摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL 199 I VOL. 6 205 Application of a High Resolution Inductively Coupled Plasma Mass Spectrometer to the Measurement of Long-lived Radionuclides Chang-Kyu Kim,* Riki Seki and Shigemitsu Moritat Department of Chemistry University of Tsukuba Tsukuba 305 Japan Shin-ichi Yamasaki and Akito Tsumura Division of Water Quality Science National Institute of Agro- Environmental Sciences Tsukuba lbaraki 305 Japan Yuichi Takaku Marubun 8- 1 Odenma-cho Nihonbashi Chuo-Ku Tokyo 1 70 Japan Yasu hito IgarashiS Division of Radioecology National Institute of Radiological Sciences 3609 Isozaki Nakaminato lbaraki 3 7 7 - 72 Japan Masayoshi Yamamoto Low Level Radioactivity Laboratory Kanazawa University Tatsunokuchi lshikawa 923- 12 Japan Some long-lived radionuclides such as '"c 226Ra 232Th 237Np 238U 239Pu and 240Pu were measured using high resolution inductively coupled plasma mass spectrbmetry (HR-ICP-MS).By using HR-ICP-MS with an ultrasonic nebulizer the detection limits of these nuclides were 0.0024.02 pg ml-l and the sensitivities were 10 times better than those obtained using HR-ICP-MS without the ultrasonic nebulizer. More accurate isotopic data were also obtained using HR-ICP-MS than with quadrupole ICP-MS at lower concentrations of the analyte because of the improvement in counting statistics that can be obtained with HR-ICP-MS due to the greater efficiency of ion trans- mission. A comparison of the measurement of the 240Pu to 239Pu ratio is shown. Keywords High resolution inductively coupled plasma mass spectrometry; long-lived radionuclide detection limit It is important in the 'post-Chemobyl era' to measure long- lived radionuclides in our environment correctly and precisely.Information on the behaviour of these nuclides in the environ- ment is indispensable in estimating potential radiation effects on humans. Conventional radiometric methods are however time consuming and tedious owing to the low concentrations and low specific activity of the nuclides. For instance when measuring 237Np at the global fallout level in the soil by ct- spectrometry it takes a week or more to obtain several counts of ten resulting in extremely low sample throughput. This is typical of analyses of long-lived radionuclides in the general environment therefore more sensitive and accurate methodo- logy is needed.Inductively coupled plasma mass spectrometry with a quad- rupole mass analyser (ICP-MS) has been applied to the deter- mination of many long-lived radionuclides. These studies although successful possess some weaknesses. For example there are a number of background molecular species which exist in the mass range below 80 and thus analyte peaks in this area may overlap because the resolution of the convention- al ICP-MS device is limited to unit mass only. The detection limits are not sufficient in some instances. In order to measure long-lived radionuclides in environmental samples directly without chemical separation new analytical techniques involv- ing higher sensitivity and lower detection limits than can be achieved with ICP-MS are still required.Recently high resolution ICP-MS (HR-ICP-MS) using a double focusing mass analyser to enable mass measurement * Present address Korea Institute of Nuclear Safety P.O. Box 16. t Present address Power Reactor and Nuclear Fuel Development Cor- Daeduk-danji Daejon 305-353 Korea. poration Tokai lbaraki 3 19- 1 I Japan. To whom correspondence should be addressed. at high resolution whilst also achieving high sensitivity was developed for the determination of ultra-trace element^.^.' In the present paper an HR-ICP-MS instrument was used for the Table 1 Operating conditions for HR-ICP-MS ICP cotidtiom- R.f. powerfkW Coolant gas flow-rate/] min-' Auxiliary gas flow-rate/l min-l Solution upt ake-rate/ml m in-' Carrier gas flow-ratel1 min-' Without ultrasonic nebulizer With ultrasonic nebulizer Ultrzrsonic nebulizer- conditions- Solution uptake-rate/ml min-' Heater temperature/'C Condenser temperature/'C Load coil to aperturelmm Aperture di ame t er/mm High resolution magnet supply/A Accelerating voltagefkV Interface ~mditiom- Double focusing mass analyser.conditions- 1.2 0.5 1 . 1 0.5 2.0 2 120 1 7 I 2.4 4.0 13 Element Data acquisition MCA* Dwell time/ Channel for Number of mass range/u channels ms one peak sweeps search Tc 98.8-99.3 50 640 24 5 Ra 225.4-226.6 50 640 24 5 Th 231.4-232.6 50 640 24 3 U 237.4-238.8 50 640 24 3 Pu 238.4-240.6 SO 640 24 3 Np 236.4-237.6 50 640 24 5 * MCA multi-channel analyser.206 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL 1991 VOL. 6 Collector )Magnet( slit I Collector Fig.1 A schematic diagram of the HR-ICP-MS arrangement measurement of long-lived radionuclides as a preliminary ex- periment on the direct measurement of radionuclides in envi- ronmental samples. Experimental Instrumentation The HR-ICP-MS assembly (PlasmaTrace VG Elemental Winsford Cheshire UK) consisted of an ICP a sampling in- terface an electrostatic analyser (ESA) a magnetic sector and detectors. A schematic diagram is shown in Fig. 1. Ions pass through a narrow adjustable slit and are injected into the field of the ESA which is an energy focusing device. The ESA in combination with a magnetic sector focuses ions with different energies onto the same point on the collector. The spectrometer has the capability of adjustable resolution by changing the width of the source slit and the collector slit.An ultrasonic nebulizer (USN) (Applied Research Labora- tories Ecublens Switzerland) was used in order to increase the amount of sample reaching the plasma. This nebulizer was of a continuous feed type. The sample solution was delivered to the nebulizer by means of a Gilson Minipuls peristaltic pump (Gilson Villiers-Le-Bel France). Aerosols generated at the transducer surface were transported by Ar gas to the heating tube and the condenser for desolvation and then to the ICP. The operating conditions of the HR-ICP-MS instrument are given in Table 1. The measurements were performed not in the high-resolution mode but in the relatively low resolution mode (MIAM = 400; the slits were opened) in order to obtain a greater ion transmission.Spectra were obtained by scanning the ESA. Reagents The nitric and hydrochloric acids used for preparing the solu- tions were of super analytical reagent grade (Tama Pure AA- 100 Tama Chemical Kawasaki Japan). Standard solutions of T c (5.5 pg ml-I; 3.5 mBq ml-I) 226Ra (1.36 pg ml-I; 50.3 mBq ml-I) 232Th (1 .O pg ml-I; 4.0 nBq ml-I) 237Np (2.0 pg ml-I; 0.052 mBq ml-I) 238U (1.0 pg ml-I; 12.4 nBq ml-I) and 239Pu and 240Pu (total concentration 20.8 pg ml-I; 72.5 mBq ml-I) were prepared by successive dilution of the stock solution with 1 mol dm-3 HNO,. De-ionized water (Milli-Q Japan Milipore Kita-Shinagawa Tokyo Japan) with a resistivity >14 MR cm was used to prepare the solutions. Results and Discussion Mass Spectra Figs. 2 4 show the comparative mass spectra for 'Vc 226Ra 232Th ?37Np ?W 239Pu and 240Pu with both concentric nebuliz- ers and USNs.The spectrum for each nuclide was clear but the peaks did not have a typical flat-topped shape. With a sector type mass spectrometer working in the low-resolution mode a flat topped mass peak is usually obtained when a con- stant ion beam is achieved. It is difficult however to obtain a constant ion beam using ICP-MS because of the intrinsic short-term fluctuations of the ICP.* At a low counting rate the effects of these fluctuations may be greater resulting in the degradation of the peak shape. At higher counting rates a flat topped mass peak is obtained. Calibration however can usually be carried out not only using peak height but also with peak area so that the peak flatness is of no concern.C 5 2 I 1 1 0 I I I - 98.9 99.0 93.1 99.2 225.8 226.0 226.2 226.4 m/z Fig. 2 Ultrasonic nebulizer was used ( c ) and ( d ) ultrasonic nebulizer was not used Mass spectra for (u) and (c) WTc ( 1.76 pg ml-'; 1.12 mBq ml-') and (h) and ( d ) "'Ra (1.36 pg m1-I; 50.3 mBq m1-I) by HR-ICP-MS. (a) and ( h )JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL 199 I VOL. 6 207 100 60 c 30 e 3 .- E O 0 ~ v) 231.8 232.0 232.2 232.4 236.8 237.0 237.2 237.4 Q c -5 100 OJ 10 iij - (D 50 5 0 0 I I 237.8 238.0 238.2 238.4 236.8 237.0 237.2 237.4 mlz Fig. 3 HR-ICP-MS. (a) (h) and (c) ultrasonic nebulizer was used (6) ultrasonic nebulizer was not used Mass spectra for (a) *j2Th ( I .O pg ml; 4.0 nBq ml-') (c) 23xU ( 1 .O pg rn1-I; 26 pBq ml-I) and ( h ) and ( d ) '"Np (2.0 pg ml-I; 0.052 mBq ml-') by .- m Q c .E 40 - m 0 2 iij 20 0 238.8 239.0 239.2 239.4 239.8 240.0 240.2 240.4 Fig.4 Mass spectra for 23'Pu and 240Pu (20.8 pg ml-'; 72.5 mBq mi-') by HR-ICP-MS. (a) Ultrasonic nebulizer was used ( h ) ultrasonic nebulizer was not used Detection Limits and Sensitivities Fig. 5 shows the background readings and sensitivities for some nuclides using ICP-MS and HR-ICP-MS indicating that lower background and greater sensitivity is achieved using HR-ICP-MS compared with conventional ICP-MS. In ICP-MS the relatively high background is mainly due to photons from the plasma reaching the photon sensitive detector by way of multiple reflections inside the instrument. A weak discharge in the lens system at the elevated ion-lens voltage and the pres- ence of charged species may also contribute to the back- ground.' However it is possible that this weak discharge only occurs when the wrong operating conditions are used.") In HR- ICP-MS fewer photons and ions might be scattered inside the device and because of the longer more complicated ion flight path and the use of narrow slits and lenses fewer reach the de- tectors.Therefore the continuum background of the HR-ICP- MS instrument was generally lower than that of the ICP-MS instrument. In HR-ICP-MS increased transmission of the ions can also be expected due to the acceleration of the ions by a high ion acceleration potential of 4 kV. Therefore higher sen- sitivity could be achieved by using HR-ICP-MS even with a conventional nebulizer than by using ICP-MS.The efficiency of the sample introduction into an ICP can be improved by substituting a USN which gives a rich spray of small droplets (4 pm). Hence the amount of sample reaching the plasma from the nebulizer was increased from 1% (usual with concentric type nebulizers) to about 10% with a USN."." As shown in Fig. 5 and Table 2 the sensitivity using HR- ICP-MS with a USN was increased about 10 times. In Table 2 the data are mean values of triplicate measurements for one sample. Detection limits of the order of 2-8 fg ml-' were achieved for the nuclides tested with the exception of and YJ. The relatively high detection limits for 'j2Th and 23xU were attributable to the 'memory effects' in the spectrometer or the 'high blank value' of the nuclides present in the reagents208 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL 199 1 VOL.6 Concentration HR-ICP-MS HR-ICF-MS of standard - without USN with CSN Nuclide Half-life/years solution/pg ml-' Counts Detect ion limit */ counts Detect ion 1 imi t/ pg ml-' pg ml-' "Tc 2 . 1 4 ~ 10' 5.5 'I 95 0.23 456 0.008 11' 0.14 yBq' 8' 5 pBq' ??6Ra 1.6oX1O3 1.36 I17 0.03 2683 0.006 12' 1 mBq' 27' 0.2 mBq' 23r-l-h 1.4 Ix 10"' 1 .o - - 2435 0.02 379' 0.08 nBq' 2 . 1 4 ~ 1 0 ~ 1.98 94 0.04 1814 0.002 7' 1 yBq' 6' 0.05 pBq' '3"P ?MU 4 . 4 7 ~ 10' 1 .o - - 1919 0.02 302' 0.2 nBq' 23Ypu 2.4 1 x I 04 16.76 815 0.04 1246 1 0.005 14' 10 pBq' 6' 0.1 mBq' ?40pu 6 . 5 7 ~ 1 O3 4.04 :!05 0.05 3027 0.004 3' 0.03 mBq' 13' 3 pBq' * Detection limits calculated by using the equation in references 3 and 16.t Counts of the blank solution ( 1 mol dm-j HNO,). Table 3 Comparison measurement of the 240Pu to 23'Pu ratio by ICP-MS and HR-ICP-MS using a concentration of 20.8 pg ml-' for 23'Pu and 24"Pu which equals 72.5 mBq ml-' in activity Parameter ICP-MS HR-ICP-MS HR-ICP-MS without USN with USN Operating conditions- Mass range 238-245 238.8-240.4 MCA channels 5 12 50 Channel for peak search - 24 Dwell time 80 ms 640 ms Sweeps 3200 5 2u)Pu to z7yPu ratio* 0.256 0.252 0.243 Accuracy 6.2% 4.6% 0.8% Precision t 25.4% 7.8% 2.0% * The value certified by the Oak Ridge National Laboratory was 0.241. t Relative standard deviation of triplicate measurements for ICP-MS and relative Poissoin standard deviation for HR-ICP-MS. and water used.These detection limits however show an im- portant potential for HR-ICP-MS in the analytical study of en- v ironmen tal radioactiv ities. In the current environment the radioactivity concentrations of the nuclides discussed in this paper are of the order of 1 mBq g-I or less. For a given activity of 1 mBq using an activ- ity counting technique it takes 1 x los s (about I d) to obtain 100 counts assuming a unit efficiency. A 100% efficiency in the actual measurement cannot be expected. For instance the efficiency of a-spectrometry the 2n gas flow counting method and liquid scintillation counting are at most about 20 40 and 90% respectively. Gamma-spectrometry exhibits only several per cent. of the measurement efficiency. Because the detection limits are functions of the background and the sensi- tivity of the method and low background measurements are achieved with radiometric measurement (especially in a- spectrometry using a Si detector) detection limits as low as less than one to several mBq (total activity) are usually ob- tainable.By using the measurement conditions of HR-ICP-MS de- scribed in this paper low detection limits were obtained and total solution consumption was less than 10 ml; i.e. the amount of 226Ra needed for detection was only 2 mBq (6 pCi) showing two advantageous features of ICP-MS over conven- tional activity measurements. Radium-226 has the shortest half-life among the nuclides tested. Isotopic Information Although ICP-MS gives isotopic information for the analyte at a low concentration (pg ml-I level) the accuracy and precision are not as good because of the poor counting statistics.In HR- ICP-MS which has a higher sensitivity better isotopic data can be achieved. Plutonium has accumulated in the environment from several sources and the isotopic composition of Pu in environmental samples is characteristic of its origin. Table 3 shows a com- parison of the results for the measurement of the 240Pu to 23yPu ratio by ICP-MS HR-ICP-MS without a USN and HR-ICP- MS with a USN. The deviation of the 240Pu to 239Pu ratio from the certified value provided by the Oak Ridge National Labora- tory was 0.8% by using HR-ICP-MS with a USN under the op- erating conditions described. It should be stated however that the method of counting is not the only factor affecting the accuracy and the precision of the isotopic information.Previous measurementsI3 of Pb isotope ratios at a higher concentration (50 ng ml-I) than the present Pu using HR-ICP-MS gave a minimum relative stan- dard deviation (RSD) of 2% while with ICP-MS a 0.2% RSD was obtained. The measurement time per unit mass was 30 s in both instances. This is explained by the fact that the scanning speed in HR-ICP-MS is not as fast as that in ICP-MS so that fluctuations in the ICP' may have a greater influence on the result. In determining isotope ratios all the factors affecting the measurement should be taken into consideration. In conclusion these preliminary results suggest the feasibi-JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL 199 1. VOL. 6 209 a) c - 237Np 239Pu ICP-MS HR-ICP-MS without ultrasonic nebulizer HR-ICP-MS with ultrasonic nebulizer Fig.5 Comparison of ( a ) background and ( h ) sensitivity in ICP-MS and HR-ICP-MS. wTc 1.8 pg ml-I; '37Np 2.0 pg m1-I; and 239Pu 3.5 pg ml-' lity of using HR-ICP-MS for the direct measurement of long- lived radionuclides in environmental samples. Further applica- tions in the field are expected. The authors thank Drs. R. C. Hutton and N. Bradshaw VG Elemental and T. Shimamura Marubun for their helpful dis- cussion; N. Bradshaw for providing Fig. 1; and M. Yoshida for drawing the figures. Part of this work was covered by a Grant in aid for Scientific Research from the Ministry of Edu- cation Science and Culture Japan under contract No. 63303015. Appendix Because the analytes referred to in this paper are all radio- active precautions are necessary in handling the solutions.However the amount of activity used was at the mBq level i.e. environmental level and even if the total amount was taken into the body almost no radiation hazard would be anticipated. The annual limits of intake (ALI) through inges- tion recommended by the International Commission on Ra- diological Protection (ICRP) International Atomic Energy Agency for w o r k e r ~ ~ ~ . ~ ~ are 1 x lox 7 x 104 3 x 104 3 x lo3 5 x los and 2 x 10' Bq for T c 226Ra 232Th 237N ' PY 23xU 239Pu and 240Pu respectively. Evidently the amounts used in the method described in this paper were far less than the ALI (1Os-1O'* times less) suggesting that any dis- charge was permissible.However laws regulations guide- lines etc. set by governments or recommendations by ICRP etc. should be strictly observed in handling radioactive materials. 1 2 3 4 5 6 7 8 9 10 1 1 12 13 14 15 16 References Kim C. K. Otsuji M. Takaku Y. Kawamura H. Shiraishi K. Iga- rashi Y . Igarashi S. and Ikeda N. Radioisotopes 1989,38. 15 1. Kim C. K.. Takaku Y. Yamamoto M. Kawamura H. Shiraishi K. Igarashi Y. Igarashi S. Takayama H. and Ikeda N. J . Radioonol. Nucl. Chem. 1989 132 13 1. Kim C. K. Oura Y. Takaku Y. Nitta H. Igarashi Y. and Ikeda N. J. Radioanal. Nucl. Chem. 1989 136 353. Igarashi Y. Kim C. K. Takaku Y. Shiraishi K. Yamamoto K.. and Ikeda N. Anal. Sci. 1990,6. 157. Morita S. Kim C. K.. Takaku Y. Seki R.. and Ikeda N. Int. J. Appl. Radial. Res. in the press. Bradshaw N. Hall E. F. H. and Sanderson N. E. J. Anal. At. Spec- from. 1989.4 80 1. Morita M. Ito H. Uehiro T. and Otsuka K. Anal. Sci. 1989,5609. Winge R. K. Eckels D. E. Dekalb E. L. and Fassel V. A. J. Anal. At. Speiwom. 1988,3. 849. Kawaguchi H. Tanaka T. and Mizuike A. Spe,Pc.trochim. Acta. Part B 1988,43,955. Hutton R. C.. personal communication. Fassel V. A. and Bear B. R. Spectim*hirn. Acta Part B. 1986 41 1089. Olson K. W.. Haas W. J. and Fassel V. A. Anal. Chem.. 1977. 49 632. Takaku U. unpublished results. International Commission on Radiological Protection (ICRP) Publica- tion 30 Part 1 (Annals of the ICRP Vol. 2 No. 3-4). Pergamon Press Oxford 1978. International Commission on Radiological Protection (ICRP) Publica- tion 30 Part 2 (Annals of the ICRP Vol. 4 No. 3/4) Pergamon Press Oxford 1980. Curie L. A. And. Chem.. 1968.40. 586. Paper 01039731 Recei\,ed September 3rd 1990 Accepted Decemher- 1 I th I990

