摘要:

The XXVIII Colloquium Spectroscopicurn Internationale will be held in The University of York United Kingdom June 29-July 4,1993 "his traditional biennial conference in analytical spectroscopy will once again provide a forum for atomic nuclear and molecular spectroscopists worldwide to encourage personal contact and the exchange of experience. Participants are invited to submit papers for presentation at the XXVm CSI dealing with the following topics Basic Theory Techniques and Instrumentation of- Applications of Spectroscopy in the Analysis of- Computer Applications and Chemometrics Laser Spectroscopy Environmental Samples Atomic Spectroscopy (Emission Absorption Fluorescence) Electron Spectroscopy Geological Materials Gamma Spectroscopy Industrial Products Mass Spectrometry (Inorganic and Organic) Methods of Surface Analysis and Depth Profiling Molecular Spectroscopy (UV VIS IR) Mossbauer Spectroscopy Nuclear Magnetic Resonance Spectrometry Photoacoustic Spectrometry Raman Spectroscopy X-ray Spectroscopy Biological Samples Food and Agricultural Products Metals Alloys PLENARY AND INVITED SPEAKERS The scientific programme will consist of Plenary and Invited Speakers.To date the following scientists have accepted invitations to present keynote lectures Plenary- Invited- M L Gross Lincoln NE R E Hester York C L Wilkins Riverside CA J D Winefordner Gainemille FL F C Adams Antwerp F V Bright Bufldo NY J A Caruso Ciwimri OH B T Chait New York NY R Donovan Edinburgh D E Games Swansea D L Glish Oak Ridge TN P Hendra Southampton F Hillenkamp Munster J A Holcombe Austin TX J Reffner Stagord CT B L Sharp Loughborough M Sigrist Zurich M Thompson London J C Vickerman Manchester PRE- and POST-SYMPOSIA In connection with the XXVIII CSI a number of symposia and workshops will be organized.EXHIBITION The conference will feature an exhibition of the latest instrumentation. ACCOMMODATION Accommodation has been reserved on campus and in the halls of residence although hotel accomodation in York will be available if desired. SOCIAL PROGRAMME The scientific programme will be punctuated with memorable social events and excursions of scientific cultural and tourist interest. The social programme is open to all participants and accompanying persons. For further information contact- THE SECRETARIAT XXVIII CSI Department of Chemistry Loughborough University of Technology Loughborough Leicestershire LE113TU UK.Telephone +44 (0) 509 222575; Fax +44 (0) 0509 233163; Telex 34319.The XXVIII Colloquium Spectroscopicurn Internationale will be held in The University of York United Kingdom June 29-July 4,1993 "his traditional biennial conference in analytical spectroscopy will once again provide a forum for atomic nuclear and molecular spectroscopists worldwide to encourage personal contact and the exchange of experience. Participants are invited to submit papers for presentation at the XXVm CSI dealing with the following topics Basic Theory Techniques and Instrumentation of- Applications of Spectroscopy in the Analysis of- Computer Applications and Chemometrics Laser Spectroscopy Environmental Samples Atomic Spectroscopy (Emission Absorption Fluorescence) Electron Spectroscopy Geological Materials Gamma Spectroscopy Industrial Products Mass Spectrometry (Inorganic and Organic) Methods of Surface Analysis and Depth Profiling Molecular Spectroscopy (UV VIS IR) Mossbauer Spectroscopy Nuclear Magnetic Resonance Spectrometry Photoacoustic Spectrometry Raman Spectroscopy X-ray Spectroscopy Biological Samples Food and Agricultural Products Metals Alloys PLENARY AND INVITED SPEAKERS The scientific programme will consist of Plenary and Invited Speakers.To date the following scientists have accepted invitations to present keynote lectures Plenary- Invited- M L Gross Lincoln NE R E Hester York C L Wilkins Riverside CA J D Winefordner Gainemille FL F C Adams Antwerp F V Bright Bufldo NY J A Caruso Ciwimri OH B T Chait New York NY R Donovan Edinburgh D E Games Swansea D L Glish Oak Ridge TN P Hendra Southampton F Hillenkamp Munster J A Holcombe Austin TX J Reffner Stagord CT B L Sharp Loughborough M Sigrist Zurich M Thompson London J C Vickerman Manchester PRE- and POST-SYMPOSIA In connection with the XXVIII CSI a number of symposia and workshops will be organized.EXHIBITION The conference will feature an exhibition of the latest instrumentation. ACCOMMODATION Accommodation has been reserved on campus and in the halls of residence although hotel accomodation in York will be available if desired. SOCIAL PROGRAMME The scientific programme will be punctuated with memorable social events and excursions of scientific cultural and tourist interest. The social programme is open to all participants and accompanying persons. For further information contact- THE SECRETARIAT XXVIII CSI Department of Chemistry Loughborough University of Technology Loughborough Leicestershire LE113TU UK. Telephone +44 (0) 509 222575; Fax +44 (0) 0509 233163; Telex 34319.

ISSN:0267-9477    

DOI:10.1039/JA99207FX001

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

JASPE2 7 ( 1 ) 1 N-8N 1-68 1 R-66R (1 992) February 1992 Journal of Analytical Atomic Spectrometry Including Atomic Spectrometry Updates CONTENTS NEWS AND VIEWS 1 N Editorial-Barry Sharp I N 5N Future Issues 6N Gordon Kirkbright Bursaty 6N 6N Conferences and Meetings 7N Courses Conference Reports-Julian Tyson and Mike Hinds and Steve Hill Nomenclature and Terminology Update-Judith Egan PAPERS 1 7 11 15 19 23 29 35 43 47 53 59 61 65 Determination of Trace Amounts of Cadmium by Laser Excited Atomic Fluorescence Spectrometvy-Mikhail A Bolshov Sergei N Rudnev Bruno Hutsch Evaluation of a Low-powered Argon Microwave Plasma Discharge as an Atomizer for the Determination of Mercury by Atomic Fluorescence Spectrometry-Yixiang Duan Xiangxing Kong Hanqi Zhang Jun Liu Qinhan Jin Determination of Aluminium Niobium and Vanadium in Low-alloyed Steels by Spark Ablation Coupled With Inductively Coupled Plasma Atomic Emission Spectrometry-Aurora Gornez Coedo M Teresa Dorado Lopez Jose L Jimenez Seco Isabel Gutierrez Cob0 Determination of Methylmercury Species by Capillary Column Gas Chromatography With Axially Viewed Inductively Coupled Plasma Atomic Emission Spectrometric Detection-Takunori Kato Takashi Uehiro Akio Yasuhara Masatoshi Morita Preconcentration and Inductively Coupled Plasma Atomic Emission Spectrometric Determination of Metal Ions With On-line Chelating Ion Exchange-Valerio Porta Corrado Sarzanini Ornella Abollino Edoardo Mentasti Enzo Carlini Determination of Trace Metals in Volatile Organic Solvents Using Inductively Coupled Plasma Atomic Emission Spectrometry and Inductively Coupled Plasma Mass Spectrometry-Steve J Hill James Hartley Les Ebdon Application of Radioactive Tracers for Investigation of Dysprosium and Manganese Vaporization in Electrothermal Atomic Absorption Spectrometry-Muhammad Mansha Chaudhry David Littlejohn John E Whitley Determination of Erbium by Electrothermal Atomic Absorption Spectrometry Using a brolytic Graphite Coated Graphite Tube and a Pyrolytic’ Graphite Coated Graphite Tube Lined With Tantalum Foil Held in Place by a Tungsten Spiral-Ma Yi-zai Bai Jian Sun Di-jun Determination of Rare Earth Elements by Liquid Chromatographic Separation Using Inductively Coupled F’lasrria Mass Spectrometric Detection-Diane S Braverman Evalution of the Influence of lnterferents in Flame Atomic Absorption Spectrometry and Correction of the Analytical Signal by the Limit Dilution Method-F Bosch Reig F Bosch Mossi V Peris Martinez A Pastor Garcia CO M M U N CAT1 0 N Mineral Microanalysis by Laser Ablation Inductively Coupled Plasma Mass Spectrometry-Nicholas J G.Pearce William T Perkins Ian Abell Geoff A T Duller Ronald Fuge CUMULATIVE AUTHOR INDEX INSTRUCTIONS TO AUTHORS IUPAC PUBLICATIONS ON NOMENCLATURE AND SYMBOLISM ATOMIC SPECTROMETRY 1 R UPDATE Watkins Mark Cave Environmental Analysis--Malcolm S Cresser Janet Armstrong John Dean Peter 53R References i FACSS Announcement and Call for Papers Typeset by Burgess & Son (Abingdon) Ltd (-1 Printed in Great Britain by PAGE BRoS Page Bros Norwich 0267-9&77( 19!9211-3JASPE2 7 ( 1 ) 1 N-8N 1-68 1 R-66R (1 992) February 1992 Journal of Analytical Atomic Spectrometry Including Atomic Spectrometry Updates CONTENTS NEWS AND VIEWS 1 N Editorial-Barry Sharp I N 5N Future Issues 6N Gordon Kirkbright Bursaty 6N 6N Conferences and Meetings 7N Courses Conference Reports-Julian Tyson and Mike Hinds and Steve Hill Nomenclature and Terminology Update-Judith Egan PAPERS 1 7 11 15 19 23 29 35 43 47 53 59 61 65 Determination of Trace Amounts of Cadmium by Laser Excited Atomic Fluorescence Spectrometvy-Mikhail A Bolshov Sergei N Rudnev Bruno Hutsch Evaluation of a Low-powered Argon Microwave Plasma Discharge as an Atomizer for the Determination of Mercury by Atomic Fluorescence Spectrometry-Yixiang Duan Xiangxing Kong Hanqi Zhang Jun Liu Qinhan Jin Determination of Aluminium Niobium and Vanadium in Low-alloyed Steels by Spark Ablation Coupled With Inductively Coupled Plasma Atomic Emission Spectrometry-Aurora Gornez Coedo M Teresa Dorado Lopez Jose L Jimenez Seco Isabel Gutierrez Cob0 Determination of Methylmercury Species by Capillary Column Gas Chromatography With Axially Viewed Inductively Coupled Plasma Atomic Emission Spectrometric Detection-Takunori Kato Takashi Uehiro Akio Yasuhara Masatoshi Morita Preconcentration and Inductively Coupled Plasma Atomic Emission Spectrometric Determination of Metal Ions With On-line Chelating Ion Exchange-Valerio Porta Corrado Sarzanini Ornella Abollino Edoardo Mentasti Enzo Carlini Determination of Trace Metals in Volatile Organic Solvents Using Inductively Coupled Plasma Atomic Emission Spectrometry and Inductively Coupled Plasma Mass Spectrometry-Steve J Hill James Hartley Les Ebdon Application of Radioactive Tracers for Investigation of Dysprosium and Manganese Vaporization in Electrothermal Atomic Absorption Spectrometry-Muhammad Mansha Chaudhry David Littlejohn John E Whitley Determination of Erbium by Electrothermal Atomic Absorption Spectrometry Using a brolytic Graphite Coated Graphite Tube and a Pyrolytic’ Graphite Coated Graphite Tube Lined With Tantalum Foil Held in Place by a Tungsten Spiral-Ma Yi-zai Bai Jian Sun Di-jun Determination of Rare Earth Elements by Liquid Chromatographic Separation Using Inductively Coupled F’lasrria Mass Spectrometric Detection-Diane S Braverman Evalution of the Influence of lnterferents in Flame Atomic Absorption Spectrometry and Correction of the Analytical Signal by the Limit Dilution Method-F Bosch Reig F Bosch Mossi V Peris Martinez A Pastor Garcia CO M M U N CAT1 0 N Mineral Microanalysis by Laser Ablation Inductively Coupled Plasma Mass Spectrometry-Nicholas J G.Pearce William T Perkins Ian Abell Geoff A T Duller Ronald Fuge CUMULATIVE AUTHOR INDEX INSTRUCTIONS TO AUTHORS IUPAC PUBLICATIONS ON NOMENCLATURE AND SYMBOLISM ATOMIC SPECTROMETRY 1 R UPDATE Watkins Mark Cave Environmental Analysis--Malcolm S Cresser Janet Armstrong John Dean Peter 53R References i FACSS Announcement and Call for Papers Typeset by Burgess & Son (Abingdon) Ltd (-1 Printed in Great Britain by PAGE BRoS Page Bros Norwich 0267-9&77( 19!9211-3

ISSN:0267-9477    

DOI:10.1039/JA99207BX003

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

6N JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 Conferences and Meetings Second FECHEM Conference on Edu- cation in Analytical Chemistry Analy- tical Chemistry-A Key For a Safer Future for Mankind August 31-September 1 1992 Charles University Prague Czechoslovakia The Conference organized by the Czechoslovak Chemical Society on be- half of the Working Party on Analytical Chemistry of the Federation of Euro- pean Chemical Societies will deal with all aspects of education in analytical chemistry and will consist of Invited Plenary lectures contributed oral and poster presentations a panel discus- sion and a section devoted to audio- visual aids and the use of computers for education in analytical chemistry. For further information please con- tact Dr. J. Barek Department of Analytical Chemistry Charles Univer- sity Albertov 2030 12840 Prague 2 Czechoslovakia. Telephone + 42 2 292051 or 297541; Fax +42 2 29 1958; E-mail NEMEC @CSEARN.

ISSN:0267-9477    

DOI:10.1039/JA99207006Nc

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 7 Evaluation of a Low-powered Argon Microwave Plasma Discharge as an Atomizer for the Determination of Mercury by Atomic Fluorescence Spectrometry Yixiang Duan Xiangxing Kong Hanqi Zhang Jun Liu and Qinhan Jin* Department of Chemistry Jilin University Changchun 130023 China A low-powered argon microwave plasma torch discharge is introduced into atomic fluorescence spectrometry (AFS) for the first time. The configuration and the characteristics of the plasma for AFS are described in this paper. Some factors influencing the determination of mercury such as the flow rates of the carrier gas and the plasma gas are discussed in detail. The detection limit for mercury by this method is shown to be 3 ppb and the relative standard deviation for a solution concentration of 1 pg ml-i is 2.5% (n=11). The proposed method is relatively free from background interference and gives a large dynamic range.Keywords Microwave plasma torch; atomic fluorescence spectrometry; mercury determination Work on the use of an inductively coupled plasma (ICP) as an atomization cell for atomic fluorescence spectrometry (AFS) has developed rapidly since Montaser and Fassel' suggested that the ICP might be a promising atomization cell for AFS in 1976. In 1981 a commercial ICP-AFS instrument manufactured by the Baird Corporation became available and its performance was described by Demem2 The microwave-induced plasma (MIP) has received con- siderable attention as an atomization cell for atomic emission spectrometry (AES) and atomic absorption spec- trometry (AAS)3*4 during the past decade since the introduc- tion of the Beenakker c a ~ i t y ~ and great success has been achieved using both techniques.Since atomic fluorescence is a hybrid phenomenon of atomic emission and atomic absorption it possesses the characteristics of both AES and AAS but is not limited by the characteristics of either of the techniques. The MIP may also be used as an atomization cell in AFS. More recently Perkins and Long6 reported the first use of a low-wattage MIP as an atomization cell for AFS measurements in which a TMolo cavity was used to produce the plasma and a hollow cathode lamp (HCL) as well as a continuum xenon arc lamp were used for excitation. The preliminary results for some metals were not very satisfactory.The detection limits were from the sub-ppm to ppm level. However these results were greatly improved by their later work7 with a high-efficiency helium MIP as the atomization source in which the detection limits for 14 elements were from the ppb to sub-ppm level. These results are comparable to and even a little better than those obtained with MIP-AES under the same conditions indicating that the MIP has great potential in AFS. In this paper the microwave plasma torch (MPT)* was introduced for the first time into AFS. The configuration of this torch is similar to that of the ICP torch and it has already been applied in AES.8 In the present work preliminary studies of the characteristics of the plasma and its performance as an atomizer for AFS as well as its application in the determination of mercury were carried out. Some other elements such as zinc and cadmium have been also studied with a similar system and detection limits at the ppb and sub-ppb level were achieved respec- tively.Detailed results for these two elements will be reported elsewhere. ~ ~ * To whom correspondence should be addressed. r r r I 1 I Cleaning reagent 4 I - ' Reducing 10 11 - 1 reagent - dr Fig. I Schematic diagram of MPT-AFS system 1 high-voltage supply; 2 preamplifier; 3 detection circuit; 4 computer; 5 PMT; 6 HCLs; 7 lens; 8 MPT; 9 microwave power supply; 10 concentrated H,SO,; 1 1 cold vapour generator; 12 recorder; and 13 power supply Experimental Reagents All chemicals used were of analytical-reagent grade.Water was distilled and de-ionized. The stock solution of mercury (1000 ppm) was prepared from HgC12. This solution was diluted as required for use. The SnC12 solution (about 5%) was prepared by adding 5 g of SnC12.H20 to 5 ml of concentrated hydrochloric acid heating slightly for dissolu- tion and then diluted to 100 ml. Instrumentation and Procedure The block diagram of the MPT-AFS system used is shown in Fig. 1. The apparatus used is listed in Table 1. A specially designed HCL was used in this experiment working in a pulsed mode. The radiation from the mercury HCL was focused with a suitable lens onto the atomization cell for excitation. The resulting fluorescence (at 253.7 nm) was collected at an angle of 45" with respect to the excitation beam.After amplification by a preamplifier and a8 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 Table 1 Apparatus used for MPT-AFS Component MPT Photomultiplier tube High voltage power supply Computer Dual-channel spectrometer Pulsed mercury HCL Microwave power generator ModeVsize Manufacturer - Laboratory built R106 - Beijing Instrumental Factory of Geology Apple I1 Nanjing Electric Instrument Factory XDY-I1 Beijing Instrumental Factory of Geology VEKY-AF The 12th Institute of Mechanics and Electronics Ministry of China DW-2 Beijing Instrumental Factory of Geology Table 2 Operating parameters for MPT-AFS Forward power Reflected power Observation height Current of HCL Photomultiplier tube power supply Flow rate of carrier gas (F,) Flow rate of plasma gas (F,,) Sample volume Reduction agent volume Time for measurement and integration 10-50 W ow 0-36 mm 15-90 mA 300 V 400- 1000 ml min-' 400-1000 ml min-' 2 ml 2 ml 5-15 s detection circuit the fluorescence signal was fed to an analogue-to-digital ( N D ) converter and then printed and stored using a computer.The data presented in this paper are given as background-corrected values. In order to obtain AFS profiles the background signal was subtracted by the computer from the analyte signals. The working curve and inter-element effect plots were treated similarly. Operating Conditions The operating conditions for MPT-AFS are shown in Table 2. The optimum microwave forward power is 50 W with 0 W reflected. The practical forward power was usually much lower than 80 W so it was not necessary to cool the torch with water during operation.The plasma is very easy to ignite by touching a metal rod which was insulated from the operator by a rubber tube to the top of the central tube. Sample introduction was performed by the use of a cold vapour generator (aerator). A 5% SnC1 solution was used as a reducing agent to produce mercury vapour. In order to prevent a large amount of air from entering the plasma and affecting the performance of the plasma the aerator was equipped with a septum. The sample solution was injected into the reaction vessel (aerator) through the septum. The aerator was cleaned with de-ionized water after each measurement. The areas of the fluorescence signals were recorded. The sample turnaround time is about 30 s.Results and Discussion Characteristics of the Plasma The plasma was produced using an MIP similar to that used by Jin et at.* The torch consists of three concentric copper tubes. The dimensions of the outer tube are 22 mm i.d. x 25 mm 0.d. The intermediate tube is of 5.3 mm i.d. x 5.8 mm 0.d. The inner tube is of 1.8 mm i.d. x 2.7 mm 0.d. For 3 2 4 Fig. 2 Microwave plasma torch I first bright zone; 2 second bright zone; 3 plasma tail plume; 4 crossing point; 5 central tube; 6 intermediate tube; and 7 outer tube all experiments the argon (plasma gas) flow is introduced continuously into the intermediate tube to maintain the plasma and the sample gas was introduced into the inner tube along with the carrier gas. The plasma was formed at t:he top of the torch (see Fig.2). The plasma can be optically divided into three zones first bright zone second bright zone and tail plume. The heights of different zones are about 6 5 and 13 mm respectively. The plume is a good observation zone for atomic fluores- cence because it has a very pale colour and the background emission is weak. When forward power is low (1 0 W) the plasma plume is short and there is a little tremble in the plasma. When the forward power is higher than 30 W the plasma becomes stable. Microwave leakage from the MPT discharge was in- spected with a radiation hazard meter and was found to be less than 5 mW cm-2 at a distance of 5 cm from the top of the unshielded torch. Atomic Fluorescence Profiles and Working Curves In order to determine the position of maximum analyte signal in the plasma tail plume atomic fluorescence profiles olf different analytes were studied.Fig. 3 represents the atomic fluorescence profile of a 1 ppm mercury solution at a plasma forward power of 70 W. In this profile the relative fluorescence intensity (Ir) of mercury versus the observation h,eight (H) above the top of the torch is plotted. The maximum intensity of the fluorescence was observed to occur at a height of 16 mm above the top of the torch. AfterJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 9 180 I I 1 ’-I 40 A Him m Fig. 3 Atomic fluorescence profile of mercury versus the observa- tion height above the top of the torch obtained with MPT A mercury atomic fluorescence (at 253.76 nm); and B background under the same conditions as those for fluorescence I I t - A B C D E I 1 I I VS 0 5 10 15 Fig.4 Influence of flow rate of carrier gas on the intensity of mercury fluorescence A 160; B 400; C 600; D 800; and E 1000 ml min-I. F Background this point intensity diminished rapidly in the plasma plume with the increase of height. It was also noted that at the first and second bright zones there exists a strong plasma flicker noise and spectral background. Therefore it is difficult to observe the mercury fluorescence signal in these areas so the plasma tail plume was usually chosen to measure the fluorescence signals. The linearity of the working curve for mercury extends to about three order of magnitude. As can be seen in the range shown (Fig. 3) no obvious self-absorption was observed.Influence of Flow Rate of Carrier Gas It was shown that the fluorescence signals were greatly affected by the flow rate of the carrier gas (Fig. 4). When the flow rate is too low the mercury generated can only be’ carried out of the reaction vessel slowly and incompletely. When the flow rate is too high the concentration of the mercury generated is diluted by the carrier gas and the fluorescence signal is also diminished. The optimum flow rate was found to be 600 ml min-l. The flow rate of the plasma gas is another important factor that affects the intensity of fluorescence and the shape of the plasma. With an increase in the flow rate of the plasma gas the plasma plume became elongated and the optimum observation height was altered. The optimum flow rate of plasma gas was found to be about 600 ml min-I.Influence of Microwave Power The influence of microwave power on the height of the plasma is obvious in the range of lower forward power (Fig. 5). The plasma height is increased with an increase in 700 8oo t \ 3 600 3 500 w .- c - 200 100 d 1 I 1 I I I 1 Microwave powerw 0 10 20 30 40 50 Fig. 5 Influence of microwave power on the height of plasma and the intensity of fluorescence of mercury A height of plasma; B mercury fluorescence intensity; and C background 0 15 30 45 60 75 90 105 120 ifmA Fig. 6 Influence of HCL current on the intensity of fluorescence of mercury microwave power and then levels off when the power is greater than 30 W. The influence of microwave power on the intensity of fluorescence is relatively complex.The intensity of fluorescence decreases with an increase in the microwave power. Influence of HCL Current The influence of the HCL current (i) on the intensity of the fluorescence signal is shown in Fig. 6. The intensity of fluorescence increases sharply with increasing HCL current at first and then levels off. It is obvious that the mercury HCL reaches a maximum emission intensity when the current is increased. Other Influences It has been noted that the reducing agent SnCl is easily absorbed on the wall of the cold vapour generator. It is very difficult to clean up even using a large amount of water. An oxidizing agent (1% KM,04) was used during the course of cleaning to eliminate the influence of SnCl on the system. Recovery An artificial solution containing 60 ppb of mercury and various metal ions including 1 ppm of Zn2+ Ca2+ A13+ and Cu2+ 10 ppm of Mg2+ Fe3+ Na+ K+ Cd2+ and Co2+ and 100 ppb of Pb2+ was used to test the recovery of mercury.The recovery was shown t o be more than 90% for ten measurements. No obvious interfering effect was observed.10 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 Table 3 Comparison of detection limits of several atomic fluores- cence methods for the determination of mercury Excitation Atomization Detection source cell limits (ppb) Ref. Microwave excited electrodeless discharge lamp ICP 10 1 Pulsed HCL ICP 25 9 Low pressure mercury vapour lamp ICP 0.2 10 Pulsed HCL MPT 3 This work Detection Limits The detection limit was measured under the optimum conditions and calculated according to the guidelines of the International Union of Pure and Applied Chemistry.For the calculation 1 1 blank readings were taken. The analytical sensitivities were calculated from the working curve of the element under study. The detection limit for mercury was shown to be 3 ppb. The relative standard deviation for a 1 ,ug ml-l solution was 2.5% (n= 11). A comparison of the detection limits for mercury obtained with different AFS methods is shown in Table 3. Conclusions This work shows that the MPT discharge is promising as an atomization cell for AFS. It exhibits a large dynamic range over a concentration range of several orders of magnitude and is relatively free from background interferences. Fur- thermore its detection limit for mercury is comparable to or even better than that of the ICP with the same excitation source. There is no doubt that MPT-AFS can be used in simultaneous multi-element analysis. A study on the use of HCL with MPT-AFS for a number of other elements and their simultaneous determination is underway. This work was supported by the National Natural Science Foundation of China. References 1 Montaser A. and Fassel V. A. Anal. Chem. 1976,48 1490. 2 Demers D. R. paper presented at the Pittsburgh Conference Atlantic City USA 198 1. 3 Jin Q. Zhang H. Yu S. Spectrosc. Spectral Anal. (Beijing) 1989 9(4) 32. 4 Lin X. Zhang H. Bing G. and Jin Q. ZCP Znf Newsl. 1988 42 1285. 5 Beenakker C. I. M. Spectrochim. Acta Part B 1976,31,483. 6 Perkins L. D. and Long G. L. Appl. Spectrosc. 1988 42 1285. 7 Perkins L. D. and Long G. L. Appl. Spectrosc. 1989,43,499. 8 Jin Q. Zhu C. Borer M. W. and Hieftje G. M. Spectro- chim. Acta Part B 1991 46 417. 9 Demers D. R. and Allemand C. D. Anal. Chem. 1981 53 1915. 10 Lancione R. L. and Drew D. M. Spectrochim. Acta Part B 1985 40 107. Paper I /02398E Received May 22 I991 Accepted September 19 I991

