606044g

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Determination of the Gold Content in Geogas by Resonance Ionization Mass Spectrometry W. Y. MA*, Q. HUI, M. XUE, W. X. JI AND D. Y. CHEN Department of Physics, T singhua University, 100084 Beijing, China A resonance ionization mass spectrometric technique for the by an electric field. The most efficient ionization of the excited atoms is through a discrete autoionizing state embedded above ultratrace determination of Au in geogas samples was developed. A three-step excitation scheme leading to an the first ionization limit of the atoms, because its crosssection can exceed by several orders of magnitude that of autoionizing state was used for detecting the Au content of 60 geogas samples combined with a graphite furnace non-resonance photoionization.11–13 The ions created are subsequently measured in a mass spectrometer where electrothermal atomizer. In order to increase the selectivity and detection efficiency, a linear time-of-flight mass additional elemental or isotopic selectivity is added.Time-of- flight (TOF), magnetic sector and quadrupole mass spec- spectrometer was built and tested. The experimental results are in good agreement with those obtained by neutron trometers have been used, of which the TOF mass spectrometer is particularly suitable for use in combination with pulsed laser activation analysis. The study of the Au distribution in geogas is of great interest in prospecting for gold deposits. excitation and has the advantage of a high transmission efficiency.Keywords: Gold; geogas; resonance ionization mass This paper reports on the ultrasensitive determination of spectrometry the Au content in geogas samples, based on thermal atomization of a substance in vacuum followed by resonant stepwise A recent method for prospecting for deeply concealed gold photoionization of Au atoms through an autoionizing state deposits involves the determination of the concentration of Au and combined with TOF mass spectrometry. Our purpose was in geogas.Wang et al.1 developed a rapid, inexpensive and to test if this method can be used in searching for buried gold highly efficient method for the dynamic collection of geogas deposits. To our knowledge, this is the first application of from soil that proved to be satisfactoryfor buried gold deposits. RIMS to the detection of Au in geogas. The experimental The problemis that the Au content in geogas has not previously results were also compared with those obtained by neutron been investigated because of its low concentration, lower than activation analysis (NAA). 0.1 ng l-1, and the lack of sensitive analytical methods. Using conventional methods for determining the Au content in this EXPERIMENTAL natural material, preliminary chemical enrichment must be used. The complexity of the multistage chemical treatment of RIMS System samples make such a concentration technique labour consum- The analytical spectrometer for RIMS was arranged according ing and not very reliable. The most severe problem is that to a standard scheme.11 It includes an electrothermal atomizer, until now there has been no suitable preliminary chemical sample autochanger, laser system, TOF mass spectrometer and enrichment method for Au in geogas, because the amount of signal collector; a schematic diagram is illustrated in Fig. 1. sample is very small. Hence there is an urgent need to develop A 20ml volume of sample solution was poured into a a method with high sensitivity and high selectivity for this graphite crucible and air dried, then it was heated by an natural material.electric current to about 1500 °C. To suppress the strong The concept of resonance ionization spectrometry (RIS) was background from thermal ions and electrons there were two first advanced by Hurst and co-workers2,3 and called single- D-shaped boxes at electric potentials of ±24 V above the atom detection (SAD) since a striking example of a single Cs heater.As a result, the residual background ion level was not atom was detected. Combining RIS with mass spectrometry, higher than one ion per second. Three laser beams were resonance ionization mass spectrometry (RIMS) is characterized by a very high sensitivity owing to efficient ionization and detection of the ions produced, and high selectivity owing to multiple resonant transitions and mass-selective detection. The high sensitivity and selectivity of RIMS provide an accurate measurement method for trace amounts with little or no sample preparation.This, in turn, improves the speed of the analysis. This feature is very suitable for ultratrace element determinations in small amounts of sample, such as determining the Au content in geogas and many other hard-to-obtain samples. Many studies4 –10 have demonstrated that RIMS is an extremely powerful tool for detecting trace amounts of most elements in the Periodic Table, and it has wide uses in geochemistry, chemical exploration and many other