ISSN:0267-9477    

DOI:10.1039/JA9910600205

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL 1991 VOL. 6 21 1 Direct Solid Sampling in Capacitively Coupled Microwave Plasma Atomic Emission Spectrometry Abdalla H. Ali Kin C. Ng* and James D. Winefordnert Department of Chemistry University of Florida Gainesville FL 3261 1 USA A capacitively coupled microwave plasma operating in the range between 500 and 700 W is used as an excitation source for the analysis of solid samples. National Institute of Standards and Technology (NIST) Standard Refe- rence Materials (SRMs) Tomato Leaves (SRM 1573a) and Coal Fly Ash (SRM 1633a) are used for the evaluation of the technique. The system contains a graphite electrod-up in which the solid sample is deposited. Heating of the electrode-cup vaporizes the analyte into the plasma for atomic emission spectrometry.The detection limits (defined as 30 of the background) for Mn Ca Mg Zn Cu As Rb and Pb in Coal Fly Ash and Cd Fe Cu Zn Zn Sr Rb Mg and Pb in Tomato Leaves were determined. The plasma gas used in this study was 20% nitrogen and 80% helium. Keywords Capacitively coupled micro wave plasma; atomic emission spectrometry; analysis of solids; Tomato Leaves; Coal Fly Ash In atomic spectrometry direct analysis of solids is important for several reasons. Dissolution of solids requires the use of hazardous chemicals which can be very time consuming and common methods of liquid sample introduction particularly by pneumatic nebulization are known to be inefficient (~10% sample throughput). Furthermore contamination and losses may occur in the process of dissolution.Deterioration of sensi- tivity and detection power occurs owing to the dilution and the degradation of the plasma as the atomization and excitation source by the solvent that is introduced (with the analyte) into the plasma. The resultant solution after dissolution can contain a high salt content with a potential for clogging the nebulizer. These problems are not encountered in a direct solid sample introduction approach. Also it is beneficial to have a technique that rapidly quantifies the elemental content of a sample before it is subjected to a time consuming dissolution procedure sometimes requiring the use of expensive and/or hazardous chemicals for more precise analysis. Currently arc and spark emission techniques work well for this preliminary quantification. However in the spark technique the sample must be electrically conducting or if it is non-conducting the sample must be mixed with a conducting powder.Several methods have been developed for solid sample in- troduction into inductively coupled plasmas (ICPs) which have been reviewed recently.'-* They include laser ablation electrothermal vaporization arcs sparks direct solid sample insertion powder injection and sluny nebulization. Microwave-induced plasmas which are usually operated at low powers ( ~ 2 0 0 W) have been mainly successful for gaseous samples particularly gas chromatographic eluate^.^.^ Capacitively coupled microwave plasmas (CMPs) which can provide high power levels have the capability of efficiently vaporizing liquid aerosols and atomizing and exciting analyte atoms.Hence most previous research has been focused on the analysis of liquid ~ampIes.~-~ For both types of microwave plasma very few studies have involved solid sample introduc- tion methods.x-'3 In most instances electrothermal vaporiza- tion devices were used to facilitate the introduction of solid samples into the but no work has been reported on direct solid sampling methods without analyte vapour trans- port for microwave plasmas. In this paper a new method for the rapid screening of solid samples is reported. This technique exploits the requirement for an electrode for plasma generation in the CMP and the sub- * On leave from the Department of Chemistry California State Univer- t To whom correspondence should be addressed.sity at Fresno Fresno CA 93740-0070 USA. sequent heating of the electrode this heating effect being used to advantage in the sampling procedure. A graphite electrode with a cup end was constructed into which solid powder was placed. Heating the electrode effected vaporization of the sample into the plasma enabling emission measurements to be made. The rapid heating of the electrode caused rapid vapori- zation of the sample thus producing a high transient concen- tration of the analyte. Coal Fly Ash [National Institute of Standards and Technology (NIST) Standard Reference Ma- terial (SRM) 1633al and Tomato Leaves (NIST SRM 1573a) were the materials chosen for evaluation of the technique. Experimental Instruments The experimental set-up is shown in Fig. 1 and the compo- nents are listed in Table 1.The electrode+xp system is shown in Fig. 2. The torch employed is similar to a conventional ICP torch except that the central tube is larger in diameter in order to accommodate the graphite electrode. The cavity for the gen- eration of the microwave plasma has been described else- where.6.10.1 I Sample Preparation For the determination of most elements the sample was used without further treatment. In Coal Fly Ash the concentrations of Mg Ca and Zn were so high that they were outside the linear response range of the procedure therefore dilution of array Electrodecup Diode array \ Fig. 1 atomic emission spectrometry Experimental set-up for capacitvely coupled microwave plasma212 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL 1991.VOL. 6 .o Graphite Table 1 Instrumentation for capacitively coupled microwave plasma atomic emission spectrometry 450 Instrumentlcomponents Manufacturer -Graphite electrode Diode array OSMA Model 1R4- 1024 Princeton Instruments Princeton NJ USA Spectrometer Princeton Instruments Software (I- 120) Princeton Instruments Jobin-Yvon HR1000 1 m 2400 grooves mm-I linear dispersion 0.5 nm mm-’ OSMA detector controller PC computer High voltage d.c. power supply Model 805- I A (maximum power output 5 kW) Princeton Instruments IBM Hipotronics Brewster NY. USA Magnetron Model 2M131 (frequency 2.45 GHz maximum power output 1.6 kW) Hitachi Des Plaines IL. USA Electrode-cup system Torch (Spex graphite rod; grade HPND) Three concentric quartz tubes Laboratory made Laboratory made Fig.2 mm Graphite electrode<up for the solid sample insertion. Units are in the sample was necessary in order to establish the detection limits. For this purpose the Coal Fly Ash was ground further using a procedure described previ~usly’~ and eventually passed through a 200-mesh nylon sieve. A 100 mg portion of this fine particle coal was diluted in spectroscopic grade graph- ite powder (Union Carbide New York NY USA) so as to obtain a 1% m/m Coal Fly Ash mixture. Even though grinding may increase the chances of contamination it is known to improve the homogeneity and hence the precision of the ana- lytical signals.I5 Procedure Tomato Leaves (5-10 mg) and the original or the 1 % Coal Fly Ash were deposited in the cup. The cup was placed on the top of the electrode and inserted into the central tube of the torch which held the electrode in position.There was a tight fit between the cup and the electrode in order to ensure good thermal and electrical contact. The mixed-gas (intermediate) plasma flow-rates (1 1 min-’ for N and 4 1 min-’ for He) were then adjusted. The outer He gas flow-rate was 6 1 min-I. There was no injection gas. The plasma was initially ignited at a low power (about 100 W) in order to ash the sample. When the ashing step was omitted the sample popped out of the cup causing flares in the plasma during the atomization step leading to plasma instability. Ashing in situ was first used in a direct sample insertion (DSI) ICP by AbduIlah et a[.’(‘ After about 15 s the power was raised to a pre-selected value of 400 W for Tomato Leaves and 700 W for Coal Fly Ash.Lower powers were used for liquid samples which were em- ployed solely for the identification of analyte emission lines. The observation height was 2mm above the cup and the emitted radiation was focused on the entrance slit of the spec- trometer by a system consisting of two matched quartz lenses (focal length = 4 in). The emission was monitored with a pho- todiode array. For better resolution second to fifth orders were used with a single scan (33 ms) spectrum in order to determine the detection limits. Results and Discussion Before the solid samples were introduced the graphite cup the electrode and the graphite powder were checked for impurities of the elements of interest by monitoring their emission lines.No measurable signals were observed for any of the elements. Moreover blanks were run before every measurement to ensure the absence of memories from previous runs and atmo- spheric contaminants. The limits of detection (LODs) based on 30 of some elements in Tomato Leaves and Coal Fly Ash are listed in Table 2. The background was measured at 0.1 nm off-peak. For the determination of Mg Zn and Ca in Coal Fly Ash a blank (consisting of graphite powder) was used. The low LODs obtained for the elements arose largely from an in- creased vaporization rate of analyte when the sample was diluted with graphite powder. Dilution of the sample reduced the matrix background and improved vaporization and atomi- zation efficiencies resulting in higher signal to background ratios.Similar increases in sensitivity were observed by Brenner et al.” who used an ICP for the determination of Cu and Zn in silicate materials when diluted in graphite. The addi- tion of graphite resulted in a higher rate of analyte evolution a stronger reducing environment and complete consumption of the solid sample.I8 A positive influence on the vaporization has also been achieved by adding organic halides to the plasma gas and/or chemical modifiers.19.”’ The precision of the meas- urement of the analytical signals was between 12 and 18% which considering the small amounts of samples being ana- lysed is acceptable. The presence of the majority of the elements were confirmed at multiple wavelengths and the lines were identified by using hollow cathode lamps (HCLs).Solution nebulization and aqueous solution deposition in the cup were also used when HCLs were not available or when the intensity was too low for definitive identification. When using an HCL nearby neon emission lines made identification of the elementsJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY. APRIL 1991 VOL. 6 213 Table 2 LODs (based on 30) of some elements in Coal Fly Ash (NIST SRM 1633a) and Tomato Leaves (NIST SRM 1573a). Element Vnm LOD/ng Tomato Lealyes- Cd cu Fe Mn Pb Rb Sr Zn Ca Mg Mn Rb Zn cu As Pb Mg Coal Fl-v Ash- 228.8 324.8 258.6 403.1 285.2 283.3 780.0 460.7 2 13.9 422.7 279.5 403. I 780.0 2 13.9 324.8 193.7 283.3 0.3 8 134 69 1 2 4 14 30 5 3 46 80 0.1 22 51 I33 of interest difficult and the method of deposition of liquid solu- tions required a cleaning step as there were analyte residuals present. Therefore solution nebulization was most frequently used for line identification. For the elements investigated a higher power was needed for Coal Fly Ash than for Tomato Leaves in order to observe sufficient signal to background ratios indicating that the former is more resistant to thermal decomposition than the latter.(The cup temperature increased with the power.) The duration of the emission signals depend- ed on the power the analyte and the matrix. Manganese which is present in similar concentrations in the two samples gave an emission signal in Coal Fly Ash which lasted twice as long as that in Tomato Leaves for the same amount of sample under the same operating conditions.This may have been a result of the chemical form in which the analyte exists in the sample. The system described here is simpler than the DSI-ICP system in terns of cost and operation. Unlike the DSI-ICP the electrode in the CMP is held in a fixed position by the central tube of the torch and can only be changed by moving the torch. Therefore it is not prone to poor reproducibility of posi- tioning of the cup. In DSI-ICP studies of analytical signals as a function of cup position showed that a variation of 1 mm produced signal intensity changes as large as 10%.'h.21 In DSI- ICP drying is generally carried out externally by use of aux- iliary heating devices.?' In the CMP described here both drying and ashing can be performed in situ. Unlike sample in- troduction via electrothermal vaporization (ETV) the pro- posed system performs both vaporization and excitation. Fur- thermore the dilution and loss of sample occurring during transport of the sample vapour inherent in ETV is non- existent in this CMP system. This research was supported by the National Institute of Health grant number 5-ROl-GM 38434-03.1 2 3 4 5 6 7 8 9 10 1 1 12 13 14 15 16 17 18 19 20 21 22 References Van Loon J. C. in Inductively Coupled Plasma Part 11 ed. Boumans P. W. J. M. Wiley New York 1987 ch. 2. Sneddon J. Sample Intrcxktion in Atomic Spectroscnpy Elsevier. Amsterdam 1990. Matousek J. P. Om P. J. and Selby M. Prog. Anal. At. Spectrosc. 1984.7 275. Risby T. H. and Talmi Y. CRC Crit. Re\,. Anal. Chem. 1983 14 231. Zhang Y. K. Hanamura S.and Winefordner J. D. Appl. Spec- trosc.. 1985 39 226. Patel B. M. Deaver J. P. and Winefordner J. D. Talanta 1988,35 641. Hwang J. D. Masamba W. Smith B. W.. and Winefordner J. D. Can. J. Spectrosc. 1988 33 156. Mitchell D. G. Aldous K. M. and Carelli. E. Anal. Chem. 1977 49 1235. Baur C. F. and Natusch D. F. S. Anal. Chem. 1981,53 2027. Hanamura S. Smith B. W.. and Winefordner J. D. Anal. Chem. 1983,55,2026. Hanamura S . Kirsch B. and Winefordner J. D. Anal. Chem.. 1985 57,9. Moharnad M. M. Uchida. T. and Minami S. Appl. Spectimc. 1 Q89,43.794. Gehlhausen J. M. and Camahan J. W.. paper presented at the 16th Annual Meeting of the Federation of Analytical Chemistry and Spec- troscopy Societies (FACSS) Chicago IL USA October I st-6th 1989 abstract No. 6 10. Ali A. H. Smith B. W. and Winefordner J. D. Talanta 1989 36 893. Stephen S. C. Ottaway J. M. and Littlejohn D. FreseniusZ. Anal. Chem.. 1987,328.346. Abdullah M. Fuwa K. and Haraguchi H.. Spectrochim. Acta. Part B 1984,39 1 129. Brenner I. B. Lorber A and Goldban Z. Spectr.ochim. Acta. Part B 1987.42.219. Esser P.. FreseniusZ. Anal. Chem.. 1985,322 677. Reisch M. Nickel H. and Mazurkiewicz M. Spectrochim. A(,ta Part B 1989,44,307. Kirkbright G. F. and Zhang L.-x. Analyst 1982 107 6 17. Shao Y. and Horlick G. Appl. Spectrosc. 1986. 40 386. Karanassios V. and Horlick G. Spec,trochim. Acfa Re\!. 1990 13 89. Paper- 0103905E Received August 29th 1990 Accepted December I I th I990