ISSN:0267-9477    

DOI:10.1039/JA9920700007

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL AND ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 11 Determination of Aluminium Titanium Niobium and Vanadium in Low-alloyed Steels by Spark Ablation Coupled With Inductively Coupled Plasma Atomic Emission Spectrometry Aurora Gomez Coedo M. Teresa Dorado Lopez Jose L. Jimenez Seco and Isabel Gutierrez Cob0 Centro Nacional de lnvestigaciones Metalurgicas Gregorio del Amo 8 28040 Madrid Spain A medium-voltage spark at a high repetition rate coupled with excitation in an argon inductively coupled plasma (ICP) was applied to the direct determination of Al Ti Nb and V in low-alloyed steels. Tests were conducted using Bureau of Analysed Samples reference materials series 451 /1-455/1 and 456/1-460/1. A comparative evaluation of the analytical performances obtained with spark optical emission spectrometry ICP with pneumatic nebulization and spark ablation inductively coupled plasma atomic emission spectrometry is given.The analytical parameters evaluated are the limit of detection signal to background ratio background equivalent concentration and relative standard deviation. Keywords Spark ablation inductively coupled plasma atomic emission spectrometry; low-alloyed steels analysis; optical emission spectrometry Steel making practices involve the addition of a number of elements to bring about deoxidation of the bath control of the grain characteristics and an increase of some mechani- cal properties. Several of these elements are added in very low amounts producing microalloyed steels. Requirements in quality control and analytical specifications of these microalloyed steels are becoming more and more strict with the aim of reliably establishing the influence of specific minor elements.In this study the minor elements consi- dered were Al Ti Nb and V. These four elements are employed to inhibit austenitic grain growth and at the same time A1 and Ti are used as deoxidizers and Nb and V to improve the strength of low carbon steels. Spark optical emission spectrometry (OES) is a routine method for multi-element determinations in steels how- ever its sensitivity is sometimes insufficient for the analysis of low amounts of the elements considered. Inductively coupled plasma atomic emission spectrome- try (ICP-AES) is now a method commonly utilized for the analysis of steels after dissolution. However the dissolution process is a laborious and time-consuming step which apart from introducing risks of contamination also causes dilution of the analyte and an increase in the saline concentration when recovery of the insoluble residue is necessary.This limits the power of detection and the precision. Recently many attempts have been made to produce aerosols directly from metal samples. The use of spark ablation (SA) for the direct vaporization of metal samples was first demonstrated by Human et a[.' Broekaert et af.* discussed the capabilities and limitations of different techniques for direct solid sampling in atomic spectrome- try. Raeymaekers et aL3 characterized the spark aerosol produced by electron probe micro-analysis. Lemarchand et aL4 applied SA coupled with excitation in a 1.2 kW argon ICP to the direct determination of Cr Mn Ni P Si and V in ferrous alloys.Prell and Koirtyohanns carried out transport studies using SA coupled with excitation in an Table 1 Instrumentation used for SA-ICP-AES and spark-OES Sparking unit JY-SAS- Operating parameters Sparking chamber Maximum voltage 700 V; maximum repetition rate 400 s-' Ceramic body of diameter 10 mm and height of 20 mm. (The argon transport gas enters parallel to the sample surface. The analyte vapour is brought into the ICP through a tube with a diameter of 5 mm.) Cathode Tungsten rod of diameter 2 mm. Inductively coupled plasma spectrometer J Y- 2 4- Generator Spectrometer Computer IBM 640 kbytes RAM Frequency 40.68 MHz power 1500 kW 0.640 m Czerny-Turner monochromator. Grating 3,600 lines mm-l; linear dispersion 0.4 nm mm-'; and practical resolution 0.01 3 nm Optical emission spectromter ARL 3560 OES- Generator Spectromter Unisource 10 pF; 20 pH; 100 Hz; voltage= 300-550 V 1 .O m Paschen-Runge. Concave grating 1080 lines mrn-l; photomultiplier tubes 3.5 mm long with ten diodes and a glass or quartz side window Attenuators Computer PDP 11/23 digital 60 with 41 positions for each12 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL.7 Table 2 Selected working parameters in SA-ICP Sampling spark- Voltage 700 V Repetition rate 400 s-l Resistance Time Gas flow Carrier gas pressure Pre-spark 0 $2; integration 2 R Pre-scanning 10 s; pre-integration 10 s; transfer 15 s; Permanent carrier gas 1.8 1 min-'; analysis gas 0.55 1 min-' 3x lo5 Pa analysis time 0.5 s per point (9 points for each peak) ICP- Power 900 W Gas flow Plasma observation height 13 mm Plasma gas 14 1 min-'; sheath gas 0.3 1 min-' Table 3 Composition (Oh) of tested BAS reference materials Carbon #Steels (residual series) BASNo.C Si Mn P S A1 Ti 45111 45211 45311 45411 45511 4561 1 457/1 45811 45911 4601 1 0.05 1 0.323 0.160 0.376 0.598 0.101 0.324 0.247 0.523 0.452 0.1 16 0.055 0.34 0.31 0.25 0.24 0.05 1 0.54 0.58 0.10 0.62 1.30 1.38 0.80 0.40 0.20 0.30 0.49 0.97 0.67 0.009 0.035 0.044 0.06 1 0.052 0.018 0.010 0.032 0.054 0.043 0.0 14 0.01 7 0.026 0.047 0.055 0.023 0.042 0.033 0.057 0.0 12 - 0.105 - 0.03 1 - 0.073 - 0.0 10 - 0.022 0.009 - 0.111 - 0.023 - 0.028 - 0.012 - Nb - - - - - 0.006 0.022 0.052 0.014 0.066 V - - - - - 0.022 0.17 0.108 0.080 0.060 ICP and they calculated the limit of detection (LOD) and the relative standard deviation (RSD) for several elements in iron- and aluminium-based alloys.Spark ablation fulfils the requirements for a direct metal solid sampling technique in plasma spectrometry. The aim of this work was to test the analytical capabilities of the SA-ICP technique for the determination of Al Ti Nb and V in low-alloyed steels. A second objective was to compare the analytical performances of the coupled SA-ICP technique with those provided by spark-OES and ICP with pneumatic nebulization. Experimental Instrumentation The instrumentation used is described in Table 1 and the selected working parameters in Table 2.Samples The Bureau of Analysed Samples (BAS) reference materials Carbon Steel (residual series) 45 1 / 1 -460/ 1 were used. The composition of these samples is shown in Table 3. Selection of Analytical Lines The selection of analytical lines was based on the study of spectral interferences in an ICP with pneumatic nebuliza- tion carried out to determine these same elements in steels.6 The analytical lines selected were Al 396.152 nm; Ti 337.280 nm; Nb 309.415 nm; and V 309.310 nm. Results and Discussion The objective of sampling ablation is to achieve the input of a high amount of analyte into the plasma with a view to obtaining improved sensitivity and a high power of detec- tion. The amount of analyte that reaches the plasma increases with both the voltage and repetition rate; how- ever the particle size is increased by an increase in the applied voltage and not by increasing the repetition rate.An increase of the particle size decreases the plasma stability *and reduces the signal to background (SIB) ratios. The (average size of the ablated particles should be (1 pm to guarantee their complete vaporization in the ICP and to achieve good stability and high sensitivity. In this study for the determination of residual elements in low-alloyed steels ithe maximum available voltage (700 V) and the maximum repetition rate (400 s-l) were selected with a view to obtaining greater sensitivity since the plasma stability and the S/B ratios obtained were satisfactory. The argon transport velocity employed during the analysis was 0.55 t - ID I= W to ..- Ti ' Nb ! V 396.152 337.280 309.415 309.310 Wavelengthtnm Fig.1 Scans of Al Li Ti Nb and V for A AMKO iron; B a BAS sample with the lowest content of each element; and C a BAS sample with the highest content of each element13 JOURNAL OF ANALYTICAL AND ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 c- x. - - 0 - 5.000 4.000 3.000 - - (Low) Table 4 Values of analytical parameters obtained using the three different techniques 0.100 0.080 0.060 ‘a 0.040 -z 0.020 W -! n Element Technique RSD* BECT SIBS LODg A1 Spark-OES 1.5 0.021 1.4 0.0010 SA-ICP-AES 1.8 0.004 2.2 0.00030 ICP with pneumatic nebulizer 0.7 0.005 2.0 0.00022 SA-ICP-AES 1.2 0.002 5.0 0.00015 Ti Spark-OES 2.0 0.019 1.5 0.00075 ICP with pneumatic nebulizer 0.8 0.003 6.0 0.00008 SA-ICP-AES 2.0 0.007 2.0 0.00045 ICP with pneumatic nebulizer 0.5 0.006 1.3 0.000 I0 SA-ICP-AES 1.0 0.005 3.0 0.00020 V Spark-OES 1.5 0.008 2.5 0.00048 ICP with pneumatic nebulizer 0.6 0.004 3.3 0.000 15 Nb Spark-OES 1.5 0.040 2.8 0.0020 0 $ 30.000 - ‘i 20.000 0 -ii *Values (%) calculated from the results obtained for the BAS sample with the lowest content of each element.tConcentrations (%) corresponding to intensity ratios of twice the background values. $Signal to background ratios calculated for 0.0 1% concentration levels. $Calculated as a percentage at a confidence level of 2c of the AMKO iron. /’ 45311 AMKO (Low) Q4 45211 1 min-*; this value provides transport of the analyte its complete vaporization in the ICP and plasma stability. The intensity-time profiles corresponding to the ele- ments studied show that these emissions are stable from 10 s (selected pre-spark time) to at least 90 s.This time is sufficient for the determination of four elements in a sequential spectrometer making one measurement per element with an integration time of 0.5 s per point and measuring nine points per peak. The calibration and measuring systems were studied. For calibration two terms were employed to describe the samples used to obtain the corresponding calibration graph i.e. ‘low’ and ‘high’. To make possible the analysis of samples with lower contents than those certified in the c 40.000 - reference materials used [BAS Carbon Steels (residual series)] AMKO pure iron (with certified contents of 1 ppm for each of these elements studied) was used as a low standard sample.As a high standard the BAS samples with the highest content of the corresponding element were used. Fig. 1 shows the scans around the selected analytical lines obtained from AMKO iron and from the BAS samples with the lowest and the highest contents of each element. Analytical measurements were made using the iron line at 373.487 nm as reference. The iron content in the BAS samples was between 99 and 97.5%. The RSDs of the intensity ratios corresponding to five sparks at different locations on the sample were below 1.5% for all the elements tested. /‘ 45111 (High) - - - 45711 I f’-,(Low) 0 0.02 0.04 0.06 0.08 0.10 Aluminium content (%I 1.200 1.000 - 3 ? G j 0.800 0.600 ?; > 0.400 0.200 0 n 1 I 1 I I 1 ‘0 0 0.01 0.02 0.03 0.04 0.05 0.06 Niobium content (%) I (cb X i/454JJ 1 0 0.02 0.04 0.06 0.08 0.10 Tit an i u m content (‘340 1 1.250 1.000 i $ ? 0.750 5 (High) 60.000 52.500 45.000 37.500 30.000 .O 0 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 Vanadium content (%) 0.40 0.35 0.30 cn 0.25 0 0.20 -2 0.15 0.10 0.05 0 Fig. 2 Calibration graphs for (a) Al (b) Ti (c) Nb and (6) V14 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 Using the calibration graphs for AMKO iron as a low and a BAS sample as a high standard the remainder of the BAS samples were analysed as unknowns. For the elements investigated these samples can be interpolated linearly in the calibration graphs. The same BAS samples were also analysed using spark- OES. In this instance calibration graphs were obtained from all the BAS samples using a second-degree regression.Fig. 2 shows the graphs obtained with both analytical techniques SA-ICP-AES and spark-OES plotting intensity ratios versus concentrations. The intensity ratio scales are the same for the two techniques. The graphs show that the lowest background values and higher intensity concentra- tion ratios were obtained using SA-ICP-AES. Inductively coupled plasma AES with pneumatic nebuli- zation was also used to analyse the same BAS samples dissolved using a microwave digestion system. The amount of sample employed was 0.25 g in 50 ml. For the three analytical techniques used the following analytical parameters were calculated RSD background equivalent concentration (BEC) S/B and LOD. Table 4 shows the values obtained for each element and for each technique.These data permit a comparative evaluation of the three techniques and show that SA-ICP is a valid analytical technique for the direct determination of Al Ti Nb and V as residual elements in steels. Spark sampling using a medium-voltage spark at a high repetition rate is a viable approach for solid sample analysis for the determination of Al Ti Nb and V as residual elements in steels with a sequential ICP spectro- meter. The RSD values when an iron line was used as the internal standard were below 2%. The BAS standard samples were interpolated linearly in calibration graphs obtained with a pure iron and a high sample. The calculated LODs were between 1 and 5 ppm which is approximately five times better than those provided by spark-OES and of the same level as those obtained using an ICP with pneumatic nebulization. References 1 2 3 4 :5 h Human H. G. C. Scott R. H. Oakes A. R. and West C . D. Analyst 1976 101 265. Broekaert J. A. C. Leis F. Raeymaekers B. and Zaray Gy. Spectrochim. Acta Part B 1988 43 339. Raeymaekers B. Van Espen P. Adams F. and Broekaert J. A. C. Appl. Spectrosc. 1988 42 142. Lemarchand A. Labarraque G. Masson P. and Broekaert J. A. C. J. Anal. At. Spectrom. 1987 2 481. Prell L. J. and Koirtyohann S. R. Appl. Spectrosc. 1988 42 (7) 1221. Gomez Coedo A. Dorado Lopez M. T. and Jimenez Seco J. L. Metalurgia 1979 15 98. Paper I /03540 A Received July 12 I991 Accepted September 16 I991