fields.The RIMS technique includes two processes, resonance ionization and mass analysis. In the resonance ionization process, the specific atoms are excited resonantly by laser radiation to an intermediate state in one or several steps, and Fig. 1 Schematic diagram of the RIMS system. then only the excited atoms are ionized by laser radiation or Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 (57–59) 57directed to the vacuum chamber and intersected the atomic beam perpendicularly between the two electrodes. The Au atoms in this sample were excited and ionized by laser beams through an autoionizing state. The Au+ ions created were subsequently accelerated under a dc electric field of 3.2 kV. After travelling in the field-free region of the TOF mass spectrometer they were separated from the background ions produced by other elements or molecules with the multiphoton non-resonant ionization process, and were detected with microchannel plates at the end of about a 1.5 m field-free drift tube.The current pulse signal from the microchannel plates was amplified by a fast amplifier and coincided with the delayed gate signal in synchronism with the laser pulse. The output Fig. 2 Autoionizing spectrum in the third excitation step of Au atoms signal was integrated to a charge which was converted into a leading from the 6d2D5/2 state to the autoionizing state with resonance digital form by a QDC (charge-to-digital converter) and was wavelength 588 nm.The solid curve represents a fit by a Shore–Fano finally fed into a microcomputer. The data were acquired and profile and the points represent the experimental data.14 processed automatically. The residual pressure in the chamber was about 10-6 Torr. By means of the sample autochanger, 20 samples can be For trace Au determination, the three-step scheme of measured continuously without affecting the vacuum.As a autoionizing state excitation and ionization was chosen: result, the measurements were carried out efficiently and the experimental conditions were uniform for all samples. 6s2S1/2 CCCC l1=243 nm 6p2P3/2 CCCC l2=479 nm Laser System 6d2D5/2 CCCC l3=588 nm autoionizing state The laser system consists of three tunable dye lasers pumped by an excimer laser to produce laser beams with suitable A first-step laser pulse with wavelength l1=243 nm excited wavelengths and intensities.The excimer laser (EMG202) and the Au atoms from the 6s2S1/2 ground state to the 6p2P3/2 two dye lasers (3002E and 3002EC) were obtained from intermediate state. Then a second-step laser pulse with wave- Lambda Physik (Germany). The third dye laser was made in length l2=479 nm performed further excitation of the atoms our laboratory. The excimer laser (XeCl) can produce laser to the 6d2D5/2 state. Finally, a third-step laser pulse with pulses of 400 mJ energy and 308 nm wavelength with a pulse wavelength l1=588 nm excited the atoms to an autoionizing width of 28 ns. In our experiments, the repetition rate of the state.The energies of the first-, second- and third-step laser excimer laser was 20 Hz. With non-linear processes consisting pulses in the experiment were 10 mJ, 100 mJ and 8 mJ, respectof second harmonic generation and mixing, the dye lasers ively. In the probe volume the typical diameter of the laser produce tunable light from the IR to UV region with intensity beams was about 5 mm, so the energy fluences of the laser 10 mJ–10 mJ and linewidth 0.2 cm-1.radiation were 51 mJ cm-2, 509 mJ cm-2 and 41 mJ cm-2, respectively. These values are sufficient to saturate the resonant transitions. Sample Preparation Aqueous AuCl3 Samples The samples were taken from the surface of a goldmine in the north of China, deeply concealed underground. About 10 l of Aqueous solutions were obtained by dilution of the AuCl3 geogas in soil were extracted by a gas acquirer from a hole standard solutions with deionized water.Calibration curves 50 cm deep underground and Au atoms were absorbed in a were constructed by measuring the AuCl3 standard solutions foamed plastic. Half of each piece of foamed plastic was over the concentration range 0.01–10 ng ml-1 before and after analysed by RIMS and the other half was analysed by neutron analysing the geogas samples every day because of a lack of a activation analysis, reported elsewhere in detail.14 standard geogas sample.The calibration curve given by y= The chemical processing of the sample was very simple: the 2450x+6 shows excellent linear behaviour. The AuCl3 stan- foamed plastic was ashed at 450 °C in a ceramic crucible and dard solution of concentration 0.01 ng ml-1 and a blank then dissolved in 1 ml of pure aqua regia to convert the Au sample of de-ionized water were used for determining the compounds into AuCl3.After drying, the sample can be detection limit of Au. Using the 3s criterion, where s is the preserved for several weeks. A 1 ml volume of 1% aqua regia root-mean-square error of the blank sample, the detection was added to