摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL. 6 60 1 High Speed Photographic Study of Plasma Fluctuations and Intact Aerosol Particles or Droplets in Inductively Coupled Plasma Mass Spectrometry Royce K. Winge J. S. Crain* and R. S. Houkt Ames Laboratory US Department of Energy Department of Chemistry Iowa State University Ames /A 50011 USA Cine-films of the inductively coupled plasma were taken at 3000 frames s-I while the plasma was sampled for mass spectometry. The axial channel expanded and contracted periodically at frequencies of 260-300 Hz depending on the operating conditions. The frequency of the observed fluctuations decreased as the separation between the torch and sampling cone increased. With a concentric nebulizer emission from vapour clouds surrounding the aerosol droplets or particles was observed flowing along the axial channel and into the sampling orifice.Keywords High-speed photography; plasma fluctuation; aerosol particles and droplets; inductively coupled plasma mass spectrometry Although inductively coupled plasma mass spectrometry (ICP-MS) is very sensitive and selective the precision and stability of the instruments used are not as good as those obtained with their optical predecessors. Improved preci- sion would facilitate many analytical applications of ICP- MS particularly isotope ratio measurements. Characteriza- tion of the noise sources in ICP-MS could provide the basic information necessary for such improvements. Douglas' showed that the factors affecting precision varied with signal magnitude i.e.counting statistics limited the preci- sion at low count rates whereas flicker noise and drift were more important at high count rates. Previous studies of noise power spectra in ICP-MS revealed peaks at discrete frequencies of several hundred H z . ~ ~ ~ Similar peaks are seen in noise power spectra in ICP atomic emission spectro- metry (ICP-AES).4-8 Photographic studies in this laboratory have shown that these noise peaks are caused by physical fluctuations in the plasma i.e. by vortices originating at and progressing along the boundary between the flowing plasma gases and the static ambient atm~sphere.~ This work shows that these plasma fluctuations also occur outside the sampling orifice in ICP-MS at frequencies consistent with those measured previously in the noise spectra of the ion signal.Such fluctuations have also been described in black and white photographs published recently by FurutalO in a paper which dealt primarily with precision in isotope ratio measurements. Recent studies of time-resolved emission signals from ICPs by Olesik and c o - w o r k e r ~ ~ ~ - ~ ~ and Cicerone and F a r n ~ w o r t h ~ ~ indicated that undissociated wet droplets or solid particles may be more important and persist longer in the ICP than previously thought. The present paper shows photographs of emissions from vapour clouds surrounding these droplets or particles for one nebulizer type. These photographs illustrated that the particles are readily drawn into the sampling interface. Experimental The ICP-MS system was the same as that used for previous studies of noise spectra.2 The operating conditions and facilities are identified in Table 1.For photographic clarity the plasma was retracted to sampling positions correspond- ing to 20 and 25 mm betweeen the sampling cone and the nearest portion of the load coil; normal sampling positions for analysis with this device lie in the 10-15 mm range. The framing rate of the camera was nominally 4000 frames s-l. Approximately one half of each 30.5 m roll of film was consumed however by the initial acceleration of the film to this nominal rate. The framing rate for the sequences shown in this manuscript correspond to approximately 3000 frames s-l as determined from red timing marks at 1 ms intervals along the edge of the film.Some of the sequences shown in this manuscript were edited by the authors e.g. successive prints represent every second or third frame from the original film in some instances. Thus the time intervals between the prints varied for the different figures and is indicated in each caption. A photograph of the sampling orifice is provided in Fig. 1. The copper sampling cone employed for the bulk of the work reported here was rounded in contour near the orifice in contrast to the more sharply pointed orifices in use for example on Perkin-Elmer SCIEX ICP-MS instruments. Cine-films showing the use of both tip styles revealed no differences attributable to the geometry of the orifice tip. Also the intense induction region of the plasma was masked by the shielding box thus simplifying the photo- graphy of the plasma zones of interest.In one experiment the plasma was photographed from two orthogonal directions on the same film. A mirror positioned just below the gap between the torch and the sampler (Fig. 2) permitted simultaneous photography of the two views. *Present address Los Alamos National Laboratory Group ?To whom correspondence should be addressed. CLS- 1 MS G740 Los Alamos NM 87545 USA. Fig. 1 Photograph of sampling cone602 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 1991 VOL. 6 Table 1 Experimental facilities and operating conditions Ultrasonic nebulizer (Figs. 3 4 and 6) Concentric pneumatic nebulizer (Figs. 7 and 8) ICP torch Ar flow rates Sampling cone Camera (ref. 9) Film CETAC Model U-5000 uptake rate 2 ml min-l; desolvation heater 110 "C; desolvation condensor 0 "C; yttrium concentration 900 mg 1-I; gas flow rate 1.2 1 min-l uptake rate 0.5 ml min-l; spray chamber double pass Scott-type (ref. 15) uncooled; gas flow rate 0.9 1 rnin-'; yttrium concentration 5000 mg 1-1 Standard Fassel-type (ref.15) three turn load coil grounded near torch exit Outer gas 16 1 min-l; auxiliary gas used only on ignition Ames Laboratory construction (see Fig. 1) sampling orifice 0.9 mm diameter; usual sampling position 25 mm from load coil on centre Meinhard Type C1 Fastax Model WF3; 16 mm Ektachrome No. 7239 and 7251 \ Fig. 2 Mirror arrangement for photographing the ICP from two orthogonal directions. The sampler is not shown. The camera lens is shown on the right Unless specified otherwise a continuous flow ultrasonic nebulizer with desolvation was employed.Initially a concentric pneumatic nebulizer with an uncooled Scott- type double-pass spray chamber was used. This sample introduction system yielded large droplets or particles. As indicated in Table 1 a higher concentration of sample was required to yield clear photographs with the pneumatic nebulizer. Results and Discussion Photographs of ICP As is usual in ICP-MS the plasma was operated horizon- tally. In one film the system was photographed without a sampling cone to indicate whether the vortex or eddy phenomenon observed in vertical plasmas9 also occurred in horizontal plasmas. The fluctuations in this film were similar to those observed previously except that the red eddy from the yttrium oxide (YO) emission was strongest in the upper region of the plasma as illustrated by the edited sequence of three frames in Fig.3. The onset of the red eddy is shown in the top frame as the 'C-shaped' red boundary on the upper side of the ICP. A weaker red eddy can be seen on the lower side of the plasma. The eddy is more highly developed (greater curvature) and has progressed further downstream in the middle frame. The eddy is nearly dissipated in the final frame. The vortices develop symmet- rically about the axis of a vertical p l a ~ m a ~ while convection probably contributes to the asymmetry of the vortices in a horizontal plasma. Fig. 3 also illustrates the initial radiation zone (IRZ the faint red 'tongue' at the right of the axial channel) and the normal analytical zone (NAZ the blue region just downstream from the IRZ).16 Photographs of ICP and Sampling Interface The sequence of prints in Fig.4 illustrates the time fluctuation of the plasma in the vicinity of the sampling orifice. The blue central zone corresponds to emission from the excited Y+ ion. Each frame is separated by 0.6 ms. The red eddy in Fig. 3 is not as evident as in Fig. 4 because the sampling cone blocks the downstream region where the eddy is prominent. The central region of the blue NAZ enters the sampling orifice. The diameter of the NAZ at the sampler fluctuates periodically on a millisecond time scale changing in diameter (or vertical dimension in this figure) by a factor of approximately two between the views shown in the upper and middle frames of Fig.4. For example in Fig. 4 the NAZ is narrowest in the top and bottom frames and is widest in the middle frames. A similar swelling and contraction of the NAZ was observed in a previous photographic study.9 The cine-films from which this se- quence was taken clearly show that the fluctuations start at 500 * N I 2- !! 400 - 3 P LL 300 - 2oo -+- Sampling position/mm Fig. 5 Frequency of plasma fluctuation as a function of sampling position 0 noise power spectra measured by MS (see ref. 2); A measured by counting frames with plasma adjacent to sampler; and A measured by counting frames with plasma retracted fully from the sampler. The ultrasonic nebulizer was usedFig. 3 Three prints from high-speed film of horizontal ICP retracted fully from sampling cone.The plasma is flowing from right to left. Most of the torch and induction region are hidden behind a shielding box; only the end of the torch is visible. Note blue emission from Y+ in the axial channel and red emission from YO and/or neutral Y at the outer edge of the plasma. The three frames shown are 1.2 ms apart; intervening frames have been removed. The ultrasonic nebulizer was used. Time progression is from top to bottom in all the sequences shown Fig. 4 A typical sequence of prints of the ICP during sampling. Note the fluctuation in the diameter of the axial channel. The ultrasonic nebulizer was used. Each frame is separated by 0.6 ms [to face page 6021Fig. 6 Orthogonal views of the ICP during sampling. Note that the eddy develops in the same frame when viewed from either direction.The direct view (i.e. from the side) is on the left; and the reflected view (Le. from the bottom) is on the right. Each frame is separated by 0.6 ms [to face page 6031 Fig. 8 Individual prints (not a time sequence) showing red vapour clouds from (a) a single very large particle and (b) two large particles. The concentric nebulizer was usedJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL. 6 603 the periphery of the plasma and then couple into the axial channel although this coupling is not easily seen in the still photographs reproduced here. Also the plasma fluctuations are shown much more clearly in Figs. 3 and 4 than in the recent black and white photographs of Furuta.’O The frequency of the plasma fluctuations was estimated either from noise-power spectra2 or in the present work by counting the frames and timing marks on the developed film.These frequencies are plotted in Fig. 5 for five sampling positions and also when the sampling cone is completely removed from the plasma. The frequencies decrease as the plasma is retracted from the sampler. The same trend was seen in the noise power spectra (circled data points in Fig. 9 although higher frequencies were seen in that work because the plasma was positioned closer to the sampler in order to yield useful ion signals. Fig. 5 shows that the frequencies measured from noise power spectra and the frequencies measured by counting frames are congruent on a single smooth curve. Thus the noise peaks observed in the 400-550 Hz range at the ‘normal’ sampling positions ( 10- 1 5 mm) arise from the same basic phenome- non as the plasma fluctuations shown photographically in Fig.4. With some intruments these plasma fluctuations are clearly audible with the unaided ear. Both the pitch and amplitude of the resulting whine decrease as the plasma is retracted from the sampling cone in agreement with the trends seen in this study. Simultaneous photographs of the ICP-MS system from two orthogonal directions (with the imaging system as in Fig. 1) are shown in Fig. 6. Note that the camera orientation was changed in order to optimize the framing of the orthogonal views. Also the two images on each frame are not exactly parallel because the mirror was misaligned slightly.In Fig. 6 as in Fig. 4 the NAZ swells and contracts around the tip of the sampling cone and these fluctuations are in phase in the two orthogonal directions. Several frames e.g. the bottom one show that the NAZ is symmetrical with respect to the sampler in one direction but is displaced to one side when viewed from the other direction. Considering the NAZ in the fourth and fifth frames of Fig. 6 i.e. those in which the blue NAZ is narrowest nearly all the blue Y+ ion emission from the NAZ is apparently flowing into the orifice. In other frames where the NAZ is at its maximum width or is oriented symmetrically with respect to the orifice small amounts of Y+ ion emission from the periphery of the NAZ miss the orifice and travel down the outer surface of the cone.This observation illustrates that a large fraction of the NAZ passes into the orifice as indicated by the gas dynamic calculations of Douglas and French.” Because the NAZ broadens as it progresses from the torch the fraction of the NAZ that passes into the sampling orifice decreases as the distance between the torch and the sampling orifice increases. Thus the ion signal decreases as the sampling position is moved downstream from the tip of the IRZ. As mentioned before much of the red eddy (Fig. 3) is blocked by the sampler in Figs. 4 and 6. In some frames a faint red plume from the eddy can be seen to pass downstream along the surface of the cone. All the time- dependent phenomena such as the red plume and the fluctuation in the width of the NAZ are seen much more clearly when the films are either projected or televised from videotape than from the still photographs reproduced in this paper.Emission from Intact Particles Discrete clouds of red emission travelling along the axial channel of the ICP were occasionally observed in the high- speed films with the concentric nebulizer and a 5000 mg 1-’ Y solution. The red clouds are presumably from the Y neutral atom or oxide emission in the vicinity of large undissociated aerosol droplets or solid particles as de- scribed by Olesik and c o - ~ o r k e r s ~ ~ - ’ ~ and Cicerone and Farnsworth.14 The clouds are also reminiscent of the expanding atomic vapour clouds from discrete droplets introduced into flames in early studies by Hieftje and Malmstadt.’* In the subsequent discussion the species responsible for these red emission clouds are simply referred to as ‘particles’.The present study cannot discrimi- nate between wet droplets (desolvation was not employed) and dry solid particles. A short sequence of prints in Fig. 7 illustrates these particles. The top frame does not show a particle but in the next frame a relatively sharp red ‘plume’ has appeared at the tip of the IRZ. This plume is attributed to emission from the vapour cloud surrounding a particle that is just emerging from the IRZ. Such particles are likely to be responsible for the ‘flicker’ often seen in the spatial position of the tip of the IRZ. In the third frame from the top the tip of the IRZ has retreated to its usual position and the particle has moved along the NAZ and is about to flow into the orifice.In the bottom frame the particle is gone. Between the second and third frames the particle has travelled approximately 8 mm estimated relative to the width of the torch. The flow velocity is then about 8 mm per 0.3 ms or about 27 m s-l which is in reasonable agreement with other estimates of flow velocity in the axial channel of the ICP.19920 Fig. 8(a) illustrates a large emission cloud presumably from a particularly large particle. The simultaneous pres- ence of at least two particles is shown in Fig. 8(6). These vapour clouds are elongated because of the distance travelled during the exposure time of the frame. Faint emissions from large particles in the axial channel were seen previously9 with an Ames Laboratory cross-flow nebulizer although such emissions were much less evident than in the present work.No such particles were observed in Figs. 3-5 of the present work which employed an ultrasonic nebu- lizer with desolvation. The sequences in Figs. 7 and 8 also illustrate clearly that most of the axial channel flows into the sampling orifice as described above. In a recent paper Montaser et aL2’ reported time- resolved measurements of the size and spatial position of wet aerosol droplets. This study indicated that at least in some instances individual droplets in a flowing gas stream were not evenly distributed in space. Instead some of the droplets tended to associate closely with one another in small groups of 3-5 droplets per group. Such groups of droplets passing through the plasma could also produce the red emission clouds seen in Figs. 7 and 8 and many of the time-dependent emission characteristics described by Ole- sik and co-workers’ ‘-I3 and Cicerone and F a r n s ~ o r t h .~ ~ Intact particles such as those shown in Figs. 7 and 8 are probably not desirable in ICP-MS instruments. They probably promote deposition in and plugging of the sampling and skimming orifices.22 Those that pass through the sampler might deposit on the skimmer photon stop or perhaps elsewhere in the ion lens. Such insulating deposits would accumulate charge change the effective potentials inside the ion lens and destabilize the ion beam. This effect could be one source of long-term drift in ICP-MS and might explain why the first small stop (sometimes called the ‘shadow stop’) at the base of the skimmer in Perkin-Elmer SCIEX instruments greatly reduces such drift.23 Transit of these large particles through the ICP could also contribute to noise in the signal observed.The deleterious effects of large droplets or particles such as these could be another factor that limits the maximum concentration of matrix elements that is tolerable in ICP-MS. As an extension of these studies the effect of other sample introduction systems e.g. laser ablation on particle behaviour in the ICP should be evaluated photographically.604 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL. 6 Ames Laboratory is operated by Iowa State University for the US Department of Energy under Contract W-7405-Eng- 82. This work was supported by the Office of Basic Energy Sciences.References 1 Douglas D. J. Can. J. Spectrosc. 1989 34 38. 2 Crain J.S. Houk R. S. and Eckels D. E. Anal. Chem. 1989 61 606. 3 Furuta N. Monnig C. A. Yang P. and Hieftje G. M. Spectrochim. Acta Part B 1989 44 649. 4 Walden G. L. Bower J. N. Nikdel S. Bolton D. L. and Winefordner J. D. Spectrochim. Acta Part B 1980,35 535. 5 Belchamber R. M. and Horlick G. Spectrochim. Acta Part B 1982 37 17. 6 Davies J. and Snook R. D. J. Anal. At. Spectrom. 1986 1 195. 7 Davies J. and Snook R. D. J. Anal. At. Spectrom. 1987 2 27. 8 Goudzwaard M. P. and de Loos-Vollebregt M. T. C. Spectrochim. Acta Part B 1990 45 887. 9 Winge R. K. Eckels D. E. DeKalb E. L. and Fassel V. A. J. Anal. At. Spectrom. 1988 3 849. 10 Furuta N. J. J. Anal. At. Spectrom. 1991 6 199. 11 Olesik J. W. Smith L. J. and Williamsen E. J. Anal. Chem. 1989,61 2002. 12 Olesik J. W. and Fister J. C. 111 Spectrochim. Acta Part B 1991,46 851 869. 13 Hobbs S. E. and Olesik J. W. Anal. Chem. in the press. 14 Cicerone M. T. and Farnsworth P. B. Spectrochim. Acta Part B 1989 44 897. 15 Scott R. H. Fassel V. A. Kniseley R. N. and Nixon D. E. Anal. Chem. 1974 46 75. 16 Koirtyohann S. R. Jones J. S. and Yates D. A. Anal. Chem. 1980,52 1965. 17 Douglas D. J. and French J. B. J. Anal. At. Spectrom. 1988 3 743. 18 Hieftje G. M. and Malmstadt H. V. Anal. Chem. 1968,40 1860. 19 Barnes R. M. CRC Crit. Rev. Anal. Chem. 1978 7 203. 20 Barnes R. M. and Genna J. L. Spectrochim. Acta Part B 198 1 36 299. 21 Montaser A. Clifford R. H. and Sohal P. presented at the XXVII Colloquium Spectroscopicum Internationale (CSI) Bergen Norway June 9- 14 199 1 paper C-6-5. 22 Douglas D. J. and Kerr L. A. J. Anal. At. Spectrom. 1988,3 749. 23 Boom A. W. personal communication 1991. Paper 1 I01 4 76E Received March 27th 1991 Accepted August 12th 1991