ISSN:0267-9477    

DOI:10.1039/JA9920700011

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

JOURNAL O F ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 15 Determination of Methylmercury Species by Capillary Column Gas Chromatography With Axially Viewed Inductively Coupled Plasma Atomic Emission Spectrometric Detection Takunori Kato Hokkaido Institute of Environmental Sciences N 19 W72 Kitaku Sapporo Hokkaido 060 Japan Takashi Uehiro Akio Yasuhara and Masatoshi Morita National Institute for Environmental Studies 16-2 Onoga wa Tukuba lbaraki 305 Japan Methylmercury species were determined by capillary column gas chromatography (GC) coupled with inductively coupled plasma (ICP) atomic emission spectrometry. Methylmercury species were converted into the iodide form and separated on a chemically bonded capillary column. An axially viewed ICP with an echelle monochromator was used as a highly selective and sensitive mercury detector for GC.The detection limit of methylmercury was calculated to be 3 pg as Hg at a signal-to-noise ratio of 2. The linear range was more than three orders of magnitude. The relative standard deviation of ten replicate measurements of 30 pg of methylmercury (as Hg) was 5%. Keywords Inductively coupled plasma atomic emission spectrometry; axial viewing; coupled gas chromato- graphy; mercury speciation; methylmercury determination Since a plasma emission detection method for gas chro- matography (GC) was first published,' this technique has been widely utilized in the speciation of volatile organo- metallics because of its high selectivity high sensitivity and wide linear range regardless of their molecular structure.Among the plasma detectors the microwave-induced plasma (MIP) has more often been used compared with the inductively coupled plasma (ICP) or direct current plasma (DCP) probably owing to its small size and good matching of gas flow rates. The MIP detector is very sensitive for volatile species containing mercury selenium arsenic and other element^,^-^ but its shortcoming is that the introduc- tion of solvent into the plasma results in quenching of the discharge. For this reason it is necessary to bypass the solvent vapour before it enters the cavity or to ignite the plasma after the solvent has passed through the cavity. With the ICP the plasma is maintained in the presence of an organic solvent and therefore such a procedure is not necessary.In this work the capability of ICP atomic emission spectrometry (AES) as a detector for GC was examined for the determination of alkylmercury compounds. As it has been reported that an axially viewed (AXV) ICP has a more intense analyte emission and lower background intensities than those in a conventional side-viewed (SDV) ICP,5-7 the AXV-ICP was employed in the experiment. For separation of alkylmercury compounds a short chemically bonded quartz capillary column was used because it was expected to have the least active sites to adsorb alkylmercury. Experimental Instrumentation The GC-ICP-AES system and connections are shown in Fig. 1 and the optimum operating conditions are summar- ized in Table 1. The column employed here is a chemically bonded fused-silica capillary column (3 x 0.35 mm i.d.) coated with methylsilicone (5 pm layer) (Gasukuro Kogyo Tokyo Japan).The splitless injection mode was employed. The end of the capillary column was directly introduced to the head of the inner tube of the plasma torch. To avoid Capillary 1 ICP (axial view) I I colum Heater P k ' Recorder Fig. 1 system (axially viewed) Schematic diagram of the capillary column GC-ICP-AES condensation part of the column between the GC outlet and the plasma torch was passed through a 2 mm i.d. poly(tetrafluorethy1ene) (PTFE) tube and heated with a tape heater to the maximum column temperature (1 50 "C). The capillary column in the torch was warmed indirectly by passing the argon sample gas over an electrically heated Nichrome wire. In order to determine the optimum conditions for ICP a metallic mercury reservoir was placed in the oven of the gas chromatograph to serve as a continuous and constant mercury source.In order to maintain the GC-ICP interface conditions the reservoir was inserted between the column and the transfer capillary (80 cm). For the AXV construction the torch box assembly was rotated so that the torch was in a horizontal attitude. Optical observations were made through the exhaust hole at the top of the torch box while the plasma gases were extracted through the normal side-viewing port. The monochromator employed was an echelle type (Spectra Span 111). A reduced half-sized image of the AXV plasma was focused on the entrance slit of the monochro- mator with a quartz lens (50 mm diameter f= 10 cm) or a reduced one tenth-size image of the SDV plasma was focused on the entrance slit with a quartz lens (10 mm diameter f= 5 cm).A Jeol DX300 mass spectrometer equipped with a Hewlett-Packard Model 57 1 OA gas chromatograph was used to identify methylmercury species. A methylsilicone capillary column 15 m x 0.35 mm i.d. 5 pm layer) (Gasukuro Kogyo) was used for the separation.16 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 Table 1 GC-ICP-AES instrumentation and optimum conditions Gas chromatography- Gas chromatograph Column Carrier gas He Flow rate Injector temperature 140 "C Column temperature Purge time 0.5 min Hewlett-Packard Model 5890A with splitless injector Fused-silica capillary column (3 in x 0.3 mm i.d.) coated with methylsilicone film thickness 5 pm 7.5 ml min-l (head pressure 20 kPa at 30 "C) Programmed from 70 "C (hold time 0.5 min) to 150 "C at 30 "C min-l Inductively coupled plasma atomic emission spectrometry- ICP Daini Seikosha R.f.generator Matching box Torch Plasma-Therm (Fassel type) Plasma gas Ar Flow rates Jhhelle-type monochromator Spectrametrics Spectraspan I11 Wavelength 253.7 nm Slit (width x height) Photomultiplier Amplifier 40 MHz 0.5 kW with automatic power controller unit Automatic matching unit with a vacuum-type condenser Outer 18 1 min-I intermediate 8.5 1 min-I sample 0.3 1 min-l Entrance 200 x 500 pm Exit 200 x 500pm Hamamatsu R292 at 950 V Field effect transistor input op-a.mp current-voltage converter (10 V mA-') time constant 1 s ? A With an SDV plasma the highest SIB ( 100) was obtained a t 0.5 kW r.f.power 0.7 1 min-l sample gas flow rate and observation height 17 mm above the load coil. However the highest SIB did not correspond to the highest S/N. and close to the dark current (net background 0.15 nA dark current 0.1 nA). The highest SIN (about 1000) was observed at 0.9 kW r.f. power 0.6 1 min-l sample gas flow rate and observation height 17 mm above the load coil. Under these conditions the SIB was reduced to 25. With the AXV plasma the highest S/B (550) was flow rate (Fig. 2). The mercury emission intensity corre- sponding to the highest S/B condition at the respective r.f. power was not reduced significantly by reducing the r.f. power from 1.1 to 0.5 kW and the optimum conditions for the highest S/N (about 20 000) were almost the same as that 500 - .- 0 c 2 g 300 m * C cx) Under these conditions the background intensity was low c.-0 3 0 x - obtained at 0.5 kW r.f. power and 0.3 1 min-' sample gas z - iij 100 - 1 1 I I I for the highest S/B. The flow rates of the outer and 0.2 0.4 0.6 0.8 1.0 intermediate gas had little influence on the S/B and S/N. Sample gas fIow/I min-' If we compare the signal and background intensities from the AXV- and SDV-ICP the background intensities of the two observation systems were almost the same and the signal intensity of the AXV-ICP was 20 times greater than that of the SDV-ICP at the respective optimum S/N. This improve- ment in S/N was four times greater than the reported values and might be partially owing to the characteristics of the Fig.2 Relative intensity of Hg and SIB ratio by GC-ICP-AES. and Bt o.7 kW; c and c 0.9 kW; and D and D 1.1 kW Reagents power A and A' o.5 kw; All of the reagents were of analytical-reagent grade. A stock solution was prepared by dissolving 37 mg of methyl- mercury chloride (Wako Osaka Japan) in 100 ml of benzene. Working standard solutions were obtained by diluting the stock solution with benzene to the required concentration. Results and Discussion ICP Optimization The optimum conditions were determined by considering the mercury signal-to-background ratio (SIB) mercury emission intensity and mercury signal-to-noise ratio (S/N). The mercury emission intensity was measured by introduc- ing mercury-containing gas. The gas was prepared by passage through a metallic mercury reservoir kept at 40 "C.The exact mercury concentration in the gas was not known but the concentration was kept constant (about 5 ng s-l) during the experiments. Cchelle-type monochromator.-As the vertical slit-height was limited to 500 pm which was the maximum in the monochromator used it was necessary to reduce the ICP image size to one tenth in the SDV arrangement in order to observe an emission zone of a few millimetres. On the other hand the limited slit-height was suitable for the AXV construction because the analyte emission which was restricted in the narrow centre zone of the plasma image could be selected from the surrounding bright torus-shaped b<ackground emission by the narrow slit-height. G.as Chromatograph Optimization The effect of the injection port temperature on methylmer- cury was investigated using GC-ICP-AES over the range 90-200 "C.The injection volume used was 2 pl of 3.18 pg ml-' methylmercury solution in benzene. The peak intensity of the methylmercury increased with increasing irljection port temperature up to 130 "C and remained almost constant in the range 130-150 "C. The peakJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 17 3 0 2 4 Ti me/m i n Fig. 3 Resolution of mercury species of 1 Hg; 2 benzene; 3 CH,HgX; and 4 C2H5HgX by GC-ICP-AES intensity gradually decreased above 160 "C and leading and tailing of the peak appeared. These effects might be due to the decomposition of the analyte. An alternative insert made of PTFE was checked but no significant change was observed.The injection port temperature was therefore set at 140 "C. In order to separate the methylmercury from the solvent (benzene) and to concentrate the methylmercury on the column head an initial temperature of 70 "C and a hold time of 0.5 min were chosen. Column temperature programmes covering the range from 70 "C (hold time 0.5 min) up to 150 "C with ramp rates of 10 20 30 and 40 "C min-l were studied. When the normal capillary column (25-50 m) was used at a linear velocity of 40 cm-I high sensitivity was not obtained because of peak broadening probably owing to the adsorp- tion or decomposition of the methylmercury on the column. In general the peak intensity of the analyte increases gradually with an increase in the carrier gas flow rate and with shortening of the column length.Therefore the use of a short column at a high carrier gas flow rate is useful for the rapid and sensitive determination of mercury compounds. In this study a 3 m column (2.5 m in the GC oven and 0.5 m at the GC-ICP interface) and a 7.5 ml min-l flow rate at 30 "C (head pressure 20 kPa linear velocity 1.3 m s-l) were chosen because of reduced adsorption effects short retention time (2.2 min) and high sensitivity. The theoretical plate value of the column was around 2500. Although these conditions (short column length and high flow rate) are unusual in capillary column GC they might be suitable for labile or thermally unstable compounds such as methylmercury. Separation of Mercury Species Separation was examined by using GC-ICP-AES for mer- cury vapour methylmercury chloride and ethylmercury chloride. Under the GC conditions mentioned above these species were adequately separated. The peak shape for methylmercury chloride was symmetrical and narrow (Fig. 3).However when a standard sample of methylmercury chloride and an environmental sample were injected repeat- edly a problem occurred the single peak of methylmercury was split into three peaks and the ratio of each peak was dependent on the amount of methylmercury chloride 1501 0 0 2 4 6 Retention tirne/min 1000 1 1 x10 150 200 250 300 350 mlz Fig. 4 (a) Chromatogram obtained by total ion monitoring and (6) mass spectra of CH,HgX (X=Cl Br I) obtained by GC-MS. 1 CH3HgCI; 2 CH,HgBr; and 3 CH,HgI injected. When very small amounts of methylmercury chloride (below 100 pg) were injected only one peak appeared.However when amounts of more than 200 pg were injected two or three peaks appeared. In order to identify the compounds giving rise to these peaks 3 ng of methylmercury were injected 20 times and the fractions corresponding to methylmercury peaks were collected in cold benzene (6 "C). The benzene solution was concentrated to a small volume by flushing with nitrogen gas and analysed by GC-MS. A total ion monitoring chromatogram and the mass spectrum of each peak are shown in Fig. 4. The peaks were identified as methylmercury chloride bromide and iodide. The single peak that appeared when small amounts of methylmercury (less than 100 pg) were injected was confirmed as methylmercury iodide.It was noteworthy that methylmercury iodide was identified in spite of injecting an authentic sample of methylmercury chloride. Conversion from chloride into iodide seemed to occur at the injection port and on the column. The stability constants of the methylmercury halides are known to decrease in the order iodide > bromide > c h l ~ r i d e . ~ ~ ~ Therefore the presence of even trace amounts of iodide may have given rise to the appearance of methylmercury iodide. The source of bromine and iodine was not identi- fied. This reaction also occurred when a real (atmospheric) sample was injected. It can be assumed that bromide and iodide in the sample were adsorbed or remained on the injection port or column and caused the formation of the bromide and iodide species.The splitting of the peak resulted in a deterioration of the detection limit and reproducibility. Therefore it was decided to convert all methylmercury species into the iodide. This conversion was completed by treatment with iodine dissolved in benzene. Benzene was shaken with hydriodic acid and several microlitres of the benzene layer were injected. By this treatment all methylmercury species including those which were originally due to the chloride and bromide analogues appeared at the position of the iodide species. Once this treatment had been carried out no peaks due to chlorideJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 18 t - m C a m .- 30 P9 1 rnin H Time - Fig. 5 Chromatogram of CH3HgI at a low concentration level obtained by GC-ICP-AES and bromide species appeared even though more than 200 samples were injected thereafter.Calibration and Detection Limit The calibration graph for methylmercury was linear in the range 0.006-6 ng of mercury (6 ng was the highest concentration examined). Fig. 5 demonstrates the detection of methylmercury at low concentration levels. The detection limit for methylmercury (as Hg) was 3 pg (injection volume 2 pl) at S/N=2. The relative standard deviation of ten replicate measurements was about 5% at the trace level (30 pg as Hg). Selectivity Because of the poor selectivity of the electron-capture detector for organomercury compounds many substances give false selectivities. The ICP detection system provides much better selectivity. At the 253.7 nm atomic emission line the selectivity for mercury when comparing CH3HgX with decane or undecane was well above 1 x 1 06 1.Hence environmental or biological samples can be injected with- out previous clean-up procedures. Because of the high- temperature characteristics of the ICP source atomization in the plasma is considered to be almost complete. For this reason the sensitivity is virtually independent of the chemical form (methyl- or ethylmercury). Atmospheric Samples Three air samples were collected in a residential area in Sapporo in September 1989 according to the method by Bzezinska et al.,*O using a tube (140 x 8 mm i.d.) packed with Tenax GC. The pumping speed was 2 1 min-' and the sampling time was 24 h. After the air sample had been collected the tube was connected to a nitrogen supply then the mercury compounds were thermally eluted into 0.5 ml of cold benzene in a micro-impinger by heating the tube at about 200 "C for 30 min with a nitrogen flow rate of 10 ml min-I.The benzene solution was concentrated to 50 pl by nitrogen gas flushing and then appropriate aliquots (2- 10 pl) of the benzene solution were injected into the GC-ICP- AES system. The concentrations of methylmercury in three air samples were calculated to be 17 28 and 54 pg M - ~ 1 1 I 0 2 4 Tim e/m i n Fig. 6 Chromatogram of an air sample obtained by GC-ICP-AES. Peak 1 benzene; and 2 54 pg m-3 of methylmercury (CH,HgX) respectively. No other alkylmercury compounds were de- tected. A typical chromatogram is shown in Fig. 6. Conclusions A gas chromatograph coupled with an ICP-AES detector was applied to the determination of trace amounts of methylmercury species.Methylmercury species were first converted into the iodide form by treatment with iodine dissolved in benzene. The iodide was readily separated on a short capillary column. The AXV-ICP detector offered advantages in analytical performance over the conventional SDV-ICP in optical configuration. In particular the AXV-ICP permitted a 20- fold more sensitive detection of mercury than the SDV- ICP. The method can be applied to the speciation of mercury at very low concentrations such as alkylmercury compounds in the atmosphere. The authors thank Dr. H. Ito for GC-MS analyses Mr. S. Sakai for air sampling and Dr. J. Edmunds for language correction. References 1 McCormack A. J. Tong S. C. and Cooke W. D. Anal. Chem. 1965,37 1470. 2 Talmi Y. CRC Crit. Rev. Anal. Chem. 1983 14 231. 3 Keliher P. N. Boyko W. J. Clifford R. H. Snyder J. L. and Zhu S. F. Anal. Chem. 1986 58 335R. 4 Broekaert J. A. C. Anal. Chim. Acta 1987 196 1. 5 Demers D. R. Appl. Spectrosc. 1979 33 584. 6 Faires L. M. Bieniewski T. M. Apel C. T. and Niemczyk T. M. Appl. Spectrosc. I 985 39 5. 7 Kawaguchi H. Tanaka T. and Mizuike A. Bunseki Kagaku 1984 33 129. 8 Talmi Y. Anal. Chim. Acta 1975 74 107. 9 Morita H. Sakurai H. and Shimomura S. Bunseki Kagaku 1982 31 314. 10 Bzezinska A. Van Loon J. Williams D. Oguma K. Fuwa K. and Haraguchi H. Spectrochim. Acta Part R 1983 38 1339. Paper 0/05823H Received December 31 I990 Accepted August 22 I991