ISSN:0267-9477    

DOI:10.1039/JA9910600211

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL 199 I VOL. 6 215 Evaluation of a 13.56 MHz Capacitively Coupled Plasma as a Detector for Gas Chromatographic Determination of Organotin Compounds Degui Huang and Michael W. Blades* Department of Chemistry The University of British Columbia 2036 Main Mall Vancouver British Columbia V6T IY6 Canada A demountable parallel plate capacitively coupled plasma source operating at 13.56 MHz has been developed as a gas-chromatographic detector for the determination of organotin compounds. The effect of operating power and gas flow-rate on analyte emission intensity has been measured and spatial emission characteristics of the plasma have been evaluated. The detection limits for Me,Sn Me,SnCI and Pr,Sn were 0.079 0.190 and 0.168 ng s-l re- spectively.Keywords Capacitiwely coupled plasma; gas chromatography; organotin compounds In recent years metal and non-metal speciation studies have increasingly attracted the interest of analytical chemists because of the importance of trace elements in toxicology and environmental science. One means of obtaining species speci- fic information is through the use of chromatographic separa- tions. A variety of atomic emission sources have been developed as chromatographic detectors since these sources can provide element specific information about each eluting peak. Plasma sources in particular the inductively coupled plasma (ICP) direct current plasma (DCP) and microwave- induced plasma (MIP) have been extensively investigated for this purpose. The coupling of gas chromatography (GC) with an ICP was described by Windsor and Denton' for simultaneous multi- elemental analysis of organic and organometallic compounds. Microwave-induced plasmas have been investigated for many years.The first coupling of an MIP with GC was reported by McCormack et al. in 1965.* Since then many examples of the use of this methodology have been developed including the use of capacitively coupled microwave plasmas (CMP)3 and surface wave sustained plasmas (~urfatron).~ Various types of samples have been determined such as pesticide residues,' haloforms in drinking water6 and organometallic compounds.' Organotin compounds have been used widely as biocides cat- alysts and polymer stablizers and their effects on the environ- ment are causing concemR A knowledge of the concentration chemical form and distribution of these compounds provides important information on the origin and transport mechanisms.Several GC and liquid chromatography approaches incorporat- ing plasma-based detection have been developed for the deter- mination of organotin compounds. Krull and Panaro' used a system whereby the organotin compounds were separated using high-performance liquid chromatography (HPLC) followed by continuous on-line hydride generation with a DCP emission spectrometer being used to detect the effluent. Suyani et al."'de- scribed the use of helium microwave-induced plasma mass spectrometry for capillary gas chromatographic detection of or- ganotin compounds. Uchida et al." recently reported a capaci- tively coupled helium microwave plasma as an excitation source for the determination of organotin compounds. They used a CMP in which microwaves were generated using a magnetron and conducted through a coaxial waveguide to the CMP excita- tion source.A tubular tantalum electrode sample injector was employed for the CMP in order to achieve high sensitivity and a more stable discharge. The analytical performance of this plasma source for the determination of inorganic tin and butyltin was evaluated by interfacing the helium CMP to a gas chromato- graph. The analytical merit compared well with helium MIP systems although electrode contact with the plasma introduces the possibility of contamination. * To whom correspondence should be addressed. One of the problems of using an MIP as a GC detector is that materials deposit on the walls of the discharge tube.Noticeable deposits can be found for long-chain hydrocarbons and other oxygen-free compounds. This problem is even more acute with samples containing inorganic compounds that are prone to forming refractory species. Besner and Huberti2 in- vestigated the effect of dopants on tin emission in a helium MIP. The helium plasma was doped with various liquid and gaseous materials and sulphur hexafluoride was found to give the best results. Recently a novel parallel plate capacitively coupled plasma (CCP) which can be used as an emission spectrometric detector for gas chromatography has been described.I3 Using this CCP a helium or argon plasma can be generated at atmospheric pres- sure at frequencies of 0.20 or 27.18 MHz and at carrier gas flow- rates as low as 20 ml min-'.The present paper describes the further development of the CCP as a GC detector operated at 13.56 MHz outlines some of the spectral and operational char- acteristics and characterizes its application to the determination of some environmentally important organotin compounds. Experimental Power Sup p I y An Advanced Energy Model RFX 600 13.56 MHz (Fort Collins CO USA) radiofrequency (r.f.) generator equipped with an Advanced Energy Model ATX-600 automatic impe- dance matching system was used to supply power to the CCP torch. The ATX-600 tuner was modified to include a 4-5 pH inductor in series with an output line to improve matching effi- ciency. When operated at 200 W forward power the reflected power could be maintained at less than 3 W.Spectrometric System The monochromator photomultiplier tube current amplifier and chart recorder used in this study were as described in a previous publication.'.' Gas Chromatograph The gas chromatograph was the same as that used in reference 13 except that a Supelco Model SPB-I fused silica capillary column (15 m x 0.53 mm 0.d. with a film thickness of 0.50 pm) was used rather than a packed column because of its chemical inertness and high column efficiency. For organotin compounds if a solid column support is insufficiently covered by the stationary liquid phase (e.,?. 2-5%) sample adsorption on the exposed siliceous sites becomes significant with polar solutes and peak tailing occurs.'3 The injector block was maintained at 280 "C for all experiments.216 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL 1991 VOL. 6 Plasma Torch Fig.1 shows a schematic diagram of the CCP and holder used for this work. The torch was fabricated from a section of fused silica rectangular in cross-section with external dimensions of 6 x 4 mm and internal dimensions of 4 x 2 mm. The total length of the rectangular portion of the torch was 6cm. One end of the torch from which emission was observed was open to the atmosphere and the other end was connected to a T- shaped arrangement of quartz tubes (0.d. 4 mm i.d. 2 mm) for the introduction of sample and make-up gas (see Fig. 1). The effluent from the GC was introduced into the torch through a quartz capillary tube which was sealed to the torch at one end.An additional gas inlet allowed the use of a make-up gas. The torch was placed in a vice-like clamping device and 40mm long 1.8 mm thick wafers of boron nitride were placed between the torch and the stainless-steel electrodes which were 4cm in length. These were clamped in place using Delrin and an aluminium holder which was tightened using an adjustable clamp as depicted in Fig. 1. Using this torch mount the CCP discharge could be easily assembled and different sizes of quartz tubing and electrodes could be tested. The torch was operated both with and without the presence of the boron nitride insulator between the quartz and the elec- trodes. With the former the intensity of the H I line at 686.13 nm at 150 W was almost the same as that at 100 W with the latter.It would appear that 50% of the power is lost with the former structure. However without the boron nitride insulator the electrodes became fairly hot and the Delrin sof- tened which made it difficult to maintain the integrity of the torch and holder. For this reason the torch was operated with the boron nitride strips for all the experiments described in this paper. Helium was used for both the carrier and make-up gases. Transfer Interface For the transfer line between the chromatograph and the CCP the capillary column was enclosed in a copper tube (70 x 0.32 cm o.d.) around which was wound heating tape and envel- oped with glass wool and cotton tape. The temperature was controlled by using a Variac rheostat to adjust the voltage to the heating tape.The transfer line was maintained at a temper- ature of 280 "C for all experiments. Support rod Plasma torch I Make-up gas / Adjustable screw db inlet Boron nitride clamp- Top view Stainless steel 0 *' - Delrin End-on view Fig. 1 Schematic diagram of the discharge tube and support structure Data Acquisition Except when indicated otherwise the working conditions were as follows the r.f. power supply operated at a forward power of 150 W with a reflected power of 2 W in auto-matching mode. The make-up helium gas flow-rate was 200 ml min-I. The monochromator wavelength setting for Sn I at 284.0 nm was made by using a tin hollow cathode lamp. After the sample was injected the gas chromatographic column was maintained at the initial temperature. The solvent began to elute at a retention time of 0.61 min.As soon as the solvent was eluted the r.f. power was applied; the plasma self-ignited and the chromatograph was operated in isothermal or tempera- ture programme mode and data were collected on the chart recorder. Chemicals Ferrocene (98%) tetramethyltin (Me,Sn 99%) and trimethyl- tin chloride (Me,SnCI 99%) were purchased from Aldrich di- chloromethane (CH,C12 99.9%) from BDH and tetrapropyltin (purity unknown) was obtained from Alpha Inorganics. All the chemicals were used without further purification. Results and Discussion Helium Plasma Background A wavelength scan of the background emission from the 13.56 MHz atmospheric pressure helium CCP between 200 and 600 nm is shown in Fig. 2. The identification and assign- ment of the molecular bands were made using data obtained from reference 15.The most prominent features are those orig- inating from OH NH NO N and Nz+ similar to those ob- served in an MIP.Ih However one of the differences is that a Q,-branch with 308.9 nm (O,O) and 282.90 nm (1,O) for OH ( ? ~ + - ? I I ) was found to be more intense for the CCP compared with the R,-branch at 306.36 nm (0,O) and 28 1.13 nm (1,O) for the MIP.Ih This may be related to the difference in gas temper- ature and suggests that the gas temperature in the CCP is prob- ably lower than that for an MIP. However further experiments must be completed in order to verify this suggestion. The background features observed at different positions (vertically and axially to the discharge tube) did not change significantly.From this observation it can be concluded that the OH NH NO N and N,' molecular bands arise f!om impurities in the helium gas supply. Estes et al.' used a 5 A molecular sieve im- mersed in liquid nitrogen in order to investigate whether the molecular bands observed in an MIP were caused by back dif- fusion of air. Their results confirmed that no air entrainment occured and the bands resulted primarily from the presence of impurities. Spatial Emission Characteristics The spatial emission characteristics for He I at 447.15 nm He I at 504.77 nm and H I at 486.13 nm were measured for the helium CCP and are shown in Fig. 3. These measure- ments were made by forming an end-on image of the plasma at the entrance slit of the monochromator and translating the image across the entrance slit by moving the torch assembly.As can be seen from Fig. 3(a) the spatial distribution for all three species shows a maximum near the walls and a minimum at the centre. The spatial distributions of emission from Fe at 371.99 nm and Sn at 284.0 nm introduced as organic compounds were also measured. For these lines a relative maximum was observed at the centre as shown in Fig. 3(h). The Sn spatial distribution was obtained using the output from the gas chromatograph whereas the Fe signal was collected with continuous introduction of sublimed fer- rocene in the headspace of a sampling vial. A similar distri-JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL 1991 VOL. 6 L 217 200 250 300 350 400 450 500 550 Wavelengthlnm Fig.2 flow-rate 200 ml min-'; and camer gas flow-rate 0 ml min-' Background emission spectrum of the 13.56 MHz helium plasma from 200 to 600 nm. Input power; 100 W; reflected power 2 W; make-up gas c .r 0 1 2 .- $40 + - P) a 30 20 10 I I 1 0 1 2 Distance from left side wall/mm Fig. 3 (a) Spatial distribution of emission from A H 1 486.13 nm; B He I 504.77 nm; and C He I 447.15 nm without sample. Input power I50 W; make-up gas flow-rate 200 ml min-' and carrier gas flow-rate 0 ml min-'. (h) Spatial distribution of A Sn 284.0 nm; and B. Fe 371.99 nm for a helium CCP with organic sample introduced. Input power 150 W and make-up gas flow-rate 200 ml min-' bution has been observed in a surface wave plasma (surfa- tron),".'' in which cylindrical plasma tubes were used.Richard et d . l X explained that if one-step excitation through electron collision with an atom in the ground level is assumed the distribution of emission is dependent on the distribution of the total electric field intensity ET and the electron density [n(r)] as a function of radial position 1' through the relation (1) where n,(r) is the population density of the excited atoms in level j A is a constant independent of position and k is a value dependent on the plasma medium and excited-state parameter and can be determined from theory. The magnitude of both the electric field and the electron density are spatially dependent. The electric field intensity is higher at the walls whereas the electron density decreases near the walls as a result of recom- bination losses.Although the appropriate measurements have not been made it is possible that the spatial distributions observed for the CCP are similar in origin to those for the sur- fatron. n,(r) = A n(r) E,"(I') Effect of Input Power In order to study the effect of changes in r.f. input power several injections of 20 ng of Sn (0.1 p1 of a solution of 100 ppm of Me,SnCI in CH,CI solvent) were made into the gas chromatograph and the emission intensity for Sn was measured. The intensity was not corrected dynamically for background however it was found that the background was relatively constant for these experiments. The peak intensity for the Sn I line at 284.0 nm as a function of input power is shown in Fig. 4. As stated earlier about 50% of the power is lost with the electrodes isolated from the discharge tube but this structure prevented the electrodes from becoming too hot and produced a more stable plasma From Fig.4 it can be seen that the intensity of Sn increased almost linearly with an increase in r.f. power. It is probable that the total applied r.f. power is not all delivered to the plasma since there are losses in the output inductance the dielectric ma- terial and the electrodes themselves. Therefore the actual power consumed in the discharge is less than is indicated in Fig. 4. As a result of the heating of the electrodes it was difficult to operate at powers higher than about 400 W since the heat would soften the Delrin insulators. For these reasons a torch with water cooled copper electrodes has been developed and future work will be carried out using this new design.218 .$ 50 E 40- .- f 30- .- c 3 20 a 10 J0URNA.L OF ANALYTICAL ATOMIC SPECTROMETRY APRIL 1991 VOL.6 - - - 0 100 200 300 400 500 Power/W Fig. 4 Effect of input power on emission intensity for Sn at 284.0 nm. Make-up gas flow-rate 200 ml min-I and carrier gas flow-rate 10 ml min-' 30 CI d I /x 10 lk 2bo 3 k &I 5& sbo Make-up gas flow-rate/ml min-' Fig. 5 284.0 nm. Input power 100 W and carrier gas flow-rate 10 ml min-' Effect of make-up gas flow-rate on emission intensity for Sn at 70 I /x I 0 1 I 1 I I 5 10 15 20 25 Carrier g a s flow-rate/ml min-' Fig. 6 284.0 nm. Input power. 100 W and make-up gas flow-rate 200 ml min-' Effect of camer gas flow-rate on emission intensity for Sn at Effect of Gas Flow-rate The effect of the helium make-up gas flow-rate on the Sn peak intensity is shown in Fig.5. The Sn signal increased with the gas flow-rate from 50 to 350 ml min-' reaching a maximum between 350 and 400 ml min-1 after which a decrease was ob- served. At flow-rates greater than about 250 ml m i d turbu- lence could be observed in the gas flow through the torch resulting in instability of the emission signals. Therefore a make-up gas flow-rate of 200 ml min-' was used for further experiments. The response of the Sn emission intensity as a function of gas chromatograph carrier gas flow-rate over the range 8-20 ml min-l is depicted in Fig. 6. Although an in- crease in carrier gas flow-rate caused an increase in the Sn emission intensity the chromatographic resolution degrades at flow-rates higher than about 10 ml min-I.Therefore a 10 ml min-' carrier gas flow-rate was used for all further studies. CG-CCP System Performance The stock solutions (loo0 ppm of Sn for each compound) were prepared by dissolving the relevant organotin compounds Me,Sn I R.f. o n \ R.f. off Retention time/min Fig. 7 Chromatogram of mixture of Me,Sn Me,SnCI and Pr,Sn using temperature programming. Input power 150 W; make-up gas flow-rate 200 ml min-I; and camer gas flow-rate 10 ml min-' in dichloromethane and working solutions were prepared by appropriate dilution with the same solvent. Fig. 7 shows a typical chromatogram of a mixture of Me,Sn Me,SnCI and Pr,Sn. A 0.1 pl aliquot of a 10 ppm solution of each com- pound containing 1 ng of Sn was injected.The signal was collected at 284.0 nm without background correction. The r.f. voltage was switched on after the solvent was eluted. The chromatograph was used in the temperature programme mode i.e. it was maintained at 45 "C for 1 min then raised to 260 "C at the rate of 50 "C min-I and then held at this temperature for another 2 min. As can be seen in Fig. 7 the sensitivity of the peak signal for Me,Sn is 2.8 times better than that for Me,SnCl. Serious tailing is exhibed by Pr,Sn which is be- lieved to be due to condensation of Pr,Sn on the walls of the discharge tube. Since the heating tape could not be brought close to the electrodes because of possible discharge between the electrodes and the heating tape a 'cold gap' existed in this region.Therefore condensation of Pr,Sn which has a high boiling point (222 "C) is a strong possibility. Another reason for tailing may be the deposition of tin oxide on the walls. Doping with some reagent materials can minimize this problem. I? The detection limits for Me,Sn Me,SnCI and Pr,Sn were 0.079 0.190 and 0.168 ng s-' respectively. For these values the definition of minimum detectable level used by Sullivan19 has been used. This is the mass of analyte required to produce a peak that is twice the height of the peak-to-peak noise divided by the full width at half height of the peak in seconds. Chromatographers usually (but not always) measure the peak- to-peak base line variation which is considered to be 60 as a measure of the noise.z0 For this report the peak-to-peak noise measurement was averaged over a time of period of 30 s.Conclusions The parallel plate CCP torch is a potential alternative to the MIP or CMP as a detector for GC for the determination of organotin compounds. The CCP can be operated at an r.f. input power of up to 400 W and make-up gas flow-rates ranging from 50 to 400 ml min-I. The current system is an improvement over that previously described" in that a de- mountable torch structure has been used and an automatic impedance matcher has been coupled to a 600 W 13.56 MHz r.f. oscillator. Detection limits for organotin compounds for the CCP indicate that the detection limit is superior to those obtained using a CMP" but inferior to those obtained using an MIP."' However the CCP is still at an early stage of de- velopment and there is scope for improvement and further exploration into torch geometries operating frequencies power sources and sample introduction strategies.Using theJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL 1991 VOL. 6 219 present configuration there is some power dissipation in the r.f. electrodes which leads to power loss and heating of the electrodes. A torch incorporating water cooled copper elec- trodes is currently being tested in an effort to overcome this problem. The authors thank Professor M. Fryzuk for the use of the gas chromatograph. Acknowledgement is made to the donors of The Petroleum Research Fund administered by the American Chemical Society the Natural Sciences and Engineering Re- search Council of Canada and the University of British Colum- bia for partial support of this research.References Windsor D. L.. and Denton M. B. J. Chromatogr. Sci. 1978 32 366. McCormack A. J.. Tong S. C. and Cooke W. D. Anal. Chem. 1965,37 147. Hanamura S . Smith B. W.. and Winefordner J. D. Anal. Chem. 1983,55,2026. Hubert J. Moisan M. and Richard A. Spectror*him. Aria Part B 1979.34. 1. Bache C. A. and Lisk D. J. Anal. Chem. 1967,39,786. Quimby B. D. Delaney M. F. Uden P. C. and Barnes R. M. Anal. Chem. 1967,51,875. Estes S . A. Uden. P. C. and Barnes R. M. Anal. Chem. 1981,53 133. 8 9 10 1 1 12 13 14 15 16 17 18 19 20 Thompson J. A. J.. Sheffer M. G.. Pierce R. C. Chau Y. K. Cooney J. J. Cullen W. R. and Maguire R. J. Orgonotin Com- pounds in the Aquatic Enrironment Scientific Criteria for Assessing Effects on Environmental Quality NRCC Report No. 22494 National Research Council. Ottawa Canada 1985. Krull I. S. and Panaro K. W. AppI. Spectrosc.. 1985,39,960. Suyani. H. Creed J. and Caruso J. and Satzger. R. D. J. Anal. A t . Spectrom. 1989,4777. Uchida H. Johnson P. A. and Winefordner J. D. J . Anal. At. Spec- from.. 1990,5 8 1. Besner. A. and Hubert J. J. Anal. At. Spectrom. 1988,s. 381. Huang D. Liang D. C. and Blades M. W. J. Anal. At. Spectrom. 1989,4789. Crompton T. R. Comprehensive Organometallic Analysis Plenum New York 1981 p. 487. Pearse R. W. B. and Gaydon A. G. The Identification of Molecular Spectra Chapman and Hall London 4th edn. 1976. Zander A. T. and Hieftje. G. M. Anal. Chem. 1978,50. 1257. Proud J. M. and Lussen L. H. Radiative Processes in Discharge Plasmas Plenum New York 1986 p. 38 1. Richard A. Barbeau A. Besner J. Hubert J.. Moisan M. and Sauve G. Can. J. Phys. 1988.66 740. Sullivan J. J. in Modern Practice of Gas Chromatography ed. Grob R. L. Wiley New York 1977. Quimby B. D. and Sullivan. J. J. Anal. Chem. 1990,62 1027. Paper 0103 7350 Received August 14th 1990 Accepted December 8th 1990

ISSN:0267-9477    

DOI:10.1039/JA9910600215

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL 1991 VOL. 6 Fluorination and Volatilization of Refractory Elements from a Furnace for Sample Introduction into an Inductively Coupled Using a Polytetrafluoroethylene Slurry Min Huang,* Zucheng Jiangt and Yun'e Zeng Department of Chemistry Wuhan University Wuhan 430072 China 22 1 Graphite Plasma by A method was developed to determine refractory elements by inductively coupled plasma atomic emission spectrometry (ICP-AES) with sample introduction by electrothermal vaporization. A slurry of polytetrafluoroethylene was used to form fluorides rather than carbides of the elements. In this way the refractory elements were efficiently vaporized and subsequently introduced into the plasma. The detection limits for Zr V Cr W Mo B and Ti were improved by a factor of 7-1 19 compared with those of electrothermal vaporization ICP-AES without the fluorinating agent.No memory effects were observed and adequate precision was obtained. The fluorination process is also discussed. Keywords Electrothermal vaporization; refractory elements; polytetrafluoroethylene slurry; inductively coupled plasma atomic emission spectrometry; fluorinating agent The most popular sample introduction technique used in in- ductively coupled plasma atomic emission spectrometry (ICP- AES) is pneumatic nebulization. It is simple and convenient to operate. However the technique suffers from low efficiency of sample transport and presents some difficulties with viscous high salt content solutions and micro-volumes of samples.The use of electrothermal vaporization (ETV) as a sample introduction technique for ICP-AES has been reported.I-l0 The detection limits were improved by one or two orders of magni- tude for most of the elements tested compared with those of pneumatic nebulization. However a problem occurred when refractory elements which combine strongly with carbon to form refractory carbides were determined.6 Very low sensitivi- ty and severe memory effects were encountered. Tantalum filament vaporizers have been used as a means of avoiding carbide formation,' but the tantalum metal becomes brittle after repeated heating cycles. The addition of a halogenating agent to improve the vola- tilization process was used originally in arc spectroscopy.' In previous work with ETV-ICP-AES Kirkbright and Snook6 and Satumba et al.' added trifluoromethane or chlo- rine to the injector gas and improved the detection limits of refractory elements.Ng and Caruso8 added ammonium chlo- ride (7% m/v) as a means of vaporizing the analytes as their chlorides. In the present study a polytetrafluoroethylene (PTFE) slurry was used as a fluorinating agent to promote the vaporization of refractory elements from a graphite furnace for determination by ICP-AES. The analytical characteristics of the technique and the advantages of PTFE as a fluorinating agent were ex- plored. The fluorination process and the optimization of the conditions were also investigated. Experimental Instrumentation and Operating Conditions In this study a graphite furnace WF-4 which is similar to an HGA 500 was employed as the vaporization device.The original silica windows at the two ends of the furnace were removed and replaced with two PTFE cylinders one of which was connected to the injector tube of the plasma torch viu a plastic tube (4 mm i.d. x 0.5 m). The other was blocked to * Present address Centre of Material Research and Testing Wuhan t To whom correspondence should be addressed. University of Technology Wuhan 430070 China. prohibit air from entering the graphite furnace. The analyte was vaporized and carried to the excitation source by a stream of argon gas. The instruments used in this work are listed in Table 1 and the optimized operating conditions are given in Table 2. Preparation of Slurry Sample With ultrasonic wave vibration and in the presence of OP sur- factant [RC2H40(C2H,),,H 1 very fine PTFE powder was dis- persed into the propanol medium which can be mixed with water in any ratio.The slurry containing 60% m/v PTFE is also commercially available and remains stable for a long time. The slurry sample was prepared by adding PTFE slurry to a liquid sample. Unless otherwise stated the PTFE content of the slurry sample was 1.8% m/v. Table 1 Instrumentation Plasma power Plasma generator 2 kW (Beijing Broadcast Instru- ment Factory China); frequency 27.12 MHz; power output G 2 . 0 kW; load coil 2.5 turns Monochrometer WDG 500- 1 A Type (Beijing Second Optics China); Czerny-Turner mounting with 1200 lines mm-' grating blazed at 250.0 nm; focal length 0.5 m with reciprocal linear disper- sion 1.6 nm mm-' Plasma imaged in a 1 1 ratio onto the entrance slit.Slit-width 25 pm Conventional type silica torch. Injector tube 1.5 mm i.d. Signal from photomultiplier (R456 Hamamatsu Japan) was measured using potentiornetric recor- der LZ3- 104 (Shichuang Fourth Instruments Works Shichuang Province China) Model WF-4 electrothermal device (Beijing Second Optics China) which is similar to an HGA 500 Plastic tube 4 mm i.d. x 0.5 m Spectrometer Optics Plasma torch Read-out Graphite furnace Interface Table 2 Operating conditions Incident power Observation height Carrier argon gas Coolant argon gas Auxiliary argon gas Heating cycle of graphite furnace Sample introduction 1.0 kW 15 mm (above working coil) 0.8 1 min-' 13 1 min-I 0.8 1 min-' Drying time I0 s at 100 "C; ashing time 10 s at 250 "C; atomization time 6 s at 2500 "C 20 p1 micropipette with disposable poly- ethylene tip222 B C Ik w u JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL 1991 VOL. 6 t \ A lr- 10 s H B .$ f - Fig.1 Signal profiles for Ti 20 pI sample A 1.0 pg ml-' solution without PTFE; B residual signal of the first firing; and C 1.0 pg ml-I Ti solution with 1.8% m/v PTFE intensity I x 20 10 s w t \ f- Fig. 2 Signal profiles of V 20 pl sample A 1.0 pg ml-I V solution without PTFE; B residual signal of the first firing; and C 1.0 pg ml-' so- lution with l .8% m/v PTFE intensity I x 10 Table 3 (from reference 12) Boiling points of carbides and fluorides for various elements Boiling point/"C Element Ti V Cr Mo W B Zr C Element 3400 3000 2672 4804 5927 2500 4377 3652 (sublimation) Carbide Fluoride 4820 284 3900 1000 3800 1 100 35 6000 17.5 3500 - 99 5100 800 - Procedure A 20 pl volume of the test sample was pipetted into the graph- ite furnace.The sample inlet hole was blocked using a graphite rod. After being dried and ashed the analyte was vaporized and carried into the plasma under optimized conditions. The peak height was measured for calibration. Results and Discussion Principles of Fluorination-Vaporization In the absence of PTFE the signal intensities of the refractory elements in ETV-ICP-AES were very weak and broad signal profiles with long tails were recorded as shown in Figs. 1 and 2. Furthermore severe memory effects were observed. No signal was detected for 1 .O pg ml-I of zirconium. The residual signals for titanium and vanadium were almost the same as the original ones.I I A 3s I I H I f- Fig. 3 Recorder tracings for 20 pl samples A 0.1 pg ml-I Zr in 1.8% PTFE; B residual signal of the first firing after vaporizing 20 pl of 10 pg ml-I Zr in 1.8% PTFE; and C residual signal for the second firing However the addition of PTFE changed the situation as fluorine can combine very strongly with the refractory ele- ments. In this situation the fluorinating reaction is the main re- action taking place in the graphite tube at high temperatures and the fluorides produced can be vaporized in the graphite furnace without any problem because the boiling points of fluorides are much lower than those of carbides as shown in Table 3. A sharp and intense signal profile was recorded when fluorination-vaporization was applied. The signal profiles of Ti and V with and without the presence of PTFE in the samples are shown in Figs.1 and 2 respectively. Similar results were observed for Zr Cr Mo W and B. It was found that there were no memory effects on the performance after high concentrations of these elements were vaporized. As shown in Fig. 3 the residual signal intensity is less than 0.1% for 10 pg ml-i of Zr under the experimental conditions used. The slurry was prepared prior to injection of a 20 p1 aliquot into the furnace. Another experiment in which the sample and PTFE sluny were pipetted separately into the furnace was also carried out. However the signal intensity enhancement was not sufficient owing to separation of the PTFE and the refrac- tory elements which reduced the probability of forming the fluoride.Furthermore this method is not convenient. Ashing Curves for Fluorination-Vaporization The ashing curves of the elements are shown in Figs. 4 and 5 from which the following conclusions can be drawn (i) no losses were found for Zr Ti and V in the ashing step until the furnace temperature was much higher than the de- composition temperature of PTFE (415 "C) and near the va- porization temperatures of their fluorides; (ii) losses of Ti Mo W and B began to occur at a temperature near the de- composition point of PTFE; (iii) the ashing temperature (Ta\,,) for the element whose fluoride has a vaporization tem- perature higher than 415 "C is controlled by the vaporization temperature of the fluoride and for the element whose fluoride has a vaporization temperature lower than 415 "C the ashing temperature is controlled by the decomposition temperature of the PTFE i.e.Ta,h >415 "C should be chosen. One of the most important advantages of PTFE as a fluorinating agent is its high decomposition temperature. For example the vaporization temperature of BF is as low as -99.7 "C while the ashing temperature used can be as high as 415 "C. As a result many matrices particularly organic matrices can be removed in the ashing step. Optimization of PTFE Content of the Slurry Sample The PTFE content was adjusted to 0.3 0.6 1.2 2.4 and 6% m/v in a series of samples. Fig. 6 shows that the signal in- tensity increases with increasing content of PTFE and reaches a plateau at 1.8%.However with 1.8% PTFE and using the heating cycle given in Table 2 the plasma dis- charge became unstable due to the vigorous decompositionJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL 1991 VOL. 6 223 L I 1 1 I 400 800 1200 Trh PC Fig. 4 pg ml-I; and V. 1.0 pg ml-' Ashing curves for 20 pl samples of Cr 0.5 pg rn1-I; Zr 0.5 ' 150 - m C 4- .- t $ 9 0 g Q - z 30 1 I 200 600 1200 Trh PC Fig. 5 and Mo 0.5 pg ml-' Ashing curves for 20 pl samples of Ti B and W 1.0 pg ml-I 0.6 1.8 3.0 PTFE concentration (% m/v) Fig. 6 sample of a 0.5 pg ml-' solution of Zr Effect of PTFE concentration on fluorination-vaporization; 20 pl Table 4 Detection limits Detection limit/ng This work Wavelength/ Without With Element nm PTFE PTFE Reference 6* Reference 8t Ti 334.9 2.0 0.018 - - V 309.3 2.0 0.05 - 0.3 Cr 267.7 0.6 0.05 0.05 0.2 Mo 202.0 0.5 0.008 0.1 - W 207.9 1.2 0.16 0.16 - B 249.7 1.7 0.05 0.0s - Zr 257. I 0.4 0.004 0.01 0.02 * Cr 357.9 nm; Mo 3 13.3 nm; and W 276.4 nm.t V 437.9 nm; and Cr 357.9 nm. 5 s I . t - Fig. 7 Typical signal reproducibility with 20 pl samples containing 1.8% m/v PTFE and 0.5 pg ml-I Cr of the PTFE. In order to solve this problem a higher ashing temperature but one which was still below the decomposi- tion temperature of the fluorides was used and made the de- composition of PTFE less vigorous. The elements combine with fluorine as soon as they are released from the PTFE in the ashing step and are subsequently vaporized when the furnace temperature is raised in the analytical cycle.In this way a PTFE concentration as high as 30% m/v in the sample has no effect on plasma stability. For the vaporiza- tion of samples containing a low content of the elements 1.8% of PTFE was used. Detection Limits and Precision The limit of detection is defined as the analyte concentration yielding a signal equal to three times the standard deviation of the background noise. Results obtained for ETV-ICP-AES with PTFE are similar to the work reported in references 6 and 8 in which other types of halogenating agents were used (Table 4). The limits of detection ranged from 4 to 160 pg. The proposed technique therefore has potential for the deter- mination of trace elements. With fluorination-vaporization no memory effect existed and good reproducibility was obtained.Fig. 7 represents a 3% relative standard deviation (RSD) for five replicate measure- ments of 0.5 pg ml-' of Cr. For 0.5 pg ml-' of Zr and 0.5 pg ml-1 of V the precisions were 4 and 2% respectively. Con- sidering that a sample size of only 20 p1 was employed the precision obtained is reasonable. Cali brat ion The calibration graph for Zr by using this technique is linear over a dynamic range of three orders of magnitude from 0.01 to 10 pg ml-I. Advantages of PTFE as a Fluorinating Agent Compared with other halogenating agents PTFE has several advantages. (i) Fluorine is sufficiently chemically active to combine with refractory materials. (ii) The PTFE has a high fluorine content. (iii) The PTFE contains few inorganic im- purities and does not introduce any cations as do inorganic ha- logenating agents therefore there are no consequent vaporization or spectral interferences.(;I?) The PTFE does not act as a fluorinating agent below 415OC enabling higher ashing temperatures to be used. (I?) The use of a finely dis- persed PTFE SIUKY ensures that the fluorination reaction is efficient. ( 1 1 i ) The PTFE slurry is easy to prepare in propanol and can be diluted with water in any ratio to form a stable emulsion.224 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL 199 I VOL. 6 Conclusion 4 Schmenmann S. M. Long S. E. and Browner R. F. J. A n d . At. A method using PTFE as a fluorinating agent for the deterrni- nation of the refractory elements Zr Cr Mo W B Ti and V by ETV-ICP-AES has been studied. Some advantages of using PTFE as a fluorinating agent have been demonstrated.The de- tection limits for the elements mentioned were from picogram to sub-nanogram levels and were improved by one or two orders of magnitude in comparison with the technique in the absence of fluorination. Further work on application/ interference studies and determination of rare earth elements using the fluorinating technique is being undertaken. 5 6 7 8 9 10 I 1 12 Specwom. 1987 2 687. Aziz A. Broekaert J. A. C. and Leis F. Specwochim. Acta. Part B 1982,37,369. Kirkbright. G. F. and Snook R. D. Anal. Chem. 1979,51 1938. Satumba R. T. Boots R. A. and Matousek. J. P. /CP /nf. Nend. 1987 13,22. Ng K. C. and Caruso J. A. Analyst 1983 108,476. Jiang Z.-c. and Fassel V. A. Fen.\-; Shivanshi 1987,6,6. Huang M. Xu L.-f. Jiang Z.-c. and Zeng Y. Chem. J. Chin. Univ.. 1989,7,709. Kantor T. Specrrochim. Acra. Part B . 1983,38 1483. Handbook of Chemistry and Physics ed. West R.C.. Chemical 1 2 3 Rubber Company Cleveland OH 59th edn. 1978. References Ng K. C.. and Caruso J. A. Appl. Spectrosc. 1985 39 7 19. Matusiewicz H.,J. Anal. At. Sperwom. 1986 1 171. Nixon D. E. Fassel. V. A. and Knisely R. N.. Anal. Chem.. 1974 46,2 10 Paper OlO2.548H Received June 7th I990 Acrepted September I I th I990