ISSN:0267-9477    

DOI:10.1039/JA9910600601

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL. 6 605 Minimization of Non-spectroscopic Matrix Interferences for the Determination of Trace Elements in Fusion Samples by Flow Injection Inductively Coupled Plasma Mass Spectrometry Jiansheng Wang E. Hywel Evans and Joseph A. Caruso* Department of Chemistry University of Cincinnati ML 172 Cincinnati OH 45221 USA The efficiency of various ion lens tuning strategies flow injection and the use of an internal standard has been investigated for the determination of trace elements in real samples. Spike recoveries indicated that internal standardization was always preferable if 100% recovery was to be obtained. However good recoveries were also obtained by tuning the ion lenses for a maximum ll5In+ signal in a matrix blank solution containing 100 ng g-l of In while using continuous nebulization sample introduction.Results indicated that tuning the ion lenses in the presence of the matrix yielded improved analyte recoveries when compared with tuning the lenses for a standard solution. Keywords Matrix tuning; flow injection; internal standard; inductively coupled plasma mass spectrometry Inductively coupled plasma mass spectrometry (ICP-MS) has been used for the determination of trace elements in various samples containing high matrix concentrations 1-4 in which severe matrix induced analyte signal suppression or enhancement usually occurs. Various techniques have been applied to reduce non-spectroscopic interferences. These include internal standardizati~n,~-~ isotope dilu- t i ~ n * * ~ standard additions,*-lo hydride generation,' 1*12 ion- exchange de-salting and prec~ncentration*~-~~ and flow inJection.16-19 A recent investigation*O has shown that flow injection (F'I) and re-tuning the ion lenses can be used to compensate for the suppression of analyte signal in the presence of a high concentration of matrix.The flow injection technique is considered to compensate for the matrix effects by lowering the amount of matrix-containing solution to which the nebulizer sampler and plasma are exposed thereby effectively reducing the effects of clogging. Ion-lens tuning in the presence of the matrix (such as synthetic ocean water) also reduced signal suppression in comparison with tuning the ion lenses for a standard solution. In this work the results obtained by comparing contin- uous nebulization flow injection standard and matrix tuning both with and without internal standardization for the determination of trace elements in real samples are presented.By comparing the recovery factors for samples spiked with the elements being studied the reliability of the analytical results has been evaluated. This work is intended to supplement a previous paper.*O Experimental Instrumentation All data were acquired using a commercial ICP-MS instru- ment (VG PlasmaQuad VG Elemental Winsford Chesh- ire UK). The operating conditions used for this work are shown in Table 1 and are typical of those used for routine multi-element analysis. A concentric nebulizer (Meinhard C-2 Precision Glassblowing of Colorado Parker CO USA) and a double-pass Scott-type spray chamber cooled to 6 "C by means of a refrigerated chiller (Neslab Instruments Portsmouth NH USA) were used.The spray chamber was maintained at 6 "C in order to reduce the amount of solvent vapour thereby reducing condensation on the torch elbow and to maintain the spray chamber at a constant tempera- *To whom correspondence should be addressed. Table 1 ICP-MS multi-element operating conditions ICP system- Forward power Reflected power Coolant flow rate Auxiliary flow rate Nebulizer flow rate Sample delivery rate Spray chamber temperature Sampling depth* 1350 W < 5 w 16 1 min-' 1 1 min-* 0.65 I min-' 1 ml min-I 6 "C 12 mm Mass spectrometer- Sampler nickel 0.7 mm orifice Skimmer nickel 1.0 mm orifice First stage pressure 1 .4 ~ lo2 Pa Second stage pressure < l x Pa Third stage pressure X ~ X lo-' Pa *Defined as the distance between the foremost coil of the load coil and the tip of the sampling cone. ture to improve precision. Solution was introduced via a peristaltic pump (Gilson Villiers Le Bel France) at a flow rate of 1 ml min-l into the concentric nebulizer. After each run period the nickel sampler was cleaned. Reagents Samples and Standards The sample stock solutions were obtained from the labora- tories of British Petroleum (Warrensville Research Center Cleveland OH USA). The solid samples were fused with a sodium carbonate flux (EM Science Gibbstown NJ USA) in a platinum crucible in the ratio of 0.1 + 3 (mass samples to mass flux) then dissolved in 1 + 3 HCl one part HC1 (Baker Instra-analyzed Phillipsburg NJ USA) in three parts distilled de-ionized water (DDW 18 Ma Barnstead Newton MA USA) so that the final concentration of dissolved solids was approximately 3% d m .The samples used for the analysis were prepared by diluting the sample stock solutions ten times with DDW with In (100 ng g-l) added as the internal standard. The spiked sample solutions (50 ng g-l of each element) were prepared by adding the appropriate amount of stock solutions containing Be Al V Co Ge As Se Y Rh Ba and Pb at the same time as the internal standard. The matrix blank solution was prepared by combining 7.5 g of sodium carbonate flux with 1 + 3 HCl to a total mass of 250 g and then diluted ten times with DDW so that the606 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 1991 VOL.6 final concentration of the matrix blank was 0.3% m/m Na2C03 and 0.9% m/m 12 mol dm-3 HC1. At this time In (100 ng g-l) was also added as the internal standard. Multi-element stock solutions (1 0 pg g-l) of Be Al V Co Y Rh Ba and Pb and Ge As Se Sn and Sb were prepared from 1000 pg ml-l single element stock solutions (certified atomic absorption standards; Fisher Scientific Fair Lawn NJ USA) in 5% nitric acid and 5% hydrochloric acid respectively. Dilute acids were prepared by dilution of concentrated HN03 (laboratory-reagent grade Fisher Scientific) and concentrated HCl (Baker Instra-analyzed) with DDW. Standard solutions were prepared by serial dilution of the multi-element stock solutions in 1% HN03 to concentrations of 5 20 100 and 200 ng g-l.Flow Injection System The flow injection system used for this work has been described in a previous paper.*O The system consisted of a programmable electronic controller (Machine Shop University of Cincinnati Cincinnati OH USA) a flow injection solenoid valve (Model 7 163 Rheodyne Cotati CA USA) a 4-way poly(tetrafluoroethy1ene) rotary valve (Model 5701 Rheodyne) and a 100 pl loop (Valco Instru- ments Houston TX USA). In practice the flow injected samples gave rise to signal peak widths for the analyte of approximately 60 s at half-height which allowed multi- element scans to be performed as the method of data acquisition provided that the start of each scan was timed to begin at the same point for each flow injected sample.This flow injection system allowed the timing of injections and sample volumes to be accurately controlled. Data Acquisition and Calculations All data acquisition was performed in the scanning mode using the software supplied with the instrument. The following data acquisition conditions were used mass range 8-2 13 mlz; channels 2048; sweeps of the mass range 400; and dwell time per channel 80 ps. Calibration and calculations were performed using the instrument software. Procedure Before each batch of samples was analysed the signal was monitored while a matrix solution containing 100 ng g-l of In was aspirated into the system. Initially the signal decreased as matrix was deposited on the cones until the equilibrium was reached when no further obvious decrease was observed.This process took approximately 15 min. After that the system was washed out with 1% HN03 for 5 min. Standard tuning Ion lenses were tuned for a maximum Il5In+ signal without any matrix present using a 100 ng g-l solution of In in 1% HN03. Calibration for all the elements of interest was accomplished by analysing a blank and four standard solutions containing Be Al V Co Ge As Se Sn Sb Y Rh Ba and Pb. Both blank and standards contained 100 ng g-l of In. The sample solutions were analysed in the following order (1) matrix blank containing 0.3% m/m of sodium carbonate and 2.5% HCl with 100 ng g-l of In; (2) sample; (3) sample spiked with 50 ng g-l of the standard; (4) standard blank containing 1% HN03 with 100 ng g-l of In; ( 5 ) 20 ng g-l of standard solution; and (6) wash out with 1 To HN03 for 3 min (continuous nebulization only).This procedure was performed twice first with contin- uous nebulization and then with flow injection. The purpose of steps 4 and 5 was to check the signal drift during the analysis caused by cone clogging. This procedure was repeated for each sample in succession. Each step was repeated three times within a specific procedure for both continuous nebulization and flow injection. The flow injection carrier stream was 1% HN03. Matrix tuning Ion lenses were tuned for a maximum llsIn+ signal with the matrix present using a solution of 100 ng g-l of In in the matrix blank. The blanks standard solutions and samples were analysed as described under Standard tuning. This procedure was again performed twice first with continuous nebulization then with flow injection. Calculation of Recovery Factors Average analyte signals were obtained by calculating the mean of the values found for three repetitions each of the blanks and samples.The sample spike recoveries were calculated by means of the equation Cspiked - &ample Spiked sample recovery factor = 50 where Cspikd is the analyte concentration (in ng g-l) determined in the spiked sample after calibration; csample is the analyte concentration determined in the sample; and 50 represents the concentration (50 ng ml-l) of the spike. The standard recovery factors were calculated as follows Standard recovery factor = - where Csmndard is the concentration of analyte determined in the 20 ng g-l standard solution by calibration and extrapo- lation; and 20 represents the true concentration (20 ng ml-l).20 Semi-quantitative Analysis One of the advantages of ICP-MS is that semi-quantitive analysis can be performed relatively quickly for most of the elements in the Periodic Table. The method is useful for determining the approximate concentration of analyte in unknown samples. The instrumental mass response graph was established by running a solution containing 100 ng g-l of Be Mg Co In Pb and U in 0.3% Na,CO and 2.5% HCl which matched as closely as possible the sample matrix. The elements spiked to the unknown samples were based on information obtained from semi-quantitative analysis. No significant amount of In was found in the samples. Results and Discussion Recoveries for 20 ng g-l Standard Solutions A 20 ng g-l standard solution was run after each sample analysis.The standard recovery factors were used to monitor the signal drift resulting from the deposition of solids on the ICP-MS interface. If no cone or nebulizer blockage occurred during the analysis the recovery factor should be equal to unity. Values of less than unity indicate signal suppression and values of greater than unity indicate signal enhancement. The recovery factor obtained for 20 ng g-l of the Rh standard spike as a function of time is shown in Fig. 1. As can be seen the recovery factor for continuous nebulization (curve A) decreased as more samples were analysed probably owing to a build-up of saltJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 1 99 1 VOL.6 607 I 1 1 33 66 99 132 165 198 Ti me/mi n 0.4 ' Fig. 1 Recovery factors for 20 ng g-I of Rh in 1% HN03 as a function of time A continuous nebulization without an internal standard; B flow injection without an internal standard; and C flow injection with an internal standard from the sample matrix on the sampler or skimmer cones. Although the cones had been conditioned beforehand and 1% HNOj was aspirated for 3 min before each sample was introduced cone blockage was still severe for continuous nebulization sample introduction. However for flow inj ec- tion (curve B) the recovery factors were improved to some extent because flow injection effectively reduced the amount of sample solution coming into contact with the ICP-MS interface reducing the possibility of the build-up of solid material.Further improved recoveries were achieved by using flow injection and internal standardiza- tion with In as shown in curve C. Recoveries for 50 ng g-1 Sample Spikes The unknown samples were spiked with 50 ng g-l standards to ascertain which analytical protocol gave the best re- coveries (i.e. those closest to unity). Based on these results the recoveries for trace elements in the samples could be evaluated with greater confidence. Fig. 2(a)-(d) illustrates spike recoveries for various elements covering the mass range in one sample analysed. Almost identical graphs were obtained for the other samples (results not given). The shaded bars represent the spike recoveries obtained by tuning the ion lenses with the standard solution in l0h HN03.The unshaded bars repre- 1.2 1 .o 0.8 0.6 $ 0.4 P o $ 1.2 8 1.0 5 0.2 'c a 0.8 0.6 0.4 0.2 0 Fig. 2 Recovery factors for unknown samples spiked with 50 ng g-* standards. The shaded bars represent the spiked recoveries obtained by standard tuning; the unshaded bars represent the recoveries obtained by matrix tuning. (a) Continuous nebulization without an internal standard; (b) flow injection without an internal standard; (c) continuous nebulization with an internal standard and (d) flow injection with an internal standard sent spike recoveries obtained by tuning the ion lenses in the presence of matrix (0.3Oh Na,C03 and 3% HCl). Each graph represents a different analytical protocol. Fig. 2(a) shows the recoveries obtained using continuous nebulization.As can be seen the recoveries obtained by tuning with the standard are much lower than unity indicating severe signal suppression. However by tuning the ion lenses in the presence of the matrix recoveries were much improved for most elements. In contrast when flow injection was employed [Fig. 2 (b)] no significant difference was observed between recoveries obtained by standard tuning and matrix tuning. Furthermore a comparison between Fig. 2(u) and (b) reveals that in general lower recoveries were obtained using flow injection compared with continuous nebulization when matrix tuning was utilized while the reverse was true when standard tuning was employed. The explanation for this is 2-fold. Two factors affect recovery namely the tuning conditions of the ion lenses and the concentration of sample matrix.For the situation where standard tuning was employed the effect of sample matrix concentration was predominant resulting in better recoveries using flow injection compared with con- tinuous nebulization owing to the effective dilution of the sample by the former technique. For the situation where matrix tuning was employed the effect of the lens tuning conditions was predominant. As all ion lens tuning was performed with a continuous signal (ie. using continuous nebulization) the resulting conditions may not have been optimum for a transient flow injection signal where the matrix was effectively more dilute. Hence better recoveries were obtained using continuous nebulization. Figs. 2(c) and (d) contain an analogous set of plots to Figs.2(u) and (b) except that this time an internal standard was used. For both continuous nebulization and flow injection very little difference was observed regardless of whether ion lenses were tuned in the presence of the matrix or for an aqueous standard. However in general recovery factors much closer to unity were obtained compared with those found without using an internal standard. The best recover- ies were obtained for lo3Rh and 138Ba in all samples. This was thought to be because the internal standard used was llsIn. These three elements are closest in terms of mass compared with 59C0 89Y and *08Pb so that if other factors such as the extent of ionization had a negligible influence this result would be expected and has been observed by other worker^.^*^ From this point of view excellent recover- ies could be obtained if more internal standards were used to cover the whole mass range.Based on the spike recoveries the analytical procedures were arranged in order of increasing recovery in Table 2 so Table 2 Analytical methods used and arranged in ascending order of spike recoveries from top to bottom Method Conditions 1 2 3 4 5(a) 5(6) 5(c) 5(d) Continuous nebulization without internal standard but Flow injection without internal standard but with Flow injection without internal standard but with Continuous nebulization without internal standard Flow injection with internal standard and standard Flow injection with internal standard and matrix Continuous nebulization with internal standard and Continuous nebulization with internal standard and with standard tuning standard tuning matrix tuning but with matrix tuning tuning tuning standard tuning matrix tuning608 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 1991 VOL.6 Table 3 Results for the determination of 59C0 Io3Rh and 208Pb. All values are concentrations in ng g-l. (Methods as defined in Table 2) S9C0 in sample lo3Rh in sample *O*Pb in sample Method of choice 1 2 3 4 1 2 3 4 1 2 3 4 12.9 3.63 2.00 0.23 13.3 4.24 2.40 0.32 16.6 5.00 2.66 0.38 19.3 6.00 3.46 0.50 19.2 6.21 3.50 0.53 19.7 6.70 3.76 0.59 22.8 6.86 4.12 0.53 20.8 7.90 4.54 0.56 27.5 19.2 10.5 13.9 32.4 25.1 13.8 19.5 39.1 28.1 15.1 20.8 44.3 28.5 16.8 27.2 46.6 36.6 20.0 31.2 46.6 37.4 21.2 32.0 48.2 35.9 21.4 30.6 47.9 37.6 22.2 30.9 1.00 0.34 - 1.29 0.81 - 1.70 0.31 - 1.63 0.15 - 1.90 1.16 - 1.96 0.43 - 1.82 0.68 - 1.68 0.21 - that the method that gave the best recoveries is at the bottom.Methods 5(a)-(d) were grouped together as there were no significant differences among the recoveries ob- tained using these methods. The results obtained for several trace elements in the fusion samples were then arranged in order of analytical method in exactly the same manner and are shown in Table 3. It is evident from Table 3 that in general the analyte concentrations determined in the fusion samples have an apparent increasing trend from top to bottom with methods 5(a)-(d) giving similar results. This trend is true for all of the four samples from low mass 59C0 to high mass 208Pb in the same order as that predicted from Table 2.Various methods of analysis adopted (e.g. matrix tuning flow injection and internal standardization) compensated to various degrees for the matrix effects in the high matrix solutions. The spiked sample recoveries were improved so that the concentrations of trace elements in the samples increased according to corresponding methods of analysis. Hence for example the concentration of Co in sample 1 is most likely to be correct if the results obtained using methods 5(a)-(d) are chosen. Where there is no available internal standard then method 4 would be the best choice as it still yielded good recoveries. The concen- trations of 5 9 C ~ lo3Rh and 208Pb obtained by corresponding methods of analysis from samples 1-4 are also listed in Table 3.In samples 3 and 4 zo8Pb was not found i. e. zo8Pb was not present at a level that was within the detection capabilities of the ICP-MS instrument. Conclusions Various techniques such as matrix tuning flow injection and internal standardization were evaluated by a compari- son of spiked sample recoveries. Based on these results the method of choice for the determination of trace elements in unknown samples should always include internal standard- ization regardless of whether continuous nebulization flow injection standard or matrix tuning is employed. In the absence of a suitable internal standard the next best alternative was found to be continuous nebulization and ion lens tuning in the presence of the matrix. numbered ES 03221 and ES 04908.They also thank the National Institutes of Health Shared Instruments Grants Program for providing the VG PlasmaQuad through grant number S10 RR02714 and BP America for providing partial support. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 References Vaughan M.-A. and Horlick G. J. Anal. At. Spectrom. 1989 4 45. Date A. R. Cheung Y. Y. Stuart M. E. and Jin X.-H. J. Anal. At. Spectrom. 1988 3 653. Hall G. E. M. Park C. J. and Pelchat J. C. J. Anal. At. Spectrom. 1987 2 189. Hutton R. C. Bridenne M. Coffre E. Marot Y. and Simondet F. J. Anal At. Spectrom. 1990 5 463. Vandecasteele C. Nagels M. Vanhoe H. and Dams R. Anal. Chim. Acta 1988 211 91. Thompson J. J. and Houk R. S. Appl. Spectrosc. 1987 41 801. Beauchemin D. McLaren J. W. and Berman S. S. Spectro- chim. Acta Part B 1987 42 467. Garbarino J. R. and Taylor H. E. Anal. Chem. 1987 59 1568. McLaren J. W. Mykytiuk A. P. Willie S. N. and Berman S. S. Anal. Chem. 1985 57 2907. Beauchemin D. McLaren J. W. Mykytiuk A. P. and Berman S . S. Anal. Chem. 1987 59 778. Janghorbani M. and Ting B. T. Anal. Chem. 1989,61 701. Burguera M. and Burguera J. L. Analyst 1986 111 171. Thompson J. J. and Houk R. S. Anal. Chem. 1986,58,2541. Heitkemper D. Creed J. and Caruso J. A. J. Anal. At. Spectrom. 1989 4 279. Beauchemin D. Siu K. W. M. McLaren J. W. and Berman S. S. J. Anal. At. Spectrom. 1989 4 285. Hutton R. C. and Eaton A. N. J. Anal. At. Spectrom. 1988 3 547. Dean J. R. Ebdon L. Crews H. M. and Massey R. C. J. Anal. At. Spectrom. 1988 3 349. Vickers G. H. Ross B. S. and Hieftje G. M. Appl. Spectrosc. 1989,43 1330. Vaughan M.-A. Horlick G. and Tan S. H. J. Anal. At. Spectrom. 1987 2 765. Wang J. Shen W.-L. Sheppard B. S. Evans E. H. Caruso J. A. and Fricke F. L. J. Anal. At. Spectrom. 1990 5 445. Paper 1/02262H Received May 13th I991 Accepted July 30th 1991 The authors acknowledge the National Institute of Environ- mental Health Sciences for grant support through grants

ISSN:0267-9477    

DOI:10.1039/JA9910600605

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL. 6 609 Determination of Uranium and Thorium in Aluminium with Flow Injection and Laser Ablation Inductively Coupled Plasma Mass Spectrometry Peter van de Weijer Peter J. M. G. Vullings Wilhelmina L. M. Baeten and Wim J. M. de Laat Philips Research Laboratories P. 0. Box 80.000 5600 JA Eindhoven The Netherlands In order to determine uranium and thorium at the sub-ng g-l level in aluminium the limit of detection (LOD) for continuous-flow nebulization inductively coupled plasma mass spectrometry (ICP-MS) is not sufficient when a sample solution with the usual maximum concentration of 1 mg ml-l is used. Therefore two alternative sample introduction techniques have been used flow injection (FI) and laser ablation (LA).With FI-ICP-MS the achievement of sub-ng g-l detection limits is hampered by the presence of 'spikes'. Although these spikes are also present with LA it is possible to obtain a 0.2 ng g-l LOD for uranium and thorium. This LOD is achieved artificially by rejecting all measurements containing spikes. Keywords Inductively coupled plasma mass spectrometry; orifice deposition; flow injection; laser ablation; spike phenomenon As the dimensions of integrated circuits become smaller their susceptibility to alpha-particle induced damage ('soft errors') increases. Reduction of the exposure to alpha particles requires materials of extremely high purity with respect to elements from the natural uranium and thorium radioactive decay series. The required concentrations for uranium and thorium are below 1 ng g-l.With continuous- flow nebulization inductively coupled plasma mass spectro- metry (ICP-MS) the usual 1 mg ml-l sample concentra- tion' results in sub-pg ml-l levels of uranium and thorium in solution. These concentrations are below the detection limits attainable (2 pg ml-l for uranium and 4 pg ml-l for thorium2). Chemical preconcentration might solve this problem but this is time consuming and at the ng g-l level there is a chance of contamination. Therefore an attempt was made to measure uranium and thorium in samples of aluminium which is used for wiring in integrated circuits with flow injection (FI) and laser ablation (LA) ICP-MS. Experimental The work described was performed with a PlasmaQuad PQ I1 Plus ICP-MS instrument (VG Elemental). The operating conditions are given in Table 1.During all experiments the Table 1 Typical operating conditions of the ICP-MS system Plasma- Power 1250 W Argon outer gas flow rate Intermediate gas flow rate Mass spectrometer- Sampling depth Sampler nickel Skimmer nickel Expansion pressure 2 . 3 ~ lo2 Pa Intermediate pressure t l x Pa Analyser pressure 2.4 x lo-' Pa 13.5 1 min-l 0.6 1 min-' 10 mm beyond load coil 1.0 mm orifice (Nicone) 0.75 mm orifice (Nicone) Liquid sample introduction- Nebulizer flow rate Sample uptake rate Nebulizer Meinhard Solid sample introduction- Nd:YAG laser energy Laser pulse width 10 ns Laser focus Repetition rate 10 Hz Camer flow rate 0.74 1 min-l 0.8 ml min-l 250 mJ pulse-' 3 mm (glass) or 10 mm (Al) below sample surface 0.95 1 min-l sensitivity of the ICP-MS instrument was at its specified value (2 x lo5 counts s-l for a 100 ng ml-l solution of llSIn).For survey analyses the ion lenses were tuned to an element in the mid-mass range. For the determination of uranium and thorium they were tuned to uranium. All calibrated glassware was soaked in 10% v/v nitric acid overnight. Standard solutions of uranium and thorium were prepared by dilution of commercially available standard solutions containing 1 mg ml-l of the elements (supplied by Spex Industries). Hydrochloric acid and nitric acid were Suprapure re- agents obtained from Merck. The water was ultrapure according to the International Organization for Standard- ization 3696 Class 1 water. For FI the aluminium samples were dissolved in a 1 + 3 + 5 mixture of nitric acid (65%) hydrochloric acid (37%) and water.This acid mixture also served as the wash solution between injections. For LA the samples were etched for 2 min in the same mixture in order to remove surface contaminants and an additional pre-ablation time of 2 min was used. The standards used to obtain the sensitivity factors for the elements in aluminium were Pechiney A1 raffine A-99 (1 199) No. 8928,8930 and 893 1 Pechiney Alliage Al-Cu A- U2GN 9143 and Alcoa SA2049/19. These five standards contained 18 elements in the range from 1 ng g-l to 10 mg g-l. For uranium and thorium a separate high-purity aluminium sample (R06) prepared in-house was used. The uranium and thorium contents in this sample as measured in this department by neutron activation analysis3 (NAA) were found to be 25 and 52 ng g-l.Results and Discussion Continuous-flow Nebulization ICP-MS As a test of the ICP-MS apparatus used the limit of detection (LOD) (3dsensitivity) of uranium and thorium using continuous-flow nebulization of a solution containing 100 pg ml-l of uranium (no matrix) was determined. By using 50 s integration times a 2 pg ml-l detection limit for both uranium and thorium was obtained. This is compar- able to the values reported in the literature.2 The 2 pg ml-l LOD in solution corresponds to a 2 ng g-l LOD in a solid if one assumes a sample solution containing 1 mg ml-l of the solid sample. Aluminium however is a difficult matrix in terms of orifice blocking. In Fig. 1 the uranium signal from a solution containing 100 ng ml-l of uranium and 1 mg ml-I of aluminium is shown.The signal decreases over time owing to the orifice becoming blocked by A1203. If it is assumed that the transport efficiency of the sample intro-610 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL. 6 0 10 20 Ti me/mi n 30 Fig. 1 238U signal from a solution containing 100 ng ml-l of U and 1 mg mi-1 of Al. Dwell time per channel is 5 s duced by the nebulizer is 1% the aluminium flow into the plasma is 0.13 pg s-l. Although the time scale of Fig. 1 exceeds the normal analysis time in continuous-flow nebuli- zation ICP-MS it demonstrates the severe problem of orifice blocking. It indicates that the usual sample concen- tration of 1 mg ml-l is still too high when analysing aluminium with continuous-flow nebulization ICP-MS.Flow Injection ICP-MS The idea of using FI was to inject higher concentrations of aluminium but with less orifice bl~cking.~ The samples were introduced by the use of a home-made injection loop. The transient signals were recorded using the single-ion moni- toring facility. The injection volumes were varied from 50 to 1000 pl. The resulting signals are shown in Fig. 2. A 250 pl injection volume was selected for the remainder of the experiments as the amplitude of the flow injection peak approaches the steady-state value of continuous-flow nebulization at this value. With this injection volume an LOD for both uranium and thorium in solution of 4 pg ml-l (no aluminium matrix) was obtained. Then the signal for 100 ng ml- of uranium was measured in the presence of 1 2.5 5 and 10 mg ml-l of aluminium as a function of time.The time interval between two injections was 2 min resulting in a wash time of approximately 1 min. As shown in Fig. 3 the only acceptable signals were those at 1 and 2.5 mg ml-* of aluminium. However the LOD of uranium in aluminium would only be 2-4 ng g-l under those circum- stances. Therefore the wash time was increased from 1 to 3 min. As a result the signal decrease was reduced substan- tially (Fig. 4). A similar observation has been made by Douglas and These workers found that for a solution containing 10 mg ml-l of calcium a wash time of four times longer than the analysis time is sufficient to remove all of the deposits on the sampler. Time/s Fig.2 238U signal from a solution containing 100 pg ml-I of U in single-ion monitoring mode for five different injection volumes A 50; B 100; C 250; D 500; and E 1000 pl I A t 4- f 0 I 0 10 20 30 40 Time/min Fig. 3 238U signal from a solution containing 100 ng m1-l of U and 1-10 mg ml-l of Al A 1; B 2.5; C 5; and D 10 mg 1-l. Each data point represents the height of the peak after injection. The time interval between the injections is 2 min resulting in a wash time of about 1 min t 4- 0 0 10 20 30 40 Time/mi n Fig. 4 Effect of wash time on the 238U signal from a solution containing 100 ng ml-l of U and 10 mg ml-' of Al A 1; and B 3 min If the slight decrease of the uranium signal over time for the 3 min wash time is accepted a satisfactory LOD for uranium in the solid (0.4 ng g-l) could be obtained.However when an effort was made to establish the LOD in the presence of 10 mg ml-l of aluminium a value of 20 pg ml-l (Le. 2 ng g-l in the solid) was found. The increase in the LOD caused by the presence of aluminium which prohibits sub-ng g-l measurements is due to spikes in the mass spectrum. These spikes are shown in Fig. 5. The spikes are single-channel events that only take place with a direct aluminium flow into the plasma. Their presence is independent of the mlz value even when no atoms or molecules are to be expected. These spikes were only observed when injecting aluminium solutions. They were not observed for solutions containing 10 mg mi-' of Na Mg Zn or Mo. For aluminium these spikes have also been observed by Makishima et aL6 A possible explanation for the presence of the spikes could be photon scattering on A1203 clusters in the expansion stage.These clusters could be present in that region due to occasional release of pieces of A1203 from the sampling cone when aluminium matrices are being injected. In order to test this explanation a stainless-steel wire was mounted at the back of the sampling cone (Fig. 6). This wire could serve as a continuous source of photon scattering which would result in an increase in the background signal. However the presence of the wire did not cause a change in the background level while the indium signal from a 100 ng ml-l solution was measured (although the sensitivity decreased by a factor of 3). Therefore the spikes are notJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL.6 61 1 0 10 Tirne/min 20 Fig. 5 (a) Signal at mlz 238. The first three peaks are the response to three injections of a solution containing 100 pg ml-I of U. After 10 min three injections of a blank solution have been performed which show no response at mlz 238. (6) Signal at mlz 238. The first three peaks are the response to three injections of a solution containing 100 pg ml-l of U and 10 mg ml-I of Al. After 10 min 3 injections of a solution containing 10 mg ml-I A1 have been performed which do show a response at mlz 238. (c) Signals at rnlz 230. The first five peaks are the response to five injections of a solution containing 100 pg ml-l of U and 10 mg ml-' of Al. After 10 min five injections of a solution containing 10 mg ml-I of A1 have been performed which show a similar response to the first five injections.Sensitivities are the same in (a) (b) and (c) induced by photon scattering in the expansion stage. In order to test the possibility of photon scattering from other regions of the mass spectrometer the neutral and the ion beam were blocked with a quartz window behind the skimmer. In this instance no spikes (or background) could be observed while a 10 mg ml-l solution of aluminium was nebulized. A quartz window behind the quadrupole but in front of the multiplier had the same result. Thus the spikes are not induced by photons unless their wavelength is so short (for example at the resonance radiation of argon) that they are absorbed by the quartz window.De-tuning of the lens stack had no influence on the number of spikes thereby eliminating the possibility of (regular) ions. Therefore neutral particles (e.g. aluminium atoms or aluminium oxide molecules) clusters of neutral particles or photons of short wavelengths are the most likely explanation for the presence of spikes. (a) Fig. 6 (a) Side and (b) bottom view of the sampling cone with a 1 mm diameter stainless-steel wire to simulate spikes Laser Ablation ICP-MS The amount of aluminium flowing into the ICP when ablating aluminium samples is 0.3 pg s-l. This number is derived from the mass loss of the sample if a 10% efficiency of the sample introduction is assumed. Despite the larger aluminium flow with LA in comparison to that with continuous-flow nebulization a slower decrease in signal is observed.This can be seen by comparing Fig. 7 with Fig. 1. There are two reasons for this slower decrease. First the ablation process is interrupted after each 3 rnin period for 1 min in order to move the laser spot to a fresh part of the surface of the sample. In this way a decrease in the signal due to cratering is prevented. This means that there is a 1 rnin wash time whereas Fig. 1 corresponds to a continuous flow. Secondly there is reduced oxide formation due to the absence of water resulting in less orifice blocking. The ion signal with LA sample introduction however is far less stable than the signal with nebulization of a solution. As the problem of orifice blocking is less severe with LA it appeared to be a suitable technique for both semi-quantita- tive survey analysis and full quantitative analysis of aluminium samples.The response curve in semi-quantita- tive survey analysis of the elements in aluminium is different from that obtained by nebulizing a solution or by ablating a glass sample. This is shown in Fig. 8 in a qualitative way (the experimental conditions are not exactly the same for the three measurements). The difference in the response curves between glass and aluminium is probably due to the higher heat conductivity of aluminium in comparison with that of glass.7 As a result of this difference the temperature during ablation of glass is higher than during the ablation of aluminium. For aluminium the gasification is influenced by evaporation effects resulting in higher sensitivity factors for more volatile elements (such as lead and magne~ium).~ It is surprising however that the sensitivity for the even more volatile cadmium is so low.For glass gasification is dominated by the ablation process resulting in a gas-phase composition that reflects the composition of the solid. 0 10 20 30 40 Ti m e/rn i n Fig. 7 Signal for 238U from an A1 standard which contains approximately 1 ,ug g-l of U. In order to avoid ablation from a deep crater the laser ablation was interrupted for 1 min after each 3 min period and the laser moved to a different place on the sample612 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 1991 VOL. 6 0 100 200 mtz Fig. 8 Response curves obtained with the relative sensitivity factors obtained for the solution at a nebulizer flow of 0.74 1 min-'. (a) Solution (without a matrix) nebulizer flow 0.74 1 min-I; (b) NIST SRM 612 Trace Elements in Glass nebulizer flow 0.95 1 min-l; and (c) aluminium Pechiney 8930 nebulizer flow 0.95 1 min-' The response curve in Fig.8(c) is not suitable for performing semi-quantitative survey analysis of elements in aluminium. The high mlz portion of the curve is dominated by the highly volatile lead. This could lead to unrealistic values e.g. for the actinides. Therefore the relative sensitivity factors for a number of elements using six aluminium standards were determined. Table 2 provides a comparison of these sensitivity factors with those obtained from the solution using the same carrier gas flow rate (0.95 1 min-l). Both sets of results are normalized to manganese.By using 1 min scans the concentration of elements can be measured in the range from 100 ng g-l to 0.1 g g-l; zinc was used as an internal standard. Fig. 9 gives an example of such measurements. The precision of the measurements with laser ablation ICP-MS is substantially lower than that of continuous-flow nebulization ICP-MS. Defining the blank signal as the signal with laser off is not correct. The laser pulse not only results in a signal from the sample but also in a signal from other parts of the sample introduction system (including any memory) which is probably induced by the shock wave of the laser-induced plasma. As an illustration the signal for 52Cr which was corrected for the laser-off signal is shown in Fig.10. At low concentrations of chromium a contribution from Arc occurs which results from ablation of the poly(tetrafluoroethy1ene) bottom of the sample chamber. In order to test the detection limit of LA-ICP-MS two aluminium samples were used in which uranium and thorium were measured by NAA:3sample R06 contains 25 k 3 ng g-l of uranium and 52 k 10 ng g-l of thorium; and sample R07 contains 0.6k0.3 ng g-l of uranium and 1.3k0.1 ng g-l of thorium. The R06 was used as a standard whereas R07 was considered to be a sample. A peak-jumping procedure was used in dual mode; the analogue mode was used to measure the aluminium matrix J 0 1 2 3 4 5 6 7 Log(Cu concentrationtng g-') Fig. 9 Plot of signal for Cu using Zn as an internal standard as a function of the concentration in A1 standards A 63Cu; and B T u Table 2 Relative sensitivity factors (intensity ratios of the signals normalized to concentration and abundance) for a number of elements as obtained in 1 min scans of six aluminium standards compared with the sensitivities for solutions (without aluminium) at the same plasma conditions (nebulizer flow 0.95 1 min-I) Relative intensity factor Relative intensity factor Solution Solution without Aluminium without Aluminium Element aluminium standard Element aluminium standard Be Mg Ti V Cr Mn Fe c o Ni c u 0.18 0.95 0.92 0.93 0.93 1 .oo 1.25 1.03 0.82 0.66 0.24 1.53 0.52 0.53 0.6 1 1 .oo 0.55 0.4 1 0.35 0.38 Zn Ga Zr Cd Sn Sb Pb Bi Th U 0.32 0.97 0.68 0.48 0.78 0.3 1 0.6 1 0.49 0.43 0.4 1 0.62 1.08 0.42 0.7 1 0.98 0.57 1.93 1.47 0.58* 0.59* *Obtained from peak jumping.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 1991 VOL.6 613 o 1 2 3 4 5 6 7 8 Log(Cr concentration/ng 9-7 Fig. 10 Plot of signal for T r using Zn as an internal standard as a function of the concentration in Al standards which served as an internal standard. The pulse-counting mode was used to record the uranium and thorium signals. The peak-jump parameters are shown in Table 3. The results given in Table 4 show the sub-ng g-l detection capability of LA-ICP-MS. These results were obtained at the beginning of the attempts to measure uranium and thorium in aluminium at such low concentrations. At that time measurements were not hampered by spikes. After many hours of introducing aluminium into the ICP system by both LA and FI the measurement of uranium and thorium in R07 by laser ablation was repeated; the spikes were then observed.However uranium and thorium could be measured at the ng g-l level when rejecting those measurements which included spikes in one of the five digital-to-analogue converter (DlA) steps (see Table 5). The detection limit as determined at mlz 230 while ablating the aluminium sample or at mlz 232 and 238 with the laser off is 0.2 ng g-l for both elements. This of course is a rather unconventional way of determining the LOD. It is how- ever the only way that our purpose could be achieved which is a sub-ng g-l measurement of uranium and thorium. If the spiked measurements were not rejected the result of the measurement would be unrealistically high.If the spiked blanks at mlz 230 are not rejected the calculated detection limit can be a factor 2-5 higher. The LOD calculated at mlz 232 and 238 with laser off is not hampered by spikes as there is no flow of aluminium into the plasma. In view of the unconventional determination of the detection limits of LA-ICP-MS it can be concluded tenta- tively that the performance of LA-ICP-MS is better than that of FI-ICP-MS. Rejection of spiked measurements in FI-ICP-MS is not possible as one measurement lasts about 1 min even if smaller injection volumes are chosen. In a 1 min period there is always at least one spike. A shorter measurement could be obtained by a synchronized peak- jump measurement during the FI peak. However in that instance the required LOD cannot be obtained.The intention was to measure uranium and thorium in aluminium samples that contained approximately 1 0 mg g-l of copper and 10 mg g-l of silicon. Unfortunately standards for this matrix were not available. In order to test whether the sensitivity factors of the elements were influ- enced by the presence of copper and silicon the following experiment was performed. By using the high-purity alumi- nium sample R06 as a standard an effort was made to measure the concentration of a number of elements in an Table 3 Peak-jump parameters for the determination of uranium and thorium in aluminium Quadrupole settle time 10 ms Peak jumping dwell time Number of points per peak Number of sweeps per peak Detector type Dual 20 ps for Al 20 ms for U and Th 5 100 Peak jumping D/A steps 5 Table 4 Comparison of the results obtained by LA-ICP-MS with those obtained with NAA (concentrations % la in ng g-') Element NAA LA-ICP-MS U 0.6 % 0.3 0.7 k 0.2 Th 1.3 k 0.1 1.3 k 0.2 Table 5 Comparison of the results obtained by LA-ICP-MS while rejecting measurements with spikes with those obtained with NAA (concentrations k la in ng g-l) Element NAA LA-ICP-MS U 0.6 k 0.3 0.8 % 0.3 3 out of 6 runs Th 1.3k0.1 1.1 k0.3 4 out of 6 runs Table 6 Comparison of the concentration for a number of elements in Al-Si-Cu as obtained with LA-ICP-MS using an aluminium standard with those obtained by NAA (concentrations % la in ng g-') R06 by Al-Si-Cu by Al-Si-Cu by Element NAA NAA LA-ICP-MS s c 7625 55k3 77+ 15 Cr 150k 10 97k5 103 f 8 As 170k20 620+ 120 520 k 60 108% 13 Sb 180k20 60%6 Hf 4.3 k 0.5 1.6k0.2 1.1 %0.2 Th 52% 10 <0.2 (0.3 U 25k3 (1 (0.3 Al-Si-Cu sample. The concentration of these elements is known by measurement with NAA.3 The results presented in Table 6 show that the concentrations recovered are almost within experimental error.Apparently the presence of 10 mg g-l of copper and silicon does not have a dramatic effect on the aluminium matrix in terms of the sensitivity factors of the impurity elements. Conclusion The measurement of uranium and thorium in aluminium is hampered by the presence of spikes. At the same aluminium flow into the plasma spikes occur more often with sample introduction as aqueous solution than by LA suggesting that A1203 is involved. By rejecting measurements with spikes it is possible to measure sub-ng g-l concentra- tions with LA-ICP-MS. The LOD for both elements is 0.2 ng g-*. References 1 2 Hieftje G. M. and Vickers G. H. Anal. Chim. Actu 1989 Date A. R. and Gray A. L. Applications of Inductively 216 1.614 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 1991 VOL. 6 Coupled Plasma Mass Spectrometry Blackie Glasgow 1989. Hanssen J. M. G. Jansen R. M. W. and Jaspers H. J. J. personal corn m unicat ion. Hutton R. C. and Eaton A. N. J. Anal. At. Spectrom. 1988 3 547. Paper 1 /00380A Douglas D. J. and Kerr L.A. J. Anal. At. Spectrom. 1988 3 Received January 28th 1991 749. Accepted July 30th 1991 6 7 Makishima A. Inamoto I. and Chiba K. Appl. Spectrosc. 1990,44 91. Hager J. W. Anal. Chem. 1989 61 1243. 3 4 5