ISSN:0267-9477    

DOI:10.1039/JA9920700015

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 19 Preconcentration and Inductively Coupled Plasma Atomic Emission Spectrometric Determination of Metal Ions With On-line Chelating Ion Exchange Valerio Porta Corrado Sarzanini Ornella Abollino Edoardo Mentasti and Enzo Carlini Department of Analytical Chemistry University of Torino Via P. Giuria 5 10125 Turin Italy An on-line preconcentration method utilizing a microcolumn of XAD-2 resin functionalized with 1 -(2- thiazolylazo)-2-naphthol has been developed. Preconcentration factors of 1 25 were easily obtained for injection times of 5 min. The detection limit ranged between 2 ng I-l for Mn and 40 ng I-' for Ni. The resin has been used to preconcentrate Cd Cu Fe Mn Ni and Zn from river water and Antarctic sea-water (Ross Bay) prior to their determination by inductively coupled plasma atomic emission spectrometry.The precision of the technique is around 10% relative standard deviation at concentrations below the pg I-' level and 5% for higher concentrations. Keywords Sea- water; on-line enrichment; chelating resin; inductively coupled plasma atomic emission spectrometry; preconcen tra tion The determination of ultratrace levels of metal ions in freshwater samples usually requires a preconcentration step andor separation of the analyte from the matrix before the instrumental analysis. There are at present two principal methods of preconcentration i.e. off- and on-line proce- dures. With off-line preconcentration the enrichment manifold is completely separate from the measurement instrument and all chemical preconcentrations are con- ducted independently away from the spectrometer.In the second procedure the enrichment manifold is connected directly to the spectrometer and the preconcentration and measurement cannot be considered as two separate tech- niques as the specific instrumentation performs the chemi- cal pre-treatment of the sample and detection of the analyte. The separation of the metal ions from the liquid sample in both of these methods can be obtained in several ways however one of the most commonly used approaches consists of allowing the sample to flow through a column packed with an active material that is capable of retaining the analyte. The selective retention of metal ions can be obtained by two different procedures. (i) A ligand which can interact with the analytes is added to the sample; the resulting complex species are then retained by the station- ary phase of the column.(ii) The complexing ligand is immobilized on the stationary phase which then systemati- cally retains the metal ions as the sample flows through the column in an ion-exchange fashion. Both of these techniques1-14 have found widespread application in off- and on-line modes as they can provide the necessary detection power in particular for the case of sea-water analysis. Sturgeon and co-workers112 used 8- hydroxyquinoline or diethyldithiocarbamate and C1 silica in an off-line system for the determination of various metal ions in sea-water. RGiiEka and Arnda14 demonstrated the possibility of using the same ligands and a solid substrate in an on-line system with excellent results.On the other hand Sturgeon et a1.,8 Marshall and Mottolag and McLaren el al.,1° chose silica immobilized 8-hydroxyquinoline as an active solid substrate for off- and on-line systems. They showed that this solid is particularly useful for the precon- centration and matrix isolation of trace metals. In the present work a column of chelating resin has been used as part of an on-line manifold. As the active solid substrate 1 -(2-thiazolylazo)-2-naphthol (TAN) loadedI5 on XAD-2 a styrene-divinylbenzene copolymer was uti- lized. After uptake by the column the metal ions were then eluted directly into the nebulizer of the inductively coupled Fig. 1 Section of the enrichment column 1 PTFE tube ( 1.5 mm Ld.); 2 Tygon tube (1.3 mm id.2.0 mm 0.d.); 3 Tygon tube (2.1 mm Ld.); 4 PTFE net; and 5 resin plasma (ICP) atomic emission spectrometer by concen- trated acids. This arrangement allowed preconcentration and determination of Cdl* Cull Fell Mn" Nil1 and Znll in river water and sea-water with high precision and accuracy. Experimental Reagents and Apparatus High-purity water (HPW) was produced with Millipore Milli-Q equipment which was supplied with de-ionized water from a mixed-bed twin ion-exchange column. All the acids and ammonia solutions (E. Merck Darms- tadt Germany) were purified using sub-boiling distillation apparatus (IS. Kurner Rosenheim Germany). Concentrated metal standard solutions (Titrisol E. Merck) were diluted as desired for the standard additions and for method evaluations.Amberlite XAD-2 resin (Serva) 50- 100 pm was purified according to the following procedure portions of the resin were placed into poly(propy1ene) Bio-Rad Econocolumns and repeatedly washed with methanol 2.0 mol dm-3 HCl and 0.1 mol HN03 and HPW in this order. The microcolumn was prepared from a piece of Tygon tubing (2.1 mm i.d. 30 mm long). The resin was held in place by two balls of poly(tetrafluoroethy1ene) (PTFE) net (pore size 75 pm). The connection between the 1.5 mm i.d. PTFE tubing and the columns was achieved by using two other Tygon tubes (2.0 mm o.d. about 7 mm long). A section of the column is shown in Fig. 1. The resin bed was 25 mm long for a total volume of solid substrate of 0.1 ml. The TAN (Fluka Buchs Switzerland) was used as a sequestration agent.The XAD-2 resin was loaded in situ with TAN by flowing a 1.0 x rnol dm-3 solution of TAN (water + MeOH 50 + 50 v/v) through a micro- column packed with the required amount of purified20 JOURNAL OF ANALYrICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 n7 Waste To ICP plasma fl Eluent Bufferd- I 0.5 ml min-' Sample 10.0 mI min-' Table 1 Typical preconcentration sequence Sequence Time/s Flow ratelm1 min-' Start - - Conditioning 20 6.0 Switch valve 1 - - Sample injection 180 10.0 Switch valve 1 - - Washing 20 6.0 Switch valve 2 - - Elution 60 1 .o Switch valve 2 - - End - - Fig. 2 Schematic diagram of the preconcentration manifold resin.15 After loading the ligand the column was washed with water and subsequently with 2.0 mol dm-3 HCl in order to remove any metallic impurities.The present column design did not show on use any channelling problems which can arise due to the reduced swelling and shrinking of the fuctionalized adsorbent during uptake of the trace metals from aqueous samples. Water from the river Po was collected in the centre of the City of Turin and immediately filtered on a 0.45 pm cellulose membrane filter. It was then frozen (-20 "C) without the addition of any reagents. As part of the activities of the Italian 'Progetto Antar- tide-Impatto Ambientale' associated with an expedition during the Antarctic summer of 1987-1 988 this operation unit received sea-water samples which had been filtered though 0.45 pm membranes a few hours after collection and then kept frozen at -20 "C.Acidification of the samples before preconcentration was required. Hence 2.0 ml 1-1 of concentrated ultrapure nitric acid were added to both the river water and sea-water samples. An atomic emission ICP spectrometer (IL Plasma 300) was used. All the instrumental parameters were the same as reported in previous work.I6 Off-line background correction on one side of the emission line was utilized. As the instrumentation used did not have a program for flow injection the emission was registered by discrete sampling and the peak areas were computed with a Lotus 123 spreadsheet program. For some measurements electrothermal atomic absorp- tion spectrometry (ETAAS) with Zeeman-effect back- ground correction (Model Zeeman 5 100 Perkin-Elmer equipped with an HGA-600) was used.Perkin-Elmer pyrolytic graphite coated graphite tubes were normally employed. Preconce n t rat ion Procedure All sample manipulations and preparations were conducted under a laminar flow fume hood. A schematic diagram of the manifold is given in Fig. 2. Valve 1 facilitates the alternate flow of the sample and the washing solution whereas valve 2 enables the column to be encorporated as a loop. While the column is being loaded a flow of the eluent is maintained to the nebulizer. The sample and washing solutions were buffered on-line through connection at a T-junction of the sample washing and buffer solution lines. A cartridge packed with Chelex 100 was inserted in the buffer h e to remove all metallic impurities from the buffer solution which was the main source of contamination.The samples and washing solution were pumped by a peristaltic pump at flow rates of 10 and 6 ml min-l respectively. The buffer solution was pumped by another peristaltic pump at 0.5 ml min-l. The final pH of the samples and of the washing solution was 8.4k0.3. The metal ions retained on the column were eluted with 2.0 mol dm-3 HC1 + 0.1 mol dm-3 HN03. The column was first conditioned to a suitable pH value by flowing the washing solution through after which valve 1 was switched on and the sample injected. The column was then washed again in order to eliminate any sample remaining in the line and in the column. By switching on valve 2 the metal ions were eluted from the column. A typical preconcentration sequence is reported in Table 1.For the determination of the metal ion concentration measuring the area of the elution peak was preferred to peak height measurements. The peak area was correlated to the concentration both with the standard additions method ,and with calibration against a steady-state signal of a :standard solution prepared in 2.0 mol dm-3 HCl + 0.1 mol dm-3 HN03. The algorithm used for the calculation 'was where c,=concentration of the analyte (ng ml-I); A=peak area of analyte emission signal (counts min); T= reading nnterval (min); F=eluent flow rate (ml min-l); Cstd concen- 1:ration of the standard solution (ng m1-l); &d=emission of the standard (counts rnin); Sbkg= background emission (counts rnin); and V,=volume injected (ml). Results and Discussion Performance of Method A s previously reported,15 an efficient chelating ion-ex- change resin can be prepared with a commercial adsorbent resin XAD-2 or XAD-4 and a water insoluble complexing agent TAN or 1 -(2-pyridylazo)-2-naphthol (PAN).The resin can be functionalized in a short time < I h and maintains a high efficiency even after treatment with concentrated acids. The chelating resin was able to retain selectively many transition metal ions such as Cu" Ni" ;SnI1 Cd" Fe" CoI1 and other ions such as U022+ and AP while Call or Mg" are not retained. The batch capacity of the resin was about 0.1 mmol g-l of Cul* for particles with dimensions of between 20 and 50 mesh but in this work an adsorbent resin with smaller particles and conseqeuntly with a higher loading capacity was used.The pH was chosen (8.4 k 0.3) after considering that at lower values complexation and retention may be incom- plete and at higher values (especially at pH>9.2) the performance of the chelating resin decreases ra~id1y.l~ The low internal diameter of the column did not cause a.ny problems in terms of sample flow resistance. It wasJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 21 400 1500 A r I v) +- 200 P t PI tn .- 1000 5 8 v) C U > C 500 2 n 30 " 0 10 20 Time/s Fig. 3 Elution profile for A Mn and B Fe obtained from water from the river Po Table 2 Recovery of elements from HPW river water and sea- water Recovery yield (Yo) Element HPW Sea-water River water Cd 92 100 100 c u 103 98 105 Fe 106 92 96 Mn 90 93 93 Ni 97 99 99 Zn 101 105 100 *Standard deviation was always within 10%.found that the maximum flow rate was around 10 ml rnin-l and this could be maintained for a high number of preconcentration cycles. When the flow did start to decrease slowly this was probably because of a long-term tightening of the resin bed. The elution profiles for Fe and Mn are given in Fig. 3. It can be seen that Fe gave larger peaks compared with Mn. The peak width for Fe is just over 20 s which is equivalent to 0.4 ml; this means that with an injection volume of 50 ml a preconcentration factor of 125 could be obtained. In terms of concentration efficiency,ll defined as the product of the preconcentration factor and the sampling frequency (Le. the number of samples analysed per minute) a value of 18 was obtained. This result is much higher than the one reported by Beauchemin and Berman14 and is only slightly lower than those reported by Fang et al." obtained with a dual column system.The efficiency of the chelating resin was evaluated in an off-line mode.15 In previous work,15 recoveries from HPW with functionalized XAD-2 with a greater particle size proved to be incomplete. With the smaller XAD-2 resin this problem was not observed and the ions were totally recovered from HPW river water and sea-water (see Table 2). Analytical Blanks Absolute blank values were evaluated for 50 ml samples. For each metal no sample solution was pumped during the preconcentration sequence but all the other steps were the same. This procedure allowed the evaluation of the contri- bution of metal ions present in the reagents used.The results are reported in Table 3. All data refer to three times Table 3 Absolute blanks and detection limits for the proposed method; sample volume 50 ml Element BlanWng Detection limithg 1-I Cd 0.4 c u 0.6 Fe 0.6 Mn 0.1 Ni 2 Zn 0.6 8 12 12 2 40 12 Table 4 Analytical data for the adopted enrichment procedures for the analysis of a river water sample Concentration/pg l-l* Element ETAASt XAD-2 +TAN XAD-2 +OX Cd 0.064 0.03 k 0.01 0.033 f 0.004 cu 1.5 1.30 f 0.10 1.3 +_O. 1 Fe 5.4 5.9 k 0.7 5.6 t- 0.4 Mn 0.82 0.80 +_ 0.05 0.87 k 0.05 Ni 5.4 5.5 k0.3 5.8 f 0.3 Zn NDS 8 8.2 k 0.2 *Mean of at least five determinations. ?Standard deviations within 10%. SND not determined. the noise of the baseline but no background peaks were observed in this instance.For this reason the results are shown without standard deviations. The insertion of the Chelex column in the buffer solution line significantly reduces the amount of metal ions intro- duced with the buffer or avoids large blank values for certain metals due to accidental contamination of the buffer. This is particularly valid for Zn which usually gives high blank levels and is often subject to contamination from the laboratory environment. The detection limits reported were calculated from the blank value. However it must be pointed out that since no blank signal was observed an increase in the sample volume can further decrease the detection limit. Analytical Results Results for the analysis of river water samples are given in Table 4 as mean values of at least five determinations made on samples from different bottles.The river-water data can be compared with the results from direct analysis by ETAAS and with those obtained with a different on-line preconcentration method obtained using 8-hydroxyquinol- ine when the ligand is added directly to the sample.' Calibration was always obtained both with standard addi- tions and external calibration even if total recovery was assumed. This was effected in order to account for any unexpected degradation of the column which could have given the wrong results when using external calibration and to avoid a change in sensitivity during the long analysis time required for some elements (Cd) which would have affected the standard additions results.The accuracy of the values found is acceptable with the only exception being for Cd for which there is agreement between the precon- centration data but they differ from the ETAAS concentra- tions. No plausible explanation for this has been found. The results for the Antarctic sea-water analysis are reported in Table 5. The values for the concentrations of the metals studied are in the same range as others found with22 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 Table 5 Analytical data for the analysis of two Antarctic sea-water samples (1987-1988 compaign). Sample A (SW 31) 75" 06' 24" south 165" 29' 06" east. Sample B (SW 46) 75" 29' 0 0 south 165" 3 1' 48" east Concentration/pg 1-' Element A B - * 0.020 2 0.0 10 Cd c u 0.30 f 0.02 0.25 2 0.04 Fe 0.29 f 0.03 0.34 2 0.02 Mn Ni 0.43 f 0.04 0.49 2 0.04 Zn 0.34 f 0.02 0.20 2 0.0 1 - * 0.01 3 2 0.005 *Not determined in this sample.different preconcentration methods.I7 Preconcentration from the same sample performed with the XAD-2-oxine method7 and the procedure reported here gave similar results. The precision of the analysis of sea-water (see Table 5) is fairly good but is not as good for elements present at very low concentrations (e.g. Cd) as for those for which the ratio of the concentration to the detection limit of the plasma is low (e.g. Ni). Conclusions The present work confirmed that on-line preconcentration is one of the best methods of sample pre-treatment in atomic spectrometry. The procedure developed requires very few sample manipulations and reagent additions and for this reason very low blank levels are obtained.More- over the concentration efficiency of the method which combines a preconcentration factor and analysis time was about 18 one of the highest reported in the literature. The functionalized XAD-2 can easily be substituted with a different active substrate such as silica immobilized 8- hydroxyquinoline or a more selective reagent. An adaptation of the present procedure for use with ETAAS and ICP mass spectrometry should be fairly easy especially for the determination of Cd" CuII Fell Mn" Nil1 and Zn" in sea-water where these elements are present at very low concentrations. The financial support from Minister0 dell'Universita e della Ricerca Scientifica e Tecnologica (MURST Rome) and from the Italian National Research Council (CNR Rome) is kindly acknowledged.1 2 3 4 5 6 7 8 9 10 1 1 12 13 14 15 16 1 '7 References Sturgeon R. E. Berman S. S. and Willie S. N. Talanta 1982 29 167. Sturgeon R. E. Willie S. N. and Berman S. S. Anal. Chem. 1985 57 6. Sarzanini C. Mentasti E. Gennaro M. C. and Marengo E. Anal. Chem. 1985 57 1960. RiiiiEka .I. and Arndal A. Anal. Chim. Acta 1989 216 243. Abollino O. Mentasti E. Porta V. and Sarzanini C. Anal. Chem. 1990,62 21. Fang Z. Sperling M. and Welz B. J. Anal. At. Spectrom. 1990 5 639. Porta V. Sarzanini C. Mentasti E. and Abollino O. Anal. Chim. Acta submitted for publication. Sturgeon R. E. Berman S. S. Willie S. N. and Desaulniers J. A. H. Anal. Chem. 1981 53 2337. Marshall M. A. and Mottola H. A. Anal. Chem. 1985 57 729. McLaren J. W. Mykytiuk A. P. Willie S. N. and Berman S. S. Anal. Chem. 1985 57 2907. Fang Z. Xu S. and Zhang S. Anal. Chim. Acta 1987 200 35. Pail S. C. Whung P. Y. and Lai R. L. Anal. Chim. Acta 1988,211 251. Pai S. C. Anal. Chim. Acta 1988 211 271. Beauchemin D. and Berman S. S. Anal. Chem. 1989 61 1857. Sarzanini C. Porta V. and Mentasti E. New J. Chem. 1989 13 463. Porta V. Sarzanini C. and Mentasti E. Mikrochim. Ada 1989 111 247. Mentasti E. Porta V. Abollino O. and Sarzanini C. Ann. Chim. (Rome) 1989 79 629. Paper I /O I5 6 7B Received April 3 1991 Accepted August 29 I991