ISSN:0267-9477    

DOI:10.1039/JA9910600221

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL 1991 VOL. 6 225 High-performance Liquid Chromatography-Atomic Absorption Spectrometry Interface for the Determination of Selenoniocholine and Trimethylselenonium Cations Application to Human Urine Jean-Simon Blais Alexis Huyg hues-Despointes Georges Marie Momplaisir and William D. Marshall Department of Food Science and Agricultural Chemistry Macdonald College 2 1 1 1 1 Lakeshore Road Ste-Anne de Bellevue Quebec H9X- 1 CO Canada The operation of a prototype high-performance liquid chromatography-atomic absorption spectrometry (HPLC- AAS) interface based on thermochemical hydride generation (THG) was characterized for the determination of sel- enonium compounds. Methanolic solutions of analytes containing selenium were nebulized by a thermospray effect pyrolysed in a methanol-oxygen kinetic flame in the presence of excess of hydrogen and atomized in a micro-diffusion flame maintained at the entrance to an unheated quartz T-tube.Factorial models for predicting the performance of the interface-detector combination at different levels of five interface operating variables indicat- ed that the interface is compatible with both reversed- and normal-phase HPLC eluents and that variations in the five operating parameters within relatively wide ranges does not affect the analyte response appreciably (less than 50% variation in response). Co-injection of trimethylselenonium iodide [(CH,),Sel] with a 1 0-fold excess of other potential interferent onium ions did not affect the THG process significantly. A modified apparatus was used to study the composition of the gases produced from the pyrolysis of (CH,),Sel or SeO in the presence of either H or He. The product gases were condensed acidified and channelled through two consecutive trapping solutions to recover SeIV and hydrogen selenide (H,Se) separately from the product mixture.The analysis of these trapping solutions by HPLC-AAS demonstrated that in the presence of H both (CH,),Sel and SeO were thermochemical- ly reduced to H,Se whereas SeIV was the major product in post-pyrolysis atmospheres of He. A rapid isocratic HPLC separation was developed for the determination of selenoniocholine and trimethylselenonium cations and applied to human urine. Recoveries of both analytes when present at levels of between five- and ten-times the normal background level of total Se were 77% or better. The low cost high reproducibility and robust nature of this system make it a good candidate for the routine determination of several selenium compounds including other selenonium cations selenoamino acids and SeIV organometalloid species. Keywords High -performance liquid chroma tograp hy-a tomic absorption spectrometry; the rmochemical hydride generation; human urine; selenoniocholine; trimethylselenonium Several studies have demonstrated that the trimethylselenoni- um cation [(CH,),Se+] is a major metabolite of selenium (as selenide selenite or selenate) in mammals.’“ In controlled metabolic studies (CH,),Se+ has been determined as 7sSe by y- radiation counting or by autoradiography after ion- exchange purification and precipitation as the Reineckate paper chromatography3-” or high-performance liquid chromatography (HPLC).6.x,y This organometalloid has also been determined indirectly by wet oxidation and fluorimetric quantification of Sel” as the 2,3-naphthalenediamine deriva- tive.’@I2 However this approach does not discriminate between (CH,),Se+ and other potential selenonium metabo- lites such as selenoniobetaine [(CH,),Se+CH,COOH] or selenoniocholine [(CH3)2Se+CH2CH20H].Neutron activation analysis of ion-exchange chromatographic eluates provides both selectivity and a low limit of detection,’,-’-” but because of its technical complexity and high cost this technique may not be readily available to all researchers. High-performance liquid chromatography coupled with de- tection by atomic absorption spectrometry (HPLC-AAS) repre- sents a valuable tool for the determination of metals and metalloids.16 However the requirements for the successful on- line coupling of HPLC and AAS instruments are demanding.The atomization cell e.,?. kinetic flame electrothermal furnace or plasma must be capable of handling voluminous flows of organic or aqueous eluents (typically 0.1-3 ml min-I of liquid is vaporized to about 400-700 1 min-I of gases at normal detector operating temperatures). Ideally the interface module should permit the detection of low- to sub-nanogram amounts of trace elements emerging from the HPLC column yet it should remain robust and inexpensive to construct and operate. Post-column hydride generation (using sodium tetrahydrobo- rate and dilute acid) circumvents problems associated with sample introduction in conventional AAS techniques.This ap- proach has been developed successfully for the determination of ar~enic’~.’~ and tin” using electr~therrnally~~~~~ or flameIx heated quartz tube atomizers. Nevertheless this technique is limited to the reducible physico-chemical forms of the ele- ments which can be volatilized as stable hydrides. Several bio- genic organometalloid compounds are fairly inert and are not predicted to react with reducing agents to form volatile hydride derivatives. Included among these are compounds in which the metalloid occurs in its lowest oxidation state such as selenoni- um species selenoamino acids”) and selenonucleosides.2’.22 Recently a thermochemical hydride generator was devel- oped as an interface for HPLC-AAS and optimized for the de- termination of low-nanogram amounts of arsenobetaine arsenocholine and tetramethylarsonium cations.’ This on-line interface was based on thermospray nebulization of the HPLC methanolic eluent pyrolysis of the analyte in a methanol- oxygen flame thermochemical derivatization using excess of hydrogen and cool diffused flame atomization of the product(s) in a quartz cell mounted in the AAS optical beam.A thermoc hemical hydride generation (THG) mechanism was suggested by indirect evidence. In this paper direct evidence supporting the gas-phase hydride generation mechanism for selenium is presented and a method for the determination of selenoniocholine and (CH,),Se+ in human urine is reported.Experimental Reagents and Standards All solvents were ‘distilled in glass’ grade or ‘pesticide analy- sis’ grade (Caledon Georgetown Ontario Canada). Certified226 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL 1991 VOL. 6 ACS-reagent grade hydrochloric and acetic acids were used. Triethylamine was purified gold-label grade and other chemicals were laboratory-reagent grade or better (Aldrich Chemicals Milwaukee WI USA). Water was doubly dis- tilled and deionized. The synthesis purification and charac- terization of trimethylselenonium iodide [(CH,),SeI] and (2- hydroxyethyl)dimethylselenonium (selenoniocholine) tetra- phenylborate standards have been described elsewhere.’? Naphtho[ 2,3-c-][ 1,2,5]selenadiazole (naphthalenepiazselenol) and bis(2,4-dinitrophenyl) selenide were prepared and purified as described previou~ly.’~.” Stock solutions of (CH,),SeI ( 1.01 x lo4 g ml-I) and selenoniocholine tetraphenylborate (2.06 x lo4 g ml-I) were prepared in methanol and stored at -40 “C. The addition of 10% (v/v) acetone was necessary to dissolve selenoniocholine tetraphenylborate.Dilution of these stan- dards in methanol containing 1% (v/v) acetic acid and 0.05% (v/v) triethylamine provided working standards. Instruments The instrumentation used for this study consisted of an HPLC system (Beckman Fullerton CA USA Model 100 A pump) an autosampler (LKB Stockholm Sweden Model 2157) and an atomic absorption spectrometer set at 196.0 nm (Philips Cambridge UK Model PU9100) which was equipped with a high energy selenium hollow cathode lamp (Photron Super Lamps System Victoria Australia) and a deuterium back- ground correction system.The optimization experiments (without chromatography) were performed with deuterium background correction. Because the background correction almost tripled the background signal of the AAS detector the chromatographic calibrations were performed without deuteri- um background correction. Narrow-bore stainless-steel tubing (i.d. 0.007 cm) was used as a post-injector. The silica transfer line (i.d. 50 pm) was connected to the HPLC tubing through a capillary reducing union (Chromatographic Specialties Brock- ville Ontario Canada). HPLC Conditions Naphthalenepiazselenol and bis(2,4-dinitrophenyl) selenide were separated on a Nucleosil C column (0.46 x 15 cm 3 pm particles CSC Montrkal Canada) using 100% methanol as the mobile phase (0.5 ml min-I). For the interface optimiza- tion the selenonium standards (100 p1 injections) were separ- ated on a cyanopropyl bonded phase [5 pm silica support 0.46 mm i.d.x 15 cm LC-CN (Supelco Bellefonte PA USA)] with methanol (0.65 ml min-I) containing 0.05% (v/v) triethylamine and 1% (v/v) acetic acid. For the determination of these compounds in urine diethyl ether (29% v/v) and tri- methylsulphonium iodide (0.2 mg ml-I) were added to the mobile phase. THG Interface The construction of the quartz interface for the HPLC-AAS method was as described previously.’3 Briefly it consisted of a quartz T-tube (an upper optical tube and a lower analytical- flame tube) with an added side arm which met the analytical- flame tube at an angle of 45”.This side arm contained a pyrol- ysis chamber which was fitted with separate inlets for 0 and H,. During the operation of the THG the liquid HPLC column eluate was thermosprayed into and combusted in the upstream region of the pyrolysis chamber. The product gases were reacted with H2 and entrained through a small O2 supported an- alytical flame maintained just below the optical beam of the spectrometer. [Caution The thermospray region of the inter- face must be heated to normal operating temperature prior to turning on the HPLC pump. Vapours of methanol (the result of an unsuccessful ignition of the thermospray) must not be allowed to accumulate in the interface.] Optimization The interface was optimized for the detection of (CH,),Se+ using a response surface methodology (half-replicate 2s com- posite design) as described previously.’3 In this approach 2n + 1 (n = number of independent variables = 5) data points were added to the half-replicate 2s factorial design (which is suitable only for estimating linear and interaction effects) allowing the fitting of a second-order model (to include quadratic effects) to search for the optimum response.Thus (j x 25) + [(2 x 5 ) + I ] = 27 data points were required. The five variables studied were the flow-rates of (i) 0’ (OT 500-4300 ml min-I) and (ii) H (1.00-2.40 1 min-I) to the pyrolysis chamber (iii) 0 (OA 100-240 ml min-I) to the analytical flame (iv) HPLC mobile phase flow-rate (0.3&1 .OO ml min-I) and (v) the percentage of diethyl ether (040%) or water (040%) in the methanolic mobile phase which also contained 1 % (v/v) glacial acetic acid and 0.05% (v/v) triethylamine.The peak area of the atomic ab- sorption signal for (CH,),SeI (2 pg) was recorded in triplicate for each of the 27 operating conditions (HPLC column removed) spaced equally across the domain of the ranges described earlier. The AAS response to 1.2 nmol of trimethyl- selenonium iodide selenoniocholine tetraphenylborate sele- nomethionine selenium dioxide and sodium selenate standards were also recorded under these same operating conditions. The possible interference with the detection of Se by other orga- nometallic/metalloid cations was evaluated by co-injecting 1.2 nmol of (CH,),SeI together with a I - or 10-fold molar excess of interferent [(CH,),AsI (CH,),SI or (CH,),PbCI].Cal- ibration graphs for selenoniocholine and (CH,),Se+ were ob- tained by HPLC-THG-AAS analysis of sequential dilutions of a fresh standard. The limit of detection (LOD) was determined from these calibration graphs using a first order error propaga- tion model with base-line noise normally di~tributed.?~ Trapping Apparatus A modified interface (Fig. 1 ) was used to study the products of reaction between the crude pyrolysis gases and either Hz or He. In operation the sample dissolved in methanol was nebu- lized by the thermospray effect combusted in an atmosphere of O2 and the crude pyrolysis products were reacted with either Hz or He.The products were acidified cooled and passed through two scrubbing solutions to trap Se’” and HzSe separately. The design of this modified interface was similar to the THG interface described except that the pyrolysis chamber tube and the quartz outlet tube were smaller (4mm i.d. x 6 mm 0.d. and 2 mm i.d. x 4 mm 0.d. x 45 cm respec- tively). In addition to the gas inlets for 0 and H a third inlet to the pyrolysis chamber was added 2.5 cm downstream from the H inlet to permit the introduction of 2 ml min-’ of HCI ( 1 mol dm->) using a peristaltic pump [Eyela (Tokyo Japan) Model MP-31. The outlet of the modified THG ‘train’ was in- serted into a water-cooled condenser [E 1.27 0.d. x 37 cm copper tube equipped with a water inlet and outlet and sealed with brass Swagelok unions (J 1.27 x 0.64 cm)].One end of a polytetrafluoroethylene (PTFE) tube (F 2.48 mm i.d. x 4 mm 0.d. x 50 cm) was thermally sealed to the quartz exit transfer line and the other end was connected to a 250 ml Erlenmeyer filtering flask (G trap 1 ) which contained 190 pmol of 2,3- naphthalenediamine in 150 ml of 1 mol dm- HCI. The gases emerging from trap 1 were channelled iio a PTFE tube (I) to a second trap (H trap 2) containing 18 mmol of NaHCO and 500 pmol of l-fluoro-2,4-dinitrobenzene (FDNB) dissolved in 150 ml of dimethylformamide and water [7 + 3 (v/v)]. The 2,3 -naph t halened iam i ne and the 1 -fl uoro-2,4-dini trobenzene solutions were used to convert Se’” into naphthalenepiazsele- no1 and to transform H,Se into bis(2,4-dinitrophenyI) selenide respectively.The trapping solutions were stirred magnetically. Experiments were carried out by injecting 20 volumes (0.2 ml) of (CH,),SeI dissolved in methanol (0.537 pmol ml-I) orJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL I99 I VOL. 6 227 D 0 H HCI I I Water E H 4 -G Fig. 1 Thermochemically generated hydride trapping apparatus consist- ing of A capillary transfer line; B silica guide tube; C thermospray heating coil surrounding a thermospray-pyrolysis quartz tube 4 mm i.d. x 14 cm with inlets (2 mm i.d. x 3 cm) for 02 H and 1 rnol dm-3 HCI; D Swagelok reducing unions to position the transfer line and the guide tube within the thermospray tube; E condenser jacket (1.27 x 37 cm); F and I (4 mm 0.d.) PTFE transfer lines; G and H 250 ml filtering flasks contain- ing 2,3-naphthalene diamine (trap I ) and 1 -ftuoro-2,4-dinitrobenzene (trap 2); and J modified Swagelok fittings to provide a watertight seal between the condenser and the silica exit tube (2 mm 0.d.x 45 cm) SeOz (0.955 pmol ml-I) into the system via an HPLC injection valve under the following conditions methanol flow-rate 0.5 ml min-I; heating element current 6 A; O2 flow- rate 600 ml min-'; H2 flow-rate 1.7 I min-'; and I rnol dm-3 HCI flow-rate 2 ml min-I. At the termination of the experiment the trapping solution from trap 1 was stirred for 30 min then extracted three times with 50 ml of benzene. The organic phases were combined washed three times with 50 ml of 1 rnol dm-3 HCI dried with generous amounts of Na,SO filtered through a Whatman No.1 filter-paper and the filtrate was evaporated to dryness under vacuum. The residue was diluted to 1 ml with methanol. Excess of FDNB in the trapping solution from trap 2 was reacted at room temperature with 1 mmol of glycine for 2 h diluted to 400 ml with 0.01 rnol dm-.' NaOH solution and ex- tracted with three successive 50 ml portions of benzene which were combined and back-extracted three times with 100 ml of 0.01 rnol dm-.' NaOH solution. The benzene extract was treated as described for the first trapping solution. The final ex- tracts were analysed (20 p1 injections) by HPLC-THG-AAS. (Caution Benzene is highly toxic and appropriate precautions should be taken.) Isolation of Selenonium Standards From Urine Urine (10 ml) which had been spiked with 10 pg each of sele- noniocholine tetraphenylborate and trimethylselenonium iodide was diluted with 100 ml of absolute ethanol and chilled in a dry ice-acetone bath for 20 min according to the method of Kraus et aLx The resulting precipitate was separated from the liquid by refrigerated (-15 "C) centrifugation at about 40008 for 15 min.The supernatant was evaporated to dryness and the residue dissolved in 10 ml of water was applied to the head of an anion-exchange column (0.5 x 10 cm of Dowex 2x8). The column was washed with additional distilled water to give a total volume of 30 rnl of column eluate which was acidified to pH 3 with HCI. The acidified aqueous solution was extracted four times with liquified phenol ( I x 10 and 3 x 5 ml). The phenol extracts were combined and back-extracted three times with water ( 1 x 10 and 2 x 5 ml). The phenol layer was diluted with 75 ml of diethyl ether then extracted three times with 5 ml of distilled water to recover the analytes.The aqueous washes were combined and back extracted with 3 x 5 ml of diethyl ether. The aqueous phase was evaporated to dryness and the residue was re-dissolved in methanol and re-concentrated to 1 ml. Aliquots (50 pl) were analysed by HPLC-AAS. Results and Discussion Post-HPLC Hydride Generation The prototype thennochemical hydride generation HPLC- AAS interface was developed2? as an analytical tool for study- ing the fate of potential biogenic arsonium and selenonium compounds. The HPLC-inductively coupled plasma (ICP) spectrometry,2N HPLC-ICP mass spectrometry29 or HPLC- graphite furnace AAS") instruments which have been used to determine arsonium compounds would probably be very suit- able for the analysis of selenonium species.However their higher purchase price and operating costs limit their availabil- ity to many researchers. As was observed previously for arso- nium compounds,2' the severe spectral interference of the kinetic flame at low AAS wavelengths precluded the use of a direct thermospray-micro-atomizer interface3' for the detec- tion of trace amounts of Se compounds. Alternative detection techniques for Se were sought. The THG interface23 was con- sidered to include at least three steps the thermospray induced nebulization pyrolysis and atomization of the organometallic analyte;" a postulated THG using hydrogen radicals; and dif- fusion-flame atomization.'