ISSN:0267-9477    

DOI:10.1039/JA9910600609

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 1991 VOL. 6 615 Flow Injection On-line Separation and Preconcentration for Electrothermal Atomic Absorption Spectrometry Part 2.* Determination of Ultra-trace Amounts of Cobalt in Water Michael Sperling Xuefeng Yint and Bernhard Welzt Department of Applied Research Bodenseewerk Perkin-Elmer GmbH 0-7770 Uberlingen Germany Ultra-trace amounts of cobalt in sea-water were determined by flow injection on-line sorbent extraction-precon- centration coupled with electrothermal atomic absorption spectrometry. With sodium diethyldithiocarbamate as the complexing agent and C,,-bonded silica reversed-phase sorbent as the column material a 42-fold enhancement of peak area compared with that for direct 40 PI sample introduction was obtained with a 120 s preconcentration and 5.6 ml of sample.The atomization signal was directly proportional to the preconcentration time. An enhancement factor of 210 and a detection limit of 1.7 ng I-’ could be achieved with a 10 min sample loading time (28 ml). Hence the extremely low cobalt concentration of 4.0 ng I-’ in National Research Council of Canada certified reference material NASS-1 Open Ocean Seawater could be determined within 12 min. Results obtained for sea-water and estuarine water certified reference materials agreed with the certified values with a relative standard deviation of 5.0%. Keywords Sea-water; ultra-trace cobalt determination; on-line solid-sorbent extraction; electrothermal atomic absorption spectrometry The determination of cobalt in sea-water is complicated by various factors.The most important of these are the very low concentrations of analyte element and the high total salt content of the matrix. Many methods such as coprecipita- tion co-crystallization solvent extraction column extrac- tion and electrolysis have been developed for the precon- centration of trace metals in sea-water. Liquid-liquid extraction of chelated metal ions is one of the most widely used techniques for the preconcentration of trace metals from water. A variety of chelating agents have been used such as ammonium pyrrolidin-1-yldithio- formate [pyrrolidine dithiocarbamate (APDC) ],2-5 diethyl- ammonium diethyldithiocarbamate (DDC)6 or different mixtures of these reagent~.~~’-l Because large volume extractions are impractical both from a theoretical and a physical point of view an upper limit is placed on the sample preconcentration factor that can be achieved with single-stage separations.A 30- to SO-fold preconcentration of trace metals from a 1 1 sample of sea-water could not be attained by simple extraction into an organic solvent such as 4-methylpentan- 2-one (isobutyl methyl ketone IBMK) not only because of the unfavourable magnitude of the distribution coefficients of the chelates,12 but primarily because the solubility of IBMK in sea-water necessitated the use of at least 2 ml of organic phase for 100 ml of aqueous phase.4 ‘Clean’ separations were very difficult because a thin band of emulsion formed during phase separation between the two liquid phases. This phenomenon resulted in a variation in the volume of the IBMK extract through the carryover of water present in the em~lsion.~ Another disadvantage of liquid-liquid extraction was the instability of many metal carbamates in organic solu- tion,2*10J1J3 which limited the time available for analysis after extraction.Also the analytical response of organo- metallic compounds was often different from that of in- organic salts making the determination difficult unless organometallic standards and the same solvent were ~sed.41415 *For Part 1 see ref. 43. ?On leave from Shandong Provincial Institute of Environmental Protection Shandong Provincial Environmental Monitoring Centre Jinan China. $To whom correspondence should be addressed. An additional problem with electrothermal atomic ab- sorption spectrometry (ETAAS) was the low surface tension of IBMK making sample delivery difficult. The IBMK tended to creep along the length of the furnace tube limiting the sample volume sever el^.^ Storage of the extracts in open cups on the autosampler was problematic because the analyte concentration increased during analysis owing to solvent evaporation.These and many other problems with the analysis of organic extracts by ETAAS16 were avoided when the metals were back-extracted into an aqueous solution. Unfortu- nately back-extraction was often slow and inefficient for metals such as cobalt copper and iron.” C~precipitation~*l~-~~ has the advantage that most ele- ments except for the alkali and alkaline-earth metals are efficiently concentrated and the procedures are generally simple.However it has disadvantages in that the coprecipi- tation reagent introduces a new matrix and that impurities from the large excess of coprecipitation reagent and from the alkali used for pH adjustment are a serious source of contamination. Very often the precipitate is difficult to filter calling for lengthy ageing procedures20 or centrifuga- tion which is cumbersome when large volumes are in- volved. Some of the problems can be overcome by flotation techniques. 19921+23 Preconcentration by coprecipitation for determination by ETAAS is hampered additionally by the large excess of precipitant which can cause severe matrix effects and background absorption.1° Because of these problems column extraction procedures have attracted increasing attention.These techniques have two potential advantages over liquid-liquid extraction procedures (i) a relatively high enrichment factor and (ii) the ability to treat large sample volumes in a closed system hence reducing the risk of contamination. The column techniques applied to the preconcentration of cobalt from sea-water can be classified according to the types of solid- phase materials used (i) ligand-immobilized ma- teria1;3-5*7-9*24-36 (ii) ligand-impregnated ~orbent;~’J~ and (iii) reversed-phase s ~ r b e n t . ~ ~ . ~ ~ In the first two examples metals are usually eluted from the column by dissociating them from the complexing agent by using mineral acids. In this process most metals are easily released except for a few such as cobalt and chromium.It is known that cobalt(I1) complexes are easily616 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL. 6 oxidized by dissolved oxygen or by their own ligands in the aqueous and organic phases and that the resulting trivalent complexes are too inert to be easily dissociated even by strong acids. Low recoveries of cobalt reported by several worker^^^^^ were caused by incomplete elution of such inert cobalt species and by incomplete complexation.30~40 Chelex- 100 has been widely used as sorbent material for preconcentration of trace metals from sea- water,4*5*7~9~2a*27*30*31-35~36 because it is a strong chelator and removes metal ions from most naturally occurring chelates in sea-water. However the resin does not remove metals held in organic and inorganic colloids which could be present even after ultrafiltration leading to low recoveries from natural water^.^ While a high column capacity results in an improved tolerance to chemical interferences it can also lead to incomplete separation from the matrix.Alkali and alkaline-earth ions occupy the resin sites not occupied by transition metals and are co-eluted by acids. The salts formed frequently impair instrumental techniques such as ETAAS,34*36-41 inductively coupled plasma mass spectro- metry (ICP-MS)24+2* and ICP atomic emission spectrometry (AES).26 Column capacity must be optimized in order to balance the tolerance capacity for interferences in the sample-loading and elution and determination stages. For sea-water calcium and magnesium had to be removed from the resin before elution of the trace metals by well- contrived washing p r ~ c e d u r e s ~ ~ ~ ~ ~ J ~ which were essential for the success of the method.The process of preconcentra- tion was slow because flow rates of less than 2 ml min-' had to be used in order to avoid incomplete retention.30 Elution from the resin was also slow with severe tailing such that a minimum of 30 ml of acid were required for complete e l ~ t i o n . ~ Consequently only low concentration factors could be achieved unless very large sample volumes were used. In addition to this problem swelling and contraction of the resin created difficulties in maintaining column flow rates necessitating frequent operator intera~tion.~Jl Another widely used column material is immobilized 8- An efficient preconcentra- tion and separation method for a number of metals utilizing silica-immobilized 8-hydroxyquinoline and an HN03-HCl mixture to elute the trace metals from the column prior to their determination by ETAAS was described by Sturgeon et ~ 1 1 .~ ~ and Willie et aZ.33 However this procedure required a relatively large acid volume of 10 ml in order to elute the sequestered metals from the column. Consequently a large sample volume (500-900 ml) was required for the analysis of uncontaminated sea- water in order to obtain the necessary sensitivity and it was difficult to attain enrichment factors greater than 100 with less than 1 1 of sample. For cobalt quantitative recovery could not be obtained. When metal-ligand complexes are formed in the aqueous phase and collected with a sorbent-packed ~ o 1 ~ m n ~ ~ ~ ~ ~ organic solvents can be used for elution overcoming the elution problems mentioned above and allowing for the selection of the functional group from a wider range of reagents.A serious drawback of these flow-through methods based on solid-sorbent extraction was the loss of analyte element by precipitation and adsorption of the complexes on to the container walls.40 All such methods invariably increase sample manipula- tion and the relatively large amounts of reagents and the container surfaces coming into contact with the sample often give rise to unacceptably high and/or random proce- dural blanks. This requires extensive purification of re- agents rigorous cleaning of laboratory ware and sample preparation under clean-room condition^.^^ The sample volume available for analysis is often limited e.g.owing to the high costs of water sampling. However the need for large enrichment factors persists requiring the develop- ment of preconcentration techniques that demand low sample volumes and yet provide high enrichment factors with minimal contamination. In only four papers7~25+28~40 are detection limits reported that are sufficiently low for the determination of cobalt in unpolluted open ocean sea-water by atomic spectrometry. In one of these papers,28 ICP-MS is used as the instrumen- tal system for the determination whereas the others used ETAAS.7*25*40 Most of the procedures start with 500-1000 ml volumes of sea-water in order to achieve the large enrichment factors necessary for the determination of this element.In addition all the methods used are off-line and require a number of procedural steps which makes them lengthy and requiring a class 10 (or at least class 100) clean- air working environment. This means that none of the procedures is straightforward and routinely applicable in a normal laboratory environment. Interest in flow injection (FI) AAS has increased not only for sample introduction but also as a technique for sample pre-treatment such as analyte preconcentration and separa- tion from the bulk of the matrix. On-line coupling of a FI-preconcentration system with ETAAS42,43 offers solu- tions to most of the problems discussed above. The closed system allows for processing of high sample volumes with a minimum of wetted surface hence reducing the risk of contamination and analyte loss by sorption on container walls.The integrated system permits fully automated operation avoiding time-consuming manual work which also enhances reproducibility and precision. The FI system can work in parallel with the graphite furnace such that the graphite furnace cycle time can be used for sample processing consequently the over-all processing time is not increased. The combination of liquid-liquid extraction principles with sorption on a solid-phase column permits an extensive selection of functional groups to be used. The possibility of choosing a functional group for chelating and a suitable solid-phase material for sorption provides the high selectivity required in order to separate ultra-trace amounts of transition metal ions from the bulk of the alkali and alkaline-earth elements in the sea-water matrix.The very high sample-to-eluate volume ratio provided by on-line sorbent extraction together with the high efficiency of sample introduction offers the potential of sufficiently high preconcentration factors to enable the determination of elements in the low ng 1-1 range. This work describes investigations into whether the combination of NaDDC (sodium diethyldithiocarbamate) as the chelating agent and c18 reversed-phase material as the sorbent which has been applied successfully to the determination of a number of elements in sea-water and related samples,43 could also be used for the precise determination of ultra-trace amounts of cobalt in sea-water.The aim was to reduce sample consumption increase the speed of analysis and improve the reliability of the determination. Experimental A Perkin-Elmer Zeemad3030 atomic absorption spectro- meter with an HGA-BOO graphite furnace and AS-60 autosampler equipped with a cobalt hollow cathode lamp operated at 35 mA was used throughout this work. The wavelength was set to 240.7 nm with a spectral slit-width of 0.2 nm. Pyrolytic graphite coated electrographite tubes with pyrolytic graphite platforms were used exclusively; the graphite furnace temperature programme is shown in Table 1. The atomization signals were recorded by high-resolution graphics and printed out with a Perkin-Elmer PR-100 printer. Integrated absorbance was used exclusively for determinations.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 1991 VOL.6 617 Table 1 Graphite furnace temperature programme for the deter- mination of cobalt in sorbent-extraction eluates with use of pyrolytic graphite coated electrographite tubes with a pyrolytic graphite platform Time/s Temperature/ Argon flow Step "C Ramp Hold rate/ml min-' 1 90 5 30 300 2 1000 5 20 300 3 2550 0 5 0 (Read) 4 2650 1 5 300 A Perkin-Elmer Model FIAS-200 FI accessory for AAS was used for the preconcentration of cobalt from standards and samples. The rotation speeds of the pumps and the (a) Ethanol HNO DDC Sample W ( C) v C Ethanol -1 W n P2 HNO Sample PC DDC - W ( e) P1 sequence of operation of the pumps and valve were programmed by an Epson PC+ computer working inde- pendently from the spectrometer.Tygon pump tubes were used for all aqueous solutions and solvent-resistant Verdo- prene pump tubes for ethanol. The manufacturing proce- dure for the conical sorbent extraction-preconcentration microcolumns with 15 pl of sorbent material has been described elsewhere.42 Small-bore (0.35 mm id.) poly(tet- rafluoroethylene) tubing was used for all connections. The FI manifold and the sequence of its operation are shown in Fig. l(a-e). The duration and function of each sequence are summarized in Table 2. A complete cycle of preconcentration and eluate introduction into the graphite furnace consisting of seven stages typically took 207 s with a sample loading period of 120 s (corresponding to 5.6 ml of sample).( b ) Ethanol DDC Sample HN03 & U P1 Ethanol .t W W DDC Sample P1 . W GF v 1 Fig. 1 Flow injection manifold and sequence of operation for sorbent extraction preconcentration ETAAS. For details see text. PI and P2 peristaltic pumps; C conical column ( 15 pl packed with RP-C18 sorbent); PC precolumn (500 pl packed with RP-C18 sorbent); W waste; and GF graphite furnace. (a) Sample loading sequence. (b) Column rinsing sequence. (c) Pre-elution to waste. (dj Analyte elution into graphite furnace. (e) Column cleaning. The two additional sequences (sequences 4 and 6 in Table 2) in which the autosampler arm is moved into and out of the graphite tube respectively are not shown here618 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 1991 VOL. 6 Table 2 Sequence of operation of the on-line sorbent extraction-preconcentration system for the detemination of cobalt by ETAAS Time/ Pump I/ Pump 2/ Pumped Stage of Sequence S ml min-' ml min-' Injector medium operation 1 [Fig. I(@] 120 2.8 0 Fill Sample 1.5 NaDDC 2 [Fig.I(@] 25 2.8 0 Inject 0.5% HNO 3 [Fig. l(c)] 8* 0 1.5* Inject Ethanol 4 10 0 0 Inject - 6 10 0 0 Inject - 5 [Fig. l ( 4 ] 4 0 0.7 Inject Ethanol 7 [Fig. l(e)] 30 0 0.8 Inject Ethanol Total 207 s *To be optimized for maximum sensitivity. Load sample Wash column Elute into capillary Capillary into furnace Inject sample Tubing into waste port Elute to waste A sample loading period of 10 min corresponding to 28 ml of sample was used for the determination of the lowest cobalt concentration in the National Research Council of Canada (NRCC) certified reference material (CRM) NASS- 1 Open Ocean Seawater.The principle of eluate zone sampling whereby a 40 pl portion of the eluate containing the most concentrated fraction of the eluted analyte is introduced directly into the graphite furnace through the capillary of the autosampler arm has been explained in detail el~ewhere.~*q~~ The over-all efficiency of this sampling strategy for cobalt was found to be approximately 35%. All reagents were of analytical-reagent grade (Merck) and doubly de-ionized water was used throughout. A 0.05% m/v NaDDC solution was prepared in a buffer solution (0.06 mol dm-3 ammonia + 0.03 mol dm-3 acetic acid; pH 9) and purified on-line by passing the solution through a high- capacity (500 pl) precolumn filled with the same material as the preconcentration column (see Fig. 1).A working stock standard solution (0.100 mg 1-l) was prepared by step-wise dilution of a 1000 mg 1-l stock solution with 0.2Oh m/v nitric acid. Cobalt reference solutions (0.025-0.20 pg 1-l) were prepared by adding a suitable volume of working stock standard solution to 100 ml of 0.2% nitric acid with use of a diluter (TAM Dispenser) and balance (Mettler PM 2000) instead of calibrated flasks in order to reduce the risk of contamination. The NRCC CRMs used in this work were CASS-2 Nearshore Seawater NASS- 1 Open Ocean Seawater and SLEW-1 Estuarine Water. Results and Discussion Optimization of Analytical Parameters The dependence of the extraction efficiency on the pH of the solution is one of the important parameters that can have a significant influence on the over-all performance of the solid-sorbent extraction method.If a narrower pH range must be maintained slight changes in the acidity from sample to sample can result in a deterioration of the extraction efficiency and hence in an increased variability of the results. The efficiency of preconcentration expressed as the integrated absorbance of a cobalt reference solution was studied as a function of the pH by measuring the signal after 1 min preconcentration and the pH of the column eMuent in sequence 1. For this purpose cobalt reference solutions (0.2 pg 1-I) were prepared in nitric acid of different concentrations before on-line preconcentration. No change was observed in the pH range 2.5-9.The effect of the concentration of the chelating agent (NaDDC) on the efficiency of the on-line sorbent extraction of cobalt was investigated by using the same reference solution (0.2 pg 1-l Co) and a 1 min preconcentration time. For the investigated concentration range of 0.02-0.1% m/v NaDDC there was no influence on the integrated absorbance signal for cobalt. While the lowest concentra- tion of the chelating agent would be sufficient for complexa- tion of the traces of cobalt typically found in sea-water 0.05% NaDDC was used for all further experiments. Other heavy metals which can also form complexes and hence consume NaDDC,40*44 can be present in sea-water samples. The higher concentration of the chelating agent could therefore avoid possible interference from other heavy metal ions.Loss of complexed metals on container walls etc. as reported by other was avoided by the on- line formation of the complex and use of the carefully designed manifold with a short connection between the confluence point of sample with the NaDDC solution and the column. There is a direct relationship between the loading flow rate and the sampling frequency so that high flow rates are more desirable to achieve a high sample throughput. However high sampling flow rates have been reported to impair the efficiency of some solid sorbents. In the system discussed in this paper with C18 as the sorbent material no deterioration of the column efficiency was found with flow rates increasing up to about 5 ml min-l. The loading flow rate was only limited by the back-pressure produced by the column which eventually impaired the precision of the liquid volume transported per unit time.A total flow rate of sample and complexing agent of 4.3 ml min-l was therefore used throughout this work which resulted in relatively short loading times and good precision. The importance of rinsing all tubes and the column carefully before elution has been discussed in detail previ~usly.~~ It has also been shown that the direction of rinsing has a decisive influence. Rinsing in a direction counter to the direction of loading was more efficient than rinsing in the same direction even when much longer times were used.43 The reason for this is that with a reverse flow in the rinsing step not only are the non-adsorbed constituents of the matrix removed but possibly even those concomi- tant elements that are retained on the column. Retention of concomitant elements occurs either because they also form DDC complexes or because of the ion-exchange capacity of the sorbent material.Depending on the pH of the rinsing solution and the stability of the complexes formed more weakly bound elements could be removed while more strongly bound elements would be retained on the column. When the column was rinsed in the direction of sample loading less strongly bound elements migrated along theJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL. 6 619 < 0.10 I + 2 0.08 P 2 c m -! Fig. 2 Effect of pyrolysis temperature (see Table 1 step 2) on the integrated absorbance of 0.2 pg 1-l cobalt reference solution after 1 min preconcentration column as in chromatography while they were removed from the column very effectively when the direction of flow was reversed.The most important parameter in this context is the kinetic stability of the analyte-DDC complex and the strength of sorptive bonding to the column in acid solution. Consequently careful optimization of the pH is essential for efficient rinsing. For this purpose the solid-sorbent column was rinsed with nitric acid in concentrations ranging from 0.02 to 2% d v . The CoDDC complex was found to be stable and was retained quantitatively on the column even when 2% nitric acid was used for rinsing. This result is essentially in agreement with those of Isshiki and Nakayama,"o who reported that cobalt was retained on solid-sorbent columns with a wide range of ligands even when 1 mol dm-3 hydro- chloric acid was used for rinsing.Nitric acid (0.5%) was used in all further experiments to remove the sea-water matrix. This concentration was found to be very effective and gave low background signals in the ETAAS detection step. When ETAAS is used for determinations the volatility and thermal stability of the analyte-DDC complexes in ethanolic solution are important. Many chelate complexes and other organometallic compounds are known to sublime undecomposed. l6 In ETAAS this could result in analyte loss during pyrolysis. In order to investigate the volatility of the cobalt complex the effect of pyrolysis temperature on the absorbance of ethanolic CoDDC extracts was ascertained.For this purpose the column eluate was dispensed by the FI system into the cavity of a pyrolytic graphite platform mounted in the graphite tube. Use of the platform results in improved atomization conditions because spreading of the sample in the tube is restricted. The results in Fig. 2 show that the integrated absorbance of cobalt remains constant up to a pyrolysis temperature of 1500 "C indicating that the thermal stability of cobalt from the chelate-ethanol solu- tion is almost the same as that of cobalt in aqueous solution in a graphite furnace (this is in good agreement with results obtained by Komhek and co-worker~).~~~~ The high stabil- ity offers the possibility of separating the analyte element from more volatile concomitants in the pyrolysis stage and hence of increasing the specificity and reducing background absorbance.A pyrolysis temperature of 1000 "C was found to be ideal and was used throughout this work for all analytical applications. Performance of the Sorbent Extraction-Preconcentration System The performance of on-line sorbent extraction-preconcen- tration ETAAS for cobalt is summarized in Table 3. Because of the low cobalt concentration typically found in sea-water a preconcentration time of 120 s was used and a 42-fold enhancement in peak area compared with that for Table 3 Performance of the on-line sorbent extraction-precon- centration system for cobalt in sea-water Preconcentration time/min Enrichment factor Characteristic concentratiodng 1-I Detection limit (30)/ng 1-l Precision (% RSD) Sample throughputh-l Sample consumptiodml Reagent consumption per sample/ml Ethanol 0.05% NaDDC 2 42 5.8 6.4 5* 5.6 0.7 3 17 *Using SLEW- 1 Estuarine Water (n = 9).tUsing NASS-1 Open Ocean Seawater (n=4). 10 210 1.2 1.7 1 ot 5 28 0.7 15 0.2 I 1 0 1 .o 2.0 3.0 Time/s Fig. 3 Superimposed atomization signals for cobalt in different aqueous reference solutions after sorbent extraction preconcentra- tion with a 2 min loading time A 25 ng 1-l; B 50 ng 1-I; and C 75 ng 1-l 40 pl direct introduction was obtained (corresponding to an over-all efficiency of 35%). An analytical curve was established by using matrix-free cobalt reference solutions in the range 0.005-0.1 pg l-l and the linear regression equation was y= -0.00006 +0.758x with a correlation coefficient of 0.99997 (three superimposed atomization signals for cobalt reference solutions are shown in Fig.3). Because the cobalt concentration in NRCC CRM NASS-1 was lower than the detection limit obtained with a 2 min sample preconcentration further experiments were carried out in order to evaluate the column capacity. The integrated absorbance signal was found to be directly proportional to the preconcentration time at least up to 10 min and the standard deviation for the blank did not increase much with preconcentration time. The mean integrated absorbance values (n = 10) of the blank for 2 and 10 min preconcentrations were 0.00 1 6 and 0.002 1 respec- tively. Typical blank signals for 2 and 10 rnin preconcentra- tion as displayed by the instrument are shown in Fig.4(a and b). With a sample loading time of 10 min correspond- ing to 28 ml of sample an enrichment factor of 2 10 and a detection limit of 1.7 ng 1-l (30) could be achieved. 1 1 0 1 .o 2.0 3.0 Time/s Fig. 4 Atomization signals for 0.2% nitric acid blank solution after on-line sorbent extraction using different loading times (a) 2; and (b) 10 min620 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL. 6 Table 4 Determination of cobalt in sea-water and estuarine water standard reference materials by on-line sorbent extraction-precon- centration ETAAS Certified reference Certified Found?/ CASS-2 5 0.025 & 0.006 0.027 & 0.004 SLEW-1 9 0.046 & 0.007 0.053 f 0.003 NASS- 1 $ 4 0.004 & 0.00 1 0.0039 k 0.0004 material Replicates value*/pg 1-I Pg I-' *The uncertainties represent 95% confidence limits.tMean values and standard deviations are given. 10 min preconcentration. 0 1 .o 2.0 3.0 Time/s Fig. 5 Atomization signals for cobalt in certified reference materials after sorbent extraction preconcentration with 2 min loading time (a) CASS-2; and (6) SLEW-1 The accuracy of the proposed method for the determina- tion of low concentrations of cobalt in samples with a high total dissolved solids content was tested by the analysis of NRCC CRMs CASS-2 NASS-1 and SLEW-1. The samples which were acidified for conservation purposes to pH 1.6 by the supplier were used without further treatment. The results shown in Table 4 agree well with the certified values with precisions that are more than adequate for such low cobalt concentrations.Typical cobalt atomization signals are shown in Fig. 5(a and b). The background absorbance signals for sea-water samples are low and are not higher than the background signals obtained from aqueous standard solutions. The background absorbance detected by the instrument is in essence due to incomplete resolution of the Zeeman line splitting which produces a slight overlap of the cr components of the absorption profile with the emission line. Zeeman-effect background correction was not necessary for this applica- tion although the Zeeman 3030 was the instrument used for this study. Conclusion Combining a micro-scale FI preconcentration system on- line with ETAAS results in a powerful integrated hybrid system. The most outstanding advantage is the greatly improved detection limit achieved by preconcentration and matrix separation. The totally closed system together with on-line purification results in very low and reproducible blanks allowing the determination of ultra-trace concentra- tions of elements even in laboratories not equipped with clean-room facilities.The automated sample processing avoids time-consuming manual work and operator interac- tion thereby ehancing reproducibility and precision. Solid- sorbent extraction offers high preconcentration and selec- tivity for the separation of ultra-trace amounts of metals from sea-water matrices. Direct introduction of the eluate helps to overcome the problems with the analysis of organic extracts in ETAAS as discussed in the introduction.Processing samples and standards by the same system avoids calibration problems caused by a different response from organic and aqueous solutions and results in excellent accuracy. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 References Leyden D. E. and Wegscheider W. Anal. Chem. 1981 53 1059A. Tao H. Miyazaki A. Bansho K. and Umezaki Y. Anal. Chim. Acta 1984 156 159. Allen E. A. Bartlett P. K. N. and Ingram G. Analyst 1984 109 1075. Sturgeon R. E. Berman S. S. Desaulniers A. and Russell D. S. Talanta 1980 27 85 Smits J. Nelissen J. and van Grieken R. Anal. Chim. Acta 1979 111 215. Chakraborti D. Adams F. Van Mol W. and Irgolic K. J. Anal. Chim. Acta 1987 196 23. Boniforti R. Ferraroli R. Frigieri P. Heltai D.and Queirazza Q. Anal. Chim. Acta 1984 162 33. McLeod C. W. Otsuki A. Okamoto K. Haraguchi H. and Fuwa K. Analyst 1981 106 419. Sturgeon R. E. Berman S. S. Desaulniers J. A. H. Mykytiuk A. P. McLaren J. W. and Russell D. S. Anal. Chem. 1980 52 1585. Danielsson L.-G. Magnusson B. and Westerlund S. Anal. Chim. Acta 1978 98 47. Kinrade J. D. and van Loon J. C. Anal. Chem. 1974 46 1894. Brooks R. R. Presley B. J. and Kaplan I. R. Talanta 1967 14 809. Hulanicki A. Talanta 1967 14 137 1. Betz M. Gucer S. and Fuchs F. Fresenius 2. Anal. Chem. 1980 303 4. Karwowska R. Bulska E. Barakat K. A. and Hulanicki A. Chem. Anal. (Warsaw) 1980 25 1043. Volynsky A. B. Spivakov B. Ya. and Zolotov Yu. A. Talanta 1984 31 449. Magnusson B. and Westerlund S. Anal. Chim. Acta 1981 131 63.Shan X.-q. Tie J. and Xie G.-g. J. Anal. At. Spectrom. 1988 3 259. Caballero M. Lopez R. Cela R. and Perez-Bustamante J. A. Anal. Chim. Acta 1987 196 287. Akagi T. Fuwa K. and Haraguchi H. Anal. Chim. Acta 1985 177 139. Hiraide M. Ito T. Baba M. Kawaguchi H. and Mizuike A. Anal. Chem. 1980 52 804. Hudnik V. Gomiscek S. and Gorenc B. Anal. Chim. Acta 1978 98 39. Hiraide M. Yoshida Y. and Mizuike A. Anal. Chim. Acta 1976 81 185. Beauchemin D. and Berman S. S. Anal. Chem. 1989 61 1857. Nakashima S. Sturgeon R. E. Willie S. N. and Berman S. S. Fresenius Z. Anal. Chem. 1988 330 592 Van Berkel W. W. Overbosch A. W. Feenstra G. and Maessen F. J. M. J. J. Anal. At. Spectrom. 1988 3 249. Fang. Z. Xu S. and Zhang S. Anal. Chim. Acta 1987 200 35. McLaren J. W. Mykytiuk A.P. Willie S. N. and Berman S. S. Anal. Chem. 1985 57 2907. Marshall M. A. and Mottola H. A. Anal. Chem. 1985 57 729. Hartenstein S. D. RbiiEka J. and Christian G. D. Anal. Chem. 1985,57,21. Fang Z. RbiiEka J. and Hansen E. H. Anal. Chim. Acta 1984 164 23. Malamas F. Bengstsson M. and Johansson G. Anal. Chim. Acta 1984 160 1. Willie S. N. Sturgeon R. E. and Berman S. S. Anal. Chim. Acta 1983 149 59. Sturgeon R. E. Berman S. S. Willie S. N. and Desaulniers. J. A. H. Anal. Chem. 1981 53 2337.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL. 6 62 1 35 36 37 38 39 40 41 42 Mykytiuk A. P. Russell D. S. and Sturgeon R. E. Anal. Chern. 1980,52 128 1. Kingston H. M. Barnes I. L. Brady T. J. Rains T. C. and Champs M. A. Anal. Chem. 1978,50 2064. Braun T. and Abbas M. N. Anal. Chim. Acta 1980,119 1 13. Stella R. Ganzerli Valentini M. T. and Maggi L. Anal. Chern. 1985,57 1941. Plantz M. R. Fritz J. S. Smith F. G. and Houk R. S. Anal. Chern. 1989,61 149. Isshiki K. and Nakayama E. Anal. Chern. 1987 59 291. Hydes D. J. Anal. Chem. 1980 52 959. Fang Z. Sperling M. and Welz B. J. Anal. At. Spectrom. 1990 5 639. 43 Sperling M. Yin X. and Welz B. J. Anal. At. Spectrom. 1991 6 295. 44 RGiiEka J. and Arndal A. Anal. Chim. Acta 1989 216 243. 45 Komdrek J. and Sommer L. Talanta 1982 29 159. 46 Komdrek J. Kolcava D. and Sommer L. Collect. Czech. Chem. Commun. 1980,45 3313. Paper 1 /01839F Received April I9th 1991 Accepted July 2nd 1991