ISSN:0267-9477    

DOI:10.1039/JA9920700019

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 23 Determination of Trace Metals in Volatile Organic Solvents Using Inductively Coupled Plasma Atomic Emission Spectrometry and Inductively Coupled Plasma Mass Spectrometry Steve J. Hill James Hartley and Les Ebdon Plymouth Analytical Chemistry Research Unit Polytechnic South West Drake Circus Plymouth Devon PL4 8AA UK A desolvation system has been designed to help facilitate the introduction of volatile organic solvents into inductively coupled plasmas. The system utilizes Peltier coolers to reduce the temperature of an interface placed between a heated spray chamber and the plasma torch. The interface consists of a drilled aluminium block that acts as a heat-sink and through which pass glass cooling tubes. The Peltier coolers are mounted around the block sandwiched between the block itself and a series of copper cooling plates.The optimum conditions necessary for the determination of trace amounts of metals in diethyl ether are discussed and the results obtained using these conditions compared with those obtained using a conventionally cooled mini-spray chamber. Detection limits using the system in conjunction with continuous nebulization were found to be in the low to sub-pg I-' range for inductively coupled plasma atomic emission spectrometry and inductively coupled plasma mass spectrometry. Flow injection techniques have also been used to reduce the amount of volatile organic solvent entering the plasma. In this instance the volatile solvent (diethyl ether) was carried in a stream of less volatile solvent (2-ethoxyethanol) to the plasma.The effects of varying the sample loop volume have been investigated and the results compared with those obtained using continuous flow nebulization. The results indicated that in addition to achieving an enhancement in plasma stability smaller sample loops (50 PI) facilitate faster flow rates which result in better shaped transient peaks and reduced memory effects. Keywords Organic solvent; inductively coupled plasma mass spectrometry; inductively coupled plasma atomic emission spectrometry; desolvation; flow injection Inductively coupled plasma atomic emission spectrometry (ICP-AES) and inductively coupled plasma mass spectro- metry (ICP-MS) have been utilized increasingly for the determination of trace metals in a wide variety of organic solvents.1-5 Typical applications have included the determi- nation of trace metals in lubricating oils,6 crude oils7 and various solvents used in high-performance liquid chromato- graphy.8 However present techniques often give rise to operating difficulties and a number of more fundamental studies have identified a range of potential problems associated with the introduction of organic solvents into the p l a ~ m a . ~ * ~ J ~ These difficulties include an increase in back- ground emission deposition of carbon on the torch and sampling cones of the ICP mass spectrometer and a decrease in plasma stability. In order to alleviate many of these problems cooled spray chambers 19279~10 a reduction in sample uptake rate^,^*^ increased forward power2 and the addition of a low flow of oxygen into the nebulizer gas to prevent carbon deposition,* have been suggested.A range of desolvation systems have been designed to remove a large percentage of the organic solvent (up to 80°/0) prior to its reaching the plasma.11-16 During the important condensation stage these desolvation systems have been cooled by employing a range of methods which include pumped ice-cold water,' pumped cooled ethy- lene glycol and water l 3 membrane gas-liquid separators14J5 and thermoelectric devices.16 However to date none of these designs have been utilized for the introduction of particularly volatile solvents which require much lower temperatures during the condensation stage than those previously reported.The use of flow injection techniques to facilitate the introduction of discrete samples into a plasma have been reported widely particularly when there has been a require- ment to reduce mass dependent interference^,'^ when analyte samples containing high solids or acid content are used18 and only small amounts of sample are available such as in biological work.19 However little work has been Fig. 1 Schem tic diagram of desolvation system. A Coola t water from mains supply; B coolant water to waste; C aerosol from spray chamber; D desolvated aerosol to plasma torch; E aluminium block; F Peltier coolers; G copper cooling plate attached to back surface of Peltier coolers; and H desolvated solvent to waste reported on the introduction of a volatile solvent into the carrier stream of a different solvent.The determination of trace metals in volatile organic solvents using ICP-AES and ICP-MS are reported here. A novel desolvation system was constructed and placed between the spray chamber and injector tube of the torch. This consisted of an aluminium block through which passed glass connecting tubing as shown in Fig. 1. The aluminium block was cooled by thermoelectric devices (Peltier coolers). These thermoelectric devices were cooled by water flowing over copper plates. This proved to be appropriate for the desolvation of volatile organic solvents such as diethyl24 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 ether because of the low temperature achievable. In addition flow injection techniques were used for the injection of discrete slugs of volatile solvents into a carrier stream of a less volatile solvent thereby facilitating an increase in plasma stability an increase in sample through- put and a reduction in carbon deposition.Experimental Instrumentation The instruments used in this study were an inductively coupled plasma mass spectrometer (VG PlasmaQuad 2; VG Elemental Winsford Cheshire UK) and an inductively coupled plasma atomic emission spectrometer of the rapid sequential type ( S 3 5 Plasmakon Kontron Spectronanalytik Eching Germany). The temperature controlled spray chamber (‘system 1 ’) was of a Scott type double-pass design insulated to allow circulation of the cooling fluid. Temperature control was achieved by means of an open reservoir of propanol in which a recirculation pump was immersed with a heating element (Techam Tempunit Techne Cambridge UK) to pump the cooling fluid.Cooling was achieved by using a refrigeration unit (Grant CCI5 Grant Instruments Bar- rington Cambridge UK) or by the addition of liquid nitrogen. In the later studies (‘system 2’) an ARL type spray chamber (ARL Luton Bedfordshire UK) was used and heated using heating tape (Electrothermal Engineering Southend-on-Sea Essex UK). This was used in conjunction with the aluminium condenser system which was cooled using four thermoelectric cooling devices (Peltier Coolers Melcrar CPI.4- 127-06L Trenton NJ USA) and mounted between water-cooled copper plates and an aluminium heat-sink. The heat-sink measuring l o x 3.5 x 3.5 cm was drilled to accommodate two 12 cm long glass cooling tubes of 5.0 mm 0.d.and 4.5 mm i.d. The gap between the aluminium and the glass was filled with a conducting paste (zinc paste). The complete device is shown schematically in Fig. 1. The flow injection manifold used was a six-port switching valve supplied by PS Analytical (Sevenoaks Kent UK). Simplex optimization experiments were performed using a software package developed at Plymouth and described elsewhere. Reagents and Standards Working solutions of the metals were prepared from Spectrosol cyclohexylbutyrate salts of the metals (Merck Poole Dorset UK). Procedure Both desolvation systems described above were optimized by the variable step-sized simplex procedure for the determination of trace metals in diethyl ether and shown to have an efficiency of 70% for the removal of the solvent.The removed solvent when analysed showed levels of the analyte below the detection limit (0.6 ng ml-1 for 63Cu). The results obtained using the desolvation system were com- pared with those obtained using a cooled mini-spray chamber for both ICP-AES and ICP-MS. In order to prevent carbon build-up on the torch (ICP-AES) and cones and torch (ICP-MS) a low flow (5-6% v/v) of oxygen was introduced into the nebulizer gas flow. Results and Discussion Inductively Coupled Plasma Atomic Emission Spectrometry Optimization The system described above was optimized using the variable step-size simplex procedure for the introduction of trace metals into diethyl ether. The criterion of merit used was the signal-to-background ratio (S/B) and the parameters optimized were the forward power the viewing height above the load coil the nebuilizer gas flow rate and the spray chamber or desolvation device temperature. A parti- cular line was selected to represent atomic lines of a low to medium ionization potential (‘soft’) and a line with a high first or second ionization potential (‘hard’) according to the criteria of Boumans and Lux-Steiner*O (‘soft’ Cu I 324.754 nm and ‘hard’ Mn I1 257.610 nm see Table 1).Univariate searches around the optima were performed to illustrate the importance of each parameter on the sensitivity (Fig. 2). The optimum conditions obtained for the forward power [Fig. 2(a)] appeared to contradict earlier work by other w ~ r k e r s ~ ~ ~ * ~ J ~ which showed that organic solvents often require an increase in power compared with aqueous solutions.However in previous work in these laboratories Ebdon et a1.I found that a decrease in power was required for a wide variety of solvents and cmcluded that it was the effect of including other parameters in the optimization particularly the combined effects of lower carrier gas flow rate and spray chamber temperature that resulted in a much reduced solvent load to the plasma. The results obtained here confirm those reported by Ebdon et a1.I The nebulizer gas flow rate [Fig. 2(b)] proved to be a critical parameter. A lower optimum gas flow rate can be expected when using the desolvation interface (system 2) owing to the longer residence time in the interface and a consequent increase in solvent removal.However the residence time of the analytes in the plasma and the effect on plasma cooling appear to be more critical. The viewing height also proved to be important for this work [Fig. 2(c)]. The relationship between the parameters particularly forward power and the nebulizer gas flow rate greatly affects the optimum viewing height however the well documented use of lower viewing heights for hard lines compared with soft lines is supported by this study using organic solvents. The temperature of the spray chamber or the condensa- tion section of the desolvation system (system 2) [Fig. 2(d)] does not reach an optimum for the introduction of diethyl ether. There is a steady increase in the S/B with a decrease in temperature for both the hard and the soft lines.Further work on this parameter was prevented by the inability to maintain a constant temperature below - 40°C. Detect ion li rn its The detection limits were calculated (Table 2) for the two spectral lines using the cooled spray chamber (system 1) and the system employing Peltier coolers (system 2) using the equation where c,= detection limit (RSD) = relative standard deviation of the background; and c,= the analyte concentra- tion.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 80 25 ( 6) - B 80 70 50 30 10 60 * 40 - (d) - - - 'Q B 1 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 - v) Forward power/kW 60 40 100 I - - 80 1'" 20 ' I I I 0 10 20 30 40 50 60 70 Heightlmm Fig. 2 A Cu 11. (4 (a) Normalized SIB versus forward power for A Cu I; and B Mn 11.(b) Normalized S/B versus nebulizer gas flow rate for I; B Mn 11; C Cu I (desolvated); and D Mn I1 (desolvated). (c) Normalized SIB versus height above load coil for A Cu I; and B Mn Normalized S/B versus temperature for A Cu I; B Mn 11; C Cu I (desolvated); and D Mn I1 (desolvated) Table 1 Optimum conditions for the introduction of trace metals in diethyl ether by ICP-AES System Line Parameter Optimum value System I (cooled spray chamber) Cu I Forward power/kW Height above load coil/mm Nebulizer gas/l min-l Temperat ure/"C Height above load coil/mm Nebulizer gas/] min-l Temperature/"C Mn I1 Forward power/kW System 2 (system employing Peltier coolers) Cu I Forward power/kW Nebulizer gas/l rnin-' Temperature/"C Height above load coil/mm Height above load coil/mm Nebulizer gas/l min-' Temperat ure/OC Mn I1 Forward power/kW 1.38 43.26 1.25 1.73 14.15 0.88 - 40 - 40 I .38 1.35 43.26 1.73 14.15 0.9 1 - 40 - 40 Table 2 Detection limits obtained under optimum operating conditions by ICP-AES System Line Detection limit/ng ml-I System I (cooled spray chamber) Cu I 10 Mn I1 15 System 2 (system employing Peltier coolers) c u I Mn I1 1 2 For both the spectral lines investigated there is a marked improvement in sensitivity when system 2 is employed.This is owing to an increase in the stability of the plasma because of a decrease in the solvent load. In addition the use of system 2 also enabled an increase in the uptake rate thereby increasing the amount of analyte entering the plasma and decreasing the significance of the effects of the pulsing of the peristaltic pump.Flow injection studies The possibility of using flow injection as a method for26 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 70 60 Table 3 Detection limits obtained using ICP-AES employing the desolvation system and flow injection Sample loop size/pI Line Detection limit/ng ml-l 300 c u I 5 100 c u I I Mn I1 3 50 c u I 0.6 Mn I1 1.5 . . introducing small volumes of diethyl ether into a stream of 2-ethoxyethanol was also evaluated. This solvent was selected as carrier because of its lower vapour pressure than diethyl ether and the intrinsically high level of oxygen. The effects of using different sized sample loops were investi- gated and the results are shown in Table 3.Decreasing the volume of the volatile solvent entering the plasma using smaller sized sample loops improved the sensitivity for both the analytes studied. By lowering the amount of volatile solvent entering the plasma the stability 70 50 30 10 of the plasma was increased considerably which also enabled an increase in uptake rate. In addition there was a decrease in memory effects aided by the more rapid throughput of the solvent. ( C) - - - Inductively Coupled Plasma Mass Spectrometry Optimization The desolvation system described above with slight modifi- cation such as the lengthening of the connecting tubing was attached to the ICP-MS instrument and evaluated for the introduction of diethyl ether. The system was optimized by the variable step-size simplex procedure for both Cu and Si (Table 4).The S/B was used as the criterion of merit and the parameters optimized included the forward power the nebulizer gas flow rate the outer gas flow rate and the temperature of the spray chamber or desolvation device. In order to indicate the importance of the optimum experi- mental parameters on the sensitivity univariate searches ~ Table 4 Optimum conditions for the introduction of trace metals in diethyl ether by ICP-MS System Element Parameter System 1 c u Forward power/kW (cooled spray chamber) Nebulizer g a d min-I Outer gad1 min-' TemperaturePC System 2 (system employing Peltier coolers) c u Forward power/kW Nebulizer gas/] min-I Outer gad1 min-' Temperat ure/"C Nebulizer gad1 min-' Outer gad1 min-' Temperature/"C Si Forward power/kW Optimum 1.91 0.75 17.75 2.00 0.7 1 17.58 1.86 0.67 17.29 - 40 - 40 - 40 50 ' 1 I I 1 1 I 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 .v) Forward power/kW Outer gas/l min-1 Tern pe ra t u rep C Fig. 3 (a) Normalized S/B versus forward power using system 2 for A Cu; and B Si. (b) Normalized S/B versus nebulizer gas flow rate using system 2 for A Cu; and B Si. (c) Normalized S/B versus temperature using system 2 for A Cu and B Si; and ( d ) Normalized S/B versus outer gas flow rate using system 2 for A Cu; and B SiJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 27 were performed around the optimum by holding all the other parameters constant (Fig. 3). The determination of Si was found to be possible when using the desolvation system because of the large decrease in interference resulting from the polyatomic ion CO+ at mass 28.The isotopic ratios obtained using the peaks at mlz 29 and 30 were found after use of background correction to be consistent with the ratio expected and the results were reproducible. During the studies described above using ICP-AES it was noted that the optimum conditions for use with organic solvents required a lower forward power than those for use with aqueous solutions. However when using ICP-MS an increase of the forward power [Fig. 3(a)] is required in order to reach an optimum. This is probably due to the spatial temperature phenomena being less important in ICP-MS. The carrier gas flow rate [Fig. 3(b)] is still of critical importance giving sharply defined optima but the actual flow rate is slightly less than that found to be appropriate for use with aqueous samples.This variation is probably due to the increased solvent removal resulting from the increase in residence time in the condenser experienced with lower flow rates. The temperature of the condensation stage of the desol- vation system moved towards an optimum value at lower temperatures [Fig. 3(c)] and consequently no optimum value was obtained. A value of -40 "C was therefore selected this being the minimum value of temperature that could be obtained using the present system. Detection limits The detection limits obtained for ICP-MS using both system 1 and system 2 were calculated for the two analytes (Table 5). Using the cooled spray chamber i.e.system 1 it was only possible to obtain a detection limit for Cu since the interference of Co+ on Si prevented use of the RSD of the blank. The equation used to calculate the detection limit is shown above. Clearly there is a marked improvement in sensitivity between the two systems owing to an increase in the stability of the plasma resulting from the decrease in solvent load. In particular the determination of Si at low ng ml-' levels was possible. Flow injection studies Flow injection was again used as a method of introducing a small volume of the analyte into a carrier stream of a less volatile solvent (2-ethoxyethanol). As with the ICP-AES studies the effects of using different sized sample loops were investigated; the results are shown in Table 6. Again the stability of the plasma increased with a decrease in solvent loading facilitating an increase in the sample uptake rate to produce better shaped peaks with less tailing (Fig.4) and a decrease in sampling time resulting from decreased memory effects. Table 5 Detection limits obtained for Cu and Si in diethyl ether by ICP-MS using optimum operating conditions System Element c,lng ml-1 System 1 System 2 (cooled spray chamber) c u 27 (system employing Peltier coolers) c u 1.2 Si 1 .o Table 6 Detection limits obtained by ICP-MS employing flow injection System Element Sample looplpl c,lng ml-I System 2 (system employing Peltier coolers) Si 50 0.7 100 0.9 300 1.4 c u 50 0.5 100 0.7 300 1.1 1 I 0 10 20 30 40 Time/s Fig. 4 Profiles of flow injection peaks obtained for the injection of 80 pg ml-I of Si in diethyl ether into a carrier stream of 2-ethoxy ethanol.Sample loop size A 50; and B 30 p1 Conclusion The desolvation system described can be used with both ICP-AES and ICP-MS to determine quantitatively the levels of trace metals in volatile organic solvents. Removal of much of the solvent entering the plasma results in increased plasma stability and detection limits that are comparable to those obtained during aqueous operation. The use of flow injection may further decrease the amount of solvent entering the plasma to provide even better detection limits together with the reduction of memory effects which facilitates more rapid sample throughput. The authors would like to thank Johnson Matthey Techno- logy Centre for supporting this work and the Science and Engineering Research Council for the provision of a studentship to J.H. References 1 Ebdon L. Evans E. H. and Barnett N. W. J. Anal. At. Specctrorn. 1989 4 505. 2 Hutton R. C. J. Anal. At. Spectrom. 1986 1 259. 3 Barret P. and Pruszkowska E. Anal. Chern. 1984,56 1927. 4 Nygaard D. D. Schleicher R. G. and Sotera J. J. Appl. Spectrosc. 1986 40 1074. 5 Boorn A. W. and Browner R. F. Anal. Chern. 1982 54 1402. 6 Algeo J. D. Heine D. R. Philips H. A. Hoek F. B. G. Schneide M. R. Freelin J. M. and Denton M. B. Specctro- chim. Acta Part B 1985 40 1447. 7 Brown R. J. Spectrochim. Acta Part B 1983 30 283. 8 Gast C. H. Kraak J. C. Poppe H. and Maessen F. J. M. J. J. Chromatogr. 1979 185 549. 9 Maessen F. J. M. J. Kreuning G. and Balke J. Spectrochim. Acta Part B 1986 41 3.28 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 10 Kreuning G. and Maessen F. J. M. J. Spectrochim. Acta Part B 1987 42 677. 1 1 Uchida J. Masamha W. R. Uchida T. Smith B. W. and Winefordner J. D. Appl. Spectrosc. 1989 43 425. 12 Brotherton T. J. Pfannerstill P. E. Creed J. T. Heitkemper D. T. Caruso J. A. and Pratsinis S. E. J. Anal. At. Spectrom. 1989 4 341. 13 Tsakahara R. and Kubota M. Spectrochim. Acta Part B 1990 45 581. 14 Gustavsson A. Spectrochim. Acta Part B 1988 43 917. 15 Backstrom K. Gustavsson A. and Hietala P. Spectrochim. Acta Part B 1989 44 1041. 16 Weir D. G. J. and Blades M. W. Spectrochim. Acta Part B 1990 45 615. 17 Vickers G. H. Ross B. S. and Hieftje G. M. Appl. Spectrosc. 1989,43 1330. 18 Hutton R. C. and Eaton A. N. J. Anal. At. Spectrom. 1988 3 547. 1.9 Dean J. R. Ebdon L. Crews H. M. and Massey R. C. J. Anal. At. Spectrom. 1988 3 349. 20 Boumans P. W. J. M. and Lux-Steiner M. Ch. Spectrochim. Acta Part B 1982 37 97. Paper 1 /039 74A Received July 31 1991 Accepted October 23 1991