? " The last technique has been re- ported to provide an absolute atomization of H,Se.'3 The atomization process is considered to be mediated by reactions with hydrogen radicals which are generated in the active zone of the diffusion flame.This spatially limited cloud of free radi- cals which does not extend to the AAS optical beam virtually eliminates spectral background noise. The thermochemical H2Se generation mechanism of the in- terface was corroborated by the fact that no AAS signal was observed in the absence of the diffusion flame or in the absence of post-thermospray H,. A confirmation of the ther- mochemical derivatization of (CH,),SeI and SeO to H2Se was obtained by chemical-trapping experiments using the apparatus presented in Fig.1. The hot product gases from the pyrolysis chamber were reacted with either H or He then mixed with 1 rnol dm-3 HCl cooled to room temperature in a condenser (E) and channelled to an acidic solution containing 2,3-naphthalenediamine (G trap I ) which converted Se'" into naphthalenepiazselenol.2h Pyrolysis products which were carried through the acidic trap 1 were channelled to an alka- line solution (H trap 2) containing FDNB to convert H,Se into bis(2,4-dinitrophenyI) selenide. After the trapping experi- ments excess of FDNB was reacted with glycine to form the base-soluble 2,4-dinitroanilinoacetic acid.M The organo- soluble content of trap 1 was extracted from the acid solution into benzene and the residue obtained after solvent removal was dissolved in methanol.The postulated bis(2,4- dinitrophenyl) selenide derivative" formed in trap 2 was ex- tracted from the basic aqueous solution with benzene evapo- rated to dryness and the residues were re-dissolved in methanol. The extracts from both traps were analysed directly by HPLC-THG-AAS. Chromatograms of the standards and of the products isolated from the trapping solutions after ther- mospray pyrolysis of (CH,),SeI are presented in Fig.2. The recoveries of naphthalenepiazselenol and bis(2,4- dinitrophenyl) selenide when the pyrolysis products of (CH,),SeI or SeOz injections were reacted with H2 or He are recorded in Table I . These results indicate that both SeO and (CH,),SeI are converted into H,Se but only in the presence of Hz. As a result of the high gas flow-rates occurring in the trapping apparatus a quantitative recovery of the products was not anticipated.Under an inert post-pyrolysis atmo- sphere (He) a portion of injected selenium (Se'") was re- covered in trap 1 but an appreciable amount remained deposited in the condenser tube as metallic red selenium. In both instances a portion of the naphthalenepiazselenol was en-228 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL 1991 VOL. 6 r- o! Time - Fig. 2 HPLC-THG-AAS chromatograms resulting from the trapping ex- periments with (CH,),SeI. Chromatogram (a) bis(2,4-dinitrophenyl) sele- nide (3.94 min) and naphthalenepiazselenol (5.28 min) standards; chromatogram (h) pyrolysis performed under He atmosphere; chromato- gram (c) pyrolysis performed under H atmosphere. Chromatograms A and B are extracts from trap 1 (Se" specific) and trap 2 (Se" specific) re- spectively Table 1 Amounts and percentage recoveries of naphthalenepiazselenol (NPSe) and bis(2.4-dinitrophenyl) selenide [(DNP),Se] from chemical traps after the pyrolysis of (CH,),SeI or SeO in the presence of H or He Analyte Atmosphere Trap Amount Recovery Amount Recovery of (%I of (%I NPSe/ (DNP),Se/ pmol pmol (CH,),SeI* H 1 NDt - ND - 2 ND - 0.67 31.2 He 1 0.36 16.7 ND - 2 0.20 9.3 ND - - ND - I ND - 1.13 29.6 H? 2 ND 2 0.11 2.9 ND - He 1 0.78 20.4 ND * 2.15 pmol injected.t ND = none detected. $ 3.82 pmol injected. supply to the pyrolysis chamber by dinitrogen oxide (produc- ing a white flame >1665 "C) resulted in a complete loss of the AAS analyte (500 x LOD) signal which may reflect a rapid thermal degradation of the hydride at this higher post- thermospray temperature.Replacing the thermospray 0 by air decreased the AAS response by about 40%. In this instance a massive air flow-rate (2 I min-I) was required to maintain the thermospray flame and the lower response may be attributed to a lower residence time of the H,Se in the analytical flame. Under optimum conditions for selenonium compounds the in- terface resulted in equivalent responses for selenomethionine selenoniocholine and selenium(1v) oxide (Table 2) however sodium selenate (Na,SeO;-) was less efficiently derivatized and detected with the THG procedure. The relative AAS response of Se [as (CH,),SeI 1.2 nmol] co-injected with an equimolar amount and a 10-fold molar excess of potential interferents As (as tetramethylarsonium iodide) S (as trimethylsulphonium iodide) and Pb (as trimeth- yllead chloride) are presented in Table 3.In each instance the response to selenium remained virtually unaffected. As was the situation for the earlier thermospray-micro-atomizer inter- face,,' this THG device was not compatible with alkaline earth metals (degradation of the quartz) halogenated solvents (reduced response) or a high proportion of water (>60%) in the HPLC mobile phase (disruption of the thermospray). Table 2 Relative AAS responses to Se compounds of equimolar amounts (1.2 nmol) recorded under optimum conditions Anal yte Relative response (%) Trimethylselenonium iodide Selenomethionine Selenoniocholine Selenium(1v) oxide Sodium selenate 100 f 2* 99 k 2 104+3 101 + 2 18k I * Standard deviation based on three replicate analyses.Table 3 nmol) recorded under optimum conditions Effect of potential interferents on the response to (CH,),SeI (1.2 Analyte Interferent Relative amount Response (molar ratio) (%I (CH,),SeI - - 100f2* l00f 1 10 9 6 k 2 (CH3),AsI 1 (CH,),SI 1 99 f 0.5 10 9 4 k 2 10 95 k 0.8 (CH,),PMII 1 97 k 2 trained into trap 2. These data corroborate the postulated THG mechanism which is most probably mediated by hydrogen radicals. The fact that no Selv was detected in trap I in an H atmosphere suggests a direct thermochemical derivatization of an oxidized species (SeIV in this instance) to its hydride. One mechanism which may explain this phenomenon is initiated by hydrogen radicals H'+OSeO 4 OH'+SeO' ( 1 ) SeO'+H + HSe'+OH' (2) HSe'+HZ 4 H,Se+H' (3) Such a reaction sequence is considered to occur in a hot spa- tially limited volume around the H inlet.In this hypothetical process the temperature-sensitive final product (H,Se) has to be stabilized rapidly by the cooling effect of the massive flow- rate of Hz. This supposition has been corroborated by addition- al experiments with the THG-AAS interface. Replacing the 0 * Standard deviation based on three replicate analyses. Optimization of the THG Interface The interface operating parameters were optimized using a multivariate methodology based on a half-replicate 25 compo- site These experiments were carried out by record- ing the instrumental response to (CH,),SeI under different combinations of interface operating parameters which includ- ed the flow-rates of oxygen (OT) to the pyrolysis chamber; hydrogen to the pyrolysis chamber; oxygen (OA) to the analyt- ical flame; and the HPLC mobile phase.A fifth variable was introduced into the model to mimic typical normal- or re- versed-phase HPLC eluents (040% diethyl ether or 040% water in a methanolic mobile phase). Both mathematical models [reversed-phase (RP) and normai-phase (NP) eluents] resulting from the statistical analysis of the data were fairly ac- curate with average relative deviations between the observed229 JOURNAL OF ANALYTlCAL ATOMIC SPECTROMETRY APRIL 1991. VOL. 6 and predicted responses of 8.5 and 5S% respectively. The linear regressions of the observed on the predicted response were correlated at r = 0.9164 and 0.9425 for RP and NP models respectively.Unmodelled variations in the perfor- mance of the interface at extreme parameter values resulted in some outlier predictions. These unmodelled variations were thought to be caused by a rapid accumulation of carbon depos- its (which could be removed by increasing the OA flow-rate temporarily) that occurred for 3040% diethyl ether and at 500 ml min-I OT and the necessity to re-adjust the position of the capillary to obtain a stable thermospray for 40% water and at 500 ml min-I OT. The accuracy of the models was consid- ered sufficient to estimate the effect of individual variables and to determine optimum parameters visually using surface- response plots.The effects of the five variables were character- ized by plotting two selected variables versus response while keeping the three others constant at the centre of the design. The curvature of these plots (Fig. 3) provided valuable infor- mation for a tentative characterization of the effects. In the NP model (methanolic mobile phase containing diethyl ether) the combination of mobile phase flow-rate and thermospray oxygen (OT) flow-rate [Fig. 3(a); proportion of diethyl ether = 20% H flow-rate = 1.7 1 min-I and OA = 170 ml min-'1 predicted a maximum response at intermediate values (OT = 650 ml min-I; mobile phase flow-rate = 0.65 ml min-I). A minimum response occurred at low OT and high mobile phase flow-rate; these conditions resulted in a short thermospray flame and a slow accumulation of carbon deposits in the THG combustion chamber. A similar pattern was ob- served when the predicted response was plotted as a function of the diethyl ether content in the mobile phase and as a func- tion of OT [Fig.3(h)]. In this instance the THG-AAS re- sponse was decreased by about 40% (relative to maximum) at high diethyl ether content and low OT reflecting an incom- plete combustion of the mobile phase. When combined with a proper level of OT the presence of diethyl ether appeared to be beneficial to the response. Thus the optimum response for (CH,),Se+ was obtained when the OT mobile phase flow-rate and proportion of diethyl ether were at intermediate values close to the centre of the design. Relative to the NP eluent the presence of water in the mobile phase was generally detrimen- tal to the response. In the presence of 20% water a maximum response was observed at low mobile phase flow-rate and high OT [Fig.3 ( c ) ] . The last condition resulted in a relatively cool post-thermospray atmosphere. The combined effects of pro- portion of water and OT somewhat corroborated this observa- tion [Fig. 3(d)]. The selenonium analyte was most efficiently derivatized in a cooler (high water content) post-thermospray environment. Optimum THG parameters for the analysis of selenonium analytes were determined from surface response plots of OT versus H flow-rate at different levels of analytical oxygen (OA) with the two other variables fixed at optimum chromato- graphic values (mobile phase flow-rate = 0.65 ml min-I; pro- portion of diethyl ether = 0%) for the selenonium compounds. These plots are presented in Fig.4. At low OT the thermo- spray flame was short and the response was generally lower (with a maximum occurring around a 1.7 1 min-I H flow- rate) presumably reflecting an inefficient pyrolysis of the analyte. At high OT and low H flow-rate this flame was vigo- rous but a minimum response was still observed most prob- ably because of a rapid consumption of H by excess of 0,. Increasing the flow-rate of H resulted in higher responses up to a maximum after which a secondary H,-O flame was ignited at the H inlet tube thereby affecting the efficiency of the THG. The detrimental effect of this secondary flame cor- roborated previous observations that excessive post- thermospray temperatures were detrimental to the sensitivity of the THG-AAS. Again the maximum response occurred at intermediate levels of H and thermospray 0 flow-rates.The second-order interaction between H and OA (level of analyti- cal oxygen not presented) corroborated the previously report- ed characteristics of this atomization mechanismJ3 which requires a fuel-rich flame. A response minimum observed at a 51 < 46 v I 2 41 36 \ Y 8 31 0 2 6 21 v) 57 a 37 Fig. 3 Surface response plots of the predicted variation in peak area with (u) and (c) oxygen flow-rate (OT) to the pyrolysis chamber and with the HPLC mobile phase flow-rate and ( h ) and (d) with the proportion of eluent modifier (diethyl ether or water) \~ers~ts OT. The other variables were maintained con- stant at the centre of the experimental design230 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL I99 1 VOL.6 49 2 45 f 41 2 3 37 t! 0 33 * 3 29 CL 25 1. 49 v) a 45 * 41 37 0 Y 3 33 a 29 1 Fig. 4 Surface response plots (normal-phase model) of the predicted vari- ation in response with hydrogen (H2) and O2 (OT) flow-rates to the pyroly- sis chamber at different flow-rates of O2 (OA) to the analytical flame. A OA = 170 ml min-I; B OA = 205 ml min-I; C OA = 240 ml min-I. Both mobile phase flow-rate and diethyl ether content were maintained constant at 0.65 ml min-' and 0%. respectively low OA resulted from an appreciable reduction in the volume of the analytical cool diffusion flame. From these surface response plots the optimum operating parameters were determined to be mobile phase flow-rate = 0.65 ml min-I; proportion of diethyl ether = 0%; OT = 725 ml min-I; Hz flow-rate = 2.03 1 min-I; and OA = 170 ml min-I.Over the ranges of operating parameters studied the difference between low and high responses was generally less than 50%. Thus the performance of this interface was only moderately affected by appreciable variations in the levels of the five vari- ables. This valuable characteristic was reflected in the fairly high reproducibility of the instrument. HPLC of Selenonium Compounds Strong cation-exchange chromatography (sulphonate based stationary phase) appeared to be the method of choice for sep- arating selenonium compounds.x However this approach was incompatible with the THG interface because of the high pro- portion of water required in the mobile phase.With optimum flow-rates of 0.24.5 rnl rnin-I a micro-bore (0.21 cm i.d.) strong cation-exchange column coupled with post-column methanol enrichment of the eluate would have been desirable I I I 0 3 6 9 Ti me/m i n Fig. 5 HPLC-THG-AAS chromatogram of A selenoniocholine; and B trimethylselenonium standards (500 ng of each) recorded under optimum conditions in this particular instance. However this packing was not com- mercially available in a micro-bore format. The trimethylselenonium and selenoniocholine cations were separated on a cyanopropyl stationary phase with a methanolic mobile phase containing a silanol masking agent [triethyl- amine 0.05% (v/v) and acetic acid 1 % (v/v)]. A chromatogram of these standards recorded under optimum THG conditions is presented in Fig.5. Although the two selenonium analytes were totally resolved the (CH,),Se+ was eluted as a broader peak which substantially decreased its LOD. The addition of up to a 500-fold (m/m) excess of ammonium acetate in the standard solution did not appreciably affect the background signal nor the retention times of the analytes. Linearity Reproducibility and Limits of Detection The LOD for each analyte was calculated from the correspond- ing calibration graph under optimized conditions using a first order error propagation model .24 The linear calibration models were highly correlated [ I - = 0.9996 and 0.9986 for selenonioch- oline and (CH,),Se+ respectively] in the concentration range studied (100 ng - 2.5 pg as selenonium salt).The calculated LOD of each analyte (as free cation) was selenoniocholine = 31.3 ng and trimethylselenonium = 43.9 ng. Subsequent studies with the same column indicated that the addition of tri- methylsulphonium iodide to the mobile phase improved the chromatography of these analytes appreciably which lowered the LODs substantially. The short-term reproducibility of the THG interface (based on three replicate analyses) for different concentrations of each analyte is reported in Table 4 The long-term reproducibility (6 h n = 6 ) recorded at 10 x LOD was selenoniocholine tetraphenylborate = 3.1 % and (CH,',,SeI = 5.8%. Analysis of Human Urine The THG technique was then applied to the determination of selenoniocholine and (CH,),Se+ in human urine.Urine ( 10 ml) which had been spiked with 1 mg I-' each of selenoniocho- line tetraphenylborate and trimethylselenoniurn iodide was de-salted with ethanol according to the method of Kraus et a/.' After centrifugation the supernatant was evaporated toJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL I99 I VOL. 6 23 1 Table 4 Reproducibility of the HPLC-THG-AAS response to various analyte levels recorded under optimum conditions Amount/ Selenoniocholine Trimethylselenonium Pt.2 tetraphenylborate iodide 0.1 10.7 11.6 0.5 2.5 1.4 I .o 1.9 2.8 2.5 0.6 1.2 RSD (%)* RSD (5%) * Relative standard deviation based on three replicate analyses. 0 3 6 9 12 Time/min Fig. 6 HPLC trace of selenoniocholine (at 7.05 min); and trimethylsele- nonium (at 8.39 min) standards which had been spiked (at 0.173 and 0.315 pg ml-' of Se respectively) into human urine.The solvent front peak appears at 3.29 min. The cyanopropyl column was eluted isocratically with 0.65 ml min-' of mobile phase containing methanol (70% v/v) diethyl ether (29% v/v) glacial acetic acid ( I % v/v) triethylamine (0.01 o/c v/v) and trimethylsulphonium iodide (20 mg per 100 ml) dryness and the residue was re-dissolved in water. The result- ing aqueous solution was passed through an ion-exchange column then acidified to pH 3 and extracted four times with liquified phenol. The phenol phases were combined back- extracted with water then diluted with diethyl ether. Finally the analytes were recovered by extracting the phenol-diethyl ether mixture with water.The water extracts were combined washed with diethyl ether evaporated to dryness and the residue was re-dissolved in methanol and analysed by HPLC- AAS using the cyanopropyl column and a mobile phase con- sisting of methanol [70% (v/v)] diethyl ether [29% (v/v)] and glacial acetic acid [ 1 % (v/v)] containing triethylamine [O.Ol% (m/v)] and trimethylsulphonium iodide (0.2 mg ml-I). Recoveries the average of three replicate determinations (k one standard deviation) were 77 f 4 and 85 f 0.4% for (CH,),Se+CH2CH,0H and for (CH,),Se+ respectively. The spiking level [173 and 315 ng ml-I of Se for (CH,),Se+CH,CH,OH and (CH,),Se' respectively] was delib- erately chosen to be from five- to ten-fold higher than back- ground levels for total Se encountered in normal human urine8 [30-50 ng ml-l of Se of which some 10% is considered to be (CH,),Se+]. Neither analyte was detected in several samples of control urine although the LODs using the same model as above and the modified solvent system were 5 and 7 ng (as Se) for (CH,),Se+CH2CH,0H and (CH,),Se' respectively.No difficulties with the chromatography (Fig. 6) or with the detec- tion of these analytes at this spiking level were encountered. The final extract could have been further concentrated and 100 pl injections of sample could have been used without compro- mising the method. Conclusion The concentrations of selenonium metabolites in biological samples remain to be determined. In order to provide a method with an LOD for Se in the 1-20 ng g-' range the ex- traction protocol to be developed for these analytes should be designed to allow the treatment of larger samples (5-25 g) and concentration of the final extract to less than 1 ml.Since the cool diffused flame atomizer has been shown to provide picogram sensitivites at gas flow-rates exceeding 5 I min-1,32.33 it is considered that the performance of this interface is limited by the efficiency of the hydride generation and hydride transport processes. This THG process appeared to be affected by high post-thermospray temperatures hence it is reasonable to anticipate that further research on this aspect will result in sub-nanogram LODs for Se and other hydride forming elements. Financial support from the Natural Science and Engineering Research Council of Canada is gratefully acknowledged. 1 2 3 4 5 6 7 8 9 10 1 1 12 13 14 1s 16 17 18 19 20 21 22 23 24 2s 26 27 References Byard J.L. Arch. Biochem. Biophys. 1969. 130 556. Palmer I. S. Fischer D. D.. Halverson A. W. and Olson 0. E.. Biochim. Biophys. Acta 1969 177 336. Palmer 1. S. Gunsalus R. P. Halverson A. W. and Olson 0. E. Biochim. Biophys. Arm 1970 208 260. Kiker K. W.. and Burk R. F. Am. J. Physiol. 1974,227,643. Nahapetian. A. T. Janghorbani M. and Young V. R. J . Nutr. 1983 113,401. Sun X. F.. Ting B. T. G. and Janghorbani M. Anal. Biochem. 1987,167,304. Ganther. H. E. Kraus R. J. and Foster S. J. Methods Enzvmol. 1987,143 195. Kraus J. K. Foster J. S. and Ganther H. E.. Anal. Biochem. 1985 147,432. Hoffmann. 1. L. and Connell. K. P. Arch. Biochem. Biophys. 1987 254 534.Watkinson J. H. Anal. Chem. 1966,38,92. Olson 0. E. Palmer 1. S. and Cary E. E. J . Assoc. Off. Anal. Chem.. 1975,58 117. Janghorbani M. Ting B. T. G. Nahapetian. A. and Young V. R.. Anal. Chem.. 1982,54 1188. Blotcky A. J. Hansen. G. T. Opelanio-Buancamino L. R. and Rack E. P. Anal. Chem. 1985,57 1937. Blotcky. A. I. Hansen G. T. Borkar N. Hebrahim A.. and Rack E. P. Anal. Chem. 1987,59 2063. Blotcky A. J. Ebrahim A. and Rack E. P.. Anal. Chem. 1988 60 2734. Ebdon L. Hill S. and Ward R. W.,Ana/yst. 1987 112. I . Ricci G. R.. Shephard L. S. Colovos G. and Hester. N. E. Anal. Chem. 1981,53,611. Haswell S . J.. O'Neil P. and Bancroft K. C. C.. Anal. Chim. Actu 1985.169 195. Bums D. T.. Glockling. F. and Harriott M. A n u / w 1981. 106,921. Stadtman. T. C. Ad\*. Eti:Fmo/. 1979.48. 1. Chen C. S. and Stadtman T. C.. Proc. Natl. Amd. &I. 1980 77 1403. Whitter. A. J. Stai. L.. Ching. W. M. and Stadtman T. C. Bio(,heni- istry 1984 23 4650. Blais. J. S. Mornplaisir. G. M. and Marshall W. D.. Anal. Chem. 1990.62. 1161. Foley. J. P.. and Dorsey J. G. Cht.omatogr.aphia 1984. 18.503. Huyghues-Despointes A. M.Sc. Thesis McGill University. 1990. Baytield R. F. and Romalis L. F. Atid. Biochent.. 1985. 144 569. Ganther H. E. and Kraus. R. J . A w I . B i o c h 3 m . . 1984. 138. 396.232 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL 1991 VOL. 6 28 Shiomi K.. Kakehashi. Y. Yamanaka H. and Kikushi T. App/. Or- ganomet. Chem. 1987 1 177. 29 Beauchemin D. Bednas M. E.. Berman S. S. McLaren. J. W. Siu K. W. M. and Sturgeon R. E. Anal. Chem. 1988,60,2209. 30 Lawrence J. F. Michalik P. Tam G.. and Gonacher H. B. S.. J . Agi'ic. Food Chem. 1986 34. 315. 31 Blais J. S. and Marshall W. D. J . A n d . At. Spectrom.. 1989. 4,271. Paper 01024070 32 Siemer D. D. Koteel P. and Jawiwala V. A n d . Chem. 1976 48 Received May 30th 1990 836. Accepted November 20th 1990 33 Dedina. J. and Rubeska I. Spectrochim. Actu Purr B. 1980 35 119. 34 Bunnett J. F. and Hermann D. H. Biochemistry 1970,9 816. 35 Hill W. J. and Hunter W. G. Technometrics 1966 8 571.