ISSN:0267-9477    

DOI:10.1039/JA9910600615

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 I VOL. 6 623 Slurry Sampling and Fluorination-Electrothermal Vaporization Inductively Coupled Plasma Atomic Emission Spectrometry for the Direct Determination of Molybdenum in Food Hu Bin Jiang Zucheng* and Zeng Yun'e Department of Chemistry Wuhan University Wuhan 430072 China A method for the direct determination of Mo in various types of food by fluorination-electrothermal vaporization inductively coupled plasma atomic emission spectrometry has been developed and optimized. Slurry samples were prepared by ultrasonic wave vibration after pre-treatment and direct injection into the graphite furnace using polytetrafluoroethylene as a fluorinating agent. The procedure was applied to the determination of Mo in National Institute of Standards and Technology Standard Reference Materials 1567 Wheat Flour 1568 Rice Flour and 1577 Bovine Liver.The values found were in reasonable agreement with the certified values with a detection limit of 0.7 ng mi-l and an RSD of 3.2% (n-10) at a concentration of 0.1 pg mi% The proposed procedure has been applied successfully to the analysis of various types of food samples; the recovery ranged between 92 and 105%. Keywords Molybdenum determination; fluorination-electrothermal vaporization; polytetra fluoroethylene slurry; inductively coupled plasma atomic emission spectrometry Molybdenum is an essential trace element required by both plants and animals.' The importance of Mo in animal nutrition has been recognized for over 40 years. The antagonistic effects on the metabolism of Cu in ruminant animals has attracted much attention in the past,2 but recent findings have indicated that Mo itself can have important direct effects on the biological processes control- ling growth and reproductive perfo~mance.~ It has also been shown from studies with patients receiving total parented nutrition that Mo is an essential element for man,4 the major source for man of Mo being in food.There is a need therefore for a convenient method of assessing the Mo status in man. Although there are a number of techniques available for the determination of Mo in biological materials recent interest has been focused on the use of electrothermal atomic absorption spectrometry (ETAAS) and inductively coupled plasma atomic emission spectrometry (ICP-AES).However the formation of stable non-volatile carbides of Mo on the graphite surface is a serious drawback to the use of ETAASSd Barium difluoride has been proposed as a chemical modifier for the determination of Mo in serum' and milk.* When using this method the appearance temperature of Mo is lowered the signal is increased and matrix interference effects are partly overcome but the memory and signal tailing effects still remain especially with solid sampling.* Electrothermal vaporization (ETV) has developed into an important tool for trace element analysis in recent years as it combines the advantages of both ETAAS and ICP-AES. Aziz et aL9 have successfully coupled a commercially available graphite furnace (Perkin-Elmer HGA-74) to an ICP torch and determined Cd Pb Mn and Zn in two National Institute of Standards and Technology (NIST) Standard Reference Materials (SRMs) and commercially available serum samples. Matusiewicz and BarneslO de- scribed a method for the determination of Al Li Au Os Pd Pt and Ru in body fluids at therapeutic levels by ICP- AES with introduction of 5 pl samples by ETV.Recent developments of this technique have been reviewed by Ng and Carusol and Matusiewicz. l2 However this approach retains the disadvantage of electrothermal atomization with respect to the formation of ~ _ _ _ ~ ~ _ _ _ _ _ _ ~ ~____ *To whom correspondence should be addressed. refractory carbides by elements such as Ti Zr V Cr Mo W Nb Ta Hf B Si and rare earth elements,13-15 which leads to a decrease and sometimes complete suppression of the analyte signals.Improved methods have been reported that include conversion of the analytes into volatile hal- i d e ~ ~ ~ - ~ ~ or graphite tubes coated with a layer of compacted metal carbides to prevent carbon from reacting with the elements under investigation. l6 A previous paper from this laboratory17 described a method for the direct determina- tion of B in a plant sample by fluorination and ETV-ICP- AES using polytetrafluoroethylene (PTFE) as a fluorinating agent; the detection limit for B was as low as 24 pg. The present study was carried out to develop an accurate precise and rapid method for the determination of Mo in various types of food by slurry sampling fluorination-ETV- ICP-AES using PTFE as a fluorinating agent.Experimental Apparatus The experimental details for the inductively coupled plasma and electrothermal vaporizer have been described previ~usly.~~ A commercial 2723 MHz Ar ICP source (Beijing Broadcast Instrument Factory Beijing China) with a 2 k W plasma generator was interfaced to a WDG 500- 1 A monochromotor (Beijing Second Optics Beijing China). The output of the photomultiplier (R456 Hama- matsu Japan) was amplified and registered on a strip-chart recorder (L23- I04 Sichuan Fourth Meters Shanghai China). Table 1 ETV-ICP-AES operational parameters Wavelength Incident power Carrier gas (Ar) flow rate Coolant gas (Ar) flow rate Auxiliary gas (Ar) flow rate Observation height Entrance slit-width Exit slit-width Drying temperature Ashing temperature Atomization temperature Sample volume 202.030 nm 1.0 kW 0.5 1 min-' 16 1 min-I 0.8 1 min-' 12 mm 25 pm 25 pm 100 "C ramp 15 s hold 15 s 380 "C ramp 10 s hold 20 s 2100 "C 3 s 10 jd624 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL.6 A WF-1 type heating device with a matching graphite furnace (Beijing Second Optics) was used as the electrother- mal vaporization device. The instrument operating condi- tions and wavelength used are given in Table 1. Reagents A stock solution of Mo with a concentration of 1 g 1-l was prepared by dissolving 0.1500 g of MOO (Specpure Shanghai Reagent Factory China) in 10% v/v ammonia solution (Suprapur Shanghai Reagent Factory) and diluting to volume with doubly distilled water. A 60% m/v slurry of PTFE (viscosity 7 x 10-,-1 5 x lo- Pa s) was supplied by the Shanghai Institute of Organic Chemistry.Except for Na2Si03 (analytical-reagent grade Shanghai Reagent Fac- tory) the other reagents (NaC1 KCl CaCl CuCl FeCl AlCl and MgC1,) were all of Specpure grade. Doubly distilled water was used throughout. Preparation of Slurry Samples Samples of pollen and spinach were dried at 105- 1 10 "C for at least 24 h and garlic was shelled and washed with tap water followed by doubly distilled water and air dried. The samples were crushed and 1 .OOOO g of the spinach and garlic and 0.5000 g of the pollen were accurately weighed into acid-washed crucibles. The materials were charred in a muffle furnace at 600 "C for 2 h then quantitatively tranferred into acid-washed PTFE bottles together with agate spheres and 5 ml of a slurry containing varying amounts of PTFE depending on the sample (pollen spinach or garlic) were added accurately.The bottles were agitated on a flask shaker for 1 h to reduce the particle size. The addition of Mo to the slurry samples was achieved by pipetting 0 10 30 and 100 pl of a 1 pg ml-l Mo solution into 0.1 0.5 and 0.5 ml slurries of the pollen spinach and garlic respectively. The mixtures were diluted to 1 ml and dispersed with an ultrasonic wave vibrator for 15 min after which the bottles were shaken vigorously prior to sampling. This results in slurry samples containing 0 10 30 and 100 ng ml-l of added Mo respectively and 6% m/v of PTFE. Powder samples (milk powder 1.0000 g and wheat flour 0.5000 g) were mixed with 2.5 ml of water then heated to 50 "C and stirred.Samples (0.5 ml) of these slurries were taken and diluted to 1 ml with the appropriate amounts of PTFE slurry and various volumes of the Mo standard solutions. The mixtures were dispersed with an ultrasonic wave vibrator for 20 min after which the bottles were shaken vigorously prior to sampling. The resulting slurry samples contained 0 10 30 and 100 pg 1-l of added Mo and 6% m/v of PTFE. Procedure After igniting the plasma the gas flow rate power and viewing height were adjusted to the conditions given in Table 1. The details of the drying ashing and atomization stages of the electrothermal vaporizer are also given. The appropriate wavelength was selected by aspirating a Mo solution. The pneumatic nebulizer was then disconnected from the plasma torch and replaced by the ETV system.A 10 p1 volume of sample was then deposited into the furnace and the drying and ashing steps were initiated. After ashing was complete the injection hole of the graphite furnace was sealed with a graphite rod and the vaporizing sequence was repeated according to the conditions described in Table 1. The desolvated vaporized sample was carried into the plasma by the Ar carrier gas. The transient emission intensity for the Mo line selected was recorded by the strip- chart recorder. A calibration graph was constructed using peak height measurements. Results and Discussion ICP Discharge Parameters The ICP discharge parameters were established using a standard solution of 0.1 pg ml-l of Mo containing 6% m/v PTFE and the signal-to-background ratios were used to take the measurements. The results showed that 1.0 kW power a carrier gas flow rate of 0.5 1 min-l and a 12 mm observation height were the optimum conditions.Optimization of the Graphite Furnace Programme Experiments were carried out to determine the best temper- ature and times for the various drying ashing and atomiza- tion steps. Optimum drying conditions were required to provide a smooth even evaporation of the solvent (water) with no spluttering; a drying temperature of 100 "C was used. Proper choice of the ashing temperature is very impor- tant. When the ashing temperature is lower than the decomposition temperature of any organic compounds present it can lead to interferences from the organic matrix.However when the ashing temperature is higher than the temperature at which the analyte vaporizes decreases in the emission signal of Mo may occur. In order to optimize the ashing and atomization temperatures ashing and atomiza- tion curves were constructed for samples containing 0.1 pg ml-* of added Mo and the optimum concentration of fluorinating agent; the results are shown in Figs. 1 and 2. It was seen that when PTFE was present decreases in the Mo analytical signal occurred at less than 400 "C whereas without PTFE no decrease was found at temperatures of more than 1240 "C. This indicated that on addition of PTFE Mo reacted with it in the graphite furnace and vaporized in the form of the volatile MoF (boiling-point 36 "C). The optimum ashing temperature was 380 "C.It is evident from Fig. 2 that the presence of PTFE greatly influenced the vaporization behaviour of Mo and the vaporization reached a plateau at 1500 "C. This showed that the fluorination reaction between Mo and PTFE was complete. By contrast in the absence of PTFE no plateau was found in the temperature range tested. From Figs. 1 and 2 it can also be seen that with fluorination-vaporiza- tion the Mo emission signal intensity is much more intense than that without the use of PTFE as a fluorinating agent. In this experiment 2100 "C was chosen as the atomization temperature for the determination. Optimization of Amount of Fluorinating Agent Different amounts of PTFE were added to a series of 500 p1 sample suspensions containing 0.2 pg ml-l of Mo. The mixtures were diluted to 1 ml with water and subjected to I A 1 I I I 200 600 1000 1400 i a TemperaturePC Fig.1 Ashing curves for Mo using ETV-ICP-AES for the introduction of 10 pl of slurry sample into the graphite furnace A 0.1 pg ml-I of Mo in 6% m/v PTFE; and B 1 pg ml-* of Mo without PTFEJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL. 6 .- a9 .!6- 0 .- w .- .- U k > 3 - - 625 f A' / z c 6 E 3 - .- P *; .- *.' - a K Fig. 2 Atomization curves for Mo using ETV-ICP-AES for the introduction of 10 pl of slurry sample into the graphite furnace A 0.1 ,ug ml-1 of Mo in 6% m/v PTFE; and B 1.0 pg ml-1 of Mo without PTFE -Y I I 1 I I PTFE concentration (% m/v) Fig. 3 Effect of PTFE concentration on fluorination-vaporiza- tion; 10 pl sample 0.1 p g ml-I Mo the furnace programme. The Mo emission signal was accentuated by PTFE at concentrations up to 5% d v .The maximum emission intensity achieved with this concentra- tion remained constant up to the highest amount studied (10%) (Fig. 3) but the stability of the plasma discharge decreased markedly and actually extinguished at high PTFE concentrations owing to vigorous decomposition of the PTFE in the plasma. The optimum concentration of PTFE chosen was 6%. Matrix Interferences The influences of the matrix on the fluorination-vaporiza- tion of Mo were investigated. The interfering elements were Al Ca Cu Fe K Mg Na and Si with a Mo concentration of 0.1 pg ml-l. It was found that by using PTFE as fluorinating agent amounts of Ca Cu K Mg and Na of up to 5 g 1-1 and A1 of up to 2 g 1-l did not affect the Mo vaporization process.Iron did not interfere with the determination of Mo at a concentration below 1 g 1-l. However when the amount of Fe was in excess of 2 g I-l the emission intensity of Mo was enhanced markedly. This is due to the Fe 202.050 nm spectral interference caused by evaporation of Fe. When the Si content was more than 1 g l-l the Mo signal decreased significantly. This is because a large amount of Si reacts with the PTFE and inhibits the vaporization of Mo. Memory Effects When using the optimum operating conditions no memory effects were apparent for 10 pl of a 10 pg ml-l solution of Mo (Fig. 4). C U 3s U B W I - r Fig. 4 Recorder tracings for 10 pl samples A 0. I pg ml-I of Mo in 6% m/v PTFE; B residual signal of the first firing after vaporizing 10 p1 of a 10 pg ml-1 of Mo in 6% m/v PTFE sample; and C residual signal for the second firing Mo concentrationlpg mr' Fig.5 Calibration graph for Mo obtained at 202.030 nm using 6% m/v PTFE as the fluorinating agent Calibration In order to obtain a calibration graph standard solutions containing 0.0 1- 10 pg ml-l of Mo with 6% m/v PTFE were subjected to the furnace programme. The results are shown in Fig. 5. As can be seen the graph is linear over a concentration range of three orders of magnitude. Detection Limit Precision and Accuracy According to the recommendations of the American Chemical Society Committee of Environmental Improve- ment the detection limit the lowest concentration level that can be determined to be statistically different from a blank is defined as three times the within-batch standard deviation of a single blank determination corresponding to a 99Oh confidence level.The detection limit for Mo with fluorination-ETV-ICP-AES is 0.7 ng ml-l at a concentra- tion of 0.01 pg ml-l but the detection limit for Mo without PTFE is 30 ng ml-l. The detection limit was improved by approximately two orders of magnitude by using fluorina- tion-vaporization. The relative standard deviation (RSD) of this method obtained for ten replicate determinations at a concentration of 0.1 pug ml-l was 3.2%. Table 2 Concentration of Mo in NIST SRMs obtained by fluorination-ETV-ICP-AES Reference material Found value Certified value SRM 1567 Wheat Flour 0.43 k 0.02 0.40 SRM 1568 Rice Flour I .48 -t 0.10 1.60 SRM 1 577 Bovine Liver 3.40 f 0.17 3.50 (PPm) (PPm)626 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL.6 Table 3 Results for determination of Mo in various types of food obtained by standard additions and slurry sampling fluorination-ETV- ICP-AES. All of the results are the means of triplicate analyses Amount of Mo/pg 1-I Sample Pollen Milk powder Wheat flour Garlic Spinach Content Before Amount After Recovery in sample/ 54.8 50.0 107.5 105 5.48 k0.21 addition added addition (%I C(g g-I 10.8 10.0 20.0 92 0.054 k 0.0 1 42.0 40.0 79.5 94 0.42 k0.06 21.8 20.0 41.2 97 0.22 kO.05 18.0 20.0 36.6 92 0.18 k0.04 To study the accuracy of the method NIST SRMs 1567 Wheat Flour 1568 Rice Flour and 1577 Bovine Liver were used. The results are given in Table 2.The agreement with the certified values was very good. The accuracy of the method was also determined by measuring the recovery of standard additions of Mo to the samples. The recovery ranged from 92 to 105% (Table 3). Sample Analysis The proposed method was applied to various types of food samples for the determination of Mo. The results obtained using the standard additions procedure are shown in Table 3. Conclusions The results of this study show that the use of PTFE as a fluorinating agent not only enhances the sensitivity for the determination of Mo by ETV-ICP-AES but also eliminates memory effects. By using slurry sampling the procedure provides a means for determining M o in solution or solid samples which can reduce sample preparation time de- crease analyte losses due to volatilization and contamina- tion.Furthermore any element capable of forming a fluoride that is more volatile than the original form of the analyte present after drying could potentially benefit from this approach. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 References Underwood E. J. Trace Elements in Human and Animal Nutrition Academic Press New York 1977. Mason T. Toxicology 1986,42 99. Phillippo M. Humphries W. R. Atkinson T. Henderson G. D. and Garthwaite P. H. J. Agric. Sci. 1987 109 321. Abumrad N. N. Schneider A. J. Steel D. and Rogers L. S. Am. J. Clin. Nutr. 1984 34 2551. Muller-Vogt G. Wendl W. and Pfundstein P. Fresenius Z. Anal. Chem. 1983 314 638. Sneddon J. Ottaway J. M. and Rowston W. B. Analyst 1978,103 776. Ericson P. McHalsky M. L. and Saselskis B. At. Spectrosc. 1987 8 101. Wagley D. Schmiedel G. Mainka E. and Ache H. J. At. Spectrosc. 1989 10 106. Aziz A Broekaert J. A. C. and Leis F. Spectrochim. Acta Part B 1982,37 369. Matusiewicz H. and Barnes R. M. Acta Chim. Hung. 1988 125 777. Ng K. C. and Caruso T. A. Appl. Spectrosc. 1985 39 719. Matusiewicz H. J. Anal. At. Spectrom. 1986 1 171. Kirkbright G. F. and Snook R. D. Anal. Chem. 1979 51 1938. Ng. K. C. and Caruso J. A. Analyst 1983 108 476. Huang M. Jiang Z. and Zeng Y. Gaodeng Xuexiao Huaxue Xuebao 1989,5 288. Ng. K. C. and Caruso J. A. Anal. Chim. Acta 1982,143,209. Hu B. Jiang Z. and Zeng Y. Fresenius J. Anal. Chem. 199 1 340,435. Paper 0/058 12B Received December 28th I990 AcceDted June 5th. 1991