ISSN:0267-9477    

DOI:10.1039/JA9920700023

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 29 Application of Radioactive Tracers for Investigation of Dysprosium and Manganese Vaporization in Electrothermal Atomic Absorption Spectrometry Muhammad Mansha Chaudhry and David Littlejohn* Department of Pure and Applied Chemistry University of Strathclyde Cathedral Street Glasgow G 7 IXL UK John E. Whitley Scottish Universities Research and Reactor Centre East Kilbride G75 OQU UK Radioactive tracers lasDy and s6Mn have been used to investigate the vaporization of Dy and Mn in a graphite furnace. The retention and distribution of the analytes after atomization in pyrolytic graphite coated electrographite (PCG) tubes have been measured and the efficiencies of Dy and Mn vaporization have been compared for wall platform and probe atomization in total pyrolytic graphite (TPG) tubes.Manganese is vaporized almost completely (95-1 00%) at 2500 "C by each of the atomization modes with little retention at the point of initial deposition. Manganese is redeposited at the very ends of the PCG tube and the amount of redeposition increases with the age of the tube. Total pyrolytic graphite (TPG) exhibits lower redeposition of Mn than does PCG and the deposition is mainly on the outside of the tube. Dysprosium is almost totally retained in PCG and TPG tubes. Vaporized Dy is redeposited very rapidly mainly at the centre of the tube and causes a memory effect in subsequent heating cycles. About 60% of the Dy is vaporized from the probe at 2750 "C whereas only 7% is vaporized from the platform.The difference in the vaporization efficiencies is reflected in the atomic absorption spectrometric sensitivities for both procedures. Wall atomization gives the best sensitivity for Dy but is only 1.5-fold better than for probe atomization. Some comments are given on the possible mechanisms of Dy vaporization and atomization but mass spectrometry studies are required to give a clearer explanation of the phenomena. Keywords Radioactive tracer wall platform and probe electrothermal atomization; atomic absorption spectrometry; pyrolytic graphite coated electrographite; total pyrolytic graphite Radioactive tracers have been used by several workers to study the behaviour of analyte elements during the various heating stages of an electrothermal atomization programme and to investigate the retention of the analyte in the graphite tube.Veillon et a1.l studied the behaviour of Cr in a graphite furnace using the Y r radiotracer. After atomiza- tion it was found that the retention of Cr in a pyrolytic graphite coated graphite tube was less than that in an uncoated tube. They attributed the lower retention to reduced carbide formation. In a similar study Krivan and Arpadjan2 showed that most of the Cr retained in the furnace after atomization was deposited at the ends of the graphite tube. On this occasion the workers concluded that formation of stable carbides of Cr did not occur. Shcherba- kov et aL3 used different shaped graphite rods to provide different thermal profiles in a graphite rod atomizer and investigated the effect of rod shape on atomization using the radioisotopes IlOAg 56Mn and 65Zn.The distribution of the atomized material was studied by radiography. It was shown that shaped rods exhibited a higher degree of vaporization of Mn compared with ordinary graphite rod atomizers. Whitley et aL4 used the 76As radiotracer to investigate the efficiency of arsenic hydride deposition in a graphite tube. The tube was cut into several segments and the radioactivity of each segment was counted to discover the distribution of the deposited As. Most of the As was deposited at the ends of the tube where the hydride gas was introduced. In this study radiotracers were used to investigate the efficiency of vaporization and the subsequent distribution of Dy and Mn for wall platform and probe atomization in a graphite tube furnace.The elements were selected for their different atomization characteristics and suitability for the production of radioisotopes as indicated in Table 1. The vaporization of Mn a relatively volatile element should *To whom correspondence should be addressed. Table 1 Data for radiotracers Parameter 56Mn "j5Dy Half-life/h Target abundance (%) Thermal neutron capture cross-section of target/barn Gamma intensity (%) Other isotopes produced Detection limit/ng 2.6 2.3 100 28 13 900 99 4 None 165mDy 0.04 0.06 not be greatly affected by differences in the mode of atomization. In contrast Dy is more difficult to vaporize in a graphite tube atomizer probably owing to the formation of refractory carbides. Hence the activity of the graphite surface and the mode of vaporization are likely to have significant effects on the efficiency of desorption of the Dy atoms.Many of the rare earth elements are incompletely vaporized in electrothermal atomic absorption spectrome- try (ETAAS) and L'vovs has recommended the use of a tantalum platform in a tantalum-lined tube to improve the characteristic mass values obtained for the rare earth elements. Both 165Dy and 56Mn have half-lives of a few hours (Table 1) which makes them suitable for analyte retention experi- ments in electrothermal atomization. The half-lives are sufficiently long to allow the radioisotopes to be detected with the required precision throughout the length of the experiments but are sufficiently short so that the radioac- tivity decays after several hours (e.g. 24 h) and permanent contamination is avoided.A high specific activity (activity per unit mass) can be achieved for Mn due to the high target abundance and high gamma intensity of the radioisotope. It is therefore possible to use a mass of radiotracer which is similar to that required for conventional ETAAS measure-30 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 ments. The lower target abundance and gamma intensity of Dy are compensated for by its high neutron capture cross- section. Accordingly the mass of the Dy radiotracer required is similar to that detected by ETAAS. Although 165mDy is produced as a side-product this radioisotope decays within a few minutes. Hence the Dy and Mn radioisotopes are sufficiently isotopically pure to allow the use of a low resolution but highly efficient NaI(T1) detector.Experimental Instrumentation Atomic absorption spectrometer A Philips SP9 single beam atomic absorption spectrometer SP9 graphite furnace PU9095 video furnace programmer and SP9 computer were used. The instrument was placed inside the fume cupboard of a radiochemistry laboratory. The following graphite tubes platforms and probes sup- plied by Philips Scientific (now Unicam) were used (i) pyrolytic graphite coated electrographite (PCG) tubes (30 x 5 mm i.d.); (ii) total pyrolytic graphite (TPG) slotted tubes (28 x 6.5 mm i.d.) and TPG unslotted tubes (30 x 5 mm id.). The TPG tubes had detachable end C-rings for location in the electrical contact jaws; (iii) TPG platforms (1 5 x 3 mm); and (iv) PCG and electrographite ridge probes (35 x 4 mm).The design and operation of the ridge probes have been described elsewhere.6 Radiotracer Preparation A 250 pl aliquot of a 100 p g 1-1 solution of Dy or Mn was sealed in a polyethylene vial and irradiated in the reactor at the Scottish Universities Research Reactor Centre (SURRC) East Kilbride at a nominal flux of 2.3 x 10l2 n s-l cm-* for 1 h. Dysprosium was given a cooling time of 30 min to allow the short-lived isotope 16SmDy to decay. The isotope purity of both tracers was established by high resolution gamma spectrometry. Procedure A 10 or 30 pl aliquot of the radiotracer solution (equivalent to 1 or 3 ng of Dy or Mn respectively) was injected onto the wall platform or probe and the drying phase of the heating programme initiated (Table 2).The tube platform or probe was removed from the instrument and the radioactivity counted in a well-shaped NaI(T1) detector. When approxi- mately 3000 counts were accumulated in the photopeak of the gamma-ray spectrum (93 keV for Dy and 840 keV for Mn) the counting was stopped the tube platform or probe was fitted back into the furnace and the rest of the heating programme executed. The radioactivity that remained in the tube or on the platform or probe was measured after completion of the heating programme. The retention of the analyte was calculated as the ratio of the remaining radioactivity to the initial activity expressed as a percen- tage applying decay correction.' This was assumed to be equivalent to the percentage mass of the analyte retained on the graphite.On some occasions the tube or probe was cut into several segments and the radioactivity of each segment was measured to establish the distribution of retained Dy or Mn on the graphite substrate. The AAS signals for Dy and Mn were measured at 421.2 and 403.1 nm respectively at bandpasses of 0.2 and 0.5 nm respectively. The Mn line selected is ten times less sensitive than the more commonly used line at 279.5 nm. Results and Discussion Retention of Mn and Dy in a PCG Tube The retention of Dy and Mn was measured after each of Table 2 Temperature programmes for wall platform and probe atomization of Mn and Dy Phase* Wall Platform Probe Dry Temperat ure/"C Hold time/s Ramp No.? Temperature/"C Hold time/s Ramp No.? Temperat ure/"C Char Pre-heat Hold time/s Ramp No.? Temperature/"C Atomize Hold time/s Ramp No.? Cleaning$ 120 250 500 60 60 60 6 6 6 800 800 800 30 30 30 3 3 3 - 2500 (Mn) 2750 (Dy) 2 0 - - 2500 (Mn) 2500 (Mn) 2500 (Mn) 2750 (Dy) 2750 (Dy) 2750 (Dy) 5 5 5 0 0 0 *Oxygen-free nitrogen was used as the purge gas; gas stop applied at the pre-heat and atomize steps. t Ramp Nos.0 3 and 6 represent heating rates of >2000 200 and 20 "C s-I respectively. $ After the measurements of radioactivity the cleaning was carried out by manual selection of the clean facility which employs a short heating phase at maximum temperature. four replicate heating cycles (Table 2) for one deposition of the analyte onto the tube wall. The masses of radiotracer used were 1 ng of Dy and 3 ng of Mn.The Mn AAS signal was reduced to zero after the first cycle as shown in Fig. 1. However even after the fourth cycle a considerable amount of Mn radioactivity remained in the tube. The Dy radiotracer measurements showed that almost all of the deposited Dy remained in the tube after the first heating cycle. The AAS signals in Fig. 2 are for 6 ng of stable Dy and were obtained in a separate experiment from the radio- tracer measurements. 80 60 C .- w 40 tT 20 - 0.8 < P) C m - 0.6 ' 2 L2 0 - 0.4 2 0.2 - 0 C C - I L I 0 1 2 3 4 5" No. of atomizations Fig. 1 Retention of Mn from A radioactivity measurements; and B integrated absorbance values. Measured after repeated wall atomizations at 2500 "C of a single depositon of 3 ng of analyte in a pyrolytic graphite coated graphite tubeJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL.7 31 120 100 c 80 5 C C Q) .g 60 cT 4 0 - c 20 - - - - - A - - - - - 0.8 In 0.6 5 C m + s 0.4 Q U E c 0 0.2 g I""(0 0 1 2 3 4 5 No. of atomizations Fig. 2 Retention of Dy obtained from A radioactivity measure- ments after repeated wall atomizations at 2750 "C of a single deposition of 1 ng of analyte in a pyrolytic graphite coated graphite tube; and B the integrated absorbance measurements for 6 ng of stable Dy obtained in a separate experiment m ( b ) 100 8o t I I I 1 1 I I 0 15 30 45 60 75 90 105 No. of injections and atomizations Fig. 3 Effect of ageing of pyrolytic graphite coated graphite tube on (a) the integrated absorbance signals; and (b) the retention of 1 ng of Mn after atomization at 2500 "C.A Tube 1 and B tube 2; errors in retention values were 3 4 % based on counting statistics Variation in the Retention of Mn With Ageing of the PCG Tube The retention of Mn was determined after every 15th atomization at 2500 "C of freshly deposited analyte. As shown in Fig. 3 the AAS signals for Mn were reasonably constant during the period of the test. However the retention of Mn increased from about 18% at the start to 40% after 105 atomizations and from 23 to 50% when the experiment was repeated with a new tube. It seems therefore that although the atomization efficiency remained the same the mass of redeposited Mn increased with the age of the tube which might have been owing to changes in the temperature gradient along the tube surface or to degradation of the pyrolytic graphite coating.The experi- ment was not performed for Dy as almost 100% retention was observed with new PCG tubes. Distribution of Retained Mn and Dy in PCG Tubes The graphite tube was sectioned in order to identify the distribution of Dy or Mn after atomization of the radio- tracer deposited at the centre of the tube. A quarter of the tube was cut from each end and the centre part was halved ( a ) 30.0+0.5 t- - - - - - 14.6f0.3 0.5k0.1 I t I 1 I t I ' I 27.6+0.6 11.9f0.4 I 1 cm - Fig. 4 Distribution of (a) Mn; and (b) Dy in pyrolytic graphite coated graphite tube. Based on the mass of the analyte at different sections of the tubes expressed as a percentage of initial mass deposited obtained from radioactivity measurements horizontally to determine the retention close to the deposi- tion point and the extent of redeposition on the rest of the middle part of the tube.A schematic representation of the percentage distribution of Mn for a single experiment is given in Fig. 4 with similar data for Dy. Very little Mn was retained at the centre of the tube but there was major deposition at the ends. Similar observa- tions have been made by Kolb el aL8 when Ge was heated in a graphite tube at 2300 K and the surface was examined by electron microprobe analysis. When the PCG tube was cut to isolate the end contact rings it was found that the deposition of Mn was mainly at the very ends of the tube which explains the persistence of radioactivity from a single deposition of the 56Mn radiotracer after multiple heating cycles.When the distribution of Dy was investigated the major area of radioactivity after atomization was close to the point of initial sample deposition. The radioactivity mea- surements indicate that at least 35% of the injected mass of Dy is vaporized during the first atomization event and contributes in part or totally to the AAS signal for Dy. The remainder of the Dy is converted into a less easily vaporized species most probably a carbide which makes a smaller contribution to the AAS signal during the second or subsequent heating cycles (see Fig. 2). From the data presented in Fig. 4 it seems that the rate of redeposition of the vaporized Dy must be far faster than the diffusional transfer of Dy along the PCG tube.There are two anomolies in the data presented in Fig. 4 that require explanation. The summation of the retained Dy is greater than 100°/o. This was probably caused by the introduction of small errors due to variations in the efficiency of the well-shaped NaI(T1) detector when used to measure the radioactivity of graphite pieces of different shapes and sizes which take-up a different proportion of the detector volume. Also there was a difference in the mass of Dy or Mn redeposited at either end of the PCG tube. This was probably because of a difference in the types of contact jaws at either side of the atomizer. During the period of this work the furnace head was fitted with contacts on the left side which were broader by about 2 mm than the jaws on the right side of the atomizer.This caused a slightly unsymmetrical temperature gradient across the tube which was reflected in the unsymmetrical deposition of the32 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 radiotracers. The broader left-hand contacts were required to accommodate the slightly shorter TPG slotted tubes that were made especially for the probe experiments described later. Vaporization and Retention of Dy and Mn in TPG Tubes Total pyrolytic graphite is an alternative tube material to PCG. A number of papers have compared the atomization and vaporization characteristics of both types of graphite and other tube materials such as glassy A number of advantages have been claimed for TPG includ- ing a faster heating rate than PCG better vaporization of refractory elements and in the SP9 furnace a lower temperature gradient from the tube centre to the ends." The percentage retention of Mn in a 5 mm i.d.TPG tube and a 6.5 mm i.d. slotted TPG tube were compared with the percentage retention in two PCG tubes. With the PCG tubes the contact rings are fabricated as part of a one-piece cuvette so that even when the rings are cut off it is not easy to discriminate between material redeposited internally at the very ends of the tube or externally on the rings. However with the TPG tubes the contact C-rings are fitted over the tube after manufacture and can be removed without cutting the graphite cylinder. When the C-rings were 'removed from a 5 mm i.d. TPG tube it was discovered that a total of about 20% of the initial mass of 1 ng of 56Mn had been redeposited on the side of the C-rings outside the tube (Fig.5). The contact ring regions of the PCG tubes contained more retained Mn than the TPG tube (see Figs. 4 and 5). As both types of tube were heated with optical sensor temperature control to 2500 "C and have the same dimensions it might have been expected that the diffusional loss of Mn through the tube ends would have been similar for both cuvettes and that the extent of external redeposition of Mn would be similar. It is possible therefore that the difference in the amount of redeposited Mn at the contact ring regions is due to greater internal deposition of Mn at the ends of the PCG tube which might be indicative of a greater temperature gradient than in the TPG tube.Hence the results support the observations of de Loos-Vollebregt et al. who measured the temperature distribution across PCG and TPG tubes heated to equilib- rium temperatures of 2100 and 2850 K and reported that the temperature distribution in the TPG tubes was more homogeneous than in the PCG tubes. Very little radioactivity was found in any part of the slotted TPG tube that is normally used for probe atomiza- tion indicating that there was negligible deposition of vaporized Mn either inside the tube or externally on the removable C-rings. It is possible that less Mn was lost through the tube ends because of the presence of the slot in the middle of the tube. Comparison of Wall Platform and Probe Vaporization of Dy and Mn Values for the percentage retention of Dy and Mn on a TPG platform in a PCG tube are given in Table 3.The mass of radiotracer deposited was 1 ng for both elements. The Mn experiment was repeated three times using separate tubes of comparable age and condition. When the results in Fig. 4 and Table 3 are compared it is clear that the vaporization of Dy from a platform in a PCG tube is much less than from the wall. Only about 6% of the radioisotope was vaporized from the platform and deposited on the tube wall. The amount of Mn deposited on the tube following vaporization from the platform was similar to that observed for direct wall atomization. Reasonable agreement was achieved between the results for the repeated Mn experiments. 2 ( 4 1 ' 9 & -I - - I ' +I 6.1f0.5 J r - I ! I I I I (b) n n U U 1 cm i Fig.5 Distribution of Mn in different types of tubes based on mass of Mn at different sections of the tube expressed as a percentage of initial mass (1 ng) deposited obtained from radioactivity measurements (a) pyrolytic graphite coated graphite tube; (b) total pyrolytic graphite tube; and (c) slotted total pyrolytic graphite tube Table 3 Percentage retention of Mn and Dy on a TPG platform and the wall of a PCG graphite tube Platform Wall Radiotracer atomization atomization Close to deposition Total tube* Mn 5.3k0.2 41.2k0.8 0.5k0.1 44.4k0.8 39.8 k 0.8 46.2 k I .O Platform Total tube* point 4.9 k 0.2 35.0 f 0.8 - 4.7k0.2 35.0k0.7 - DY 91.7k 1.3 5.6k0.1 73.32 l.0f 103.2+ 1.5f * Different tubes and platforms of comparable age used for each set of measurements. ?Data from Fig.4. The vaporization of Dy and Mn from PCG probes inserted into a pre-heated slotted TPG tube was also investigated. When I ng of 56Mn was deposited onto a PCG probe and atomized at temperatures from 2 100 to 2900 "C the percentage retention of Mn on the whole probe was found to be 10-209'0 as indicated in Fig. 6. Although the amount of Mn retained on the probe decreased with temperature from 2 100 to 2500 "C it remained constant at about 10% at higher temperatures. It was expected that the efficiency of vaporization of Mn from the probe at the temperature studied would be close to 100%. On further investigation it was found that the amount of Mn retained at the point of sample deposition (ie. the probe head) was negligible as indicated in Fig. 7.The majority of the radioactivity measured when the whole probe was placed in the NaI(T1) detector was due to redeposited Mn on the probe stem and the part of the probe head beyond theJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 33 I I I I 2100 2300 2500 2700 2900 Atomization temperaturePC Fig. 6 Retention of Mn on pyrolytic graphite coated graphite probe as a function of atomization temperature. Based on radio- activity measurements; 1 ng of 56Mn initially deposited (a) 5.3f0.4 L I 5.1f0.5 I I 3.6f I 0.4i0.3 I 0.4 I I 1 ( b ) 52.5f1.2 I 0.7f0.1 i 5.lf I 40.5i1.0 I 0.3 I I 1 1 cm Fig. 7 Retention of (a) Mn; and (b) Dy on pyrolytic graphite coated graphite probes after atomization at 2500 and 2750 "C respectively.Based on radioactivity measurements; 1 ng of 56Mn and 165Dy deposited initially on probe head droplet-containing ridge. When the radioactivity in the slotted TPG tubes was counted it was found to correspond to 5.3+0.4% of the initial mass of Mn deposited on the probe. In previous probe studies it has been suggested that a significant amount of the vaporized material was lost through the slot in the tube wall.13 It might have been expected therefore that a greater amount of Mn would have been deposited on the graphite probe stem. The results given in Fig. 7 for Dy also suggest that losses of vaporized material through the slot were less significant than expected. When a comparatively new PCG probe was sectioned it was found that about 4Oo/u of the deposited analyte was retained on the probe head. Only a small amount of Dy (about 6%) was found on the part of the probe head beyond the ridge and on the stem.When the tube was placed in the detector it was found that 52.4-+ 1.2% of the Dy had been deposited on the wall. When the experiment was repeated with a used PCG probe 58.8% k 1.1% of the Dy was retained on the total probe with 36.7% + 1 .O% redeposited on the tube. The results support the previous observation (Fig. 4) which indicated that the rate of redeposition of Dy onto the graphite tube is far greater than the loss of the element by diffusion or gas expansion either through the slot or out of the ends of the tube. A summary of the Dy and Mn retention figures for the 0.6 A I I 1 8 16 24 32 40 0 Amount of dysprosiumhg Fig.8 Integrated absorbance AAS calibration graphs for Dy using three modes of atomization A wall; B probe; and C platform at 2750 "C various modes of atomization in TPG slotted tubes is given in Table 4. It is clear that vaporization of Dy from a probe is more efficient than from a platform which was confirmed by atomic absorption measurements (Fig. 8). The AAS characteristic mass values for Dy based on measurements of integrated absorbance were calculated to be 53.3 79.3 and 359 pg for wall probe and platform atomization respectively. The values based on peak height absorbance were 104 191 and 1060 pg respectively. Other workers14 have reported a characteristic mass value of 45 pg for Dy based on peak height absorbance measurement with wall atomization. A comparison of the Dy characteristic mass values for platform and probe atomization indicates that the mass of Dy atomized from the probe is about five times that atomized from the platform.This corresponds reason- ably well although not exactly with the information given in Table 4 concerning the mass of Dy retained on the probe and platform. From the percentage retention values given in Table 4 it appears that the vaporization efficiencies of Dy from a platform and a probe in a slotted tube at 2750 "C are 7 and 60% respectively. As the characteristic mass calculated using integrated absorbance measurements for Dy with wall atomization is slightly greater than the probe atomization value it seems that at least 60% of the Dy is vaporized from the wall of a TPG slotted tube.It was difficult to estimate the amount of Mn or Dy retained at the point of deposition in a TPG tube. With the PCG tube it was possible to cut the cylinder of graphite into segments as described under Procedure. However when this was attempted with TPG the tube tended to fragment as it could not be easily cut. Hence there is no information in Table 4 for the amount of Mn or Dy retained and/or redeposited close to the deposition point in the TPG slotted tube. A comparison of the results in Table 4 and Fig. 4 indicates that about 20% of the Dy was redeposited on the C-ring contacts of the TPG tube whereas less than 10% of the Dy escaped from the central section of the PCG tube after vaporization. This suggests that the rate of redeposi- tion of Dy in a TPG tube is less than for a PCG cuvette possibly because of a lower degree of interaction of Dy with the TPG tube wall.Conclusions The use of radioactive tracers has allowed a study of the vaporization characteristics of Dy and Mn under different atomization conditions in a graphite furnace. There is very little retention of Mn close to the deposition point in a Philips SP9 atomizer when PCG or TPG tubes are heated34 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 Table 4 Percentage retention of Mn and Dy on TPG slotted tubes for different methods of atomization Wall Platform Probe* Tube Tube Tube PCG Tube without with TPGi with probe- with Radiotracer C-rings C-rings platform C-rings head C-rings Mn 0.6 f 0.3 1.7 f 0.5 1.3 +- 0.3 4.2 f 0.4 0.4 * 0.3 5.3 f 0.4 DY 77.0 * 1.3 97.5 f 1.4 93.3 -t 1.3 7.0 f 0.1 40.5 4 1 .O 52.5 k 1.2 * Data from Fig.7. to 2500 "C and wall platform or probe atomization is used. Redeposition of Mn takes place at the very ends of the unslotted tubes with a significant proportion on the outside of the tube on the graphite contact rings. Redeposition inside the tube at the ends seems to be less for TPG compared with PCG suggesting a lower temperature gradient along the TPG tube. Irrespective of the type of tube the redeposited Mn is not revaporized to any significant extent and therefore does not contribute to memory effects in AAS measurements. With the slotted TPG tube used in probe atomization very little Mn was retained in any part of the tube including the C-ring contacts. A significant amount of vaporized Mn could have exited via the slot but there was not a substantial amount of Mn on the stem of the probe.Dysprosium is less easily vaporized in a graphite tube than Mn. This was illustrated by the almost 100% retention of the radioisotope in slotted or unslotted PCG or TPG tubes heated to 2750 "C. There were some differences in the vaporization characteristics of Dy when wall platform and probe atomization were used. Retention of Dy at or near the point of the initial deposition was much greater with platform atomization than for wall atomization. The radiotracer measurements indicated that vaporization of Dy from a PCG probe in a slotted TPG tube was almost as good as wall atomization which was confirmed by AAS measurements. The probe seems to be a better vaporization and atomization system than the platform for the determi- nation of refractory elements such as Dy.Irrespective of the type of tube vaporized Dy tended to redeposit mostly at the centre of the tube although a greater fraction of Dy was found at the ends of a TPG tube than for a PCG cuvette. With PCG subsequent heating cycles only vaporized a small fraction of the retained Dy which was again redeposited and not removed by diffusion or gas expansion. Hence this study has shown that memory effects for Dy in AAS emanate from Dy retained at the tube centre rather than at the ends. Wie et al. l 5 have studied the atomization mechanisms of rare earth elements and have suggested that in a graphite tube Dy atoms are formed through reaction of DyO(g) with C(s).In a tantalum tube atomizer thermal dissociation of the vaporized rare earth oxides is thought to occur and the characteristic mass values are improved compared with those of electrothermal AAS.15J6 L'vov16 has proposed that rare earth elements form gaseous carbides in the graphite furnace and that partial decomposition of the carbide on the surface of unvaporized particles leads to the appearance of a carbon film causing a reduced rate of vaporization. From the results reported here it seems that the efficiency of Dy vaporization for a single deposition decreases with repeated heating cycles and that all of the element is apparently retained in the tube. Prell et a1.l7J8 have postulated that various sites of different activity exist on a graphite surface and that a variety of reactions between adsorbed analyte species (e.g. oxides) and other gaseous or condensed species (e.g.carbon and matrix components) can take place at these sites. It might be that the Dy vaporized in the first heating cycle is produced because of reactions of the oxide at one type of graphite site and that the remainder of the Dy is unvaporized because it is converted into a refractory carbide or is adsorbed more strongly as the oxide on other sites. The vaporized Dy does not travel very far along the tube and so interaction of the gaseous atoms or molecules with the tube wall must be significant. It is possible that the redeposited Dy atoms might be adsorbed on graphite sites that promote carbide formation maintaining the retention of Dy reducing the vaporization efficiency and contributing to the memory effect observed in Fig.2. Differences in the surface properties of TPG and PCG can account for the greater distribution of Dy in the TPG tube implied by the data in Table 4. A complete understanding of the vaporization and atomization of Dy requires mass spectrometry studies similar to those performed by Prell et a1.17J8 It seems likely that the mechanisms involved might be more complex than gaseous carbide decomposition and the formation of carbon shells which does not necessarily explain the poorer vaporization efficiency of Dy for repeated heating cycles when almost all the element is retained in the tube. References 1 Veillon C. Guthrie B. E. and Wolf W. R. Anal. Chem. 1980 52 457. 2 Krivan V. and Arpadjan S. Fresenius 2. Anal. Chem. 1988 329 745. 3 Shcherbakov V. I. Belyaev Yu I. and Myadoedov B. F. J. Appl. Spectrosc. (Engl. Trans.) 1981 36 893. 4 Whitley J. E. Hannah R. and Littlejohn D. Anal. Proc. 1988 25 246. 5 L'vov B. V. J. Anal. At. Spectrom. 1988 3 9. 6 Corr S. P. and Littlejohn D. J. Anal. At. Spectrorn. 1988 3 125. 7 Hoffman B. W. and van Camerik S. B. Anal. Chem. 1967 39 1198. 8 Kolb A.. Miiller-Vogt G. and Wendl W. Spectrochim. Acta Part B 1987 42 951. 9 Duncan I. S. Littlejohn D. Marshall J. and Ottaway J. M. Anal. Chm. Acta 1984 157 291. 10 Brown A. A. and Lee M. Fresenius 2. Anal. Chem. 1986 323 697. 1 1 de Loos-Vollebregt M. T. C. Bol M. and de Galan L. J. Anal. At. Spectrom. 1988 3 151. 12 Welz B. Schlemmer G. Ortner H. M. and Wegscheider W. Prog. Anal. At. Spectrosc. 1989 12 1 1 1. 13 Littlejohn D. Cook S. Durie D. and Ottaway J. M. Spectrochim. Acta Part B 1984 39 321. 14 Brodie K. Analytical Methods for Graphite Tube Atomisation ed. Rothery E. Varian Techtron Pty. Victoria Australia 1982 ch. 2. 15 Wei J. Xu T. Ni D. and Wang X. presented at the Colloquium Spectroscopicum Internationale Pre-Symposium on Graphite Atomizer Techniques in Analytical Spectroscopy Lofthus Norway June 6-8 199 1 Abstract p. 18. 16 L'vov B. V. Analyst 1987 112 355. 17 Prell L. J. Styris D. L. and Redfield D. A. J. Anal. At. Spectrom. 1990 5 23 1. 18 Prell L. J. Styris D. L. and Redfield D. A. J. Anal. At. Spectrom. 1991 6 25. Paper I /04 758B Received September 13 1991 Accepted October 8 1991