ISSN:0267-9477    

DOI:10.1039/JA9910600225

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL 1991 VOL. 6 233 Flow Injection Flame Atomic Spectrometric Determination of Aluminium Iron Calcium Magnesium Sodium and Potassium in Ceramic Material By On-line Dilution in a Stirred Chamber Vincente Carbonell Angel Sanz Amparo Salvador and Miguel de la Guardia Department of Analytical Chemistry University of Valencia 50 Doctor Moliner Street 46 I00 Burjasot Valencia Spain ~~ The on-line dilution of ceramic samples previously fused with alkaline carbonates increases the dynamic range of the flame atomic spectrometric determination of aluminium iron calcium magnesium sodium and potassium. A double-channel manifold with a simultaneous double injector and a versatile system of dilution with stirred cham- bers allows direct determination of the six elements studied in different ceramic matrices i.e.clay kaolin quartz felspar varnish stoneware and porcelain rapidly and with low consumption of the reagents. The effect of the flow injection parameters on the analytical characteristics of the flame atomic spectrometric determination has been studied and the results obtained in the analysis of real samples have been compared with those found in batch analyses. Keywords Ceramic; flow injection; determination of aluminium iron calcium magnesium sodium and potas- sium; flame atomic spectrometry The analytical determination of the elemental composition of ceramic materials is very important in order to control the characteristics of raw materials chosen for use and to test the quality of finished products.'.' Both the major and minor components of ceramic matrices must be simply but selectively determined; flame and plasma spectrometric methods are ideal for this purpose.3-s Flame atomic absorption and flame atomic emission spectrometry are currently the most widely used techniques for the determina- tion of various elements e.g.aluminium iron calcium mag- nesium sodium and potassium in ceramic samples. However problems arise because flame spectrometry has a small dynamic range and the sensitivity of the technology is too high for the determination of the minor components hence for the practical analysis of ceramic materials a series of dilu- tions is required in order to obtain absorbance or emission readings that fall within the linear range of the calibration graph.The principles of flow injection analysis have been applied in recent years in all branches of analytical hemi is try.^.^ One area in which the application of flow injection (FI) methods improves the performance of the analytical technique is atomic spectrometry."-' The use of FI in conjunction with flame spectrometry can not only provide on-line pre-concentration dilution or treat- ment of the samples but can also extend the linear calibration range. A simple way to dilute the samples is by the injection of a small sample volume using normal-" or pulse-timed solenoid injectors,I3 with the use of a high volume of the transport medium. l 4 Other strategies that have been proposed in order to extend the dynamic range of the calibration graph are based either on the treatment of the absorbance peaks," or in the use of an appropriate manifold. In the latter different systems have been proposed such as a variable dispersion manifold with di- lution coils of different sizes;I6 the use of a multi-line FI system with zone sampling;I7 a cascade dilution;Ix a gradient chamber based on zone ampl ling;'^.^^) or split-zone flow injec- tion.I On the other hand it is true that the dilution occurring in a mono-line FI system is highly reproducible. Therefore by dis- crete dilution of the samples direct analysis of the minor com- ponents using simple manifolds is possible.22-2s In this paper a versatile manifold that allows on-line dilu- tion of samples for analysis by flame atomic spectrometry is proposed and the determination of aluminium iron calcium magnesium sodium and potassium in eight different ceramic materials is described.Experimental Apparatus and Reagents A Perkin-Elmer 5000 and a Perkin-Elmer 2380 flame atomic absorption spectrometer have been used to carry out the emission and absorbance measurements. With the latter calcium iron and magnesium hollow cathode lamps were used. The experimental parameters employed are listed in Table 1 . In order to provide sample introduction and on-line dilu- tion of the samples the manifolds shown in Fig. 1 were em- Table 1 Instrumental parameters employed for the determination of aluminium iron. calcium magnesium sodium and potassium Element Parameter Al Fe Ca Mg Na K Wavelength / nm 309.3 248.;1 422.1 285.2 589 766.5 Technique FP* AA ' AA AA FP FP Flame N,GC,H Air-C2HI Air-C,H2 Air-C2H2 Air-C,H Air-C,H Burner height (arbitrary units) 6 6 5 5 5 5 Burner angle (") 180 I80 180 180 I80 180 * FP Flame photometry. AA Atomic absorption.234 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL 1991 VOL.6 I - 1 I I I ( C) ( d ) I I I I r\ - 1 R U I Fig. 1 Manifolds employed to carry out the on-line dilution of ceramic samples prior to their analysis by FAAS. P. Peristaltic pump; I. injection valves; C three-way connector; MI and M2. mixing chambers of 1 and 3 ml volume. respectively; S magnetic stirrer; and R recorder ployed. A Gilson Minipuls HP4 peristaltic pump ensured a constant carrier flow of water through a double channel. A double injector allowed the simultaneous introduction of both sample and lanthanum solution.A merging point pro- moted the mixing of both channels before the streams passed through a stirred mixing chamber. This system pro- vided on-line dilution of the samples. If a higher dilution was required two chambers were used. After dilution samples were fed to the atomization system directly and the absorbance peaks were recorded. A Rheodyne injection valve Type 50 with various fixed volume loops was used for sample introduction. A Y-shaped three-way connector (Omnifit) was used as the merging point. The mixing chambers were laboratory-built from polytetrafluoroethylene (PTFE) according to a model previously patented for the on-line treatment of samples.'h The chambers were composed of two parts a lower cylindrical portion with a 12.2 mm i.d.and a height of either 16.8 or 24.3 mm and an upper part with a slight dome which ensured Y r 1 Fig. 2 Geometry of the mixing chambers employed Table 2 Preparation of the lo00 pg I-' stock solutions Standard Masslg Salt Acid Mg 3.9 I60 MgC 1 ,.6H,O - 25 ml 1 mol dm- HCI Ca 2.4973 CaCO Fe 1.oooO Fe 25 ml concentrated HNO 25 ml concentrated HCI+ Al 1.oooO Al K 1.9070 KCl - Na 2.5420 NaCl - drops of HNO the escape of any air bubbles. Fig. 2 shows the geometry of the mixing chambers used. All the connecting tubes were of PTFE with 0.8 mm i.d. Stock solutions of magnesium calcium iron potassium and sodium (lo00 mg 1 - I ) were prepared by dissolving the appro- priate analytical-reagent grade salt in de-ionized water as summarized in Table 2 and a 1 % lanthanum solution was pre- pared from LaCI,.7HI0.General Procedure Digestion of the sample Weigh 0.3 g of sample into a platinum crucible and add 1.5 gJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL 1991 VOL. 6 235 Table 3 Normal content of aluminium iron calcium magnesium sodium and potassium in different ceramic matrices Concentration (for 0.3 g of sample in 250 ml)/mg I-' Matrix Clay Kaolin Quartz Felspar Varnish Mayolica Stoneware Porcelain Al 110 225 2 108 64 95 116 146 Fe Ca Mg Na K 7 0.9 2 4 17 4 1 0.7 2 7 0.4 0.9 0.1 1.2 0.8 1 3 0.4 17 105 1 8.7 5 10 39 4 102 4 3 9 10 2 2 7 26 2 1 1 11 34 Table 4 Effect of the manifold employed on the dynamic range of the determination of magnesium by FI-flame atomic absorption spectrometry. Manifold configurations employed ( h ) 1 ml mixing chamber; (c) 3 ml mixing chamber; (6) 1 ml plus 3 ml on-line mixing chambers; and ( e ) 3 ml plus 1 ml on-line mixing chambers.Calibration graph equations are given in absorbance units and the concentration expressed in mg I-'. The dynamic range has been established from the characteristic concentration obtained in the experimental conditions in the upper part of the calibration graph Manifold Dynamic range/ Calibration Regression mg I-' graph equation coefficient ( h ) 0.17-5 A = O.OOO9 + 0.0251. 0.9996 ( 0.13-10 A = 0.002 + 0.034~ 0.9998 (4 0.12-30 A = 0.002 + 0.036~ 0.9998 (el 0.16-50 A = 0.01 + 0.028~ 0.9997 (4 0.1 1-40 A =-0.03 + 0.039~ 0.998 of sodium carbonate (Na,CO,) or potassium carbonate (K2C0,) and 0.3 g of boric acid.Introduce the crucible into a muffle furnace at 1100 OC after 40 min allow to cool to 700- 800 "C. Transfer the contents of the crucible into a 250 ml por- celain evaporating basin add 50 ml of HCI (1+1 v/v) and stir until the solids are completely dissolved. Place the basin under an infrared (IR) lamp and evaporate to dryness at 130 "C. Add 5 ml of concentrated HCI to re-suspend the silica (SiO,) and then dilute to 100 ml with HCI (1+1 v/v). Filter using a red- band without ash Schuller filter-paper and dilute to 250 ml. Batch analysis Samples previously fused were diluted to an appropriate volume in order to obtain absorbance readings within the linear part of the calibration graph of each element. Hence di- lutions of from 1+4 to 1+99 of the previously obtained solu- tions were made.Alternatively sodium or potassium could be added as an ionization buffer and for the determination of calcium and magnesium lanthanum could be added as an in- terference buffer. Flow injection Appropriate volumes of the previously digested samples were injected into an FI manifold simultaneously with an equal volume of I % lanthanum solution. For each element in each matrix the manifold and the volume of sample injected can be modified in order to obtain a linear relationship between absorbance readings and the concentration of the element. Results and Discussion Effect of the Manifold Employed on the Dynamic Range The content of aluminium iron calcium magnesium sodium and potassium in ceramic materials obviously varies in different matrices. Table 3 summarizes the normal con- centration of each element found in various ceramic materi- als (in mg I-') for a dilution level of 0.3 g of sample in 250 ml.From these data and taking into account that the upper level of the calibration graphs in flame spectrometry corresponds to 100 mg I-' of aluminium 5 mg I-' of iron and calcium 2 mg I-' of potassium I mg I-' of sodium and 0.5 mg 1-' of magnesium it can be concluded that it is necessary to carry out different di- lutions for the determination of each element in each type of sample. One of the properties of the FI manifolds is that their com- ponents can be easily changed and so a simple manifold as indicated in Fig. 1 can be arranged in order to use ( a ) a dilu- tion coil 2 m in length without the use of a mixing chamber; ( h ) two short coils and a small ( 1 ml) stirred mixing chamber; (c') two short coils and a stirred chamber with a high volume (3 Table 5 Calibration graph data obtained for the determination of aluminium iron calcium sodium and potassium using different manifolds for the on- line dilution of samples. Dynamic range has been established from the characteristic concentration obtained in the experimental conditions in the upper part of the calibration graph Element Manifold Injected volume/pl Carrier flow/ml min-' Dynamic range Calibration graph equation* Regression coefficient 8.2 8-250 E= 0.002 + 0.00055~ 8.2 4-250 E= 0.002 + 0.00 1 I c 8.2 3.7-250 E= 0.005 + 0.012~.8.4 0.61 2 A= 0.0002 + 0.0074~ 8.4 0.1-12 A= 0.005 + 0.032~ 8.4 0.3-1 2 A=-0.0007 + 0.0 17~' 8.4 0.5-10 A= 0.006 + 0.0096~ 8.4 0.2-10 A= 0.0 19 + 0.020~ 8.4 2.2-1 26 A= 0.001 + 0.002~ 0.9996 0.9994 0.9995 0.9998 0.999 1 0.9996 0.9997 0.999 1 0.9999 Na (('1 100 8.2 0.2-14 E= 0.0 19 + 0.020~ 0.9993 8.2 2.2-140 E= 0.002 + 0.002~ 8.2 0.7-50 E= 0.0009 + 0.0064~ 8.2 0.3-20 E= 0.002 + 0.014~ 8.2 0.2-10 E= 0.003 + 0.023.0.9995 0.9996 0.9997 0.9998 * A Absorbance in absorbance units; E emission in emission units.236 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL 1991 VOL. 6 D 5 min - c- Time Fig. 3 Effect of the dilution levels on the magnesium peak shapes A 4; B 8; C 16; and D 32 mg I-'. The manifold employed in each instance in- cludes A a 1 ml volume mixing chamber [Fig. I(h)]; B. a 3 ml mixing chamber [Fig. 1 (c)]; C I ml plus 3 ml on-line mixing chambers [Fig. I(d)]; and D 3 ml plus I ml on-line mixing chambers (Fig.I(e)l ml); (6) two on-line stirred chambers the first being that with the lower volume ( 1 ml) the second with the higher volume (3 ml); and ( e ) two on-line stirred chambers the first being the higher volume chamber (3 ml) and the second the lower ( 1 ml) (see Fig. I.) The five configurations of the manifold allow different dilu- tions of the same sample and so can provide different dynamic ranges for the determination of the elemental compo- sition of a sample by flame atomic spectrometry. Magnesium has been used as a test system in order to deter- mine the effect of the FI manifold on the upper level of the cal- ibration graphs. By using an injected sample volume of 100 pI and a carrier flow-rate of 8.2 ml min-I a series of standards was injected into the manifold and the corresponding calibra- tion graphs calculated from the data obtained.Table 4 sum- marizes the equations found and shows that the dynamic range can be increased from 5 mg I-' using a dilution chamber with a 1 ml volume to 50 mg I-' using the 3 ml plus the 1 ml on-line dilution chambers. It is interesting to note that using manifold configuration (6) (in Fig. l) a good linear relationship is obtained for 0.12-30 mg I-' of magnesium but a poor relationship for 0.1 1-40 mg I-' therefore the use of the 3 ml chamber before the 1 ml chamber [Fig. l(e)] is preferable as this arrangement provides a greater sample dilution and hence a better dynamic range. The changes in the manifold can be made more easily and hence are more convenient than the use of different flame pa- rameters.They therefore allow the absorbance measurements to be performed using the optimal experimental conditions. However it is necessary to indicate that the use of greater dilu- tion levels requires a longer time delay and that corresponding- ly the peak shapes are broader (see Fig. 3). From the data in Table 3 and the results obtained for magne- sium the appropriate manifolds have been selected for the de- termination of the other five elements. The details from the corresponding calibration graphs obtained are summarized in Table 5. Effect of the Injected Sample Volume Table 5 shows that the injection of larger sample volumes im- proves the sensitivity of the analytical method.A study of the influence of sample volume on the absorbance values obtained for all the elements determined has been carried out using the most appropriate manifold configuration. The results obrained (Fig. 4) show that a dramatic increase in absorbance or emission is produced when the injected volume is increased from 100 to 500 p1 but the dynamic range can also be reduced. However this effect varies for the differ- ent elements considered. The loss of linearity in the calibration graphs are shown by the poorer regression coefficients. This effect is most marked for the manifold in which there is no di- lution chamber. Effect of the Carrier Flow-rate In order to obtain stable and reproducible results in FI-flame atomic spectrometry it is necessary to work with a pump flow- rate that is greater than the natural aspiration rate of the nebu- lizer." The analytical sensitivity in FI-flame atomic spectro- metry has been previously shown to increase when the pump flow-rate increases.2x By using the most appropriate manifold configuration for each element considered the influence of the pump flow-rate on the absorbance has been studied.Fig. 5 shows that sensitivity can be increased by increasing the carrier flow-rate so that the time taken for the analysis can be improved without reducing the dynamic range of the cali- bration graph. 1 A 100 500 Volume injected/pl 1 ooc Fig. 4 Effect of the volume of sample injected on the absorbance of A 20 mg I-' of magnesium; B I0 mg I-' of iron; F 10 mg I-' of calcium and on the emission of C 5 mg I-' of sodium; D 200 mg I-' of aluminium; and E 10 mg I-' of potassium I I 6 9 0- 3 Carrier flow-rate/mI min ' Fig.5 Effect of the carrier flow on the absorbance of F. iron (10 'mg ml-I); C . calcium (26 mg ml-I); and D magnesium (20 mg m1-I). and o n the emission of A aluminium (250 mg mi-'); B. sodium (8 mg ml-I); (and E. potassium (I0 mg ml-')JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL 1991 VOL. 6 237 Analysis of Real Samples Samples of ceramic materials of different types have been treated as described under General Procedure and analysed by both flame atomic spectrometry using conventional off-line di- lution of the samples with continuous aspiration into the flame and the proposed FI procedure. The data obtained are given in Table 7; both procedures provided similar results.However the FI method is less time consuming avoids the accumulation of salts in the nebulizer because of the dilution of the samples and the use of the merging point significantly reduces the lan- thanum consumption and hence reduces the cost of the analy- sis. Table 6 Analytical parameters of the FI determination of Al Fe Ca Mg Na and K in ceramic materials. Sensitivity has been calculated from the slope of the calibration graph Element Manifold Sensitivity LOD*/ RSDt mg 1-I (%) A1 ( 0 ) 0.55 a.u.e. mg-' 1' 1.5 0.84 Fe ( 0 ) 0.92 cm mg-' 1 0.09 0.63 Ca ( h ) 0.15 cm mg-I 1 0.31 1.34 Mg (6) 0.47 cm mg-'l 0.17 0.69 Na ((') 0.37 cmmg-' 1 0.24 0.75 K ( c ) 0.1 I cm mg-' 1 0.73 2.12 * LOD limit of detection fork = 2.t RSD relative standard deviation. t a.u.e. arbitrary units of emission. Use of the Dilution Profile for Calibration Purposes The use of dilution chambers has been shown to allow the use of an absorbance-time profile for a single standard as a contin- uous calibration Fig. 6 shows as an example the absorbance-time profile of a concentrated standard of magnesium and the absorbance peaks corresponding to various other standards. The first part of the absorbance profile cannot be used for calibration pur- poses but the absorbance values corresponding to different times in the tailing part of the graph can be used. The relationship between concentration and time corre- sponds to an exponential equation c=c,[ l-exp(-ut/V )] ( 1 ) where c is the effluent concentration at time f; c is the concen- tration of the calibration standard; u is the flow-rate; and V is the volume of the manifold (including the mixing chamber).' However a series of practical problems related to the exact determination of the experimental parameters disturbs the use of the theoretical dilution profiles.Empirical approaches of the Analytical Parameters The analytical parameters such as sensitivity limit of detec- tion and precision have been established for each of the elements considered using the most appropriate manifold configuration and FI parameter values. These data are sum- marized in Table 6. The sensitivity has been determined from the slope of the re- gression line of the calibration graph. The limit of detection corresponds to the concentration of the element in the digest- ed solution which provides an absorbance value equal to twice the standard deviation of the blank readings.The preci- sion has been derived from the relative standard deviation of ten measurements of a single sample containing a concentra- tion corresponding to the central part of the dynamic range. The data obtained show that the proposed procedure gives adequate sensitivity and precision. Table 7 Li2C0 Analysis of ceramic samples. ( 1 ) Samples fused with K,CO,; (2) samples fused with Na2C0,; (3) certified value; and (I) samples fused with Concentration found (%) CaO Na20 Sample* FI Batch FI Batch FI Batch FI Batch FI Batch FI Batch Clay sI (1) 29.17 27.25 (2) 29.18 27.56 0.17 0.15 0.19 0.15 0.30 0.33 0.30 0.33 0.41 0.48 - - 1.08 1.07 1.10 1.07 - - 1.88 1.85 - - Clay s2 (4) Kaolin (4) 34.01 33.82 0.17 0.1 1 0.23 0.34 0.82 0.90 0.14 0.15 0.13 0.11 5.72 4.01 0.32 0.31 0.35 0.37 0.12 0.13 0.04 0.02 0.18 0.13 0.06 0.04 Felspar sI (2) 18.95 17.30 0.51 0.49 0.05 0.06 11.40 11.24 0.13 0.14 Felspar s2 ( 1 ) 17.59 17.32 (2) 17.10 17.30 0.72 0.55 0.69 0.52 0.05 0.03 0.05 0.02 2.67 2.92 - - 0.17 0.10 0.13 0.1 1 - - 9.91 11.03 Felspar (3) 17.7 0.54 0.04 11.20 2.83 0.10 9.58 10.48 - - Varnish (4) 0.46 0.50 0.13 0.13 0.41 1.12 Stoneware s ( I ) - - 0.35 0.23 0.16 0.21 0.54 0.62 1.17 1.12 Stoneware s2 ( 1 ) 18.80 18.02 (2) 18.46 17.38 0.25 0.28 0.27 0.28 0.21 0.23 0.21 0.23 0.66 0.65 - - 1.13 1.12 1.15 1.19 - - 2.36 2.49 Porcelain s (4) 22.83 23.60 1.05 1.10 0.13 0.20 3.01 2.56 0.87 0.96 0.30 0.31 Porcelian s2 ( 1 ) 25.82 24.90 (2) 27.48 25.65 1.31 1.16 1.30 1.16 0.17 0.18 0.17 0.19 1.37 1.34 - - 0.33 0.26 0.33 0.28 - - 4.10 4.50 * s Sample 1; s2 sample 2.238 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL 1991 VOL.6 - Time A A J 5 min A' I I C ~ Fig. 6 Absorbance-time profiles of A a concentrated magnesium stan- dard (40 mg I-' of magnesium) and B C and D corresponding to standards containing 30 20 and 10 mg ml-' of magnesium respectively Table 8 Equations of dilution obtained for each manifold Manifold Element Maximum Equation of Regression standard concen- dilution coefficient tration/mg ml-' ( h ) Ca I26 c = 1.08~0.91' 0.996 (($1 Na 14 c. = 1.14 x 0.92' 0.96 (4 Mg 30 c = 2.15 x 0.96' 0.992 (el K 140 c = 1.12~0.96' 0.990 type proposed by Olsen et alez9 can be used more easily.The empirical relationship between the absorbance at each time value of the dilution profile using the most concentrated standard and the experimental absorbance values found by discrete injection (in the same manifold) of standards with a known concentration provides a dilution equation c=AB' (2) in which A and B are empirical coefficients where A is directly proportional to the concentration of the standard injected and B depends on the manifold volume and the flow-rate employed and t is the time (in seconds). Using this empirical equation the dilution absorbance- time profile can be used as the calibration graph to relate the absorbance values obtained to the concentration of the sample. The different dilution equations obtained for each manifold configuration considered are shown in Table 8.It has been previously reportedz5 that the same empirical equa- tion for the dilution profile could be used for different ele- ments when using the same manifold with the same injected volume and carrier flow while taking into account the differ- ence between the concentration of the standard used for cali- bration. Conclusions The studies carried out demonstrate that the appropriate selec- tion of a simple manifold permits a rapid determination of a range of elements at different concentration levels in the same sample without off-line dilutions and that this approach can be useful in applied analyses. 1 2 3 4 5 6 7 8 9 10 1 1 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 .2 7 .2 8 29 References Bennet H.and Hawley W. G.. Methods of Silicate Analysis. Aca- demic Press London 2nd edn. 1965. Singer F. and Singer S. S. Industrial Ceramic Chapman & Hall London 197 1. Buckley F. Ramsey M. H. Rooke J. M. Hughes H. and Norman P. J. Anal. At. Specworn. 1988,3 203R. Voinovitch I. Analyse des Sols Roches et Ciments Masson Paris 1988. Astraitsis J. Zachariadis G. A. Dinitrakonidi E. D. and Limeonov V. Fresenius Z. Anal. Chem. 1988,331,725. R;%ka J. and Hansen E. H. Flow! Irtjection Analysis Wiley New York 1988. Valcarcel M. and Luque de Castro M. D. Flow Injection Analysis. Principles and Applications Ellis Horwood Chichester 1987. Tyson J. F. Analyst 1985 110,419. RiiiEka J. Fresenius Z. Anal. Chem. 1986,324. 745. Tyson J. F. Anal. Chim. Acta 1988,214,57.Flow Injection Atomic Spectroscopy ed. Burguera J. L. Marcel Decker New York 1989. Burguera J. L. Burguera M. and Alarcon 0. M. J . Anal. At. Spec- ti'orn. 1986 1.79. Burguera J. L. Burguera M. Rivas C. de la Guardia M. and Sal- vador A. Anal. Chim. Acta 1990,234,253. Basson W. D.. and van Staden J. F.. Fresenius Z. Anal. Chem. 1980 302. 370. Tyson J. F. Analvst 1984 109 3 19. Tyson J. F. Mariara J. R. and Appleton J. M. H. J. Anal. At. Spec- trom. 1986 1,273. Reis B. F. Jacintho J. Moratatti J. Krug F. J. Zagatto E. A. G. Bergamin F. H. and Pessenda L. C. R. Anal. Chim. Acta 1981 123 22 I . Whitman D. A.. and Christian G. D. Talanta 1989,36,205. Toei J. Anal. Lett. 1988 21 1633. Garn M. B. Gisin M. Gross H.. King P. Schmidt N. and Thommen C. Anal. Chim. Acta 1988,207,225. Clark G. D. RhziEka J. and Christian G. D. Anal. Chem. 1989 61 1773. RkiEka J. and Hansen E. H. Anal. Chim. Acta 1978 99,37. Stewart K. K. and Rosenfeld A. G. Anal. Chem. 1982,534,2368. Gisen M. Thommen. C. and Mansfield K. F. Anal. Chim. Actu 1986,179 149. de la Guardia M. Carbonell V.. Morales. A. and Salvador A Fresenius Z. Anal. Chem. 1989,335975. de la Guardia M. Morales A. Carbonell V. and Salvador A. Spanish Pat. 9002644 1990. Brown M. W. and RkiEka J. Analjist. 1984 109 1091. Carbonell V. de la Guardia M. Salvador A. Burguera J. L. and Burguera. M. Microc~hem. J. 1989,40 233. Olsen. S. RiziEka. J. and Hansen E. H. Anal. Chim. A m 1982 136. 101. Paper- 0104 I86F Recei\?ed September- 13th I990 Accepted December- loth 1990