ISSN:0267-9477    

DOI:10.1039/JA9910600623

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL. 6 627 Cold Vapour Atomic Absorption Method for the Determination of Mercury in Iron(iii) Oxide and Titanium Oxide Pigments Using Slurry Sample Introduction lgnacio Lbpez Garcia Maria Jesus Vizcaino Martinez and Manuel Hernandez Cordoba* Department of Analytical Chemistry Faculty of Chemistry University of Murcia 30071 Murcia Spain A simple and rapid method for the determination of mercury in commercial iron(ii1) oxide and titanium oxide pigments combining a slurry sampling procedure with the cold vapour technique is described. The samples are suspended in water containing 0.02% m/v sodium hexametaphosphate and mercury vapour is generated from a hydrochloric acid medium by adding sodium tetrahydroborate(iii). A detection limit of 5 ng g-1 of mercury is achieved.Calibration can be performed using standard aqueous mercury solutions. Excellent agreement is found between the results of the slurry sampling procedure and those obtained for iron(iii) oxide pigments using lengthy conventional acid dissolution procedures. Keywords Mercury determination; iron(rrr) oxide pigment; titanium oxide pigment; slurry sampling; cold vapour In recent years the interest of many researchers has focused on the use of suspensions or slurries as a means of introducing solid samples into the atomizer of an atomic absorption spectrometer. Most of the papers to date have dealt with the determination of a number of elements in a wide range of materials including soils coal biological samples and inorganic pigments using electrothermal atom- ization.This is because for flame atomization purposes the atomization efficiency is markedly dependent on several factors for example a sufficiently low particle size is critical and comparatively few application~l-~ have been reported using this approach. Haswell e? aL5 have reported a procedure for the determination of cold acid-soluble arsenic in various matrices by slurrying samples in hydrochloric acid prior to hydride generation. Recently Madrid e? aP published a detailed paper on the determination of lead in foodstuffs and biological samples using hydride generation from slurried samples. Apart from the papers already mentioned as far as we know there are no reports dealing with the generation of mercury vapour from slurried samples.Interest in slurry- based procedures has increased as the length of time required for some dissolution procedures can be great and there is always the risk of analyte loss in pre-treatment of samples. The aim of this work was to study the determina- tion of mercury through the generation of mercuIy vapour from slurries prepared from iron(@ oxide or titanium oxide pigments. These commercial products meet the essential requirements for low particle size and their mercury contents are severely restricted by law. The procedure reported here gives a fast determination which is useful for routine analyses by avoiding dissolution steps. Experimental Apparatus A Perkin-Elmer Model 300 atomic absorption spectro- meter a Model MHS-10 hydride generation system and a Hewlett-Packard 3394-A integrator were used.Some exper- iments were performed using a Perkin-Elmer Model 1 lOOB atomic absorption spectrometer and the MHS-10 hydride generation system. All the measurements were made at the mercury wavelength of 253.7 nm using a conventional * To whom correspondence should be addressed. mercury hollow cathode lamp (Perkin-Elmer) operated at 4 mA with a band width of 0.7 nm. Background correction was not used. Reagents All reagents were of anlytical-reagent grade and were used without further purification. Doubly distilled water was used throughout. A stock solution containing 1000 pg ml-l of mercury was prepared by dissolving 1.3535 g of mercury(rr) chloride in 50 ml of concentrated hydrochloric acid and diluting to 1 1 with water.From this stock solution working standard solutions were freshly prepared by appropriate dilution with a solution containing 1% v/v nitric acid and 0.002% m/v potassium permanganate. Sodium tetrahydroborate was obtained from Fluka and used as a 4% m/v solution in water containing 1% m/v sodium hydroxide. This solution was prepared daily and filtered before use. Sodium hexametaphosphate (HMP) was obtained from Fluka. Slurry Procedure Although the pigments studied are sold as powders with low particle size in order to assure homogeneity the samples were ground using a ball mill for 10 min and then dry sieved using a 45 pm sieve (325 mesh). The small fraction (less than 0.5%) containing larger particles was discarded. A 0.5 ml volume of 2% m/v HMP solution was added to an accurately weighed amount of sieved sample (1-5 g) the solution was made up to 50 ml with water and stirred magnetically for a minimum of 10 min.While the suspen- sion was being stirred an aliquot (1-5 ml) was placed into the vessel of the hydride generation system. Concentrated hydrochloric acid [4 ml for iron(1n) oxide slurries or 1.5 ml for titanium oxide slurries] was added and the solution was made up to 10 ml with water. The plunger of the MHS-10 system was depressed for 8 s (this delivers 4 ml of the tetrahydroborate solution) and the peak height was mea- sured. Aqueous standard solutions of mercury (25-200 ng) and a reagent blank was used for calibration under the same experimental conditions. Aciddissolution Procedure For comparison purposes samples of iron(rxr) oxide pig- ments were analysed as follows 2-10 g of sieved sample628 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL.6 0.05 were weighed in a 100 ml covered conical beaker and 30 ml of a mixture of concentrated nitric and hydrochloric acids (1 + 3 v/v) were added. The samples were heated gently (50-60 "C) until total dissolution. It was necessary to add several 5 ml portions of the acid mixture in order to compensate for evaporation. After dissolution the liquid was made up to 100 ml using 6 mol dm-3 hydrochloric acid. Aliquots of this solution were placed into the reaction vessel the acidity was adjusted to 4.8 mol dm-3 hydro- chloric acid in a final volume of 10 ml and the mercury content was determined using the standard additions method.A reagent blank was also analysed to correct the results. - C n- I D m I I I I I I I Results and Discussion Previous experiments have shown that when tin(@ chloride was used as the reducing agent the profiles of the absorbance-time signals obtained from slurried samples were wider and lower than those obtained when sodium tetrahydroborate was used. Therefore all subsequent exper- iments were carried out using sodium tetrahydroborate solution. From an experimental point of view the effect produced by acids must be carefully considered as several reports have indicated significant differences in the generation of mercury vapour depending on the nature and concentra- tions of the acid media used. In practice sufficient acid must be present in the reaction vessel to neutralize sodium tetrahydroborate and to provide an excess as the reduction does not take place in an alkaline medium.A number of 2% iron(Ir1) oxide slurries were prepared as described under Experimental and the effect of sulphuric nitric and hydrochloric acids was studied. Different amounts of the acids were added to aliquots of the slurried samples in order to produce in a final volume of 10 ml the concentrations indicated on the x-axis of Fig. 1. Next 4 ml of the sodium tetrahydroborate solution were added using the hydride generation system and the absorbance-time profiles for mercury were obtained. When sulphuric or nitric acid was used very similar peak areas were obtained. However the peak heights obtained from samples in sulphuric acid media were higher than those found when nitric acid was used.This difference appears to be in agreement with the results of Koirtyohann and Khali17 who found that sulphuric acid but not nitric acid has a significant effect on the partition constant K defined as the ratio of the concentration of mercury in air to that in a liquid. Furthermore it has been suggested that the oxidiz- ing properties of nitric acid hamper the reduction of the determinant,* thus leading to wider and lower peaks. When hydrochloric acid was used the peak areas were also very similar to those found in sulphuric and nitric acid media but the peak height was the highest obtained. As can be seen from Fig. 1 the peak height is markedly dependent on the hydrochloric acid concentration and this behaviour is different from that shown by aqueous standard mercury(r1) solutions (curve D).The noticeable decrease in the peak height for the slurried samples at low hydrochloric acid concentrations could be attributed to the chemical form of mercury that is present within the iron(@ oxide samples. On the other hand the values for the reagent blanks were lower in hydrochloric than in sulphuric acid media (absor- bances of 0.018 and 0.071 respectively). Therefore a 4.8 mol dm-3 hydrochloric acid medium was selected as being adequate to generate mercury vapour from the iron(1rr) oxide slumed samples. A similar study was carried out using hydrochloric acid media for titanium oxide samples. In this instance 5% slurries were used and nearly constant peak areas were obtained within a wide range of acidities.As shown in Fig. 2 0.20 I I 0.15 z e 0.10 z 9 (0 5 I -7 * A 0 1 2 3 4 5 6 7 8 Hydrochloric and nitric acid concentration/mol dm-3 I I I I I I I 0 0.5 1 1.5 2 2.5 3 3.5 4 Sulphuric acid concentration/mol dm-' Fig. 1 Effect of acid concentration on generation of mercury vapour from slurried iron(@ oxide samples A hydrochloric acid; B sulphuric acid; C nitric acid; and D as for A but for aqueous mercury solution B 0 - - * w - I I 0 1 2 3 4 5 6 Hydrochloric acid concentration/mol dm-' Fig. 2 Effect of hydrochloric acid on generation of mercury vapour A slumed titanium oxide sample; and B reagent blank 0.20 I 0.15 Q C ; 0.10 9 z - / B A maximum peak heights were obtained for 1.5-2.5 mol dm-3 acid concentrations and so a 1.8 mol dm-3 hydrochloric acid medium was chosen.The effect of sodium tetrahydroborate concentration was studied using different solutions in the 0.4-6% m/v range. The solutions being studied were delivered to the reaction vessel which contained the slurried sample at the chosen acidity conditions in a total volume of 10 ml by depressing the plunger of the hydride generation system. When the tracing on the recorder indicated that the maximum signalJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL. 6 629 Table 1 Slopes of the standard additions calibration graphs obtained from iron(rr1) oxide sample Slurry Acid dissolution Sample P(%) Vt/ml Slope$/ 1 0-3 A ng-' m$/g Vt/ml Sl0pe$/10-~ A ng-I 1 2 5 1.78 f 0.05 2 5 1.78 f 0.04 2 2 2 1.82 k 0.08 2 5 1.82 f 0.03 3 2 2 1.85 k 0.04 2 5 1.88 ? 0.05 4 2 2 1.80 & 0.05 1 4 1.79 f 0.04 5 2 5 1.86 k 0.1 2 5 1.82 f 0.07 9 5 1.78 f 0.09 6 9 5 1.72 k 0.07 * Slurry percentage.$Mean of three valueskSD (standard deviation); slope for aqueous mercury 1.78 x $ Mass of iron(m) oxide taken in the acid-dissolution procedure. Volume of suspension or solution added to reaction vessel. A ng-I. Table 2 Slopes of the standard additions calibration graphs obtained from titanium oxide samples Original samples Calcined samples Sample P(%) I/t/rnl Sl0pe$/10-~ A ng-' P(%) Vf/ml Slope$/ 1 0-3 A ng-* 1 10 5 1.55k0.04 10 5 1.54 & 0.09 2 10 5 1.49 f 0.06 10 5 1.49 f 0.10 3 5 2 1.56 k 0.06 10 5 1.50k0.08 4 10 5 1 S O f 0.05 10 5 1.55f0.07 * Slurry percentage. t Volume of suspension added to reaction vessel.t Mean of three values k SD; slope for aqueous mercury 1.54 x 1 0-3 A ng-'. had been attained the plunger was released. As can be seen in Fig. 3 a 4% m/v solution of the reducing agent was necessary in order to obtain the maximum peak height. Similar results were obtained for slurried titanium oxide samples. Calibration Sensitivity and Repeatability In order to perform an optimum calibration it was necessary to ascertain whether mercury was entirely or partially removed from the slurried solids and the effect of varying the mass of solid introduced into the reaction vessel. Mercury vapour was generated using different volumes of a 2% m/v iron(rI1) oxide slurry prepared from a sample containing 0.91 ,ug g-l of mercury. As 4 ml of concentrated hydrochloric acid must be used in a final volume of 10 ml in the reaction vessel six experiments were performed using 1,2 3,4 5 and 6 ml of the slurry.A plot of the peak height against the volume of slurry gave a straight line showing a correlation coefficient r= 0.9995. Another set of experiments was performed using slurries prepared from an iron oxide sample with a very low mercury content (0.065 pg g-*). Five slurries covering the 2-10% range were prepared and 5 ml of each were placed in the reaction vessel. A straight line (r=0.9987) was again obtained suggesting the absence of a matrix effect due to the mass of solid present in the reaction vessel. Similar experiments were carried out using slurries prepared from titanium oxide samples and straight lines were also obtained. Because the maximum content of mercury in these samples is fixed by law at 2 fig g-l slurries containing more than 10% m/v of solid were not prepared as the procedures were considered sufficiently sensitive.These experiments suggest that a calibration using aque- ous solutions of mercury should be valid. To confirm this six iron(1n) oxide samples from two manufacturers were dissolved in acids and their mercury content determined as described under Experimental. It is important to point out that when the analysis was carried out on these solutions a practical problem was encountered for final hydrochloric acid concentrations of (3 mol dm-3 in the reaction vessel. Under these experimental conditions a dark precipitate was seen when the reducing agent was added and the slopes of the standard additions graphs were different to those obtained when aqueous standards of mercury were ana- lysed.The problem was avoided by using a 4.8 mol dm-3 hydrochloric acid medium. A number of slumes were prepared from these samples and standard additions Cali- bration graphs using several volumes of the slurried samples were again obtained. As can be seen in Table 1 where the results are summarized the slopes of the standard additions calibration graphs both for solutions and slurries are very similar and virtually identical to the slopes of calibration graphs for aqueous solutions of mercury proving that direct calibration with aqueous standards is valid. A different approach was followed in order to prove that a calibration for the analysis of titanium oxide pigments can also be made using aqueous standards.No reliable procedure with the exception of those based on the volatilization of mercury is available for the determination of the analyte. In order to overcome this the samples were calcined at 400 "C for 2 h. Slurries were then prepared from these freshly calcined samples and applying the procedure reported here no measurable amounts of mercury were found. Next standard additions calibration graphs were obtained by using both the previously calcined samples and the original titanium oxide pigments. The slopes of these graphs which are summarized in Table 2 demonstrate that as for iron oxide samples direct calibration with aqueous standards is also valid.Tables 3 and 4 show that the results for the determination of mercury content obtained using the proposed procedure agree with those found using acid dissolution for iron(xn) oxide samples and the indicated approach for titanium oxide.630 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL. 6 Table 3 Comparison of the results for mercury obtained from six iron(n1) oxide samples analysed by the acid dissolution and slurry procedures n = 3 Sample Acid dissolution/ Slurry 1 0.169 0.170 2 0.860 0.905 3 0.600 0.596 4 0.765 0.789 5 0.510 0.507 6 0.065 0.070 Pg g-' In order to evaluate the repeatability of the procedure four different iron(r1r) oxide samples were taken and five 4% m/v slurries were prepared from each one. Next ten successive measurements were obtained for each of the 20 slurries.The relative standard deviations (RSDs) were in the 1.6-6.4% range. The same method was followed for titanium oxide samples using 10% m/v slurries and RSD values in the 1.4-7.1 % range were obtained. The detection limit for mercury calculated on the basis of 20 for 5% m/v iron(rr1) oxide slurries using 4 ml of suspension in the reaction vessel was 5 ng g-l. A similar figure (7 ng g-l) was found when using 6 ml of 10% m/v titanium oxide slurries. Table 4 Comparison of the results for mercury obtained from four titanium oxide samples analysed by the proposed method n=3 Contenthg g-' Sample Aqueous calibration Standard additions 1 12 2 24 3 501 4 67 10 26 506 65 In spite of the fact that the results given in Tables 3 and 4 indicate very low mercury contents all of them below the limit imposed by law it is surprising that such levels can exist in commercially available samples which are submit- ted to thermal treatment during the industrial process used to obtain them.Serious risks of contamination by mercury during the storage of samples have been reported? and so additional experiments were carried out to ascertain whether any of the analyte is released during such a process. The experiments showed that when iron oxide pigments were vigorously shaken in cold 0.3 mol dm-3 nitric acid solution containing 0.0 1 mol dm-3 potassium permanga- nate about 75% of the total mercury was released into the solution suggesting that a great part of the analyte is easily available as a consequence of superficial contamination. The authors are grateful to the Spanish Comisi6n Intermin- isterial de Ciencia y Tecnologia (CICYT) (Project 87-0053) for financial support. 1 2 3 4 5 6 7 8 9 References Langmyhr F. J. and Wibetoe G. Prog. Anal. At. Spectrosc. 1985 8 193. Sample Introduction in Atomic Spectroscopy ed. Sneddon J. Elsevier Amsterdam 1990 ch. 3. Baxter D. C. and Frech W. Fresenius 2. Anal. Chem. 1990 337 253. Miller-Ihli N. J. Fresenius 2. Anal. Chem. 1990 337 271. Haswell S. J. Mendham J. Butler M. J. and Smith D. C. J. Anal. At. Spectrom. 1988 3 731. Madrid Y. Bonilla M. and CAmara C. J. Anal. At. Spectrom. 1989 4 167. Koirtyihann S. R. and Khalil M. Anal. Chem. 1976,48 136. Hon. P. Lau O. and Wong H. Anal. Chem. 1983 108 64. Welz B. Atomic Absorption Spectrometry VCH Weinheim Paper No. I /02 7200 Received June 7th 1991 Accepted August 5th 1991 1985 pp. 244-248.