ISSN:0267-9477    

DOI:10.1039/JA9920700029

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY. FEBRUARY 1992 VOL. 7 35 Determination of Erbium by Electrothermal Atomic Absorption Spectrometry Using a Pyrolytic Graphite Coated Graphite Tube and a Pyrolytic Graphite Coated Graphite Tube Lined With Tantalum Foil Held in Place by a Tungsten Spiral Ma Yi-zai and Bai Jian Institute of Analysis and Measurement Chinese Research Academy of Environmental Sciences Beijing 100012 China Sun Di-jun Research Center for Eco-Environmental Sciences Academia Sinica P. 0. Box 934 Beijing 100U83 China A new atomizer using a pyrolytic graphite coated graphite tube (PGT) lined with tantalum foil and held in place by a tungsten spiral (WTaPGT) was used for the determination of Er by electrothermal atomic absorption spectrometry (ETAAS). The application of WTaPGT and PGT for the determination of Er by peak area and peak height measurement are discussed.The characteristic masses of Er atomized in the WTaPGTs and PGTs were 13.3 and 19.4 pg respectively at an atomization temperature of 3000 K which were close to the value of 17 pg measured by L'vov using a Ta platform. The acids HN03 HCI H2S04 and HC104 at concentrations of 1 mol dm-3 and lmg m1-I solutions of the chloride or nitrate salts of K Na Ca Mg Al Fe La Ce and Y caused no matrix interferences. The WTaPGT atomization can be used for the direct determination of Er in 10 mg ml-l geochemical and water sediment samples after dissolution. The results showed good agreement with reference values. This paper shows that modern graphite furnace techniques provide the possibility of developing a standardless method for the determination of Er in varied and complex matrices with an accuracy of 10-1 5%.Keywords Erbium in geochemical samples; pyrolytic graphite coated graphite tube lined with tantalum foil; interference; electrothermal atomic absorption spectrometry; tungsten spiral L'vov' first suggested the use of a graphite tube lined with Ta in order to prevent the diffusion of vapour through the porous graphite wall. L'vov and Pelieva2 used an HGA-76B graphite tube lined with Ta foil. Their results showed that for 32 of the 40 elements studied a significant increase in sensitivity was achieved when a Ta foil lining was used. A considerable reduction in memory effect and lower atomi- zation temperatures were also reported for the majority of these elements.Wahab and Chakrabarti3 have reported the use of HGA-76B graphite tubes lined with Ta or W foil for the determination of Y.3 Graphite tubes lined with a thin layer of Ta or W foil suffer from deformation problems the Ta and W foil gradually become fragile with use and tend to crack which makes further determinations impossible because of the appearance of leaks between the Ta or W foil and the inner surface of the graphite tube. A new atomizer using pyrolytic graphite coated graphite tubes (PGTs) lined with Ta foil and held in place by a W spiral (WTaPGT) has been developed by Ma and W U . ~ - ~ ~ The WTaPGT can be used at a high atomization tempera- ture (2900-3000 K) and with a long atomization time (10-20 s) for more than 200-300 firings without deforma- tion and disintegration.This paper deals with the direct determination of Er in geochemical and water sediment sample solutions with PGT and WTaPGT atomization by measurement of both peak absorbance and integrated absorbance in different matrices and using different instrumental conditions. The possibility of developing a standardless analytical method for the determination of Er is also discussed. Experimental Apparatus A Model WFD-Y3 (now known as WFX-ID) atomic absorption spectrometer (Beijing Second Optical Instru- ment Factory) equipped with a WFX-1 graphite furnace and a Model 056 chart recorder with two pens (one for peak absorbance measurement and the other for integrated absorbance or temperature measurement) was employed using the resonance line at 400.8 nm under gas stop conditions.The spectral bandwidth was set at 0.4 nm and the lamp current at 5.0 mA. Eppendorf microlitre pipettes (1 0 and 20 pl) fitted with disposable poly(propy1ene) tips were used for sample introduction. All temperature measurements were performed on the laboratory-built graphite furnace pyrometer Type MT- 1 ,8*9 which uses a Si photocell and a PbS detector to measure the radiation from the graphite tube and record the tempera- ture dependent radiation curves. The temperature of the graphite wall was calibrated by using three method^.^.^ (i) A W ribbon lamp BW-1600 (Chinese Research Academy of Metrology) was used for the range 900-2200 "C by changing the lamp current. (ii) The melting-points of different metals such as Pb Sn Au Pt Mo Nb and Ta were used giving an error of less than 1%.(iii) A Pt-Rh thermocouple was used to calibrate in the range 900-1 500 "C. An argon sheath flow rate of 2.5 1 min-l and carrier gas flow rate of 0.5 1 min-l were used. The PGTs were made in the laboratory and the pyrolytic graphite coating (0.2 mm in thickness) was made by the Institute of Metal Academia Sinica. The final size of the PGTs used was inner radius 2.8 mm outer radius 4.2 mm and length 28 mm. Preparation of the WTaPGT The construction of the WTaPGT is shown in Fig. 1. The WTaPGT was made in two steps. Firstly a graphite tube lined with Ta foil was prepared by the insertion of Ta foil of a suitable size according to the inner diameter of the PGT (x3=5.6 mm). The piece of foil (0.2 mm in thickness) cut into 18y mm (y=2.88x3= 16.1 mm) was wrapped around a steel rod or drill (x2=0.87x3=4.9 mm).Secondly a tungsten spiral made by wrapping a 0.5 mm tungsten wire around a steel rod or drill (xI=O.62x,=3.5 mm) was inserted into the Ta foil lined graphite tube taking care to36 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 I 2 mm H 7 m 28 mm -1 29 mm 2.35 mm H 1-18 mm+ Fig. 1 Construction of the pyrolytic graphite coated graphite tube lined with tungsten and tantalum and the size of the tantalum foil. For details see text align accurately the sampling hole and the sample injection port of the PGT. The lined tube could be used only after 1-2 firings according to the following heating programme drying at a setting of 60 A (420 K) for 22 s; ashing at 250 A (1 570 K) for 18 s; and atomization at 550 A (3000 K) for 20 s.Reagents Erbium stock solution 1000 pg ml-l was prepared by dissolving 0.1 144 g of Er203 (Specpure Johnson Matthey Royston UK) in 10 ml of concentrated HN03 and diluting to 100 ml with de-ionized water. Working standards were prepared by appropriate dilution with 0.16 mol dm-3 HN03. The acids HN03 HCl H2S04 and HC104 (all 1 mol dm-3) were prepared by dilution of HN03 HC1 H2S04 and HC104 (high-purity grade Beijing Chemical China). Potassium Na A1 and Fe (all 1 mg ml-I) were prepared by dissolving a suitable amount of KCl NaCl AlC13 and FeC1 (spectroscopic grade Beijing Chemical) in 0.8 mol dm-3 HN03. Calcium Mg La Ce and Y (1 mg ml-I) were prepared by dissolving a suitable amount of CaC03 MgO La203 CeO and Y203 (spectroscopic grade Beijing Chemical) using concentrated HN03 followed by dilution with de-ionized water.Geochemical Sample and Sediment Dissolution Procedure Standard geochemical samples GSS4 GSS7 and GSR 1 and sediments GSD2 GSD5 GSD7 and GSD12 obtained from the Institute of Physical and Chemical Prospecting Institute of Analysis and Measurement of Minerals and Rocks Beijing China were used. A 250 mg amount of each sample was decomposed in a poly(tetrafluoroethy1ene) crucible with the addition of 2 ml of 67% HN03 4 ml of 35% HF and 2 ml of 72% HC104 and placed in a pressure vessel in an oven at 180 "C for 8 h. The solution was then evaporated to incipient dryness and the residue dissolved in 5 ml of 0.8 mol dm-3 HN03.The solution was filtered into a 25 ml calibrated flask. The filter containing the undis- solved residue was placed in a pyrographite crucible and ashed at 450 "C for 4 h in a muffle oven. It was then melted with the addition of 1 g of Na202 at 800 "C for 15 min. The melt was boiled for 15 min with 100 ml of 1 O/o triethanol- amine and 1 ml of 25 mg ml-l Mg(N03) solution. After being stored overnight the solution was filtered and the precipitate was washed with 2% NaOH solution 6-8 times. The filter containing the precipitate was boiled in a 100 ml beaker with 10 ml of 6 mol dm-3 HN03 and diluted with 10 ml of de-ionized water. This solution was then filtered and the filtrate was collected in a 25 ml calibrated flask. The final solutions were diluted to 25 ml with de-ionized water.The concentration of the sample solutions was 10.0 mg ml-I. Results and Discussion Selection of Analytical Parameters and Analytical Charac- teristics of the PGT and WTaPGT The effect of lamp current on the atomic absorption of 10 ng of Er at an atomization temperature of 3000 K with a PGT is shown in Table 1. The signals in both peak and integrated absorbance decreased slightly with the increase of lamp current. A range of 5-1 5 mA can be used for the determination of Er. The typical atomization signals with the PGT and WTaPGT are shown in Fig. 2. L'vov's t h e ~ r y ~ ~ . ~ ~ was used to express these signals. The total recording time constant (fRC) in our instrument is 0.20 s ( i e . the WFD-Y3 spectrometer and the 056 chart recorder).A series of calculations using L'vov's formu- lae19720 show that Tl'=T1+O. 15 s and z2'=z2+O.20 s when 71 and z2 are in the range 0.5-10 s; here 71' and r i are the values measured by experiment T~ and 7 2 are derived from formulae. As shown in Fig. 2 with PGT atomization at 3000 K 71' and z2' are 3.6 and 8.0 respectively the time of appearance of the atomization signal from the beginning of the atomization step T~, is 1.5 s. A significant memory effect was observed probably because of the formation of carbon shells on the surface of sample microparticles.21-22 The residue memory absorbance [A,(mem)] and integrated absorbance [ QA(mem)] for 10 ng of Er were 0.100 and 0.70 respectively which correspond to 32% [A,(mem)/A,] and 23% [QA(mem)/QA] of the total peak absorbance (A,) and total integrated absorbance (Ai).With the WTaGPT atomi- zation at 3000 K T,' and z2' are 3.5 and 2.1 respectively fapp is 3.0 s. The values of A,(mem) and A,(mem) for 4 ng of Er were 0.026 and 0.10 respectively which correspond to 7% [A,(mem)/A,] and 7% [Ai(mem)/Ai] of the atomization signals. The influence of atomization temperature on the absorp- tion signals of Er are listed in Tables 3 and 4. Erbium is a refractory element. A memory effect of Er with either PGT or WTaPGT atomization was observed with atomization temperatures in the range 2600-3200 K (PGT) and 2600-3100 K (WTaPGT) the effect being greater for Table 1 Effect of lamp current on atomic absorption of 10 ng of Er with PGT atomization. Ashing temperature 1570 K; and atomizing temperature 3000 K Lamp current/mA 5.0 10 15 20 Peak absorbance 0.220 0.210 0.202 0.186 ( A p ) .4 ( 4 ) 5 * 1.00 0.95 0.92 0.85 Integrated absorbance 2.90 2.80 2.73 2.61 ( QA).~/( QA* 1.00 0.97 0.94 0.90 * Ratio of absorbance at lamp current x to that at 5 mA.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 37 - ~ - 2 - 0.05 - -.A 2,' / i /. Table 2 Calculated values of the characteristic masses of Er at different atomization temperatures Er 400.8 nm; M= 167.2 g mol-I; y'= 1 .OO; and f=0.48 Atomization temperature/K 2600 2700 2800 2900 3000* 3100 3200 D/cm2 s-l 4.43 4.75 5.07 5.40 5.76 6.10 6.46 A?J 1 0-2 cm-I 7.04 7.18 7.31 7.44 7.57 7.69 7.82 a 1.19 1.14 1.09 1.04 1.00 0.96 0.93 H(a,o) for o=0.72a 0.303 0.317 0.331 0.346 0.360 0.374 0.389 L 3000 8 + E 2000 1000 rn,(cal j/bg f= 0.48 m,(cal)/pg f= 0.109 * L'vov's data from ref. 23.0.940 0.993 0.926 0.918 0.910 0.903 0.895 2.23 2.35 2.46 2.57 2.71 2.82 2.96 9.8 10.3 10.8 11.3 11.9 12.4 13.0 Oa40* 3000 Y \ $ 0 co.'otlKz3 8 1000 2 Time/s Fig. 2 Atomization signals for Er. (a) PGT 1 signal for 10 ng of Er and 2 memory signal of Er; and (b) WTaPGT 1 signal for 2 ng of Er and 2 memory signal of Er. The equation for 1 is given by l/exp [A(max)-A(mem)] atomization using the PGT and the value of Ai(mem)/Ai decreased with an increase in atomization temperature. There values are 23% (3000 K) 29% (2800 K) and 32% (2600 K) for PGT atomization and 4% (3 100 K) 7% (3000 K) 10% (2900 K) 13% (2800 K) and 14% (2600 K) for WTaPGT atomization. It is impractical to use the PGT for the determination of Er because of the serious memory effect involved.The peak absorbance of Er in WTaPGT is 3-4 times larger than that for PGT atomization. Owing to the shorter atomization time with the WTaPGT the lifetime of a WTaPGT is 3-4 times longer than that of a PGT when determining Er. Thirty years ago L'vov pointed out the possibility of developing standardless (absolute) analysis by electrother- mal atomic absorption spectrometry (ETAAS). Theoretical calculation of the characteristic mass (m,) in ETAAS was made by L'vov and co-workers for 43 elements including Er.23,24 The results they obtained are listed in Table 2. In this paper the m values of Er at different atomization temperatures are also listed in Table 2. The formula used Table 3 Effect of atomization temperature on the rnc values for 10 ng of Er with PGT atomization Atomization temperature/K Peak absorbance A -A p( mem) mp(exP)/Pg* Integrated absorbance QA - QA(mem) mo(exP)lPgt m,(cal)$.f=0.48 m,(cal)/pg f= 0.109 2600 0.086 0.058 759 3.08 2.08 21.1 2.23 9.8 2800 0.1 52 0.1 12 393 3.18 2.28 2.46 19.3 10.8 3000 0.248 0.198 222 3.07 2.27 2.71 19.4 11.9 3200 0.420 0.320 138 3.1 1 2.41 2.96 18.3 13.0 1.5 1.5 1.5 1.5 3.8 3.4 2.9 2.6 25.2 17.0 10.2 5.2 81.1 55.9 34.4 19.7 25.3 16.8 10.0 5.0 0.22 0.19 0.17 0.15 * rnp(exp) is the characteristic mass for peak absorbance measure- t rn,(exp) is the characteristic mass for integrated absorbance 4 rn,(cal) is the calculated characteristic mass for integrated ments. measurements. absorbance measurements in Table 2.xt = la + 71' + 3 72'. for the calculation of m values was that derived by L'vov:23,24 The molar mass of Er M=167.2 g mol-I y' is the coefficient estimating the effect of fine or hyperfine struc- ture and Doppler profile in a hollow cathode lamp I= 1-00 for Er f i s the oscillator strength Z(r) is the state sum of temperature T A t D is the Doppler line width H(a w ) is the Voigt integral and g is the statistical weight of the lower level. The value of r2/12 is taken from the size of PGT used in our experiments; r=2.8 mm 1=28 mm. The diffusion coefficent D for Er in argon at temperature T were determined from the relationship D=Do(T/273)" (2) The values of Do and n for Er were calculated in ref. 25 as being 0.075 cm2 s-l and 1.81 respectively.The Doppler width of the 400.8 nm (A) line of Er is calculated by the expression:38 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 Table 4 Effect of atomization temperature on the m value for 40 ng of Er with WTaPGT atomization Atomization temperature/K Peak absorbance A - Ap(mem) Integrated absorbance mp(exP)*lPg QA - QA(mem) mo(exP)t/Pg mo(cal),$ f = 0.48 mo(cal)/pg f = 0.109 *Footnotes as in Table 3. 2600 0.107 0.093 1.72 1.48 1.71 7.50 189 11.9 2700 0.142 0.124 1.62 1.40 1.80 7.89 142 12.6 2800 0. I87 0.166 1.58 1.38 1.88 8.27 106 12.8 2900 0.308 0.283 1 S O 1.35 1.97 8.65 62.2 13.0 3000 0.395 0.369 1.42 1.32 2.07 9.1 1 47.7 13.3 3100 0.416 0.400 1.34 1.28 2.16 9.50 44.0 13.8 0.143 0.143 0.147 0.152 0.155 0.156 0.63 0.63 0.64 0.67 0.68 0.69 3.0 3.0 3.0 3.0 3.0 3.0 12.0 9.0 6.0 4.5 3.5 3.0 11.0 8.0 6.4 3.8 2.1 2.0 48.0 36.0 28.2 18.9 12.8 12.0 10.8 7.8 6.2 3.6 1.9 1.8 0.22 0.21 0.19 0.18 0.17 0.16 The Voigt parameter a is calculated according to L'vov9 A ~ D x P.5 and the Lorentz width AtL= T-Ob7.K is the proportional coefficient. By using L'vov's data for Er T= 3000 K a= 1 .00,23 the calculated K is 1.49 x lo4. The magnitude of the Voigt integral H(a,w) for o=0.72a as a function of the Voigt parameter is determined based on the table by P o ~ e n e r . ~ ~ The facter g,exp( -E/KT)/Z( T) was calculated based on the table of Moore2* for the 400.8 nm line of Er transition 3H6-(3H6:1P1)7 0-24950 cm-I. The calculated rn values for Er at different atomization temperatures are listed in Table 2.According to L'VOV,~~ there should be a difference between the temperature of the gas and the temperature of the wall. It was assumed that the gas temperature is 73 K lower than the set temperature the temperature of the absorbing layer T(K)= T("C)+ 200. The experimental values of the rnc values for Er at different atomization temperatures with the PGT are listed in Table 3 and with the WTaPGT in Table 4. The data in Tables 2-4 lead to the following conclusions. (1) A comparison of the characteristic masses for inte- grated absorption rn,(cal) by theoretical calculation in Table 4 revealed that the maximum value of m,(cal)/m,(exp) with the WTaPGT at 3 100 K and the PGT at 3200 K were 0.156 and 0.162 pg respectively the average value is 0.159 pg which is 4.4 times less than the value of 0.70 pg for Yb obtained by L'VOV.