ISSN:0267-9477    

DOI:10.1039/JA9910600233

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL 1991 VOL. 6 239 Optimization of Cold Vapour Atomic Absorption Spectrometric Determination of Mercury With and Without Amalgamation by Subsequent Use of Complete and Fractional Factorial Designs With Univariate and Modified Simplex Methods George A. Zachariadis and John A. Stratis* Laboratory of Analytical Chemistry Department of Chemistry Aristotelian University Thessaloniki 54006 Greece Experimental conditions for the determination of Hg with commercially available purpose-built apparatus were opti- mized by sequential use of two multiple parameter methods based on a factorial design and a modified simplex procedure. The responses that were evaluated to determine the optimum conditions were the peak height and peak area of the Hg signal.The significance of the effects was tested using the analysis of variance (ANOVA) at a 99% level of significance. Interactions observed between the parameters were quantitatively evaluated and dis- cussed. The flow-rate of air volume of sample solution use of a desiccant and the interactions between these pa- rameters in the determination of Hg by cold vapour atomic absorption spectrometry (CVAAS) without amalgamation were optimized according to a complete 23 factorial design and a univariate method. The experi- mental design was also considered for the determination of Hg after amalgamation on an Au-Pt gauze. The flow- rate of nitrogen mass of the amalgamator trapping time releasing time and interactions between them were sta- tistically evaluated according to a fractional factorial design (half-replicate of a complete 24 factorial design) and subsequent use of the modified simplex method.This approach for partial optimization of a commercial system is rapid and has many advantages over simple univariate methods. An absolute detection limit of 0.33 ng of Hg was achieved using the amalgamation technique for a total solution volume of 50 ml. This is comparable to the limits obtained by univariate methods of optimization. An approximately 1 0-fold improvement in the detection limit was observed with this technique in comparison with the direct method. Keywords Cold vapour atomic absorption spectrometry; mercury determination; gold-platinum amalgamator; fac- torial designs; modified simplex The application of modem statistical methods of experimental optimization such as factorial designs or simplex methods in analytical procedures is rare although factorials have been in use since about 1960 and simplex methods since 1969.Massart et al.‘ have produced a comprehensive account which covers more than 30 references on applications of complete and fractional factorials in various analytical procedures in ad- dition to some for simplex methods. The original simplex method was modified in order to reach the optimum conditions sooner giving the so called ‘modified simplex’ method. Long2 has provided detailed information on the application of the simplex method in analytical chemistry while Deming and Morgan3 have reviewed the application of the method up until 1973. Factorial designs have an advantage over simplex methods in that in the region preceeding the optimum a large amount of quantitative information can be obtained about the significance of the various effects and interactions However both strategies may fail if there is more than one optimum.Also factorial methods can deal with non-quantitative para- meters (e.g. experimental circumstances or attributes) while a simplex lacks this possibility. One obvious disadvantage is the large number of experiments required when several factors are examined but this can be minimized by the use of fractional factorials. For the simplex this is not a serious limitation because incorporation of many factors does not increase the number of experiments to a large extent but the difficulty of si- multaneously changing all the factors remains.Another serious disadvantage compared with simplex methods which has been discussed previously,J is that factorial experiments cover a pre-defined region so there is a real possibility of mis- interpreting the position of the optimum. This problem appears * To whom correspondence should be addressed. in situations where the initial values of the effects are too close together to give a significant difference or are too far apart giving a large but useless significant difference. The purpose of this work was the application of the above methods to an experimental procedure and their sequential use in order to extract more information about the optimum conditions of an analytical determination. This combination was suggested by Morgan and Deming4 and was employed to avoid the disad- vantages of each method.It can be applied to other dynamic systems in addition to the cold vapour atomic absorption spec- trometric (CVAAS) determination of Hg. The analytical procedure selected for investigation was the determination of Hg by CVAAS using a commercially avail- able instrument (Perkin-Elmer Model MAS-SOA). The purpose was to investigate the combination of multiple para- meter methods in a real dynamic situation. This system proved to be ideal for such an investigation because a number of factors with obvious interactions between them affect the de- termination. In addition the method is not time consuming hence allowing easy duplication or triplication of the experi- mental results.Although the cold vapour technique in the atomic absorp- tion spectrometric determination of Hg is the most widely used method and has been optimized by many workersS-X using uni- variate methods use of the factorial design or modified simplex method has not been reported. On the other hand it is known that many factors affect the determination and interac- tions between some of these factors have been recognized. So the classical optimization strategy using a univariate method is insufficient alone to estimate and evaluate these interactions. Mercury can be determined by CVAAS without a pre- concentration step or after pre-concentration by amalgamation on a suitable noble or alloy such as Au-Pt. The Au- Pt alloy has been reported in the as being one of the best and is used in commercial instrumentation.240 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL 199 1 VOL.6 In this work a study of the MAS-5OA system without amal- gamation as it is intended to be used was carried out in addi- tion to investigations with a pre-concentration step on an amalgamator utilizing the statistical optimization methods dis- cussed above. The absolute amounts of Hg determined were in the range 10-100 ng in the former instance and 1-100 ng in the latter. Experimental Reagents All the reagents were of analytical-reagent grade (Merck pro analysi). The acids used were HNO (maximum Hg content O.OOO0005%) and HCI (Suprapur). The SnCI solution was prepared by dissolving 10 g of SnCI,.2H20 (maximum Hg content O.OOOOOl%) in 10 ml of 50% HCI and diluting to 100 ml with doubly de-ionized water.This solution was aerated for 15 min with nitrogen in order to minimize the Hg content. The desiccant used was Mg(CIO,),. A stock standard solu- tion of lo00 mg I-' of Hg" was prepared by dissolving 1.7081 g of Hg(N01)2.H20 in 5 ml of HNO and diluting to 1000 ml with doubly de-ionized water. Working solutions were prepared daily by appropriate dilutions of the stock solu- tion in 0.5% v/v HNO,. The amalgamator material was a gauze (0.25 mm mesh size with the diameter of the wire being 0.025 mm) made from an Au-Pt alloy (90% Au-10% Pt). G - J G- 1 r U U E Fig. 1 Mercury vapour trap with amalgamator. A Electrical supply ( I 10 V); B polytetrafluoroethylene (PTFE) caps PTFE fixation; C quartz tube (12 cm long 0.5 cm i.d); D thick glass wool; E Au-Pt gauze ( 1 cm long) F Ni-Cr wire spiral resistance (17 R) and G flow of nitrogen The purge and carrier gas for the determination without pre- concentration was air pre-purified to remove trace amounts of Hg by passing over a similar Au-Pt gauze.High-purity (99.99%) nitrogen purified in the same manner was used as the carrier gas in the procedure with the pre-concentration step. Instrumentation The determination of Hg was carried out using a Perkin-Elmer mercury analyser system MAS-SOA equipped with the origi- nal pump cylindrical plastic cell and mercury vapour lamp as illustrated in the manufacturer's These parameters were unmodified. The amalgamator accessory was connected to the MAS-5OA in the same manner as it can be connected to an MHS-20 system." The tube with the amalgamator is illustrated in Fig.1 and was constructed from a quartz tube ( 1 2 cm length and 0.5 cm i.d.) which was connected to the circuit liu PTFE screw caps at both sides. The Au-Pt gauze was placed in the centre of the tube with glass wool on both sides. The gauze evenly filled the interior of the tube to ensure quantitative amalgamation of the Hg vapour on the gauze. A Ni-Cr wire spiral (electrical resistance 17 R) was placed around the middle of the quartz tube and was connected to a I10 V d.c. electrical supply. After heating the resistance for 17-18 s a suitable temperature (>600 "C) was reached and the Hg vapour released from the amalgam. It was necessary to test the efficiency and repeatability of the whole trap.The temperature measurements were carried out with a Rh-Pt thermocouple (calibrated in the range 1W 1100 "C) by placing the thermocouple in the Au-Pt gauze. The capacity for Hg trapping was tested by placing a second amalgamator just after the first. The second amalgamator did not trap Hg in detectable amounts even after four subsequent cycles of measurements. The stability and repeatability of the resistance was tested because the surface of the Ni-Cr wire was gradually oxidized by atmospheric oxygen although the electrical resistance was not changed. The whole amalgamator was cooled after the determination by use of a continuous flow of nitrogen for 10 min. This time is the same as recommended by Bricker,Is although after 4 min the temperature of the alloy fell to below 250 "C.Below this temperature Welz and Schubert-JacobsI2 found that trapping of Hg was not achieved and water vapour did not condense; this observation is limited to matrix-free solutions. An altema- tive proposed by these workers was the use of fritted glass to trap the water droplets prior to the tube Hence two thick sec- tions of glass wool were placed inside the connecting tubing. The purpose of the first experiment was to test the thermal efficiency of the amalgamator in relation to the releasing time. The results are given in Fig. 2. The two graphs show the be- haviour of the amalgamator with two different nitrogen flow- rates and it is clear that at low flow-rates the required temper- ature is achieved faster. The next test was to establish the time needed to reach a temperature of 600 OC over the whole range of the flow-rates.As illustrated in Fig. 3 for a range of flow- rates of between 0.2 and 0.6 I min-' the required temperature is achieved in 17 s. 800 I 1 0 4 8 12 16 20 24 28 32 Ti me/s Fig. 2 Effect of firing time of the resistance (Ni-Cr wire) on the temper- ature of the Au-Pt gauze for two different flow-rates of nitrogen A 0.8 and B 2.0 1 min-I I" 0.2 0.6 1 .o 1.4 1.8 2.2 Flow-rate/l min-' Fig. 3 perature of 600 "C in the amalgamator Effect of nitrogen Row-rate on the time needed to achieve a tem- Good repeatability is attained after 4-5 firings and it is maintained for at least 30 firings. In practice it was found that even after 80 firings (18 s each) both the amalgamator and the resistance remained stable and did not need to be changed.All that was required was the careful cleaning of the Au-Pt gauze with hot dilute HNO and a blank firing before trapping of the Hg.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY. APRIL 199 1. VOL. 6 24 1 Procedure I Each sample contained 100 ng of Hg". The water-vapour trap was filled with 4 g of fresh Mg(C104)? and was used for eight determinations before replacement. GardneP reported some problems with this desiccant because when it becomes damp some losses of Hg occur but it is better than CaCl," and it is the desiccant usually employed for this purpose. The Hg vapour generation was performedI7 by reduction of Hg2+ with 1 ml of 10% SnClz solution. Air was used as the carrier gas in this procedure and a stable base line was achieved when air was passed through an amalgamator before entering the Dres- chel bottle.The factors affecting the direct determination of Hg can be classified into four groups chemical physico-chemical geo- metrical and mechanical. For the partial optimization in this work the factors of the first three groups were not optimized but the optimum values were taken from the available litera- ture. The main factors in the fourth group that were optimized were the flow-rate of air (A) the volume of the sample ( B ) and the presence or absence of the desiccant (C) before the Hg vapour entered the cell. Hence a complete factorial design of three factors at two levels (27 was selected" (Table l) to evaluate the significance of the above factors and possible in- teractions between them at a 99% probability level.Table 1 Table of signs in a complete 23 factorial design Effect Treatment Sum (1) a h ah c' a(* hc ahc x A - + - + - + - + I l + + - - + + .v2 + + + + .v3 AB + - - + + - - + .Y4 AC + - + - - + - + .vs ABC - + + - + - - + s7 Result v ?'2 Y3 r4 Y Y s Y7 Y s - - B C - - - - + + + + - - - - BC The experimental results were obtained in triplicate so as to allow for the possibility of estimating the interaction ABC and the residual error. Every factor is investigated at two levels so each factor loses 2-1=1 degrees of freedom. Consequently all the effects have 7 degrees in total. If there is no replication 8-1=7 degrees of freedom are available so no degrees of freedom are available for the estimation of the residual error.Replication of experimental results is then necessary in order to obtain extra degrees of freedom. If the experiments are re- peated in triplicate 24-1=23 degrees of freedom are available while the sum of the degrees for the particular effect remains as 7. So there are 23-7=16 degrees available for estimating the re- sidual error. Another approximate solution to the problem would be the incorporation of the third-order interaction into the residual error or expression of the residual error by means of this interaction based on the fact that the high-order interac- tions are usually negligible as described below. Analysis of variance (ANOVA) is a suitable test of significance for the statistical interpretation of the results. As described in the experimental section the univariate method was used to complete the optimization.In this instance the re- sponse was measured as either peak height or peak area of the Hg signal (in arbitrary units). The statistical interpretation was the same for the results obtained for peak height and peak area. Procedure I1 The Hg vapour generation was performed as in Procedure I. However in this part of the study an Au-Pt gauze was used as the amalgamator for the pre-concentration of the Hg vapour. Mertens and Althaus" stated that both nitrogen and argon were suitable as carrier gases although helium was found" to contribute less noise than other inert gases. In this procedure nitrogen after being purified through an amalgamator was used which eliminated the noise sufficiently.The flow-rate of nitrogen was kept constant during the trapping and was reduced to 0.1 1 min-' during the releasing period. This proce- dure was applied for two ranges of concentration 20-200 and 200-2000 ng 1-I. Four 'mechanical' factors were tested in this type of determination the flow-rate of nitrogen (A) mass of the gauze (B) trapping time (C) and releasing time (D). Hence a 24 factorial design should be used or a half-replicate (24- '=29. The volume of sample was not optimized and a 50 ml volume was employed throughout so that the simplex was not moved to a higher level (see below). This limitation exists because absorbance is the response used for the optimi~ation.'~ For a complete 24 design 24=16 experimental results are re- quired.The complete factorial with interactions was designed according to reference 4. The half-replicate was designed as detailed in Table 2. This is produced by incorporation of factor D to a complete 23 factorial by replacing one effect (e.g. ABC) with D . In this instance the third-order interac- tion ABC is considered to be negligible so the replacement is D=ABC. Also taken into account in this replacement is the fact that interactions of D are negligible as is seen from the procedure became the releasing time is obviously indepen- dent of the other factors. Because of the null interactions of D (releasing time) 'the aliases are confounded just theoretically ,4 and from the pairs of aliases of Table 2 only the first repre- sent the real effect. The experimental results were duplicated so as to obtain the degrees of freedom available for the esti- mation of the residual error.~~ ~ ~ ~ ~~~~ Table 2 Table of signs in a half-replicate of a complete 24 factorial ex- periment (fractional factorial) Effect Alias Treatment Sum A B C AB AC BC D Result BCD ACD ABD CD BD AD ABC Because some interactions between quantitative parameters proved to be significant the modified simplex method4 was used to complete the optimization for the two ranges of con- centrations being considered. In this instance the peak height of the Hg signal (instrument units) was the response used because this gave better reproducibility compared with the peak area. The variances were compared by use of the F-test. Results and Discussion Optimization of Procedure I The experimental conditions and results are presented in Table 3. The sequence in which the results were taken was random as shown in the second row of the table.The two levels for each factor are also described. The significance of the effects was tested with ANOVA and the results are shown in Tables 4 and 5 . In Table 4 the peak height results are presented and in Table 5 the peak area results. The mean sums of the squares are approximated in some instances. When the factor has a high value (high level +) the peak is increasing. This is expressed in the sign (+) of242 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL 1991 VOL. 6 Table 3 Experimental results of the complete two level-three factor fac- torial design in the determination of Hg without amalgamation (respons- es peak height and peak area) Parameter Treatment ( I ) a h ah ( 9 a(.hc uhc Sequence (random) 1 8 2 7 4 5 3 6 Air flow-rate A - + - + - + - + Volume B - - + + - - + + Desiccant C + + + + - - - - Peak height- I 127 125 98 96 130 126 101 100 I1 126 126 98 95 131 127 100 99 111 128 127 96 96 131 129 100 98 I 663 313 692 301 901 309 953 305 11 687 277 683 294 758 278 903 301 I11 674 240 628 274 852 297 860 296 Peak area- The low (-) and high (+) levels of the factors are the following Factor Unit Symbol (-) Value (+) Value Air flow-rate 1 min-’ A 0.6 2.0 Sample volume ml B 50 100 Desiccant - C No Yes Table 4 tion of Hg without pre-concentration Analysis of variance for peak height signals in the determina- Sum s s 2 .v3 x4 s g .v() s7 -22 -356 +34 +4 -6 4 8 (X,)* 484 126736 1156 16 36 16 64 ANOVA- Source of Degrees of Sum of Mean sum F, variation freedom squares of squares F 8.53 A 1 B 1 C 1 AB 1 AC 1 BC 1 ABC 1 Residual error 16 20.2 5280.7 48.2 0.7 1.5 0.7 2.7 15.3 Total 23 5370.0 * S significant.t NS not significant. 