ISSN:0267-9477    

DOI:10.1039/JA9910600627

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL. 6 63 1 Tandem Sources Using Electrothermal Atomizers Analytical Capabilities and Limitations Heinz Falk Spectro Analytical Instruments Tiergartenstrasse 27 4 190 Kleve Germany Tandem emission sources with electrothermal volatilization and atomization of the sample such as furnace atomic non-thermal excitation spectrometry (FANES) or furnace atomization plasma emission spectrometry are compared with other excitation sources e.& an inductively coupled plasma. The interdependence between the excitation and atomization processes which causes restrictions for analytical procedures is considered. Of particular interest for the analytical applications of tandem sources is the influence of matrix constituents on the excitation conditions.Furnace atomizers are able to form relatively high matrix concentrations in the excitation plasma which can lead to a breakdown of the initial electron energy distribution. Correspondingly depression of the analytical signals by the matrix has been found for FANES which leads to some general conclusions on tandem sources. Keywords Atomic emission spectrometry electrothermal atomizer; tandem emission source; glow discharge matrix effect The glow discharge mode of excitation is well established in atomic spectroscopy. It is characterized by narrow spectral lines and a low background.' In addition the simple operation makes it especially suited for use as a spectros- copic light source. Of the glow discharges hollow cathode lamps have become broadly applicable in spectroscopy not only in atomic absorption but also as emission In the latter the glow discharge was originally used for both atomization of the sample by sputtering action and subsequent excitation usually in the negative glow plasma.A prerequisite for this procedure is that the sample consists of an electrically conducting material or has to be ground and mixed with a conducting powder such as c ~ p p e r . ~ The coupling of atomization and excitation leads to a relatively simple construction of such an emission source; it does however force the user to find an acceptable compromise for the operating parameters. For example changing the carrier gas from He to Ar will increase the density of the sputtered material within the plasma dramatically3 but will also shift the upper limit of the excited levels towards lower energies. The excellent signal-to-noise ratio achievable when using a glow discharge as the excitation source due to the non- thermal character led to a separation of the atomization step by using a separate device forming a 'tandem source' (see Fig.I). Transportation losses that occur while trans- ferring the sample vapour from the atomizer to the excitation volume can be avoided when the tube of the electrothermal atomizer forms the cathode of a hollow cathode discharge; this technique has been named furnace atomization non-thermal excitation spectrometry (FANES).6-8 Sample introduction drying and ashing are similar to those used in electrothermal atomic absorption spectrometry (ETAAS) but atomization is carried out at reduced pressure while the negative glow plasma inside the tube is maintained by the anode voltage applied to the electrode which is introduced into the vacuum vessel.By using FANES limits of detection typical for ETAAS are reached but in a simultaneous multi-element mode covering a high dynamic range of analyte concentrations. The original FANES instruments with the tube furnace as the cathode have been modified by the introduction of a small rod cathode into the furnace forming a hollow anode discharge (HA-FANES).9-11 This approach despite some experimental peculiarities is based on essentially the same physical principles viz. thermal atomization of a dry / Plasma - 7 ~ Tubing / f Sample Furnace 9===+ Photons / Plasma Anode T Fig.1 Schematic diagram of the two basic approaches for tandem sources consisting of an atomizer coupled to an excitation plasma. (a) Spatial separation of volatilization and excitation; and (b) volatilization and excitation in the same volume residue of the sample at reduced pressure and excitation of the vapours in a glow discharge. Similar to FANES from a constructional point of view is furnace atomization plasma emission spectrometry (FAPES).l2 Here a plasma is formed inside a tube furnace at atmospheric pressure with a high frequency antenna positioned on the furnace axis. The physical nature of a high-frequency plasma at atmospheric pressure differs from a glow discharge; however because of a number of similarities FAPES will be included in the following considerations.Glow discharge tandem sources are definitely suited for use in improving the analytical capabilities of conventional one-step emission sources. It is the aim of this paper to consider the analytical implications of the physical limita- tions of such tandem sources. Separation of Atomization and Excitation Tandem sources should allow a fairly complete separation of the processes of volatilization and atomization from the632 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL. 6 h 400 I I200 g 2 350 150 g 100 n 50 0 500 1000 1500 2000 250i Temperature/K Fig. 2 Temperature influence on discharge voltage and line emission in a FANES discharge.* Conditions A 9 x lo2 Pa He 50 mA; B 5 x lo2 Pa Ar 40 mA; C He (319 nm) 9 x lo2 Pa He 40 rnA; and D Ar (45 1 nm) 10 x lo2 Pa Ar 40 mA excitation step.In other words while atomizing the excitation conditions have to be kept constant. This can be achieved by spatial separation of the atomizer and plasma which is however not very efficient owing to the transpor- tation losses (see Fig. 1). On the other hand all tandem sources that accomplish the more efficient approach in which atomization and excitation take place in the same volume such as in FANES and FAPES show an interaction between the atomization and excitation processes. Depending on the elements to be determined the furnace temperature varies between ambient and 3000 K. While keeping the pressure constant during atomization there is a decrease of carrier gas density in the atomizer which is proportional to the furnace temperature.As a result the discharge characteristics and consequently the excitation conditions are changed. More marked is that the thermo- ionic emission exhibits an exponential dependence on the temperature causing a sharp increase of thermal electrons above 1 500 K. This completely changes the character of the discharge which is then no longer self-sustained. As an example the discharge voltage for FANES is given in Fig. 2.8 The temperature dependence of the carrier gas emission lines is also shown in this figure reflecting the change in excitation conditions as a result of variations in the plasma parameters. Whereas the He line intensity decreases 5-fold during the transition from typical glow discharge conditions to a regime governed by thermal electrons the Ar intensity change is insignificant.These intensity variations are attributed to changes in the excitation conditions as the gas density is diminished by less than 15% within the critical temperature range. The He line at 319 nm with an excitation energy (Eex) of 23.7 eV is much more influenced by the temperature of the cathode than is the Ar line at 45 1 nm with an excitation energy of 14.6 eV. The high energy ‘tail’ typical for glow discharge^,^ disappears when thermo- ionic emission is dominant. Obviously the density of electrons at the low energy end remains almost constant despite a 1 0-fold decrease in electrical power consumption of the discharge when the transition from the low to the high temperature region takes place.The same effect has been reported for HA-FANES the Ar I line (420.1 nm Eex= 14.5 eV) emission is almost unaf- fected while reaching a temperature of 1500 “C but the Ar I1 line (434.8 nm Eex=19.5 eV ionization energy Ei,,=15.76 eV) shows a dramatic decrease when the thermo-ionic emission is governing the discharge.’ This behaviour of the FANES system is very important for its analytical application as most analytes have relatively low excitation energies. Consequently elements with low and those with high volatilization temperatures show acceptable degrees of excitation.8 Matrix Influences on Excitation Conditions Electron Density and Temperature In electrical discharges the density and energy distribution of the free electrons determine the excitation conditions. All of the other excitation processes such as ion recombina- tion or secondary collisions are ultimately powered by electrons.Therefore the changes in the density and energy distribution of the electrons in a tandem source will be considered as they directly affect the analytical applications. A collection of data on electron density and temperature for a variety of spectroscopic excitation sources is given in Tables 1 and 2. These data show the typical variation in electron density in low-pressure discharges of between 1 x lo1* and 1 x 1014 ~ m - ~ whereas atmospheric plasmas e.g. the inductively coupled plasma (ICP) have density variations of between 1 x lOI4 and 1 x 10l6 ~ m - ~ . However the electron temperatures in both source types are compar- able ranging between 5 000 and 10 000 K.It is worth noting that in a low-pressure discharge the plasma is not in local thermodynamic equilibrium (LTE) as can be seen from the difference between the gas and electron tempera- tures in Table 2. Usually the electron velocity distribution is not Maxwellian therefore the term ‘temperature’ has to be used with care. Consequently for a glow discharge the term ‘electron temperature’ refers to an atomic ensemble in which the population of energy levels follows a Boltzmann distribution corresponding to that temperature and are essentially excited by the aforesaid electrons. In this sense the electron temperature of non-LTE plasmas yields infor- mation about the excitation of atoms and ions in such a system. Matrix Concentration in Emission Sources The interaction of the source plasma with the matrix constituents is a major reason for changes in the excitation conditions.The matrix concentration in the excitation region is considered below. For comparison purposes the ICP will be discussed first. The concentration of the sample vapour relative to the carrier gas in the ICP is given by where Qa is the aerosol gas (+plasma gas) flow rate in mol min-I; Q is the sample flow rate in mol min-l; andf is the nebulizer efficiency. When the sample contains the matrix concentration c (g ml-l) then the matrix contribution relative to the camer gas (crel ,,,) becomes As an example the following assumptions are made Qa= 2 1 min-l of Ar; Qs= 1 ml min-l of H20 ;f (pneumatic)= 3%; f (ultrasonic)=30°/o;16 and cm= 1%.Some results are also shown in Table 3. For an ultrasonic nebulizer with a cooler it is assumed that the aerosol reaches the saturation vapour pressure of water at the cooling temperature. As the example shows the matrix concentration in the ICP is fairly low under typical operating conditions whereas the concentration of water vapour is such that it will always be an influencing factor. The solvent concentra- tion has to be reduced when using ultrasonic nebulization. If sample introduction is carried out by sample sputter- ing e.g. by using a hollow cathode discharge with a discharge current of 50 mA a typical sample introductionJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL. 6 633 Table 1 Electron density of various spectroscopic radiation sources Gas Discharge Pressure/ temperature/ current Electron Source Gas kPa K density/mA cm-* den~ity/cm-~ GD* GD HCDt MIPS MIP FANES FANES FANES ICPC'l ICPh 11 Ar Ar Ar Ar Ar Ar Ar He Ar Ar 1.4 0.53 1.2 0.26 101.3 2.0 2.0 1.3 101.3 101.3 *GD glow discharge.t HCD hollow cathode discharge. $ MIP microwave-induced plasma. 5 NR not recorded. 1 ICPc 'cool' region of ICP above the coil. 11 ICPh 'hot' region of ICP inside the coil. 500 500 500 NR 500 2 500 500 5 000 8 000 150 98 100 NR§ NR 35 35 35 - 8 x lOI4 3.5 x 10" 1 x 1013 I x 1013 I x 1014-1 x 1016 3 x 1014 4~ 1014 2 x 1013 I x 1014-1 x 1015 I x 1015-1 x 1016 Method Spectrometric Probe Probe Probe Stark Spectrometric Spectrometric Saha Spectrometric Spectrometric Reference 13 14 1 15 16 8 8 8 17 17 Table 2 Electron temperatures measured in various spectroscopic sources Discharge Gas current Pressure/ temperature1 density/ Source Gas Wa K mA cm-* GD GD HCD HCD MIP MIP FANES FANES FANES FANES ICPC ICPh Ar 1.4 Ar 0.53 Ar 0.5 Ar 2.0 Ar 3.0 He 0.7 Ar 2.0 Ar 2.0 He 1.3 He 1.3 Ar 101.3 Ar 101.3 500 500 500 1500 500 500 500 2 500 500 2 500 5 000 7 500 150 98 13 80 NR NR 35 35 35 35 Electron temperature/ K 5 000 12 000 3 200 4 000 4 500 3 400 8 000 8 300 10 000 10 300 5 500 8 000 Method Spectroscopic Probe Spectroscopic Spectroscopic Spectroscopic Spectroscopic Spectroscopic Spectroscopic Spectroscopic Spectroscopic Spectroscopic Spectroscopic Reference 13 14 18 19 16 16 8 8 8 8 17 17 Table 3 Matrix concentrations in various emission sources relative to carrier gas (calculated) Source type Expected Maximum concentration (%) tolerable sodium Solvent Matrix concentration* (%) ICP pneumatic nebulizert,$ 1.9 0.0 19 0.05 ICP ultrasonic nebulizert,$ 19 0.16 0.05 ICP ultrasonic nebulizer + coolee 1 0.16 0.05 FANES (Ar)t,§"l 0 8 0.0005 0.002 Hollow cathode lamp§ 0 0.00 1-00 1 FAPESt,$,I II 0 8 0.0 1 * 10% loss of discharge power via matrix excitation.t Sample 1 % aqueous solution. §Discharge current 50 mA; and rdp= 1. fi 10 pl sample T= IOQO K. 11 T,=5000 K n,= 1 x lOI4 ~ m - ~ discharge power= 100 W. $rdp =0.1. Ratio of expected concentration to tolerable 0.4 J 3 16 000 800 0.5-5 rate of 0.2-1 pg s-l for metals is ~btained.~ The density of sputtered material ranges from 1 x 10" to 1 x 10l2 ~ m - ~ . ~ ~ Therefore the concentration of the sample relative to the (3) carrier gas is approximately 0.00 1 -0.0 1%.In a graphite furnace the dry residue of the sample is volatilized in around 0.2 s. The maximum concentration of sample species inside the furnace is given by where fat is the atomizer efficiency; V is the sample volume; N is Avogadro's number; n is the carrier gas density; M is the molar mass; and V the atomizer volume. In a low-pressure situation such a system has a typical634 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL. 6 120 -5 - 0 20 40 60 80 100 U A Electron energyJeV Fig. 3 A Excitation cross-section of Na (589.6-589.9 nm).22 B Dissociation cross-section of H2.24 C Ionization cross-section of Ar I.23 D Ionization cross-section of Hg 123 efficiency of 1% and at atmospheric pressure about 50°/6.21 By using a 10 pl sample containing a matrix concentration of 1% (M= 50) at a pressure of 2 P a an atomizer volume of 1 cm3 and a temperature of 1000 K which applies to FANES crrl,,,=8% is achieved.For FAPES operated at atmospheric pressure the same value is obtained. Such a relatively high matrix concentration is present in the furnace for approximately 0.2 or 0.5 s for FANES or FAPES respectively at the volatilization temperature of the matrix. In Table 3 various emission sources are compared and show that a relatively high matrix concentration can occur in furnace-based tandem sources. Interaction of Plasma Electrons With Matrix Species Electrons in emission sources lose their energy mainly by excitation or ionization collisions with the carrier gas but also with the matrix species.The number of excitation impacts (k,,) per time and volume unit is where n is the atom density; n is the electron density; u is the electron velocity; E is the electron energy; A,(E,) is the excitation cross-section; and f,(E,) is the electron distribution function. The expression f,(E,) x dEe has to be dimensionless. There is a similar expression for the ionization collision rate where the excitation cross-section is replaced by the ionization cross-section. However the excitation rate has to be balanced with a corresponding collisional de-excitation rate before the emission losses can be calculated. In fact the relative population in a 2-level system can be written as n* CE - no CD+RD ( 5 ) where n* is the excited-state population; no is the ground- state population; and CE CD and RD are the rates for collisional excitation collisional de-excitation and radiative de-exci t at ion respectively .Collisional de-excitation is typically 10 times higher than radiative de-excitation in an ICP,22 but in low-pressure discharges radiative de-excitation is the dominating pro- c ~ s s . ~ ~ In eqn. (4) n can represent the carrier gas in addition to species introduced by the sample. The actual energy loss of the plasma electrons depends on the excitation and ioniza- tion functions. Fig. 3 shows some typical examples with a very steep rise at the excitation or ionization energy.24*2s For comparison a dissociation cross-section function is in- cluded in Fig. 3 as this might also contribute to the energy losses of the plasma electron^.^^ Only energies of less than about 20 eV have to be taken into account as only electron temperatures of less than 10 000 K (see Table 2) corresponding to an average electron energy of 2 eV occur.In this instance the functions shown in Fig. 3 can be replaced by a rectangle starting at the corresponding energy for excitation ionization or dissocia- tion respectively. Under this pre-supposition and the assumption that the plasma electrons follow a Maxwellian distribution eqn. (4) can be integrated as follows ke,= 4 x n x n x A, x r I exp( a) k x T x (*+ k x T I ) ] where A, is the amplitude of the steplike excitation function; k is the Boltzmann constant; T is the electron temperature; and Eo and Em are the integration limits.Assuming that the photons emitted from the excited atoms leave the plasma the power loss (Pex) by this effect becomes pex = kex rdp Eph Vso (7) where Eph is the photon energy; V is the source volume; and rdp = RD/(CD+RD). By using eqns. (5-7) the concentration of any species in the plasma which would dissipate a given fraction of the electric power applied to the plasma via excitation losses can be calculated. This has been done for Na as the matrix component where the tolerable excitation loss is assumed to be 10%. The result is shown in Table 3 which gives only an upper limit for that matrix concentration causing a notice- able influence on the plasma. The calculations for the ICP apply to the 'normal' observation height about 20 mm above the coil where depression effects by the matrix are dominant.26 The power loss given by eqn.(7) can be observed as a signal depression if this effect is not compensated for by other effects which act in the opposite direction e.g. the increase in the electron density by matrix ionization. This takes place at low observation heights in the ICP. Table 3 shows that the maximum matrix concentration to be expected under realistic analytical conditions is usually considerably higher than the tolerable amount. Only in sputter sources does the influence of the matrix on the excitation conditions in the plasma appear to be negligible. Despite the uncertainties involved in the estimate it can be seen that influences of the matrix in tandem sources such as FANES and FAPES will start at relatively low concentrations.Analytical Implications In contrast to a nebulizer-operated ICP analyte and matrix are not necessarily present in the furnace volume at the same time in electrothermal atomizers. The actual matrix concentration for the duration of the analyte peak depends on the volatilization temperatures and residence times. Residence times can vary between typically 0.1 and 0.5 s for both FANES and FAPES. As Table 3 shows the worst instance occurs when analyte and matrix peaks coincide. Here it would be expected that in FANES the matrix influences become noticeable at matrix concentrations as low as 0.001%. Indeed a depression of the Cu signal in FANES for NaJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL. 6 635 140 t 2 20 401 Y ~ 1 lo-' 1 0.01 0.1 Na concentration (%I Fig.4 Emission intensity of Cu (324.7 nm) from FANES as a function of Na concentration (20 pl sample) for analytes with different volatilities. A Al; B Fe Ni Co and Cr; and C Cu. Volatilization temperatures Na 800; Cu 800; Fe Ni Co and Cr 850-1000; and Al 1100 "C concentrations of above 0.001% has been observed8 as shown in Fig. 4. For Fe Ni Co and Cr the depression starts at one order of magnitude higher Na concentrations whereas for Al such an effect does not appear at all. Under low-pressure conditions Cu and Na atomization takes place at 700-900 "C but Al is atomized at 1200-1 500 "C. The other elements mentioned are also volatilized within that range. The matrix effect on this group of elements could have been further reduced by using a more adequate heating programme allowing more of the matrix to volatil- ize before the Fe Ni Co and Cr peaks appear.Conclusions Despite the rather crude estimates of matrix effects in emission sources carried out in this work the following conclusions can be drawn (i) the separation of the volatili- zation and atomization stage from the excitation process cannot be completely achieved for tandem sources in which both processes take place in the same volume; (ii) glow discharges are more prone to matrix influences than excitation sources when working at atmospheric pressure; (iiz) tandem sources including furnace atomizers are very efficient therefore analyte and matrix concentrations in the excitation source are relatively high; (iv) for emission systems such as those used in FANES and FAPES an adequate temperature programme is crucial to keep matrix effects at an acceptable level; and ( v ) tandem sources need elaborate procedures for analytical application which might restrict their multi-element capability for samples containing high matrix concentrations. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 References Falk H.Spectrochim. Acta Part B 1977 32 437. Grimm W. Spectrochim. Acta Part B 1968 23 443. Improved Hollow Cathode Lamps ed. Caroli S. Ellis Hor- wood Chichester 1985. Broekaert J. A. C. J. Anal. At. Spectrom. 1987 2 537. Brenner I. B. Laqua K. and Dvorachek M. J. Anal. At. Spectrum. 1987 2 623. Falk H. Hoffmann E. and Ludke Ch. Spectrochim. Acta Part B 1981,36 767.Falk H. Hoffmann E. and Ludke Ch. Spectrochim. Acta Part B 1984 39 283. Falk H. Hoffmann E. and Ludke Ch. Prog. Anal. Spectrosc. 1988 11 417. Ballou N. E. Styris D. L. and Harnly J. M. J. Anal. At. Spectrom. 1988 3 1141. Harnly J. M. Styris D. L. and Ballou N. E. J. Anal. At. Spectrom. 1990 5 139. Riby P. G. Harnly J. M. Styris D. L. and Ballou N. E. Spectrochim. Acta Part B 199 1 46 203. Sturgeon R. E. Willie S. N. Luong V. Berman S. S. and Dunn J. G. J. Anal. At. Spectrom. 1989 4 669. Patel B. M. and Winefordner J. D. Can. J. Spectrosc. 1987 32 138. Fang D. and Marcus R. K. Spectrochim. Acta Part B 1990 45 1053. Brassem P. and Maessen F. J. M. J. Spectrochim. Acta Part B 1974 29 203. Zander A. T. and Hieftje G. H. Appl. Spectrosc. 1981 35 357. Magyar B. Guide-lines to Planning Atomic Spectrometric Analysis AkadCmiai Kiiido Budapest 1982 p. 145. Mehs D. M. and Niemczyk T. M. Appl. Spectrosc. 1981,35 66. Dobrosavjevic J. S. and Marinkovic M. Spectrochim. Acta Part B 1974 29 87. Kowollik G. Investigations on Emission and Absorption in Hollow Cathode Lamps for Atomic Absorption Spectrometry at Low Currents Thesis Humboldt University Berlin 1979. Falk H. and Tilch J. J. Anal. At. Spectrum. 1987 2 527. Lovett R. J. Spectrochim. Acta Part B 1982 37 969. Griem H. R. Plasma Spectroscopy McGraw Hill New York 1964. Massey H. S. W. and Burhop E. H. S. Electronic and Ionic Impact Phenomena vol. 1 Clarendon Press Oxford 1969. Physikulisches Taschenbuch ed. Ebert H. F. Vieweg & Sohn Braunschweig 1967 p. 4 18. Sun D. Zhang Z. Qian H. and Cai M. Spectrochim. Acta Part B 1988 43 391. Paper I /01508G Received March 28th 1991 Accepted August 21st I991

ISSN:0267-9477    

DOI:10.1039/JA9910600631

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 1991 VOL. 6 637 Determination of Geographic Origin of Agricultural Products by Multivariate Analysis of Trace Element Composition* Robert S. Schwartz and Le 1. Hecking US Customs Service Research Division Office of Laboratories and Scientific Services 1301 Constitution Avenue Room 71 13 Washington DC 20229 USA Samples of orange juice pistachio and macadamia nuts were analysed for selected elements using inductively coupled plasma atomic emission spectrometry and atomic absorption spectrometry. Discriminant analysis was used to form mathematical models for predicting the geographic origins. The accuracy of these models was assessed using re-substitution and cross-validation analysis for the orange juice and macadamia nut samples and by the calibration set-prediction set method for the pistachio nut samples.Perfect prediction results were achieved for the pistachio nut samples. Re-substitution analysis results for the orange juice and macadamia nut samples indicated prediction accuracies of 96 and 98% respectively while cross-validation results indicated 88 and 78'30 respectively. Separations were visualized by performing canonical discriminant analysis involving the same elements as in the discriminant analysis and plotting canonical scores for the first two or three canonical functions. Complete resolution of all samples by geographic origin was achieved for all three commodities. Keywords Geographic origin; orange juice; pistachio and macadamia nuts; multivariate data analysis; trace element Determination of the geographic origin of imported mer- chandise is an analytically challenging problem that is currently the focus of much attention within the US Customs Service.The Customs Service enforces trade related laws rules and regulations many of which specify treatment based on geographic origin. The three commodi- ties discussed in this manuscript orange juice pistachio and macadamia nuts all require just such origin-based treatment. For orange juice the relevant government programme is the Caribbean Basin Initiative (CBI) which eliminates the tariff on products from any one of 22 designated countries in the Caribbean Basin. This situation provides obvious economic incentive for fraud by trans- shipment of non-CBI merchandise through CBI countries thus evading payment of duty.Orange juice a high-volume item carrying a duty of $0.35 per gallon of single-strength juice is of particular concern due primarily to the prox- imity of Brazil a non-CBI country which exports a great deal of this commodity to the Caribbean Basin. Of major concern therefore is the ability to differentiate CBI orange juice from that of other countries particularly Brazil. For pistachio nuts the relevant issue was the embargo on products from Iran pistachio nuts being one of the more important Iranian exports. The other major producers are Turkey and the USA particularly California so it is important to be able to distinguish pistachio nuts from these three sources. It should be noted that a significant portion of pistachio nuts imported into the USA are Californian nuts which have been processed abroad.Finally Customs interest in macadamia nuts arose from information that South African macadamia nuts were being trans-shipped to the USA through nearby African countries in violation of the ban on the importation of South African agricultural products one of the sanctions specified in the 1986 Anti-Apartheid Act. A powerful method for the determination of the geo- graphic origin of agricultural products is multivariate statistical analysis of the data provided by analytical instruments such as chromatographs and spectrometers *Presented in part at the 1990 Winter Conference on Plasma Spectrochemistry St. Petersburg FL USA 8th- 13th January 1990. which have the ability to determine more than one component at a time in a sample.If these components have sufficient discriminatory power the set of their concentra- tions will form a characteristic pattern or 'fingerprint' relating to the geographic origin of the sample. Multivariate data analysis provides the ability to detect these patterns and is essentially helpful when the number of components necessary to differentiate samples from different geographic origins increases. This methodology has been used to determine the geographic origins of honey,' olive oil,2 orange juice3 and and has also been used to determine adulteration in orange juice.* It should be noted that the orange juice study cited in ref. 3 only considered juice from Florida and Brazil. The components determined in these studies havevaried but can be broadly classified as either organic substances or elements.The use of elemental concentrations for the determination of the geographic origin of vegetable matter and foods derived therefrom has a number of advantages when compared with the use of organic substances. Firstly organic substances must be manufactured by the plant and therefore must be at least in part under genetic control so that the magnitude of the influence of geographic area in determining the levels of these substances might tend to be problematic. Elements on the other hand must be absorbed by the plant from the soil in which it is grown. It is known that the levels of elements present in plant tissue is directly dependent on the levels of these elements in their growth media,9 which in most instances is the soil.To the extent that the level of elements in the soil are characteristicofthe region it might reasonably be expected that the levelsfound in plants grown in a particular soil will reflect this. In addition elements do not decompose on storage as do organic substances produced by the plant. In this study the application ofthis methodology to orange juice is extended by considering Caribbean Basin countries in addition to Brazil. Two new applications are also presented the determination of the geographic origin of both pistachio and macadamia nuts. Experimental Instrumentation The elements B Ba Ca Cu Fe Mg Mn P and Zn were determined by inductively coupled plasma atomic emission638 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 1 99 1 VOL.6 spectrometry (ICP-AES) and K and Rb were determined by atomic absorption spectrometry (AAS). The ICP-AES mea- surements were obtained using an Instrumentation Labora- tory Model P L 100 inductively coupled plasma atomic emission spectrometer (Thenno Jarrell Ash Franklin MA USA) equipped with a standard Instrumentation Labora- tory cross-flow nebulizer and torch. Atomic absorption measurements were obtained using a Perkin-Elmer Model 5000 atomic absorption spectrometer (Perkin-Elmer Nor- walk CT USA) equipped with a single-slot burner a K hollow cathode lamp (Fisher Scientific Washington DC USA) and a Rb electrodeless discharge lamp (EDL) and EDL power supply (Perkin-Elmer). Operating conditions for the ICP-AES and AAS instruments are listed in Table 1.Reagents Water for the preparation of all solutions had a resistivity of 18 MSZ and was obtained from a reverse osmosis-de- ionization system (Millipore Bedford MA USA); nitric acid was Instra-Analyzed (J.T. Baker Phillipsburg NJ USA); caesium chloride was Hi Pure grade (Spex Edison NJ USA); lanthanum chloride was of a grade suitable for AAS analysis of alkaline earth elements (Fisher Scientific Pittsburgh PA USA); and Triton X-100 was commercially available material (lot number 75 1560 Fisher Scientific). Multi-element working standard solutions for use in ICP and AA analyses were prepared from suitable dilutions of stock solutions containing all of the elements of interest. Stock solutions were prepared from 1000 pg ml- aqueous standard solutions of each element (Fisher Scientific) for analytes with final concentrations of less than 10 pg ml-l (B Ba Cu Fe Mn Rb and Zn) and from high-purity (99.99% or higher) compounds or metals (Spex) for the analytes Table 1 Operating conditions ICP-AES- Plasma power level Gas flow Coolant Auxiliary Nebulizer Boron Barium Calcium Copper Iron Magnesium Manganese Phosphorus Zinc Observed wavelength Observation height AAS- Flame Gas flow Acetylene Air Potassium Rubidium Potassium Rubidium Observed wavelength Slit width 1.2 kW 15 1 min-l 0.5 1 min-' 0.45 1 min-l 249.77 nm 455.40 nm 317.93 nm 324.75 nm 259.94 nm 279.08 nm 257.61 nm 213.62 nm 206.20 nm 10- 14 mm above load coil (optimized for each element) Air-acetylene 2.0 1 min-l 15.5 1 min-l 404.4 nm 780.0 nm 0.7 nm 1.4 nm present at higher concentrations (Ca K Mg and P).All standard solutions were 1 mol dm-3 in HN03. Samples There were 27 orange juice samples studied all of which were obtained as frozen concentrated material having Brix values of approximately 65". (Degrees Brix are used as a measure of the concentration of an orange juice sample and is numerically equal to the % m/m of soluble solids measured as sucrose. lo Trace element concentrations were determined in the concentrates and were then adjusted to the values that would have been obtained if the juices had first been diluted to a Brix value of 1 1.8" which is the value stipulated in US regulations as that required for unconcen- trated orange juice.) These samples were obtained from commercial shipments entering the USA.The geographic origins as indicated by the documentation accompanying these samples were as follows Honduras 1; Belize 4; Brazil 12; Jamaica 8; Mexico 1; and USA 1. There were 33 pistachio nut samples studied with the following origins Afghanistan 2; California 20; Iran 6; Sicily 1; and Turkey 4. Of these 6 Californian 3 Iranian and all 4 Turkish samples had authenticated origins; all other samples were obtained from commercial shipments and origins were accepted as attributed from the accom- panying documentation with the exception of 3 samples which were determined to be Iranian by two independent methods. Finally 40 macadamia nut samples were studied with the following origins Australia 9; Costa Rica 2; Guatamala 10; Malawi 12; South Africa 3; and Zimbabwe 4.Of these 1 Australian 1 Costa Rican 3 Guatamalan 3 Malawian and 3 South African samples had authenticated origins; the remaining samples were obtained from commercial ship ments with origins accepted as attributed from the accom- panying documentation. Sample Preparation All samples were prepared for analysis by microwave- assisted digestion using an MDS-8 1 D microwave digestion system (CEM Matthews NC USA). Samples were weighed into 120 ml Teflon PFA digestion vessels and 20 ml of concentrated nitric acid was added to each sample. [Teflon PFA (perfluoroalkoxy) is manufactured from tetrafluoroe- thylene with a fully fluorinated alkoxy side chain.] Sample sizes were as follows 2 g of orange juice concentrate; 1 g of pistachio nutmeat; or 0.7 g of macadamia nutmeat.Twelve samples of orange juice or pistachio nuts or six samples of macadamia nuts were processed at one time. Each vessel was then fitted with a pressure relief valve and cap and sealed using the capping station of the MDS-8 1D system. For the orange juice samples the vessels were left unsealed for approximately 30 min to allow a preliminary exother- mic reaction to be completed. The carousel of the mi- crowave oven was then loaded with the vessels and the samples were processed using the appropriate programme as listed in Table 2. At the conclusion of the digestion programme the vessels were allowed to cool to room temperature uncapped and the solutions were reduced by evaporation to a volume of approximately 1 ml in the microwave oven. For the pistachio nuts 2 ml of 3Ooh hydrogen peroxide were added prior to evaporation which was initiated after the initial bubbling had stopped.Liquid residues were taken up in either 1 mol dm-3 HN03 for orange juice and pistachio samples or 1 mol dm-3 HN03-O. 1% m/v Cs (as CsCl) for the macadamia samples transferred into 25 ml calibrated flasks and made up to volume with the same solvent. All ICP analyses were conducted on portions of these solutions that had beenJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL. 6 639 Table 2 Power-time programme for microwave oven digestion of orange juice concentrate pistachio nuts and macadamia nuts. Values given are for time (in min)* Power level (%) Commodity 50 100 Orange juice concentrate 15 10 10 12 - Pistachio nuts 10 10 10 10 - Macadamia nuts 15 10 10 15 15 to cool for 5 min and then vented.*At the conclusion of each time peiod the vessels were allowed made 0.05% v/v in Triton X-100. The determinations of K in orange juice and pistachio nut samples were conducted on 1 0-fold dilutions of the final solution using a 1 mol dm-3 HN03-0.2% m/v La (as LaC13) diluent; determinations of K in macadamia nut samples were conducted on the final solution without modification. The determinations of Rb in all instances were conducted on the final solution without modification. All analyses were conducted versus multi- element standards prepared using the same diluents. Multivariate Data Analysis Multivariate analysis of elemental concentration data was performed on an IBM PC/XT personal computer using SASISTAT version 6 software (SAS Institute Cary NC USA).The two major techniques used in this study were discriminant analysis and canonical discriminant analysis. Discriminant analysis uses a sample set with known group membership called a calibration set to form a mathema- tical model which is used for predicting group membership of unknown samples. A related technique known as stepwise discriminant analysis was used to help select a subset of elements from amongst those determined having good discriminatory ability to form the calibration model. Canonical discriminant analysis is a dimension reduction technique that is used to visualize group separations in 2- dimensional plots. This technique uses linear combinations of the original elemental concentrations to form a new set of variables referred to as canonical discriminant functions.The number of canonical functions is significantly lower than the number of original variables the elements in this instance. Each sample can be represented by its set of scores on these functions and as most of the variance or information content is concentrated in the first two or three functions this information can be simply displayed in 2- dimensional plots of scores for each of the samples on the first two or three canonical functions. In these plots if the elements chosen are sufficiently discriminatory samples from the same area will cluster together whereas samples from different areas will be separated. There are a number of ways to demonstrate the validity of a discriminant analysis.The most straightforward way is to divide the samples into two groups a calibration set and a prediction set. The model formed from the calibration set is then used to predict the group membership of the prediction set which is then compared with actual group membership. To be successful the calibration set should include a statistically sound representation of samples from all classes of interest. Dividing the samples into two groups however reduces the number of samples available to form a statistically sound calibration set. When the number of samples available is limited the statistical soundness of the calibration model can be impaired and other validation techniques which make maximum use of the available samples to form a calibration model can be applied.Two such techniques used in this study are re-substitution analysis and cross-validation analysis. In a re-substitution analysis all the samples are used for the calibration model the predictive ability of the model is then evaluated by using it to ‘predict’ the group membership geographic origin in this instance of the samples. This however gives an optimistic estimate as the same samples contributing to the model are the ones being predicted. A more realistic estimate of predictive ability is obtained from cross- validation in which the model to be used for the classifica- tion of a given sample includes all samples except the one being classified. As none of the samples being classified contributes to the model used for classifying it the positive bias is eliminated.Results and Discussion Orange Juice Originally it was planned to use as a calibration set a group of samples of frozen orange juice concentrate supplied by the Food and Drug Administration (FDA) which had authenticated origins. This set was to be used for the determination of the geographic origin of orange juice concentrates obtained by the US Customs Service from normal commercial shipments. In practice however this worked out poorly. The calibration set formed from the FDA samples had poor predictive ability for the Customs samples especially in the important category of Brazilian juices where all 12 Customs samples were misclassified. It was found that the trace element levels for five of the seven elements used in the multivariate analysis were significantly higher for the Customs samples these were Ba Ca Mn P and Rb.The differences observed for Ba were particularly dramatic the Customs Brazilian samples averaged 0.37 ppm of Ba while the FDA Brazilian samples averaged 0.06 ppm of Ba. As mentioned later Ba is one of the more important elements in determining the geographic origin of orange juice. It was felt that these differences were most likely to be due to differences in processing the FDA samples had been processed in special non-commercial facilities. Although further study of the source of these differences was not pursued it is possible that the differ- ences in processing could have led to the differences observed in the levels of trace elements. It was then decided to use the entire set of 27 Customs samples as a calibration set taking the geographic origin from the accompanying documentation and to assess the accuracy of the calibration model by using re-substitution and cross-validation techniques.A discriminant analysis was performed on this sample set using B Ba Ca K Mn P and Rb. The order of discriminatory power for this group of elements was B> Mn> Ba>Rb>>P>Ca>K. Results of the re-substitu- tion and cross-validation analysis are shown in Table 3. It can be seen that the discriminating power of the model is good and that as expected re-substitution results are somewhat more optimistic than those of the cross-valida- tion technique. As noted in Table 3 the cross-validation Table 3 Discriminant analysis results for orange juice samples Brazilian result7 Correct result* Number False Type of analysis (%I correct positives Re-substitution 96 12 1 Cross-validation$ 88 1 1 1 *From all orange juice samples tested n=27.f n = 12. $Results adjusted for areas represented by only one sample.640 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 1991 VOL. 6 5.0 .- c C 3 2.5 c - lu .- s O - 10 results exclude samples that are the sole representatives of a single area; this is because in this type of analysis the sample being classified is excluded from the model making it impossible to classify a single sample representing one area correctly. In order to visualize group separations a canonical discriminant analysis was carried out on this sample set using the data for the same elements from the discriminant analysis.In Fig. 1 a plot of the scores for all samples on the first two canonical functions is shown; good separation is observed for all samples except for the noticeable overlap between Brazilian and Jamaican samples. It should be realized however that these plots are only 2-dimensional representations of multi-dimensional data and do not always contain all the relevant information in the data. In this instance it was found that the Brazilian and Jamaican samples could be separated according to their scores on the third canonical function. This is illustrated in Fig. 2 which - - J J J J L 10.0 1 u I 10 8 - 6 - C .o 4 1 2 - c C 0 C .- e 0 - 0 -0 0 c -2 -4 $ -6 -8 7.5 1 M I I I - T T T T cc - C C C - C - S - J I I I I 1 I -20 -15 -10 -5 0 5 10 15 $ -5.0 -7-5 i J J L L L .LL L JLL L L 8 6 8 8 H -10.0 I I I I I I I I -7.5 -5.0 -2.5 0 2.5 5.0 7.5 10.0 12.5 First canonical function Fig.1 Plot of scores on the first two canonical functions for the orange juice samples. Country codes B Belize; H Honduras; J Jamaica; L Lower Siio Paulo Brazil; M Mexico; and U USA c L 2 L .- E l 2 0 -3 t L L L L M L J R E E 6 H J 1 J J J - 4 1 J J -7.5 -5.0 -2.5 0 2.5 5.0 7.5 10.0 12.5 First canonical function Fig. 2 Plot of scores on the first and third canonical functions for the orange juice samples. Country codes as in Fig. 1 AJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 1991 VOL. 6 64 1 the excellent separation achieved for the most important Iranian category. Macadamia Nuts Owing to the limited number of standards available it was decided to combine the samples and standards into one large set rather than divide them into a calibration and a prediction set as had been done with the pistachio samples.A discriminant analysis was performed using the entire 40- sample set as a calibration model using data for the following nine elements Ba Ca Cu Fe K Mg Mn P and Zn. The order of discriminatory power for this set of elements was Zn>Cu>Ca=Mg>P=K>Mn=Fe-Ba. The validity of this nine-element model was checked by performing a re-substitution and a cross-validation analysis on the entire sample set. The results are shown in Table 5. Although excellent results were obtained for the re-substitu- tion analysis the cross-validation results were significantly poorer both in terms of the over-all percentage correctly classified and the classification of the most important South African category.However as mentioned earlier re-substi- tution would be expected to give optimistically biased results the degree of difference observed here between the re-substitution and cross-validation results for the South African samples merits additional comment. This large difference can be ascribed at least in part to there being only three South African samples in the set. In a cross-validation analysis the sample being classified is excluded from the model. This would leave the remaining two South African samples to form the model for the classification of the third. It is unlikely that just two samples will form an accurate model for the samples from a given country and such a model would not be expected to give reliable results.The one false positive was a Guatamalan sample. The estimated probability of group membership for this sample was 0.54 South African and 0.40 Guatamalan with the remainder of the probability associated with other areas. In practice such a sample which has significant probabilities of belonging to more than one country would not have been assigned to any country owing to the uncertainty. This makes the false positive classification a moot point in this instance. Group separations were visualized in a manner similar to that described for orange juice and pistachio nut samples. A canonical discriminant analysis was performed on the entire 40-sample set using the same nine elements as for the discriminant analysis.Fig. 4 shows a plot of the scores for each sample on the first two canonical functions. The samples from each country are well separated except for Malawi and South Africa where there is a small degree of overlap. This overlap is reasonable in view of the geogra- phic proximity of these two countries. While the first two canonical functions do not completely separate the samples from these countries it was found that they were resolved by their scores on the third canonical function. This is illustrated in Fig. 5 which is a plot of the scores on the first ~~ ~ ~ Table 5 Discriminant analysis results for macadamia nut samples South African result? Correct result* Number False Type of analysis (Oh) correct positives Re-substitution 98 3 0 Cross-validation 78 1 1 *From all macadamia nut samples tested n=40.tn = 3. 5 4 3 C 0 3 . 2 w- r ' E 0 f3 .- C rn -1 -2 -3 - 6 - 4 - 2 0 2 4 6 8 First canonical function Fig. 4 Plot of scores on the first two canonical functions for the macadamia samples. Country codes A Australia; C Costa Rica; G Guatamala; M Malawi; S South Africa; and Z Zimbabwe. Asterisks indicate samples with authenticated origins. Perimeters of the regions containing samples from a given country have been outlined for clarity A A G G G G' G G G G' G' G I I t I I t - 6 - 4 - 2 0 2 4 6 8 First canonical function Fig. 5 Plot of scores on the first and third canonical functions for the macadamia samples. See Fig. 4 for details and third canonical functions.It can be seen that the samples from Malawi and South Africa are now completely separated. Conclusions The applicability of multivariate analysis of trace element composition to the determination of the geographic origin of three agricultural products namely orange juice pista- chio nuts and macadamia nuts has been demonstrated. The multivariate techniques known as discriminant analysis642 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL. 6 and canonical discriminant analysis have been shown to be useful for predicting geographic origin and for helping to visualize the separation of products of different geographic origins. This approach should have wide applicability to the products in general. 5 Kwan W. O. and Kowalski B. R. Anal. Chirn. Acta 1980 122 215. 6 Frank I. and Kowalski B. R. Anal. Chirn. Acta 1984 162 241. Lebensm. Unters. Forsch. 1983 177 15. 8 Pane S. W. Food Technol.. 1986.40. 104. determination of the geographic origin of agricultural 7 Borszeki J.9 KoltaY L-9 InczedY J- and Gems E. Z. References Gilbert J. Shepherd M. J. Wallwork M. A. and Hams R. G. J. Apic. Res. 1981 20 125. Forina M. and Armanino C. Ann. Chirn. (Rome) 1982 72 127. Bayer S. McHard J. A. and Winefordner J. D. J. Agric. Food Chem. 1980,28 1306. Kwan W. O. Kowalski B. R. and Skogerboe R. K. J. Agric. Food Chew. 1979,27 1321. 9 KaTbata-Pendias A. and Pendias; H.; Trace Elements in Soils and Plants CRC Press Boca Raton 1984 pp. 51-55. 10 Oficial Methods of Analysis ed. Horwitz W. Association of Official Analytical Chemists Arlington VA 13th edn. 1980 sect. 22.025 p. 363. Paper 1 /00896J Received February 2Sth 1991 Accepted July 30th 1991