the ceramic crucible to re-dissolve AuCl3 before limit of Au obtained with the AuCl3 standard solution was detection. Only 20 ml of the liquid sample were injected into 0.003 ng ml-1. the graphite crucible for analysis. Geogas Samples For real sample analysis 32 geogas samples taken above the RESULTS AND DISCUSSION goldmine and 28 geogas samples from the surrounding area Autoionizing State of Au were analysed.Each sample was measured three times and the mean values obtained are given in Table 1. The concentration Gold atoms were excited in three steps to an autoionizing state lying 4636.5 cm-1 above the first ionization limit, which of Au in geogas samples taken above the goldmine is in the range 0.12–6 ng l-1, obviously larger than that in samples was found as reported previously.15 This autoionizing state, characterized by a strong and narrow resonance peak, has a from the surrounding area (0.02–0.1 ng l-1).The precision is about 30% for the Au content in geogas at levels as low as very large photoionization yield. Fig. 2 shows the autoionizing spectrum with a resonance wavelength of 588 nm; the solid 0.02 ng l-1 and is better than 15% for Au contents larger than 0.1 ng l-1.curve represents a fit by a Shore–Fano profile.14 58 Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12Table 1 Gold content in geogas in soil (ng l-1 geogas) In conclusion, RIMS is an extremely efficient method for the determination of ultratrace concentrations of elements. Samples from above goldmine Samples from surrounding area Significant attributes include excellent sensitivity and very high Sample no. Au content/ng l-1 Sample no.Au content/ng l-1 selectivity, which provide for accurate measurements with little or no sample preparation and allow the use of very small 1 0.14 33 0.09 amounts of sample. These merits are very important in ultra- 2 0.15 34 0.04 3 0.19 35 0.10 trace element determinations in hard-to-obtain samples and 4 0.20 36 0.10 improve the speed and accuracy of the analysis considerably. 5 0.28 37 0.09 It is clear that RIMS will be useful in prospecting for concealed 6 0.34 38 0.03 deposits and has a wide range of application in many fields. 7 0.44 39 0.03 8 0.63 40 0.04 The authors thank Professor X. J. Xie and X. Q. Wang of the 9 6.0 41 0.02 10 0.64 42 0.03 Geophysical and Geochemical Exploration Institute for pro- 11 0.38 43 0.04 viding the geogas samples and many helpful discussions during 12 0.25 44 0.07 these experiments. 13 0.18 45 0.06 14 0.14 46 0.02 15 0.18 47 0.02 16 0.25 48 0.04 REFERENCES 17 0.30 49 0.05 1 Wang, X. Q., Xie, X. J., and Lu, Y.X. Chin. Geophys. Geochem. 18 0.68 50 0.08 Explor., 1995, 19, 161. 19 0.39 51 0.06 2 Hurst, G. S., Nayfeh, M. H., and Young, J. P., Appl. Phys. L ett., 20 0.23 52 0.09 1977, 30, 229. 21 0.15 53 0.07 3 Hurst, G. S., Nayfeh, M. H., Kramer, S. D., and Young, J. P., 22 0.13 54 0.09 Rev. Mod. Phys., 1979, 54, 767. 23 0.12 55 0.06 4 Hurst, G. S., and Letokhov, V. S., Phys. T oday, 1994, 47, 38. 24 0.15 56 0.08 5 Payne, M. G., Deng, L., and Thonnard, N., Rev. Sci. Instrum., 25 0.18 57 0.10 1994, 65, 2433. 26 0.23 58 0.09 6 Kluge, H. J., Bushaw, B. A., Passler, G., Wendt, K., and 27 0.63 59 0.05 Trautmann, N., Fresenius’ J. Anal. Chem., 1994, 350, 323. 28 2.72 60 0.10 7 Saloman, E. B., Spectrochim. Acta, Part B, 1993, 48, 1139. 29 0.35 8 Saloman, E. B., Spectrochim. Acta, Part B, 1994, 49, 251. 30 0.21 9 Wendt, K., Passler, G., and Trautmann, N., Phys. Scr., 1995, 31 0.18 T58, 104. 32 0.14 10 Perera, I. K., Lyon, I. C., and Turner, G., J. Anal. At. Spectrom., 1995, 10, 273. 11 Hui, Q., Chen, D. Y., Niu, J. G., Cheng, Y., Xu, X. Y., and Zhao, Table 2 Comparison of the results obtained by RIMs and NAA W. Z., Inst. Phys. Conf. Ser., 1990, 114, 297. 12 Ma, W. Y., Hui, Q., Zhao, W. Z., Wen, K. L., Xu, X. Y., and Au content/ng l-1 geogas Chen, D. Y., Mod. Phys. L ett. B, 1991, 5, 1095. 13 Ma, W. Y., Li, L. Q., Zhao, W. Z., and Chen, D. Y., Opt. Sample no. RIMS NAA Commun., 1991, 85, 408. 12 0.25 0.24 14 Wang, X. Q., T he Study of Prospecting for Giant Gold Deposits by 23 0.12 0.12 Wide-spaced Geochemical Sampling in Combination With L SAD 44 0.07 0.06 and Other Analytical T echniques, Chinese Report of National 8th 46 0.02 0.03 Five-year Plan, 1995, 30. 52 0.09 0.10 15 Zhao, W. Z., Xu, X. Y., Ma, W. Y., Cheng, Y., Hui, Q., Wen, 59 0.05 0.06 K. L., and Chen, D. Y., Appl. Phys. B, 1991, 52, 299. 16 Braun, T., and Rausch, H., Anal. Chem., 1995, 67, 1517. It is well known that NAA is a useful method for trace Paper 6/06031E elements determinations with high sensitivity, high accuracy Received September 2, 1996 and good precision.16 Table 2 gives some results for the Au Accepted October 2, 1996 content in geogas samples obtained by our method and by NAA. The results obtained with these two methods show good agreement. Journal of Analytical Atomic Spectrometry, January 1997, Vol. 12 59

ISSN:0267-9477    

DOI:10.1039/a606031e

出版商:RSC    

年代:1997

数据来源: RSC