~* If the low f value of 0.48 for Er is replaced by the value of 0.48/4.40=0.109 the rn,(cal)/ m,(exp) values are 0.66 k 0.024 (3.6%) for the WTaPGT in the temperature range 2600-3 100 K.With PGT atomiza- tion the m,(cal)/m,(exp) values decreased considerably as the atomization temperature decreased; they were 0.7 1 (3200 K) 0.62 (3000 K) 0.56 (2800 K) and 0.47 (2600 K). The lower values of rn,(cal)lm,(exp) with the PGT might be caused by the formation of carbon shells on the surface of the particles.21J2 (2) The characteristic masses m,(exp) obtained for integrated absorbance were found to be only slightly dependent on the atomization temperature with WTaPGT atomization i.e. 12.9+0.59 pg (4.6%) for temperatures in the range 2600-3100 K and 18.8+ 1.74 pg (9.3%) with PGT atomization for temperatures in the range 2600-3400 K.The lower sensitivity and precision made the use of the PGT impractical. Integrated absorbance measurements obtained with the WTaPGT are preferable for the determi- nation of Er. (3) The characteristic masses mp obtained from peak absorbance measurements were remarkably dependent on the atomization temperature with both the WTaPGT and PGT. As can be seen from Tables 3 and 4 the absorbance ratio of Er at 3400 K to that at 2600 K is 6.9 with the PGT. The slope of the absorbance versus temperature graph for Er was about 74%/100 K. With the WTaPGT the ratio of Er a'bsorbance at 3100 K to that at 2600 K is about 3.9 with a slope of 58%/ 100 K. Because the atomization temperature clhanges over the lifetime of the PGT and WTaPGT better repeatability could be obtained by using integrated absor- bance.The characteristic mass mpl calculated from peak absor- bance obtained with a PGT lined with Ta foil (0.1 mm in thickness) at 2700 K was 60 pg,2 which is close to the value of 62.2 pg with WTaPGT atomization obtained in the present study. Because the original values given for the concentration of the Er solution were wrong the published values of rnp were corrected to be 46.5 pg (2900 K WTaPGT),5-6 285 pg (3000 K PGT)5q6 and 45 pg (2900 K WTaPGT)." These values of mp are close to the value of 47.7 pg listed in Table 4 (3000 K WTaPGT) and 222 pg listed in Table 3 (3000 K PGT). The mp at 2700 "C (2900 K.) with PGT obtained by Grobenski was 1500 pg.29 (4) The theoretical average residence time z2 can be calculated by the formula z,(cal)= N80 derived by L'VOV,~~ and are listed in Tables 3 and 4.A large discrepancy between the calculated zz(cal) and the experi- mental values was observed for example zz(exp)/zz(cal) (3000 K PGT) is 58.5 rz(exp)/r2(cal) (3000 K WTaPGT) is 11.2. This can be partly explained by the probable forma- tion of carbon shells on the surface of the sample micropar- ticles,21Jz however other further reasonable explanations need to be found. Recently as shown by L'VOV,~~ when the 0.1 mm (1 20 mg) Ta platform was used for the determina- tion of Tm At (pulse width of atomic absorption signals at the half maximum in units of time) was 0.17 s which is cllose to the z,(cal) value of 0.15 s.For the 0.2 mm (240 mg) T'a platform At was 0 . 3 1 s which nearly doubled as the mass of the Ta platform doubled. The Ta foil in the WTaPGT had a mass of 1.26 g which is ten times heavier than the 0.1 mm (1 20 mg) Ta platform used by L'vov. The use of 1.26 g of Ta foil with WTaPGT atomization results in a s2(exp) v,alue that is ten times longer than that obtained with the 0.1 mm (1 20 mg) Ta platform.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992. VOL. 7 39 - - - (5) According to eqn. (2) in the course of the total atomization time Ct = tap,+ 71 + 3r2 the absorbance de- creased to the value A3r2 =A,exp( - 3 ) = 0.05OAp which is close to the baseline. The values of X t obtained with the PGT are 19.7 s (3200 K) 34.4 s (3000 K) 55.9 s (2800 K) and 8 1.8 s (2600 K).The C t values with the WTaPGT are 12.0 s (3100 K) 12.8 s (3000 K) 18.9 s (2900 K) 28.2 s (2800 K) 36.0 s (2700 K) and 48.0 s (2600 K). At the same temperature the total atomization time with the PGT is about twice that with the WTaPGT. For practical use the atomization time is always less than 20 s so the best atomization temperatures are 3200 K for the PGT and 2900-3100 K for the WTaPGT. The integrated absorbance and peak absorbance versus ashing temperature and atomization temperature for Er with the PGT and the WTaPGT are shown in Figs. 3 and 4. The tolerable ashing temperatures for Er are 2200 K (PGT) and 2200 K (WTaPGT). The appearance temperatures Tap for Er are 2180k120 K (PGT) and 2180klOO K (WTaPGT),9 which were measured using a new type of Zeeman-effect atomic absorption spectrometer ZM-2 * with the injection of 80 ng of Er.The value of Tap is defined for an atomization time of 30 s when the absorbance rises to a value of 0.0044 f 0.0020. An ashing temperature of 1570 K (PGT and WTaPGT) and atomization temperatures of 3200 K (PGT) and 3000 K (WTaPGT) were used for the measurements. 0.3 8 m f! s 0.2 3 0.1 I * P I 0.6 0.4 8 m c 0.2 2 I I I 1 I '0 500 1000 1500 2000 2500 3000 3500 Temperature/ K Fig. 3 Effect of ashing (left-hand traces) and atomization tem- perature on the atomic absorption of 10 ng of Er with PGT atomization A integrated absorption; and B absorbance 0.5 2.5 1 I 2.0 P-3 -P 10.4 8 0 n $ 1.5 n -0 4- F 1.0 [3 C U - 0.5 0' I I I J O 500 1000 1500 2000 2500 3000 3500 Temperature/K Fig.4 Effect of ashing (left-hand traces) and atomization tem- perature on the atomic absorption of 10 ng of Er with WTaPGT atomization A integrated absorbance; and B absorbance I I 0 100 200 Number of firings Fig. 5 Dependence of integrated absorbance and absorbance for 4 ng of Er over the lifetime of a WTaPGT. A Integrated absorbance; and B absorbance Fig. 5 shows the dependence of integrated absorbance and peak absorbance for Er over the lifetime of the WTaPGT. The integrated absorbance of Er remained almost the same over 250 firings. The relative standard deviation (RSD) for integrated absorbance was found to be less than 5% and for peak absorbance less than 10%. Comparison of Interference Effects on Er Experiments were conducted in order to evaluate the extent of interference by various mineral acids and chloride and nitrate salts on Er absorption signals using PGT and WTaPGT atomization.These materials are either used for sample decomposition (e.g. mineral acids) or are present as matrix elements (Ca Mg Fe and Al) or concomitant with Er (La Ce and Y) and which generally exist in the form of chloride or nitrate salts after decomposition. Mineral acids Nitric acid HC1 H2S04 and H3P04 at a concentration of 1 mol dm-3 were investigated and no interference was observed in the integrated absorbance with HN03 HCl and H2S04 for PGT or WTaPGT atomization. The HC104 caused a remarkable decrease in the peak absorbance and integrated absorbance of Er with PGT R,=(rnp)H201 (mp)HC104 was 0.66 R,=(mo)H2d(m,),C,04 was 0.60.HOW- ever no obvious interference was observed for 1 mol dm-3 HC104 on either the peak absorbance or integrated absor- bance with the WTaPGT in which instance both R and Ri have a value of 0.99. Therefore the use of the highly acid resistant WTaPGT is preferable for the determination of Er particularly when large amounts of mineral acids and perchloric acid are used for sample decomposition. Sodium chloride and potassium chloride For some geological samples Er is only partly decomposed by mineral acids and NaOH KOH Na202 Na2C03 or K2C03 fusion can also be used. The fused material is dissolved in hydrochloric acid which results in large amounts of NaCl or KCl being formed. At the high ashing temperature used most of the NaCl and KCI were volatil- ized and no interferences were observed in either peak absorbance or integrated absorbance measurements for I mg ml-I of NaCl and KC1 with PGT and WTaPGT atomization.40 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL.7 Table 5 Matrix effect on 10 ng of Er with PGT atomization; atomization temperature 3200 K Er in matrix Er in water - md mJ TI'/ md mJ TI'/ 1 mol dm- HNO 400 17.9 3.0 18 246 17.8 3.0 1 mol dm- HCl 184 16.5 3.0 11 187 16.4 3.0 1 mol dm-3 H2S04 355 17.1 4.5 25 214 17.5 3.5 1 mol dm- HC104 383 28.6 4.0 16 251 17.0 4.0 1 mg ml-1 K (KCl) 364 16.6 5.0 20 373 16.2 3.4 1 mg ml-' Na (NaC1) 319 18.0 3.5 20 278 18.0 3.5 I mg ml-I Ca [Ca(N03)2] 159 18.6 2.2 7.8 393 19.5 4.0 1 mg ml-1 Mg [Mg(NO,),] 303 17.4 3.5 20 291 16.5 3.5 1 mg ml-1 A1 (A1Cl3) 310 16.1 4.5 20 310 16.8 3.0 1 mg mi-' Fe (FeCl,) 364 18.2 6.0 18 286 18.6 4.0 1 mg ml-1 Ce [Ce(NO,),] 326 19.6 3.0 15 180 20.0 3.5 1 mg ml-' Y [Y(N03),] 331 17.5 3.5 2.0 237 17.6 3.5 1 mg ml-1 La [La(NO,),] 333 17.0 4.0 19 314 17.4 4.0 Matrix Pg Pg s S Pg Pg s * R,=(m,)H20/(m,) matrix. 7 Ri = ( m,)H,O/( m,) matrix.- Ti/ s R,* R,i 14 0.62 0.99 11 1.02 0.99 14 0.60 1.02 16 0.66 0.60 15 1.03 0.97 17 0.87 1.00 18 2.46 1.04 20 0.96 0.95 18 1.00 1.04 14 0.79 1.03 8 0.55 1.02 13 0.72 1.00 18 0.94 1.02 Table 6 Matrix effect on 4 ng of Er with WTaPGT atomization; atomization temperature 3000 K Er in matrix Er in water rnd mJ TI'/ md mJ T I r / 1 mol dm- HNO 81.5 14.5 6.0 7.0 71.8 14.6 5.0 1 mol dm- HCl 80.4 14.2 5.0 6.9 71.8 14.2 5.8 1 mol dm- H2S04 79.3 14.5 5.0 7.0 74.3 14.8 4.5 1 mol dm- HC104 89.3 14.5 5.0 8.0 88.0 14.4 5.0 1 mg ml-1 K (KCl) 54.0 12.2 4.0 5.5 48.9 12.2 4.0 1 mg ml-1 Na (NaCl) 52.1 13.2 3.0 6.0 52.2 12.8 3.0 1 mg ml-1 Ca [Ca(N03)2] 55.5 12.7 3.0 5.0 52.2 12.8 3.0 1 mg ml-1 Mg [Mg(NO,),] 59.9 13.2 3.5 6.0 55.0 12.8 3.5 1 mg ml-1 A1 (AlCI,) 32.5 12.7 3.0 3.0 54.8 12.8 3.5 1 mg ml-1 Fe (FeCl,) 67.2 13.1 3.5 7.0 63.1 13.2 3.5 1 mg ml-1 Ce [Ce(NO,),] 57.9 11.8 3.0 5.5 59.9 12.7 4.8 1 mg ml-1 Y [Y(NO,),] 106.7 13.6 5.0 11 62.2 13.2 3.5 1 mg ml-1 La [La(NO,),] 60.3 13.5 3.5 6.0 57.7 13.2 3.5 Matrix Pg Pg S s Pg Pg s * R,=(m,)H20/(m,) matrix.7 Ri=(m,)H201(m,) matrix. 52'1 s R,* R,t 6.0 0.88 1.01 6.2 0.89 1.00 6.5 0.94 1.02 8.0 0.99 0.99 5.0 0.91 1.00 5.4 1.00 0.97 5.4 0.94 1.01 6.0 0.92 0.97 6.0 1.69 1.01 6.5 0.94 1.01 6.3 1.03 1.08 6.5 0.58 0.97 6.5 0.96 0.98 Calcium and magnesium nitrate aluminium and iron(rrr) chloride Calcium Mg A1 and Fe exist as major components in many geological samples and after decomposition they form chloride or nitrate salts in the solution.The experimental results showed that no interferences were observed in either peak absorbance or integrated absorbance for 1 mg ml-l solutions of Mg as Mg(N03)2 and Fe as FeC13 with PGT and WTaPGT atomization. With PGT atomization peak absor- bance for Er increased 146% in 1 mg ml-* Ca solution as Ca(N03)2 in comparison with that for a pure solution of Er and rlf and z2' were shorter; however the integrated absorbance for Er showed no such differences. With WTaPGT atomization both peak absorbance and inte- grated absorbance remained unchanged.The effect of A1 on the determination of Er was complicated in peak absor- bance measurements. The peak absorbance of Er in 1 mg ml-I A1 solution as AlC13 increased 15% with PGT and increased 69% with WTaPGT in comparison with that for a pure solution of Er and T~' and 7zf were shorter. The peak area for Er was unchanged with WTaPGT atomization. Lanthanum cerium and yttrium nitrate The total rare earth elements content in water sediments and geochemical samples varied from 185 to 484 ppm. The interferences of the total rare earth elements in such samples were negligible. The purpose of studying the chemical effect of La Ce or Y on the determination of Er was to establish the possibility of determining trace amounts o1f Er against high background concentrations of rare earth elements.As shown in Tables 5 and 6 no interferences were o,bserved in integrated absorbance measurements for 1 mg ml-l solutions of La as La(N03)3 Ce as Ce(N03) and Y as Y(N03)3 with either PGT or WTaPGT atomization. However low R values were obtained for peak absorbance with either PGT or WTaPGT atomization. 1)irect Determination of Er in Water Sediments and Geoche- mical Samples I n order to confirm the suitability of WTaPGT atomization fix the direct determination of Er in 10 mg ml-' dissolved water sediments and geochemical sample solutions five certified reference materials (CRMs) of water sediments GSD 2 GSD 5 GSD 7 GSD 1 1 GSD 12 and three CRMs of geochemical samples GSS 4 GSS 7 and GSR 1 were analysed using aqueous Er standards containing 20.0 50.0 100 and 200 ng ml-* under the following conditions injection volume 0.020 ml; ashing temperature 1570 K forJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL.7 41 Table 7 Determination of Er in certified reference materials of water sediments and geochemistry samples with WTaPGT atomization injection volume 0.020 ml; ashing temperature 1570 K atomization temperature 3000 K* Standard sample? GSR 1 GSD 7 GSD 2 GSD 5 GSD 12 GSS 4 GSS 7 GSD 1 1 GSD 1 1 Standard sample? GSR 1 GSD 7 GSD 2 GSD 5 GSD 12 GSS 4 GSS 7 GSD 1 1 GSD 1 1 * n = 5-1 0. Concen tratiod mg ml-' 10.0 10.0 5.0 10.0 10.0 10.0 10.0 10.0 5.0 Concen trationl mg ml-' 10.0 10.0 5.0 10.0 10.0 10.0 10.0 10.0 5.0 Reference Er contentlppm RD (O/O) from CRMs RSD (Oh) value/ PPm QA 6.5 2.30 8.00 3.10 3.10 4.50 2.70 4.60 4.60 6.43 f 0.1 1 8.00 f 0.25 3.12 2 0.03 3.12 & 0.07 4.49 * 0.05 2.70 k 0.03 4.74 +- 0.20 4.64k0.10 2.30 f 0.10 Standard 14.3 12.3 13.5 13.1 13.1 13.8 12.3 14.2 13.9 Sample 14.4 12.3 13.5 12.9 12.9 13.8 12.3 13.8 13.8 A QA A P 5.55 2 0.55 - 1 .1 - 14.6 1.90 k 0.35 0 .o - 17.4 7.66 f 0.59 0 .o -4.2 3.13 2 0.13 +0.6 + 1.0 3.54 -+ 0.22 +0.6 + 14.2 4.20 +- 0.30 -0.2 -6.1 2.82 2 0.12 0.0 + 4.4 4.97 +- 0.35 +3.0 + 7.4 4.09 k 0.74 +0.9 -11.1 mplpg H20 standard Standard Sample 5,IIS 5;rs 50.1 58.7 2.6 2.3 35.2 42.6 2.0 2.0 39.4 41.1 2.2 2.0 43.6 43.1 2.4 2.2 43.7 38.3 2.4 2.2 39.5 42.3 2.0 2.0 41.0 39.3 2.2 2.2 46.8 43.6 2.4 2.3 41.4 46.0 2.2 2.1 QA A p 1.7 9.9 4.3 19.4 3.1 7.7 1.0 4.2 2.3 6.2 1.1 7.1 1.1 4.3 4.2 7.1 2.2 18.1 Sample solution f,'/S 2.8 2.0 2.2 2.4 2.2 2.2 2.2 2.4 2.4 5;/s 2.7 2.0 2.4 2.4 2.2 2.2 2.2 2.2 2.3 TGSR 1 (rock) GSS 4 and GSS 7 (soils) are geochemical samples and GSD 2 5 7 1 1 and 12 are water sediment samples.18 s; and atomization temperature 3000 K with the WTaPGT. The percentage relative deviation from reference values of the CRMs [RD (O/O) from CRMs] were determined for both peak absorbance and integrated absorbance in order to evaluate the results and are listed in Table 7. With peak absorbance for the determination of Er RD (O/O) from CRMs was poor ranging from - 17.4 to + 14.2% with RSDs in the range 4.2-19.2%. When integrated absorbance values were used the RD (%) from CRMs were improved and satisfactory results were obtained the values obtained were in the range from - 1.1 to + 3.0% with RSDs ranging from 1 .o to 4.3%.The experimental work on characteristic mass values in peak absorbance (m,) and integrated absorbance (m,) measurements shown in Table 7 was aimed at establishing the feasibility of absolute analysis. As can be seen from Table 7 the rnp values fluctuated in the range 35.2-58.7 pg for Er in 10 mg ml-l sample solutions. The values of T]' and T~' changed considerably for different sample solutions the variations are 2.0-2.8 s for 51' and 2.0-2.7 s for 72'; mP values for sample solutions and for Er aqueous standard solution also showed differences. When integrated absorbance values were used satisfac- tory results were obtained with WTaPGT atomization. The m values were 12.3-14.4 pg and that for nine sample solutions and nine water standard solutions was 13.3 f 0.69 pg with an RSD of 5.2%.The maximum difference for m with WTaPGT atomization was only 17% which is close to the difference of the m value obtained by Slavin and Carnrick3I using different instruments for Cd Al As Pb Se and T1. Although the heating rate of the walls of the PGTs and WTaPGTs are only 1200 K s-l and no background correction and fast electronic detection were used in our instruments the m values were very stable at different ashing temperatures (500- I200 K) and atomization tem- peratures (2600-3 100 K) for different matrices including mineral acids chloride and nitrate salts rare earth element nitrate salts and for different types of geochemical and water sediment samples at a concentration of 10 mg ml-l.However we do not believe that the standard material and water sediment solutions should be discarded. The frequent check of m may uncover otherwise undetected errors including those from memory effects ashing losses back- ground and spectral interferences imperfections in the PGT or WTaPGT and inaccurate standards. Conclusion This work has shown that the m value for Er is very stable with WTaPGT atomization at temperatures in the range 2600-3100 K. The interferences caused by mineral acids including H3P04 chloride and nitrate salts of Na K Ca Mg Fe A1 and rare earth elements can be substantially reduced by using a high ashing temperature of 1570 K. Direct determination of Er in 10 mg ml-I geochemical and water sediment sample solutions has been achieved by WTaPGT atomization with peak absorbance measure- ments.This work re-confirms the relative freedom from interference of the modern graphite furnace technique for refractory elements such as Er. This work was supported by the National Natural Science Foundation of China. References 1 L'vov B. V. Spectrochim. Acta Part B 1969 24 53. 2 L'vov B. V. and Pelieva L. A. Can. J. Spectrosc. 1978 23 1. 3 Wahab H. S. and Chakrabarti C. L. Spectrochim. Acta Part B 1981 38 463. 4 Wu Z.-k. and Ma Y.-z. Zhonguo Huanjing Kexue 1981 4 65. 5 Ma Y.-z. and Wu Z.-k. Huaxue Tongbao 1982 2 22. 6 Wu Z.-k. and Ma Y.-z. Huanjing Huaxue 1982 I 228.42 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY FEBRUARY 1992 VOL. 7 7 Wu 2.-k.and Ma Y.-z. Fenxi Huaxue 1983 11 423. 8 Zhang Z.-m. Li S.-y. Ma Y.-z. and Wu Z.-k. Huanjing Kexue 1982 3 61. 9 Ma Y.-z. Wu Z.-k. Zhang Z.-m. and Li S.-y. Huaxue Tongbao 1983,5 17. 10 Ma Y.-z. Li S.-y. Zhang Z.-m. Wu Z.-k. Fen X. Su W. Sun D.-j. Huaxue Tongbao 1984 8 18. 1 1 Feng X. and Ma Y.-z. Huaxue Xuebao 1984,42 137. 12 Ma Y.-z. Li S.-y. Zhang Z.-m. Wu Z.-k. Fen X. Su W. and Sun D.-j. Spectrosc. Spectral Anal. (Beijing) 1984 4 3 1. 13 Ma Y.-z. Su W. and Sun D.-j. Fenxi Huaxue 1985,13,379. 14 Ma Y.-z. Su W. and Sun D.-j. Environ. Sci. China 1985,5 57. 15 Ma Y.-z. Su W. and Sun D.-j. Lihua Jianyang Huaxue Fence 1985 21 341. 16 Ma Y.-z. Sun D.-j. and Zhu M.-x. Fengxi Ceshi Tongbao 1986 5 55. 17 Jing S . 4 Li S.-y. Wang R.-r. Ma Y.-z. and Zhang 2.-m. Talanta 1987 34 699. 18 Ma Y.-z. He H.-k. and Yang X.-t. GraphiteFurnaceAtomic Absorption Spectrometry (Chinese) Chinese Nuclear Publish- ing House Beijing 1989 pp. 59-74 and 344-369. 19 L'vov B. V. Atomic Absorption Spectrochemical Analysis Elsevier New York 1970 pp. 1 15-1 22. 20 L'vov B. V. Pure Appl. Chem. 1970 23 11. 21 L'vov B. V. Nikelaev V. G. and Norman E. A. Zh. Anal. Khim. 1988 43 46. 22 Welz B. Curtius A. J. Schlemmer G. Ortner H. M. and Birzer W. Spectrochim. Acta. Part B 1986 41 1 1 75. 23 L'vov B. V. Nikolaev V. G. Norman E. A. Polzik L. K. and Mojica M. Spectrochim. Acta Part B 1986 41 1043. 24 L'vov B. V. Nikolaev V. G. Norman E. A. Kocharova N. V. and Romanova N. P. Zh. Anal. Khim. 1989,44 802. 25 L'vov B. V. Nikolaev V. G. Zh. Prikl. Spectrosk. 1987,46 7. 26 L'vov B. V. Opt. Spectrosk. 1970 28 1. 27 Posener D. W. Aust. J. Phys. 1959 12 184. 28 Martin W. C. Zalubas R. and Hagan L. Atomic Energy Levels-The Rare-Earth Elements National Bureau of Stan- dards Washington DC 1978 p. 316. 29 Grobenski Z. Anal. Chem. 1978 289 337. 30 L'vov B. V. Nikolaev V. G. Norichikhin A. V. Polzik L. K. Spectrochim.' Acta Part B 1988 43 1 14 1. 31 Slavin W. and Carnrick G. R. Spectrochim. Acta Part B 1984 39 27 1. Paper 0/05449F Received December 4th 1990 Accepted July 22nd 1991

ISSN:0267-9477    

DOI:10.1039/JA9920700035

出版商:RSC    

年代:1992

数据来源: RSC