ISSN:0267-9477    

DOI:10.1039/JA9910600637

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL. 6 643 Immobilized Alga as a Reagent for Preconcentration in Trace Element Atomic Absorption Spectrometry Hayat A. M. Elmahadi and Gillian M. Greenway School of Chemistry University of Hull Hull HU6 7RX UK The alga Sebnestrum capricornutum was covalently immobilized on controlled pore glass and packed in a minicolumn which was incorporated into a flow injection system for the preconcentration of trace elements prior to determination by atomic absorption spectrometry. Samples (5 ml) of Cu2+ Pb2+ Zn2+ Co2+ Hg2+ and Cd2+ were preconcentrated to give detection limits of 0.05 2.5 0.2 8.0 30.0 and 2.0 ng ml-l respectively with a sampling rate of 20 h-l. Keywords Immobilized alga; preconcentration; trace metal; atomic absorption spectrometry The ability of algae to accumulate trace metals by biosorp- tion has been known for some years.' This effect has been put to use in water treatment systems,* however only recently has it been exploited for analytical measurement methods3 Many reagents have been investigated for the preconcentration of trace metals however these usually have only one type of binding site.The possible advantage of algae is that potentially the cell wall has many constitu- ents that can be implicated in metal binding including mine and carboxyl groups from amino acids and polysac- charides sulphydryl groups and unmethylated pectin^.^ This range of binding sites means that by altering the elution conditions different metal ions can be retained preferentially.Another advantage of using these organisms is their small and uniform cell size. Most of the analytical work that has been published using algae has been carried out on material that was not immobilized. This can be a difficult and time-consuming procedure requiring several steps including washing and centrifuging. A biosensor has been reported where the alga was immobilized on a carbon paste electrode for the determination of Cu** species.5 Other workers have immo- bilized algae mainly for biotechnology applications.6 This has been either by invasive adsorption of live algae into polymer matrices or by physical entrapment in polymers. Dagnell et al.' reported the use of polyacrylamide immobil- ized alga. These workers have also developed a method of physically trapping the alga in a silica gel polymeric material.* Immobilization of chemical reagentsg and en- zymesl0 by covalent attachment to a water insoluble substrate provides very stable preparations.Controlled pore glass (CPG) has been shown to be a particularly effective insoluble substrate as it exhibits good mechanical properties in flowing streams. For enzymes,1° the CPG is first silanized and then the bifunctional properties of gluteraldehyde are used to cross-link the enzyme to the silanized glass through the lysine amino groups on the enzyme. This work describes the covalent immobilization of an alga on CPG for the preconcentration of Cu2+ Pb2+ Zn2+ Co2+ Hg2+ and Cd2+. The diversity of active binding sites of the alga means that its ability to preconcentrate is not hindered by immobilization.Experimental Reagents The green alga Selenestrum capricornutum was selected as it is a fast growing and easily cultivated species. It was cultured by the Department of Applied Biology at the University of Hull in an aerated medium and harvested after 1 week. Chu 10 medium (medium No. 10 as described by Chdl) was prepared in the laboratory and used for cultivation of the alga. This consisted of Ca(N03)2.4H20 (57.6 mg l-l) Ca(N03)2 (40 mg l-l) K2HPO4 (5.0 mg l-l) Mg!304.7H20 (25.0 mg l-l) Na2C03 (20.0 mg l-l) Na2SiOj (25.0 mg l-l) FeC13 (0.8 mg l-l) 25 mmol 1-1 HN03 and soil extract. The soil extract was prepared by placing 250 g of soil into 500 ml of water and boiling in a steamer for 2-3 h. This was filtered through a Whatman No.1 filter paper autoclaved and stored in a refrigerator for 2-3 d to allow the sediment to settle. A 10 ml volume of this solution was added to the medium before making it up to 1 1 with water and autoclaving. The alga was centrifuged and washed three times with 50 ml of distilled de-ionized water. After washing it was heat treated and then freeze dried. The CPG (CPG-240,22.6 nm pore diameter 80- 1 20 mesh) and 8-aminopropyltriethyox- ysilane were obtained from Sigma. Cadmium chloride and cobalt(@ sulphate and all of the other reagents were of analytical-reagent grade from Merck. Distilled de-ionized water was used throughout. Immobilization of the Alga A 0.1 g amount of the ground freeze-dried material was weighed into a small beaker (20 ml) then 2 ml of 0.01 mol dm-3 hydrochloric acid followed by two 5 ml aliquots of ethanol were added whilst stirring.This mixture was heated on a steam-bath for 1-2 min until all of the material appeared to go into solution. A 2 ml volume of water was then added and 10 ml of the solution were transferred into a clean beaker. The volume was made up to approximately 25 ml using phosphate buffer (0.1 mol dm-3). The solution was adjusted to pH 6 using sodium hydroxide solution. The immobilization procedure which is presented in Fig. 1 was a modified form of that used for the immobilization of enzymes on CPG.l0 The CPG was activated by taking 1 g and boiling it in 10 ml of 5% v/v nitric acid for 30 min. It was then filtered through a porous sintered glass filter washed with water and dried in an oven at 95 "C for 1 h.The CPG was then silanized with 8-aminotriethy- oxysilane solution by taking 2.5 ml of the reagent and diluting it with water to 25 ml. This solution was adjusted to pH 3.45-3.50 with 6 mol dm-3 hydrochloric acid and 5 ml of the solution were then added to the dried CPG and heated on a water-bath at 75 "C for 150 min with stirring. The resulting product was washed with water and dried at 95 "C for 2 h. This procedure was repeated twice. The aldehyde derivative was then prepared by taking 2.5 ml of an aqueous gluteraldehyde solution (50% v/v Merck) and making it up to 50 ml with phosphate buffer (pH 7). A 5 ml volume of this solution was added to the treated glass in a firmly stoppered round-bottomed flask flushed with nitrogen. The reaction was allowed to continue for at least 1 h at room temperature during which time a brown644 solution De-ionized ’ water JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL.6 3 To waste 1 W I 3 4 i 5 OEt OEt Controlled I I I glass I pore ?OH + EtO-Si-(CH&NH - C P a k ( C H z ) a N H OEt OEt (4 OEt H I I I OEt (B) A + OHC-(CH2)rCHO - CPai-(CH&-N=C-(CH2),-CHo OEt H I I C PG-o-sii( CH,)s-N = C -. (CH&- CH =N I alga B + H\N$ H / alga - OE I t Fig. 1 Reaction scheme for the covalent immobilization of the alga on controlled pore glass where ci is the initial concentration of metal ions and cf is the concentration after equilibration (in mol dm-3); m is the mass of controlled pore glass (in g) and v is the volume of solution (in cm3).Fig. 2 Electron micrograph of Selenestrum capricornutum immo- bilized on controlled pore glass coloration was observed. Then the activated glass was washed with water. A 0.5 g amount of the activated CPG was added to the 25 ml of alga in phosphate buffer (pH 6). Nitrogen was bubbled through the solution at 10 min intervals for the first hour of the immobilization process the reaction was left under nitrogen for a further hour at room temperature (with stirring) and then the solution was stored at 4” C for 2 d. The resulting immobilized algal cells were then filtered and air dried. The immobilized alga was observed by scanning electron microscopy to investigate theeffect ofthe chemical processon the integrity of the cells. As can be seen from Fig. 2 the alga was successfully immobilized and in general the cell walls were still intact showing the characteristic half-moon shape.Instrumentation and Preconcentration Procedure The flame atomic absorption spectrometer was a Varian Model AA75 and was used with an air-acetylene flame. Table 1 shows the wavelengths used for the different elements. Results were recorded on a chart recorder (Chessell Model BD4040). The flow injection (FI) manifold is shown in Fig. 3. It consisted of a peristaltic pump (Ismatec Minipuls SA 8031) and a rotary polytetrafluro- ethylene (PTFE) valve (Rheodyne 5020). All connections were made with 0.8 i.d. PTFE tubing. Two 3-way valves (Omnifit Anachem) were utilized the second being in- cluded in an effort to minimize the amount of buffer passing into the flame.The immobilized alga was packed into a glass tube 5 cm long x 2.5 mm i.d. In the procedure volumes of up to 5 ml of metal ions in buffer solution were passed through the minicolumn which was then washed with water. The accumulated ions were then released by injection of acid and transported to the flame for detection by atomic absorption spectrometry (AAS). Table 1 Conditions for preconcentration-AAS Wavelength/ Metal ion nm PH cu* + 324.8 7.5 Zn2+ 21 3.9 7.5 coz+ 240.7 8.0 Hg2+ 253.7 6.5 Cd2+ 228.8 8.5 PbZ+ 2 17.0 5.5 Evaluation of Exchange Capacity In order to evaluate the exchange capacity of the immobil- ized alga for different metals a 25 ml aliquot of a 0.025 mol dm-3 metal standard (either Cu2+ Pb2+ Zn2+ Coz+ Hg2+ or Cd2+) in the appropriate buffer was added to 0.1 g of the immobilized alga.The mixture was then allowed to equilibriate for 16 h at room temperature while stirring after which the solid was filtered off and the metal ion concentration of the supernatent liquid was determined. Fig. 3 FI manifold for the preconcentration of metal ions by the immobilized alga 1 3-way valve; 2 peristaltic pump; 3 injectionJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL. 6 645 Results and Discussion The effect of the pH of the buffer solution on the ability of the column to preconcentrate was investigated for the different metal ions (Fig. 4). Solutions of 0.1 mol dm-3 phosphate buffer and TRIS [ 2-amino-2-(hydroxymethyl)- propane-1,Idioll buffer were used to cover a pH range of 4-10. As can be seen in Fig.4 for Cu2+ Hgz+ and Pb2+ the highest sensitivity was obtained in phosphate buffer at pH values of 7.5 6.5 and 5.5 respectively. The highest sensitivities for Zn Co and Cd were obtained for TRIS buffer at pH values of 7.5 8 and 8.5 respectively. The other main factor affecting the preconcentration technique is the eluent acid that releases complexed ions from the algal surface. The concentration of acid must be limited to the lowest possible level in order to prevent degradation of the biomass. Fig. 5 shows the effect of acid concentration on the absorbance signal for hydro- chloric acid identical results were obtained for nitric acid. A 100 pl volume of 0.5 mol dm-3 acid was required to elute Cu2+ Zn2+ and Pb2+ completely. The Co2+ was not fully eluted until 100 p1 of 1 mol dm-3 acid were used whereas Cd2+ only required 100 pl of 0.1 mol dm-3 acid (sulphuric acid was also found to be suitable in this instance). Mercury was more problematic in that lower sensitivity and peak broadening with tailing was observed when it was eluted with acid (0.1-3 mol dm-3 nitric or hydrochloric acid).In order to overcome this problem thiourea was dissolved in the 100 pl of 0.1 mol dm-3 hydrochloric acid eluent so that the solution was 0.1 mol dm-3 in thiourea; the thiourea released the Hg from the column by forming a strong complex with it. The parameters affecting the FI system were also investi- gated such as flow rate and the length of the preconcentra- tion column but these factors were found to be negligible in terms of dispersion compared with the effect of the nebulizer.12 \ F k 1 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 1 0 PH Fig.4 Effect of pH on metal ion uptake A 500 ng ml-1 PbZ+; B 100 ng ml-1 Zn2+; C 80 ngml-1 Cd2+; D 50 ngml-1 Cu2+; E 250 ng ml-1 Co2+; and F 5000 ng m1-I Hg7+ Calibration Fig. 6 shows a typical calibration graph and the series of calibration traces obtained for each individual element using the procedure described with the conditions as established above. The figures of merit are given in Table 2. The limits of detection were compared with the limits of detection obtained without preconcentration by calculating the en- hancement factor. As can be seen from the Table 2 the alga was effective at preconcentrating all the elements investi- gated but was particularly good for Cu2+ ZnZ+ and Cd2+.Capacity and Recovery The effectiveness of the alga in preconcentrating metal ions was assessed further by measuring its capacity by the batch method described under Evaluation of Exchange Capacity. The results given in Table 3 show that the alga has a high uptake capacity for Cu2+ Zn2+ Cd2+ and Pb2+. The uptake for Co2+ and Hgz+ was lower. Some of these values are high compared with those obtained for chemical preconcentra- tion reagents such as silica-immobilized 8-hydroxyquinoline (230 pmol g-l for Cu2+) and 8-hydroxyquinoline-5-sul- phonic acid (6.0 mmol g-l for Cd2+).9 The recovery of the metal ions was found by comparing the signal obtained with direct injection with that obtained after preconcentration and elution with the appropriate volume and concentration of acid (Table 3).The recoveries obtained were good with only Coz+ showing a low recovery. The lifetime of the immobilized alga was 3 months if stored below 4 "C whilst not in use. Interferences Table 4 shows the interference effects of high concentrations 200 160 a 6 120 .- Q) Y m 80 2 40 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 Acid conce nt rat io n/m ol d m-3 Fig. 5 Effect of hydrochloric acid concentration on elution of metal ions A 1 10 ng ml-1 Zn2+; B 60 ng ml-l CuZ+; C 250 ngml-1 Co2+; D 5000 ng ml-l Hg2+; E 60 ng ml-I Cd2+; and F 250 ng ml Pb2+ 160 140 E 120 E 2 100 m .- 2 80 60 40 20 Y L I 1 1 L I I . 1 0 10 20 30 40 50 60 70 80 90 100 Concentration of metal ion/ng ml-' [~n'+l/ng mi ' Fig. 6 Calibration graph for a series of metal ion standards and the calibration trace for Zn ions A Co2+ ( x 10); B Pbz+ ( x 10); C Cu2+; D Hg2+ (x 100); E,CdZ+; and F Zn2+646 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL.6 Table 2 Analytical performance of the on-line preconcentration FI-AAS system for 5 ml of sample (sampling rate 20 h-l) Parameter cuz+ Zn2+ co2+ Hg2+ CdZ+ Pb2+ Linear rangel ng ml-a 5-45 10-100 55-300 650-5000 15-80 60-450 Correlation coefficient 0.9992 0.9998 0.9997 0.9988 0.9995 0.9999 RSD* (%) ( n = 5 ) 1 .5( 40) 1.3( 100) 1.4(250) 1.2(4000) 1 .5( 30) 1.1(300) Detection limit?/ Enhancement ng ml-1 0.05 0.2 factor+ 4000 lo00 8 30 2 2.5 50 167 500 80 *Values in parentheses are the concentrations of the metal ions in ng ml-I. t2a value. $Factor by which the limits of detection decrease using preconcentration as opposed to direct injection.Table 3 Exchange capacity and recovery for the immobilized alga Metal uptake Recovery Metal ion capacity/mol g-' (%I effect is also seen for Pb2+ interference with Hg2+ preconcen- tration. This behaviour needs further study but might be due to the release of ions that have not been completely recovered from previous determinations. cu2+ Zn2+ co2+ HgZ+ Cd2 + Pbz+ 9.70 8.36 2.52 1.60 11.70 11.45 1 00 98 85 96 102 96 ~~ (20 pg ml-*) of different metal ions on the preconcentration process for particular metal ions. From the table it can be seen that the interference effects for Cu2+ Pb2+ and Zn2+ are generally insignificant considering the high level of interfer- ing ions present with Co2+ and Hg2+ being affected to a greater extent.The interference effect can be explained by the differences in strength of the different ion complexes formed with the algal cell wall. Once all of the binding sites of a chemically immobilized chelating reagent are occupied then those ions that are less strongly bound can be displaced by interfering ions present in excess thus depressing the final absorbance rnea~urement.'~ This process is more complex for the alga because there is more than one type of binding site present and more selective binding occurs. From the results obtained for the interferences the relative affinities of the different metal ions for Selenestrum capricornutum in decreasing order are Hg>Cu>Pb>Zn>Co>Mg. Mercury was bound most strongly and could only be removed by introduction of the thiourea ligand to the eluent.The behaviour of Cd was anomolous. The Co2+ Hg2+ and Mg2+ ions do not interfere in the preconcentration of Cd2+ but an enhancement of the absorbance is seen as a result of the interference of Cu2+ Pb2+ and Zn2+. This enhancement Table 4 Interference effects of high concentrations of different interfering ions (20 pg ml-l) on the preconcentration process for particular metal ions Metal ion& ml-I cu2+ 100 Interfering ion cu2+ - Zn2+ -2.8 co2+ -0.0 Hg2+ -0.0 Cd2+ -0.0 Pb2+ -0.1 Mg2+ -0.0 Zn2+ Co2+ HgZ+ Cd2+ 200 200 5000 80 Change in peak height (%) -11.1 -50.0 -18.0 +lo7 - - 55.0 -27.0 + 100 -3.8 - -5.5 -0.0 -25.0 -0.0 - -0.0 -2.3 -36.0 -27.0 - -3.8 -22.7 +45.5 +lo7 -0.0 -0.0 -0.0 -0.0 Pb2 + 500 - 0.0 -0.0 -0.0 - 57.9 -2.8 - 0.0 -0.0 Conclusions The immobilized alga is shown to be an effective reagent for the quantitative preconcentration of a number of trace metals.The covalent immobilization procedure was success- ful with the column retaining activity for 3 months if stored at below 4°C when not in use. In addition to being a useful analytical reagent the immobilized alga can be used for general studies of the accumulation of metals on algal cell walls allowing rapid flow through studies. Further work will investigate the use of the method for real samples. Dr. R. Goulder and S. Lythe of the Department of Applied Biology University of Hull are thanked for cultivating the alga used in this work. 1 2 3 4 5 6 7 8 9 10 1 1 12 13 References Shumate S. E. 11 Strandberg G. W. McWhirter D. A. Parrott J. R. Bogacki G. M. and Locke B. R. Biotechnology and Bioengineeringsymposium No. 10 Wiley New York 1980 p. 27. Greene B. Hosea M. McPherson R. Henzel M. Alexander M. A. and Darnell D. W. Environ. Sci. Technol. 1986,20,627. Mahan C. A. Majidi V. and Holcombe J. A. Anal. Chem. 1989,61 624. Crist R. H. Oberholser K. Shank N. and Nguyen M. Environ. Sci. Technol. 1981 15 1212. Gardea-Torresdey J. Darnell D. and Wang J. Anal. Chern. 1988,60 72. Trevan M. D. and Mak A. L. Trenh Biotechnol. 1988,6,68. Dagnell D. W. Greene B. Henzel M. Hosea M. McPherson R. A. Sneddon J. and Alexander M. D. Environ. Sci. Technol. 1986 20 206. Darnall D. W. Gabel A. US EPA Res. Dev. [Rep.] EPA EPA/600/9-89-072 Int. Conf New Front. Hazard. Waste Manage. 3rd 1989 pp. 217-225. Devi S. Habib K. J. andTownshend A. Quim. Anal. 1989,8 159. Masoom M. and Townshend A. Anal. Chzm. Acta 1984,166 111. Chu S. P. J. Ecol. 1942 30 284. Tyson J. F. Appleton J. M. H. and Idris A. B. Anal. Chim. Acta 1983 145 159. Bysouth S. R. and Tyson J. F. Anal. Chim. Acta 1988,214 329. Paper I /02492B Received May 28th I991 Accepted July 22nd 1991