20.2 21.0 s* 5280.7 5500.7 S 48.2 50.2 S 0.7 0.7 NSt 1.5 1.6 NS 0.7 0.7 NS 2.7 2.8 NS 0.96 the sum. If the high value (+) of the factor causes a decrease in the peak this is expressed in the sign (-) of the sum. The value of the ratio F is also an indication of the power of the effect (e.g. 5500.7 describes a strong effect by factor B and 50.2 a serious effect by factor C). According to the ANOVA table of peak heights the follow- ing conclusions can be made at the 99% probability level.(i) The flow-rate of air ( A ) seriously affects the peak height ‘low flow-rate causes on increase in peak height’. This is owing to the low dispersion of the Hg vapour in the air stream when the flow-rate is low. (ii) The volume of the sample ( B ) strongly affects the peak height ‘small volume causes an increase in peak height’. As the volume of the sample decreases the aera- tion of the Hg vapour becomes spontaneous and a more con- centrated air-Hg stream is delivered to the cell. (iii) The desiccant (C) seriously affects the peak height ‘presence of the desiccant causes an increase in peak height’. It is obvious that the circulation of the dried air stream is advantageous under these conditions although the desiccant provides a me- chanical stop. (iv) There are no significant interactions Table 5 Analysis of variance for peak area signals in the determination of Hg without pre-concentration s 5 .x-s s -5766 -240 +I284 -126 -1 104 +204 -328 (.v,)’ 33246756 57600 1648656 15876 1218816 41616 107584 Sum S’ ,v* x3 X4 ANOVA- Source of Degrees of variation freedom A 1 B 1 C 1 AB 1 AC 1 BC 1 ABC 1 Residual error 16 Sum of squares 1385282 2400 68624 662 50784 1734 4483 21941 Total 23 1535910 *S significant.tNS not significant. Mean sum of squares 1385282 2400 68624 662 50784 1734 4483 1371 Fw F 8.53 1010.0 s* 1.7 NSt 50. I S 0.5 NS 37.1 S 1.3 NS 3.2 NS between the above parameters however incorporation of them in the residual error was investigated. 2 15.3+0.7+1.5+0.7+2.7 00 = = 1.05 ( 1 1 16+ 1 + 1 + ! + 1 According to equation (l) the evaluation of the effects is not affected (1.05 compared with 0.96).From the ANOVA table of the peak areas it is concluded that at the 99% probability level (a) the flow-rate of air (A) strongly affects the peak area according to the relationship ‘low flow-rate causes an increase in peak area’; (h) the volume of the sample ( B ) has no significant effect on the peak area; (c) the desiccant (C) seriously affects the peak area according to the relationship ‘existence of desiccant causes an increase in peak area’ which is related to a similar increase in peak height; and (6) the interaction between factors A and C is significant according to the relationship ‘presence of desiccant x lowering of the flow-rate causes an increase in peak area’. When a water vapour trap is used the flow-rate must be lowered in order to trap the vapours efficiently.From the analysis of variance of the peak heights it was concluded that no interactions were significant and hence the univariate method can be applied to give a better approxima- tion of the optimum region. On the other hand analysis of var- iance of the peak areas has revealed a significant interaction between the flow-rate (parametric factor) and the presence of the desiccant (a non-parametric factor). Because a non- parametric factor cannot be quantified the univariate method must be applied in a separate experiment. The experimental results of this approach are plotted in Figs. 4-7. The response (peak) was plotted on the y-axis and the factor being varied on the x-axis.In Fig. 4 the negative effect of flow-rate and the positive effect of the desiccant on peak height are shown. The optimum region is compressed at flow-rates ~ 0 . 8 1 min-’ because the peak area shows no changes in this region. The curve obtained in the presence of the desiccant is almost parallel to that with no desiccant because there are no interactions between the flow-rate and desiccant. Also the former curve is above the latter indicating the positive effect of the desiccant on the peak height. Fig. 5 il- lustrates the strong negative effect of sample volume on peak height which is demonstrated by the sudden change in direc- tion of the graphs with a negative slope. The parallel graphs again confirm the absence of interactions between flow-rateJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL 1991.VOL. 6 243 I 1 I 1 1 0 0.4 0.8 1.2 1.6 2.0 2.4 Flow-rate/l mi n-l Fig. 4 Effect of air flow-rate on peak height of 100 ng of Hg for a sample volume of 50 ml. A With desiccant; and B without desiccant w 0 25 50 75 100 125 150 Volume of sample/ml Fig. 5 Effect of volume of liquid sample on peak height of 100 ng of Hg for an air flow-rate 0.4 1 m i d . A. With desiccant; and B without desiccant 600 '""I f A loo' 1 I 1 J 0.2 0.6 1 .o 1.4 1.8 2.2 Flow-ratefl min-' Fig. 6 volume of 50 ml. A With desiccant and B without desiccant Effect of air flow-rate on peak area of 100 ng of Hg for a sample 5001 1 200 I 1 1 0 25 50 75 100 125 150 Volume of sample/rnl Fig. 7 Effect of volume of liquid sample on peak area of 100 ng of Hg for an air flow-rate 0.4 1 min-'.A With desiccant; and B without desiccant and desiccant. In Fig. 6 the strong effect of flow-rate on peak area is evident together with the interaction between the flow- rate and the presence of the desiccant. This interaction is dem- onstrated by the irregularities along the graphs. In Fig. 7 it can be seen that the effect of sample volume on the peak area of the Hg signal (arbitrary units) is not significant because the graphs show no evident gradient and must be regarded as being parallel to the x-axis. Calibration study in procedure I The base line of the spectra was more stable in the presence of the desiccant. This advantage together with an increase of peak heights and areas should be balanced against the disad- vantages of dampening the desiccant and the possibility of trapping the Hg.However it was decided that a desiccant would be used for the calibration study. The peak height appears to be free from interactions. The optimum conditions determined from the univariate experiment were flow-rate of air 0.4 1 min-I; volume of sample 25 ml; presence of a desic- cant; and signal measured using peak height. The peak height is expressed in absorbance units (AHg) and increases linearly with the absolute amount of Hg (in ng) present (XHg). The calibration graph is given by the equation The slope (sensitivity) is in good agreement with the experi- mental value of 6 ng of Hg which gave an absorbance of 1%. The detection limit is derived according to IUPAC Definitive Rules2" using the sum of the mean of eight blank measure- ments plus three times the standard deviation of these meas- urements.The absolute detection limit is 3.3 ng of Hg. The repeatability expressed as relative standard deviation from ten repeat determinations of 40 ng of Hg was 6.9%. AHg= 1 . 5 1 6 ~ 1 0 ~ + 5.886~10~X (2) Optimization in Procedure I1 The experimental conditions and the results obtained are pre- sented in Table 6. The results of the ANOVA experiments ex- pressed as peak height are shown in Table 7. The following conclusions can be made at the 99% probability level. (i) The flow-rate of nitrogen (A) strongly affects the peak height 'low flow-rate-increase in peak height'. The low flow-rate leads to a more efficient trapping of Hg vapours on the amalgamator gauze.(ii) The mass of the Au-Pt gauze (B) seriously affects the peak height 'small mass of gauze causes an increase in peak height'. A small mass of amalgamator causes a spontaneous release of Hg from the amalgam and produces a more concen- trated stream. However this behaviour depends on the actual value of flow-rate (see interactions below). (iii) The trapping time (C) seriously affects the peak height 'long trapping time causes an increase in peak height'. (iv) The interaction between the flow-rate and the mass of the gauze is significant 'low flow- rate x small mass causes an increase in peak height'. Hence with high flow-rates it is necessary to use a greater mass of amalgamator. (v) The interaction between the flow-rate and the trapping time is significant 'low flow-rate x long trapping time causes an increase in peak height'.(vi) The effect of the releas- ing time (D) above 18 s is not significant. Table 6 Experimental results of the fractional factorial (half-replicate of a two level-four factor design) in the determination of Hg after amalgama- tion on an Au-Pt gauze (response peak height) Parameter Treatment ( 1 ) ad hd ah cd ac hc abcd Sequence (random) 7 5 2 3 8 6 1 4 Nitrogen flow-rate A - + - + - + - + Mass of Trapping time C amalgamator B - - + + - - + + Releasing time D - + + - + - - + + + + + - - - - Peak height I 48 12 25 10 72 18 46 10 I1 45 15 27 8 80 14 42 15 The low (-) and high (+) levels of the factors are the following Factor Unit Symbol (-) Value (+) Value Nitrogen flow-rate 1 min-' A 0.6 2.0 Trapping time S C 60 I80 Releasing time S D 18 28 Mass of amalgamator g B 0.2 0.4244 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY.APRIL 1991. VOL. 6 270 240 v) 210 .- E 180 p 150 a Q 120 E \ c .- 90- 60- 30 I- Table 7 on an Au-Pt gauze Analysis of variance for peak height signals after amalgamation - - - - - - i- Sum .II X' .v3 .v4 .vs .Yo .v7 -283 -121 +lo7 +89 -83 -21 +25 (x,)' 80089 14641 11449 7921 6889 441 625 ANOVA- Source of Degrees of Sum of Mean sum F99 variation freedom squares of squares F 11.3 A I B 1 C 1 AB 1 A C 1 BC 1 D 1 Residual error 8 5005 915 716 495 43 1 27 39 73 Total 15 7702 *S significant. tNS not significant. 5005 550 S* 915 101 S 716 79 S 495 54 S 43 1 53 S 27 2.9 NSt 39 4.3 NS 9.1 From the analysis of variance of the peak heights it was concluded that the interactions between flow-rate and mass of Au-Pt gauze and between flow-rate and trapping time are significant hence an approximation of these parametric factors should be obtained by the modified simplex method.An approximation of the optimum conditions for flow-rate and trapping time was performed separately for 10 and 100 ng of Hg. The simplex procedures,"." with variable step sizes are shown in Figs. 8 and 9. In some instances the allo- cation of the vertices is limited by the actual readings on the flow-meter (to one decimal point). For this reason (a) flow- rates ~ 0 . 1 1 min-I were not tested; (h) some simplexes are not precisely isosceles triangles; and ( c ) the expansion or contraction factors (usually 2 or 0.5 respectively)" are vari- able.The position of the initial simplex was chosen accord- ing to the normal conditions used for the determination of Hg. However the size of this simplex was the minimum that could be considered reliable according to the significant figures of the flow-meter readings. From the optimum response surfaces given in the striped area of Figs. 8 and 9 the conditions selected to be most appro- priate are 120 s trapping time and 0.3 1 min-' flow-rate for 10 ng of Hg and 180 s and 0.2 1 min-' for 100 ng of Hg. 300 I \ 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Flow-rate/l min-' Fig. 8 Progress of simplexes to the optimum surface for nitrogen flow- rate and trapping time for 10 ng of Hg. The striped area is the maximum response surface. The vertex No. 16 is chosen as optimum.See text .- 150 120 90 60 30 0 0.1 0.2 0.3 0.4 Flow-rate/l min-' 1 0.5 Fig. 9 Progress of simplexes to the optimum surface for nitrogen flow- rate and trapping time for 100 ng of Hg. The striped area is the maximum response surface. The vertex No. 15 is chosen as optimum. See text The values for the trapping time are in good agreement with those found by Welz,'? and were selected as the minimum values on the optimum response surface on which the sim- plexes were recycled. Calibration study in procedure I1 Although the analysis of variance has shown that the factors A B and C should be optimized the mass of the amalgamator was not tested because 0.24.25 g of the gauze is the minimum amount required to fill the tube adequately and homogeneously." It should be re-stated that these small amounts apply only to the low flow-rates because for higher flow-rates of nitrogen a larger amount of amalgamator is required.I7 The releasing time chosen as the best was 18 s because a lower value is not recommended for increasing the temperature of the amalgamator as has been discussed under Instrumentation.The volume of the sample in Table 10 Optimum conditions according to the simplex procedure Parameter 1-10 ng of Hg 10-100 ng of Hg Flow-rate of N 0.3 1 min-' 0.2 1 min-' Mass of gauze 0.2 g 0.2 g Trapping time 120 s 180 s Releasing time 18 s 18 s this study was 50 ml. Finally the optimum conditions according to simplex procedure are applied as given in Table 10. Two calibration studies were required and the ranges chosen were 20-200 ng 1-I (1-10 ng of Hg absolute) and 200-2000 ng I-' (10-100 ng of Hg absolute).If the calibration for the second range is performed using the optimum conditions of the first range the graph is still linear but the sensitivity is poorer. The peak heights used to prepare the calibration graph are expressed in absorbance units. The equations of the calibration graph can be expressed as follows (3) (4) The slope (sensitivity) of equation (3) is in agreement with the experimental value of 0.8 ng of Hg which gave an absorbance of 1%. The calculation of the detection limit based on three times the standard deviation of the blank (eight repeat blank measurements) alniost corresponds to the values obtained at the 90% confidence level.'" The absolute detection limit for the concentration level 20-200 ng I-' is 0.33 ng of Hg. The rela- AHg= 1.036 x104 + 3.7 18 x10-3x~g A Hg = 1.333 x 1 0-3 + 4.062 x I O-jX,,JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL 199 I VOL.6 245 tive standard deviation was 5.5% for 6 ng of Hg absolute and 8.8% at the 10 ng level (ten repeat determinations). Conclusions The CVAAS determination of Hg is an excellent system in which to study optimization by experimental design. There are advantages of optimization procedures while using a commer- cial portable instrument for Hg determination to establish the detection limits after a pre-concentration step. One of the pur- poses of this work was to develop a sequential technique for ex- perimental designs that can be applied as rapid and efficient optimization schemes for various samples under different con- ditions when data given in the literature cannot be applied di- rectly.The sequential optimization strategy in the direct determina- tion of Hg with an MAS-5OA (procedure I) gave a detection limit of 3.3 ng of Hg. This limit was further improved by em- ploying amalgamation on an Au-Pt gauze (procedure 11) when the detection limit was 0.3 ng of Hg. To improve the de- tection limit further for the general cold vapour technique op- timization of more parameters is required e.g. the type of gauze and modification of the shape of the cell or improve- ment in the relative detection limits by increasing the sample volumes. It is recommended that modem statistical methods of exper- imental design are used in order to extract as much quantita- tive and qualitative information as possible from the procedure.Factorial designs are the first tool in this strategy in order to eliminate areas where subsequent research would not be useful and to evaluate the significance of various effects quantitatively. Care should be given to the selection of a suitable design and the number of replicate measurements required. The presence of significant interactions between the parame- ters in a dynamic method is also demonstrated for the determi- nation of Hg with and without amalgamation. Hence a simplex method is the second tool in an optimization process to reduce further the number of experiments needed. The authors express their gratitude to Bernhard Welz and Marrianne Schubert-Jacobs Perkin-Elmer Bodenseewerk Uberlingen Germany and V.Simeonov Faculty of Chemistry University Sofia Bulgaria for useful discussions and supply- ing the amalgamation material. 1 2 3 4 5 6 7 8 9 10 1 1 12 13 14 15 16 17 18 19 20 References Evaluation and 0ptimi:ation of Laboratory Methods and Analytical Procedures. eds. Massart D. Dijkstra A. and Kaufman L. Elsevier Amsterdam 4th edn. 1986 p. 23 1. Long D. Anal. Chim. Acta 1969,46 193. Deming S. and Morgan S. Anal. Chem. l973,45,278A. Morgan S . and Deming S. Anal. Chem. 1974,46 1 170. Wittmann 2.. Talanta 1981,28,271. Bouzanne M. Sire J. and Voinovitch I. Analusis 1979,7,62. Temmerman E. Dumarey R. and Dams R. Anal. Lett. 1985 18 203. Simeonov V. and Andreev G. FreseniusZ. Anal. Chem. 1983,314 761. Newton M. P. and Davis D. G. Anal. Lett. 1975,8,729. Aldrighetti F. Ar. Spectrosc. 198 1 2 13. Welz B. Melcher M. Sinemus H. and Maier D. At. Specrrosc. 1984,5,37. Welz B.. and Schubert-Jacobs M. Fresenius Z. Anal. Chem. 1988 331 324. Mertens H. and Althaus A. Fresenius Z. Anal. Chem. 1983 316 696. Model MAS-SOA Mercury Analyzer System Users Manual Perkin- Elmer Norwalk CT 1978. Bricker J. Anal. Chem. 1980,52,492. Gardner D. Anal. Chim. Acta 1980 119 167. Yamamoto Y. Kumamaru T. and Shiraki A. Fresenius Z . Anal. Chem. 1978,292,273. Fundamentals of Mathematics and Staristics. eds. Brookes C. Bette- ley I. and Loxston S. Wiley New York 1979 p. 457. Et'aluation and 0ptimi:ation of Laboratory Methods and Analytical Procedures eds. Massart D. Dijkstra A. and Kaufman L. Elsevier Amsterdam 4th edn. 1986 p. 275. IUPAC Compendium of Analytical Nomenclature eds. Irving. H . Freiser H. and West. T. Pergamon Press Oxford 1978 p. 1 17. Paper 9104392F Received October I I th I989 Accepted November 28th I990

ISSN:0267-9477    

DOI:10.1039/JA9910600239

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL 1991 VOL. 6 CUMULATIVE AUTHOR INDIEX FEBRUARY-APRIL 19911 Abell Ian 145 Abollino 0.. 119 Ah Abdalla H. 2 1 1 Apte S. C. 169 Barnes Ramon M. 57 Baxter Douglas C. 109 Beinrohr Ernest 33 Berglund. Ingemar 109 Berman Shier S. 19 Blades Michael W. 215 Blais Jean-Simon 225 Branch Simon 15 1 155 Bridenne Martine 49 Brindle Ian D. 129 Brindle Mary E. 129 Butcher David J. 9 Canals Antonio 139 Carbonell Vincente 233 Carre Martine 49 Chen Hengwu 129 Comber S . D. W. 169 Corns Warren T. 155 Dawson John B. 93 de la Guardia Miguel 233 de Loos-Vollebregt Margaretha T. C. 165 Diaz de Rodriguez Olga 49 Ebdon Les 151 155 Fang Zhaolun I79 Forbes Kimberely A.. 57 Ford Mick. 15 1 Foulkes Mike I5 1 Franks Jeff 145 Frech Wolfgang 109 Furata N. 199 Garden Louise M.159 Gardener M. J. 169 Gervais Lyne S. 41 Gunn A. M. 169 Hassell D. Christian 105 Hernandis Vincente I39 Hieftje G. M. 191 Hill Steve 155 Holcombe James A. 105 Huang Degui. 2 15 Huang Min 221 Huyghues-Despointes Alexis 225 Igarashi Yasuhito 205 Irwin Richard L. 9 Jiang Zucheng 22 1 Kibble Helen A. B. 133 Kim Chang-Kyu 205 Kluckner Paul D. 37 Le Xia-chun 129 Ledingham Kenneth W. D. 73 Littlejohn David 159 Luong Van T. 19 L’vov Boris 19 1 Majidi Vahid 105 Marot Yves 49 Marshall John 145 159 Marshall William D. 225 Mentasti E. 119 Mermet Jean-Michel 49 Michel Robert G.. 9 Millward Christopher G. 37 Miyazaki Akira 173 Momplaisir Georges Marie 225 Mora Juan 139 Morita Shigemitsu 205 Ng Kin C. 21 1 Offley Stephen G. 133 O’Neill. Peter 15 1 155 Peng Runzhong 165 Poluzzi Vanes 33 Porta V. 119 Prell Laurie J. 25 Rapta Miroslav 33 Redfield David A. 25 Reszke Edward E. 57 Rowbottom William H. 123 Salin Eric D. 41 Salvador Amparo 233 Sampson Barry 115 247 Sanz Angel 233 Sarzanini C. 119 Seare Nichola J. 133 Seki Riki 205 Singhal Ravi P. 73 Slavin Walter 191 Sperling Michael 179 Stratis John A. 239 Sturgeon Ralph E. 19 Styris David L. 25 Taddia Marco 33 Takahashi Junichi 9 Takaku Yuichi 205 Tao Hiroaki 173 Tiggelman Johan J. 165 Tsumura Akito. 205 Tye Chris 145 Tyson Julian F. 133 Uden Peter C. 57 Welz Bemhard 179 Willie Scott N. 19 Winefordner James D. 2 1 1 Yamamoto Masayoshi 205 Yamasaki Shin-ichi 205 Zachariadis George A. 239 Zeng Yun’e 22 1

ISSN:0267-9477    

DOI:10.1039/JA9910600247

出版商:RSC    

年代:1991

数据来源: RSC