ISSN:0267-9477    

DOI:10.1039/JA9910600643

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 1991 VOL. 6 647 Determination of Zinc in Human Milk by Electrothermal Atomic Absorption Spectrometry Josiane Amaud and Alain Favier Laboratoire de Biochimie C Centre Hospitalier Regional et Universitaire de Grenoble BP 21 7X 38043 Grenoble Cedex France Josette Alary Laboratoire de Chimie Analytique Faculte de Pharmacie Universite Joseph Fourier Domaine de la Merci 38700 La Tronche France An electrothermal atomic absorption spectrometric method has been developed for determining zinc in human milk. Milk samples were analysed after 1 +99 dilution with 0.1% Triton X-100. The within-run precision was found to be 4.6%. The accuracy was ascertained by recovery of standard additions and was found to be 96+ 10% (n-85 milk samples). The accuracy was also checked against National Institute of Standards and Technology Standard Reference Material 1549 Non-Fat Milk Powder.The results were 713+35 pmol g-l (n=38) (certified value 705,+34 pmol g-l). The detection limit was found to be 0.052 pmol I-'. The calibration graph was linear up to 3 pmol I-'. The choice of experimental parameters (diluent dilution rate background correction graphite furnace tube etc.) are discussed. Normal values for colostrum and transitional milk varied from 45 to 318 and from 30 to 146 pmol I-' respectively. Keywords Determination of zinc; electrothermal atomic absorption spectrometry; human milk; lactation During the first three months of life milk is the principal source of zinc. However the concentration of zinc in human milk decreases rapidly with progressive lactation.Premature and low birth weight infants are at risk from zinc deficiency owing to their limited gastric capacity imma- ture intestinal kidney and liver function reduced body stores and rapid growth.I4 Severe zinc deficiencies are easy to detect and have been reported in infants receiving intravenous nutrition or formulae with inadequate zinc concentration (less than 31 pmol 1-l) or in premature infants receiving mature human milk from a Cases of zinc deficiency in premature or full term infants fed with their own mothers' milk have also been reported.'-16 In these cases the concentration of zinc in the milk was particularly low ( 1 A7.6 pmol 1-l). Zinc is commonly determined in milk samples by flame atomic absorption spectrometry.The sensitivity of the method is inadequate for determining very low zinc concentrations however with electrothermal atomic absorption spectrometry it is possible to determine low zinc concentrations. In this work a suitable technique for the determination of zinc in human milk using a direct electrothermal atomic absorption spectrometric method is presented. Experimental Instruments and Apparatus A Perkin-Elmer Model 560 atomic absorption spectro- meter was used With a zinc hollow cathode lamp as the light source (intensity- 15 mA). The instrument was fitted with an AS 40 autosampler and an HGA 500 graphite furnace. The absorbance was measured at 2 13.9 nm with a slit-width of 0.7 nm. A three-step graphite furnace programme was used; drymg 20 s at 1 10 "C with a ramp time of 20 s; ashing 30 s at 650 "C with a ramp time of 1 s and atomization 5 s at 2200 "C with a ramp time of 2 s.The internal gas was nitrogen and the flow rate during the atomization step was reduced to 250 ml min-l. The injection volume in either standard or pyrolytic graphite coated graphite furnace tubes All of the plasticware was made of polycarbonate polyethylene or polystyrene and was soaked for 16 h in 10% v/v nitric acid then again in 10% v/v hydrochloric acid for was l0pl. a further 16 h. Prior to use the plasticware was rinsed in de- ionized water and dried in a stainless-steel oven. The subjects who gave oral consent were enrolled during the first week of lactation. A total of 112 lactating mothers providing 203 individual milk samples were selected.Of these 97 mothers lived in the urban Grenoble area while 15 lived in rural areas 82 of the mothers who agreed to participate in this study were Caucasian 25 were Arabian 4 were Asian and 1 was Negro. Of the total 20 of the mothers were from low-income 87 from middle-income and 5 from high-income families. While 64 of the mothers were primiparae 48 were multiparae. The mean age of the mothers was 26 years (range 19-39 years). The mean weights before pregnancy and before delivery were 56 kg (range 40-87 kg) and 67 kg (range 52-96 kg) respectively. The mean height was 16 1 cm (range 145- 176 cm). All of the mothers were healthy and apparently well-nourished women based on clinical observation. There was neither glucose nor albumin in their urine and the mean haemoglo- bin value at delivery was 1 14 g 1-l (range 97-1 37 g 1-I).All had uncomplicated pregnancies and delivered a single infant at term [ 39 A 2 weeks (mean ~f standard deviation) range 36-43 weeks]. Of the infants 53 were male and 59 female. All were healthy and growing well. The mean birth weight of the infants was 3293 g (range 2370-4300 g) and their mean height was 50 cm (range 46-54 cm). Samples The breast was cleaned with de-ionized water. Approxi- mately 10 ml of breast milk were hand expressed before one of the morning feeds (09.00-1 1.00 hours) into a 30 ml polystyrene bottle. Milk samples were transported to the laboratory on ice. Aliquots of the milk (1 ml) were transferred into 5 ml polystyrene tubes then frozen at - 20 "C prior to analysis.Reagents Hydrochloric acid (0.1 mol 1-l) was prepared from ultra- pure hydrochloric acid I I mo1 1-' (Prolabo Normatom).648 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL. 6 Nitric acid (0.1 moll-') was prepared from ultrapure nitric acid 15 mol 1-l (Merck). The following reagents were also used 0.05- 1 % m/v Triton X- 100 (Prolabo); 1 % m/v sodium lauryl sulphate (Sigma); 1% m/v benzyl dimethyl hexadecyl ammonium chloride (Fluka); and 0. l0h m/v histidine (Sigma). A zinc stock solution containing 75 pmol 1-l of zinc was also prepared. Procedure The preparation of the standard solutions and samples and the analyses were conducted in a filtered-air room (class 100 000). Preparation of standard solutions The stock solution was prepared from zinc powder (Merck) 4.9 mg were dissolved in 5 ml of 11 mol I-' hydrochloric acid.After total dissolution the solution was poured into a calibrated flask containing about 700 ml of de-ionized water. The volume was made-up with de-ionized water. Aliquots (5 ml) of the stock solution were transferred into 5 ml polystyrene tubes and stored at -20 "C. Working standard solutions were prepared from the stock standard solution using the same diluent as for the milk samples. The zinc concentrations used were 0.375,0.75 and 1.5 pmol l-l when the milk was diluted 1 +99. Preparation of samples A whole human milk sample was diluted in 0.1 Oh Tritron X- 100 ( 1 + 99) before injection into the graphite furnace tube. Statistical analysis The mean and standard deviation were calculated and Student's t-test was performed.The linear regression and correlation coefficient were also determined. Results and Discussion Graphite Furnace Programme The optimum conditions of the graphite furnace pro- gramme were determined. All measurements were made in triplicate. A temperature of 650 "C was the highest temperature that could be used without volatilization of the zinc. During the atomization step the temperature and ramp time affected the sensitivity. The results presented in Fig. 1 show that the lowest temperature required to achieve total volatilization of the zinc was 2200 "C. A ramp time of 2 s allowed the best sensitivity (Fig. 1). A gas flow of 250 ml min-l provided the greatest linear range. Saner and 1000 1500 2000 2500 500 Tern peratu re/"C Fig.1 Influence of atomization temperature and ramp time (RT) on the absorbance of zinc in solution (broken lines) or in milk sample (solid lines). A and B RT = 0 s; C and D RT = 1 s; E and F RT=2 s; and G and H RT=5 s Table 1 Amount of zinc in the selected diluents (n = 30 determi- nations) Diluent De-ionized water HCl 0.1 mol 1-1 HN03 0.1 mol 1-1 Triton X-100 0.1% m/v Triton X-100 1% m/v Sodium lauryl sulphate 1% m/v Benzyl dimethyl hexadecyl ammonium chloride 1% m/v Histidine 1% m/v *ND = not detectable. Amount of zinchmol 1-1 ND* 46+ 13 77+ 15 ND ND 1381- 11 92+ 19 ND Yuzbasiyanl' determined zinc in human milk by electroth- ermal atomic absorption spectrometry but prior to analy- sis the milk samples were dry ashed. They used ashing and atomization temperatures of 450 and 2000 "C respectively with a 230 ml min-l gas flow during the atomization step.Influence of Diluent and Dilution Rate De-ionized water two acids (0.1 mol 1-I nitric and hydrochloric acids) various wetting agents (1% sodium lauryl sulphate Triton X-100 and benzyl hexadecyl di- methyl ammonium chloride) and a chelating agent (0.1Oh histidine) were selected for trial. De-ionized water is the most commonly used diluent for zinc determination in milk by flame atomic absorption spectrometry. Nitric and hydrochloric acids might influence the ashing and atomiza- tion steps. Furthermore nitric acid has been used by Murphy et a!.'* and Moran et all8 However Arpadjan and Nakova19 have shown that a wetting agent is necessary for direct determination of zinc in milk by flame atomic absorption spectrometry because of the high concentra- tions of lipids in the sample.Three wetting agents were selected according to their polar properties one anionic (sodium lauryl sulphate) one cationic (benzyl hexadecyl dimethyl ammonium chloride); and one neutral (Triton X- 100). The zinc present in milk is chelated by citric acid amino acids and proteins,20-22 as a result a chelating agent was also selected. The contamination from these diluents was calculated trom an aqueous calibration graph (Table 1). The zinc concentrations of the blanks were undetectable for Triton X- 100 and 0.1 % histidine. Zinc contamination in the other diluents varied from 46 k 13 (0.1 mol 1-l HCl) to 138 2 1 1 nmoll-l(lo/o sodium lauryl sulphate) (n= 30).Zinc contam- ination was lowest in the acids and highest from the 1% sodium lauryl sulphate and 1 O/o benzyl hexadecyl dimethyl ammonium chloride. Nevertheless these contaminations were sufficiently low to allow the determination of zinc in normal human milk. The results of the recovery experiment are given in Table 2 and the within-run precision in Table 3. The recovery of standard additions increased with the dilution rate. The recovery of standard additions varied from 65 (1 +49) to 144Oh (1 + 499) (de-ionized water) from 50 (1 + 49) to 1 12% (1 + 499) (0.1 moll-' hydrochloric acid) from 37 (1 +49) to 1OOoh (1 +499) (0.1 moll-' nitric acid) from 84 (1 + 49) to 200% ( 1 + 999) ( 1 % Triton X- loo) from 37 ( 1 + 49) to 107% (1 + 999) (1 96 benzyl hexadecyl dimethyl ammonium chlo- ride) from 42 ( 1 + 49) to 143% (1 + 999) (1 Oh sodium lauryl sulphate) and from 66 (1 +49) to 126Oh (1 +999) (0.1% histidine). With a dilution of 1 +49 the linearity was insufficient for accurate recoveries.However with dilutions of 1 +499 or 1 +999 (results not shown) the recoveryJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL. 6 649 Table 2 Influence of the diluent and dilution rate on the recovery of standard additions (n = 10) replicate determinations of the same sample analysed on different days). Values between 90 and 110% are considered satisfactory Dilution rate Diluent De-ionized water HC1 0.1 mol 1-1 HN03 0.1 mol 1-I Triton X-100 1% m/v Benzyl dimethyl hexadecyl ammonium chloride 1% m/v Sodium lauryl sulphate 1% m/v Histidine 0.1% m/v *Values given + 1SD.1 +49 65+5 50+ 1 37+4 84+8 42+2 66+3 37+ 1 1 +99 82+6 74+7 57+ 1 111+7 69+ 1 66+3 70+4 1 + 199 98+ 13 9728 69+5 122+ 12 71 + 7 77+7 74+9 Table 3 Influence of the diluent and the dilution rate on the within-run precision expressed as the RSD= [ B D ] x 100 for n = 30 determinations of different dilutions of the same milk sample. Values of less than 5% are considered satisfactory Dilution rate Diluent De-ionized water HCl 0.1 mol 1-1 HN03 0.1 rnol 1-I Triton X-100 1% m/v Sodium lauryl sulphate 1% m/v Benzyl dimethyl hexadecyl ammonium chloride 1% m/v Histidine 0.1% m/v 1+49 1+99 5 10 1.5 5 4 4 2.5 5 3 3 2 4 3 3 1 + 199 10 9 6 7 4 4 4 experiments suffered from problems of contamination. Although a mean recovery of standard additions between 90 and 110% was observed with all of the diluents tested the dilution rate was different for each.Dilutions of 1 + 499 even if the recoveries of the standard additions were correct were not recommended because of the low signal- to-noise ratios and the special requirements needed to avoid contamination. As a result the relative standard deviation (RSD) (Table 3) increased with the dilution rate. The RSD varied from 5 (1 +49) to 39% (1 +999) (de- ionized water) from 1.5 (1 +49) to 30% (1 +999) (0.1 mol 1-1 HCl) from 4 (1 +49) to 24% (1 + 999) (0.1 mol 1-1 HN03) from 2.5 (1 +49) to 36% (1 +999) (1% Triton X- loo) from 3 (1 +49) to 5Oh (1 +999) (1% sodium lauryl sulphate) from 2 (l+49) to 8% (1 +999) (1% benzyl dimethyl hexadecyl ammonium chloride) and from 3 (1 +49) to 7% (1 + 999) (0.1% histidine).The precision achievable was better in the presence of wetting agents which allowed stable homogenization of the milk samples. l9 The detection limit and linearity are shown in Table 4. The detection limits were calculated according to the following criteria X+3SD where X is the mean of 30 replicate zinc determinations at the blank level and SD is the corresponding standard deviation. The detection limit depended on the diluent (Table 4) as did the accuracy and precision. The best detection limits were obtained with de- ionized water O.loh histidine and l0h Triton X-100. The linearity also depended on the diluent. The linearity was greater with de-ionized water and 1 % Triton X- 100 (Table 4).The slopes of the calibration graphs (results not shown) depended on the diluent which was in agreement with findings previously reported. l9 According to these different results Triton X-100 and a 1 + 99 dilution were chosen as optimum. This choice of diluent is in agreement with that of Arpadjan and Na- kova,19 who compared a variety of different diluents [de- ionized water 0.05-0.1 % Triton X- 100,0.05-0.2% Meriten Table 4 Influence of the diluent on the linear range. Detection limit defined as X+ 3SD for n = 30 determinations at the blank level and linearity value for triplicate determinations Detection limit/ Linearity/ Diluent nmol 1-1 pmol I-' De-ionized water HCl 0.1 mol 1-1 HN03 0.1 mol 1-1 Triton X-100 1% m/v Sodium lauryl sulphate 1% m/v Benzyl dimethyl hexadecyl ammonium chloride l0h m/v Histidine 0.l0h m/v Triton X- 100 0.1 O/o m/v 39 4 85 3 122 2 27 4 171 1.5 149 2 35 3 52 3 (nonyl phenyl polyglycolether) 0.05-0.2% saponin and 0.05-0.2% sodium dodecyl benzenesulphonate] for the determination of zinc in milk by flame atomic absorption spectrometry and selected 0.1% Meriten and 0.05% Triton X-100. According to Arpadjan and Nakova,19 a wetting agent was also necessary to homogenize and dissolve the lipids present in the milk. Effect of the Concentration of Triton X-100 The results are presented in Fig. 2. The recovery of the standard additions increased with concentration up to 0.1 % Triton X-100 and then reached a plateau. Consequently 0.1% Triton X-100 was used. Background Correction Deuterium background correction has been used by several groups of w ~ r k e r s .l ~ * * ~ t ~ ~ In this study zinc was determined L Q) 8 50 CT 0 Concentration of Triton X-100 (% m/v) Fig. 2 Influence of Triton X-100 concentration on the recovery of standard additions (mean f 1 SD) C B 1 I Time - Fig. 3 Absorbance of zinc in diluted milk with and without deuterium background correction. A background; B corrected absorbance; and C uncorrected absorbance650 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL. 6 in milk both with and without deuterium background correction (Fig. 3). The slopes of the calibration graphs were similar in diluted milk (54 with deuterium background correction and 55 without n= 3). Furthermore no differ- ence was found in precision or accuracy.The mean recovery of the standard additions measured in triplicate was 10 1 % with deuterium background correction and 100% without. The mean peak heights were found to be 96f6 mm (n=30) with deuterium background correction and 97.5 k 5 mm (n=30) without. Zinc concentrations were slightly lower with deuterium background correction (171.5+ 10.3 versus 174.5k8.7 pmol l-l n=30). This difference was not statistically significant. However the within-run precision was slightly improved without deuter- ium background correction ( 5 versus 6% n= 30). Conse- quently it was decided not to use the deuterium back- ground correction. The relatively high dilution used and the relatively low ion and low protein concentrations in milk probably explain the reduced interferences.Comparison of Standard (Non-coated) and Pyrolytic Graph- ite Coated Graphite Tubes Pyrolytic graphite coated graphite tubes for use in the furnace have a lower permeability to gases a lower porosity and a higher resistance to oxidation than standard graphite tubes. As a result pyrolytic graphite coated graphite tubes have extended lifetimes. However they are twice as expensive as standard graphite tubes. The detection limit was improved when using the standard graphite tubes (52 nmol 1-l) rather than the pyrolytic graphite coated graphite tubes (88 nmol 1-l). These results were different from those found by Sturgeon and Chakraba~=ti.~~ The slopes of the calibration graphs prepared in 0.1% Triton X-100 and in diluted milk were enhanced when using the standard graphite tubes [0.1% Triton X-100 5 1 & 2 versus 46 f 3 (n= 10); diluted milk 5 1 f 3 versus 49 + 3 (n= 30)].These results were in agreement with those found by Foote and Delves.26 No difference was observed in this work for the accuracy and precision. The recoveries of standard additions were 100 f 4% with a standard graphite tube and 107 + 5Oh with a pyrolytic graphite coated graphite 25c r L - K 3 .- 2 225 e N L 9 E k z # E 75 i2 - B 1 150 0 K 0 I I 1 75 150 225 250 Results from pyrolytic graphite coated graphite tube for zindpmol I-’ Fig. 4 Influence of the type of graphite tube on zinc concentra- tion. Regression line solid line; and line of identity broken line. The cross in the centre indicates the mean+ 1SD for n=75 milk samples tube (n=30 replicate determinations of the same milk sample).The within-run precision was slightly better with a standard graphite tube (3 versus 5% n= 30) which is in agreement with the previous The zinc concentrations were determined in 75 different milk samples using both the standard and the pyrolytic graphite coated graphite tubes. The results obtained (Fig. 4) showed good agreement between the two types of graphite tubes. A mean zinc concentration of 170 pmol 1-* with a standard deviation of 80 pmol l-l (range 46-283 pmoll-*) were obtained using the standard graphite tubes whereas with the pyrolytic graphite coated graphite tubes the mean zinc concentration was 169 k 75 pmol 1-l (range 46-296 pmol 1-l). The correlation coefficient (r) was found to be 0.90. The linear regression line had a slope of 0.96 and an intercept of 7 pmol 1-l.Nevertheless matrix interferences differed with the nature of the graphite tubes used for the analysis. The standard additions recoveries were found to be 97 k 11% (n=85) with the standard graphite tubes and 97 f 12% (n=85) with the pyrolytic graphite coated graph- ite tubes; but no correlation was found between the recoveries obtained (r=0.52). These results were in agree- ment with those found by Foote and Delves.26 According to these results standard graphite tubes were considered to be suitable for the determination of zinc in human milk. Calibration Zinc was determined in 85 milk samples by using either an external calibration procedure (standard solutions made up in 0.1% Triton X-100) or by the method of standard additions.The results presented in Fig. 5 show good agreement between the two calibration processes. Zinc concentrations were not statistically significantly different. With the external calibration the mean zinc concentration was 149f64 pmol 1-l (range 48-274 pmol I-l) whereas with the method of standard additions the mean zinc concentration was 159 f 74 pmol 1-l (range 46-289 pmol 1-I). The correlation coefficient was found to be 0.90. The linear regression line slope was 1.02 and the intercept 6 pmol l-l. Nevertheless the recoveries of standard additions varied from 8 1 to 120% (Fig. 6). Considerable variability in mineral content of human milk has been reported2’ and might explain these results. 2 0 75 150 225 250 [Zinc] by external calibration/pmol I-’ Fig. 5 Influence of the type of calibration procedure on the zinc concentration obtained from milk.Regression line solid line; and line of identity broken line. The cross in the centre indicates the mean f 1SD of n= 85 milk samplesJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL. 6 65 1 ~~ ~ Table 5 Zinc concentrations (in pmol 1-l) in human milk samples in the first days of lactation Data source Days post-partum 1 122k87 Ref. 28 n=7 Ref. 23 n= 13 Ref. 35 n= 10 This work n=6 131 k86 119+20 128k34 2 n=8 177+72 n=24 168+69 - - n=27 167k61 3 n=9 144+30 n-34 133+41 n= 10 69 + 20 n=46 134+51 4 n=9 98+ 10 n= 33 99 f 23 n = l l 88 f 26 - - 5 n=8 83k 17 n=33 82k 18 n= 10 60k 19 n=25 83 k 29 6 7 8 - n= 10 - - 73_+ 16 - - - - - - - - n= 10 - - 51 + 19 n=41 n=44 n=3 75k25 74k17 64+17 - 80 90 100110120 Recovery (%I Fig.6 Distribution of recoveries of standard additions in 85 milk samples The external calibration procedure was retained for practical reasons. This choice was in agreement with previous studies.1a~2a Precision Accuracy and Linear Range The electrothermal atomic absorption spectrometric method selected used standard (uncoated) graphite tubes and no background correction. The diluent was a 0. l0h m/v Triton X-100 solution and the dilution rate was 1 + 99. The working standard solutions were prepared in the same diluent and contained zinc concentrations of 0.375 0.75 and 1.50 pmoll-l. The precision expressed as the RSD for 30 analyses of different dilutions of the same sample was 4.6%.The accuracy was evaluated by the recovery of standard addi- tions. By using 85 different milk samples recoveries of different amounts of zinc which had been added to the milk were 97 2 10%. The accuracy was also determined by analysing National Institute of Standards and Technology Standard Reference Material 1549 Non-Fat Milk Powder. The mean value was found to be 7 13 k 35 pmol g-l (n= 38) (certified value 705 f 34 pmol g-l). The detection limit was calculated according to the criteria X+3SD where X was the mean of 30 replicate determinations at the blank level (0.1% Triton X-1 00) and SD the corresponding standard deviation. The detection limit was 52 nmol 1-l. The linear limit determined over a period of 3 d was up to 3 pmol l-I. Normal Values Zinc was determined in 203 milk samples collected in the first week post-partum.The major factor influencing zinc concentration in human milk is the stage of lactation. Factors such as age parity and anthropometric measure- ment do not influence the zinc con~entration.~~-~~ Zinc concentrations in the milk of women living in urban or rural areas are ~ i m i l a r . j ~ - ~ ~ The results are presented in Table 5. There was an increase in the concentration of zinc from day 1 to day 2. After day 2 the zinc concentration in milk declined. This instability of the zinc concentration during the first post-partum week has been reported by other workers,23*28*35 and reflects the rapidly changing physiological state of the mammary gland. The results given here are similar to those reported by Casey et af.23*28 but the zinc concentrations found are higher than those reported by Hibberd e? al.35 Differences amongst individual women were substantial.These results were also in agreement with those previously r e p ~ r t e d . ~ ~ * ~ ~ y ~ ~ Conclusion The validity of the proposed method was more than adequate for clinical and biochemical investigations. Zinc concentrations at early stages of lactation were in agree- ment with those reported by Casey et a1.,23*28 although the methods of analysis used were different. Casey e? al.23*28 used a flame atomic absorption spectrometric method and prior to analysis the milk samples were ashed in a low- temperature asher. The proposed method presents the advantage of only performing a simple dilution of the milk sample before injection into the graphite furnace. The detection limit was adequate for the determination of zinc in milk with a low zinc content.This direct method allowed rapid determinations. Problems of contamination were reduced by the use of only a single reagent before injection into the graphite furnace. Numerous determinations could be performed on the same day. References 1 Friel J. K. Gibson R. S. Balassa R. and Watts J. L. Acta Paediatr. Scand. 1984 73 596. 2 Pleban P. A. Numerof B. S. and Wirth F. H. in Clinics in Endocrinology and Metabolism ed. Taylor A. vol. 14 WB Saunders London 1985 p. 545. 3 Mendelson R. A. Bryan M. H. and Anderson G. H. J. Pediatr. Gastroenterol. Nutr. 1983 2 256. 4 Ziegler E. E. Am. J. Clin. Nutr. 1985 41 440. 5 Aggett P.J. Atherton D. J. More J. Davey J. Delves H. T. and Harries J. T. Arch. Dis. Child. 1980 55 547. 6 Ahmed S. and Blair A. W. Arch. Dis. Child. 1981 56 31 5. 7 Blom I. Jameson S. Krook F. Larsson-Stymme B. and Wraume L Br. J. Dermatol. 1981 104 459. 8 Connors T. J. Czamecki D. B. and Haskett M. I. Arch. Dermatol. 1983 119 319. 9 Courtney Moore M. E. Moran J. R. and Greene H. L. J. Pediatr. (St. Louis) 1984,105 600.652 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL. 6 10 Murphy J. F. Gray 0. P. Rendall J. R. and Hann S. Early Hum. D o . 1985 10 303. 11 Parker P. H. Helinek G. L. Meneely R. L. Stroop S. Ghishan F. Y. and Greene H. L. Am. J. Dis. Child. 1982 136 77. 12 Weymouth R. D. Kelly R. and Landsdell B. J. Aust. Paediatr. J. 1982 18 208.13 Zimmerman A. W. Hambidge K. M. Lepow M. L. Greenberg R. D. Stover M. L. and Casey C. E. Pediatrics 1982,69 176. 14 Bye A. M. Goodfellow A. and Atherton D. J. Pediutr. Dermatol. 1985 2 308. 15 Kuramoto Y. Igarashi Y. Kato S. and Tagami H. Acta Dermatol. Venereol. 1986 66 359. 16 Roberts L. J. Shadwick C. F. and Bergstresser P. R. J. Am. Acad. Dermatol. 1987 16 301. 17 Saner G. and Yuzbasiyan V. Nutr. Rep. Int. 1984,29 1181. 18 Moran J. R. Vaughan R. Stoop S. Coy S. Johnston H. and Greene H. L. J. Pediatr. Gastroenterol. Nutr. 1983 2 629. 19 Arpadjan S. and Nakova D. Nahrung 1981 25 359. 20 Blakeborough P. Salter D. N. and GUK M. I. Biochem. J. 1983,209 505. 21 Cousins R. J. and Smith K. T. Am. J. Clin. Nutr. 1980,33 1083. 22 Lonnerdal B. Am. J. Clin. Nutr. 1985 42 1299.23 Casey C. E. Hambidge K. M. and Neville M. C. in Truce Elements in Man and Animals eds. Mills C. F. Bremner I. and Chesters J. K. vol. 5 CAB 1985 p. 633. 24 Costantini S. Macri A. and Vernillo I. Riv. Suc. Ital. Sci. Aliment. 1981 10 231. 25 Sturgeon R. E. and Chakrabarti C. L. Anal. Chem. 1977,49 90. 26 Foote J. W. and Delves H. T. Analyst 1982 107 1229. 27 Atkinson S. A. Radde I. C. Chance G. W. Bryan M. H. and Anderson G. H. Early Hum. Dev. 1980 4 5 . 28 Casey C. E. Hambidge K. M. and Neville M. C. Am. J. Clin. Nutr. 1985 41 1193. 29 Feeley R. M. Eitenmiller R. R. Benton Jones J. and Barnhart H. Am. J. Clin. Nutr. 1983 37 443. 30 Higashi A. Ikeda T. Uehara I. and Matsuda I. Tohoku J. Exp. Med. 1982 137 41. 31 Picciano M. F. Calkins E. J. Gamck J. R. and Deering R. H. Acta Paediatr. Scand. 1981 70 189. 32 Rajalakshmi K. and Srikantia S. G. Am. J. Clin. Nutr. 1980 33 664. 33 Clemente G. F. Ingrao G. and Santaroni G. P. Sci. Total Environ. 1982 24 255. 34 Coni E. Falconieri P. Ferrante E. Semeraro P. Beccaloni E. Stacchini A. and Caroli S. Ann. 1st. Super. Sunita 1990 26 119. 35 Hibberd C. M. Brooke 0. G. Carter N. D. Haug M. and Haner G. Arch. Dis. Child. 1982 57 658. Paper 1 /O I2 781 Received March I8th 1991 Accepted August 16th I991

ISSN:0267-9477    

DOI:10.1039/JA9910600647

出版商:RSC    

年代:1991

数据来源: RSC