Flow Injection On-line Reductive Precipitation Preconcentration With Magnetic Collection for Electrothermal Atomic Absorption Spectrometry S. SELLAb , R. E. STURGEON* a , S. N. WILLIEa AND R. C. CAMPOSc aInstitute for NationalMeasurement Standards, National Research Council of Canada, Ottawa, Ontario, Canada K1A 0R9 bDepartment of Analytical Chemistry, Universidade Federal Fluminense, Niteroi/RJ, Brazil cDepartimento De Quimica da PUC/Rio, 22453 Rio de Janeiro, Brazil A flow injection on-line preconcentration system coupled to an forming elements.15 The presence of palladium, used as a coprecipitant, is also attractive in that it serves as a matrix electrothermal atomic absorption spectrometer for the determination of trace metals in saline media is described.The modifier for subsequent ETAAS determinations of the more volatile analytes. Detection limits in the ng l-1 range have filterless, magnetic collection of coprecipitated analytes is based on the tetrahydroborate reductive precipitation of added been reported following the processing of large volumes of sample (i.e., 900 ml), suYcient for the determination of elements iron and palladium in alkaline medium at a sample flow rate of 1.7 ml min-1.The precipitate is dissolved in a 20 ml volume of interest in uncontaminated seawater. It is desirable to implement this procedure on-line using FI technology, thereby of mixed acid and transported direct to the graphite furnace.With the exception of Cr (33% recovery in seawater), reducing sample and reagent consumption while enhancing throughput. FI manifolds for collection of inorganic precipi- recoveries of Ag, As, Bi, Cd, Co, Cu, Mn, Ni, Pb, Sb and Tl averaged 84% from deionized water and 67% from coastal tates invariably utilize some form of in-line filter, often a stainless steel HPLC screen18 or membrane filter.19 To date, seawater with a frequency of 12 measurements h-1. The sensitivity of the graphite furnace technique can be enhanced filterless on-line coprecipitation systems have exclusively relied on the use of knotted reactors and organic precipitants.To over 400-fold (for an 11 ml sample volume) compared to a standard 20 ml injection volume. Detection limits in the low pg the best of our knowledge, this is the first report on the implementation of an on-line filterless system based on the (absolute) or pg ml-1 (relative) range can be reached. Results, presented for the determination of As, Cd, Cr, Cu, Mn and Ni collection of an inorganic precipitate. in 1 ml volumes of CASS-3 NRCC Certified Reference Material seawater, are not statistically diVerent (t-test, 95% EXPERIMENTAL confidence limit) from the certified values for these elements.Apparatus Keywords: Flow injection preconcentration; magnetic collection; reductive precipitation; seawater; electrothermal A Perkin-Elmer (Norwalk, USA) Model 5000 atomic absorpatomic absorption spectrometry tion spectrometer fitted with continuum source background correction, an HGA 500 furnace and an AS 60 autosampler was used in combination with a Perkin Elmer Model FIASFlow injection (FI) approaches for electrothermal atomic 400 flow injection unit (equipped with 2 software controlled absorption (ETAAS) have traditionally been more diYcult to pumps and an injection valve).Perkin-Elmer hollow cathode implement than those associated with continuous sample intro- or electrodeless discharge lamps (As, Se, Sb) were used as line duction atomic spectrometric techniques.The advent of com- sources under the manufacturers’ recommended conditions of mercialized dedicated hardware and software, however, has current and power. The FIAS manifold is schematically illusserved to alleviate this problem and most applications of trated in Fig. 1. Teflon separatory funnels served as reagent FI–ETAAS are associated with on-line trace element precon- and sample reservoirs.The unit was also fitted with a 12 V 3- centration.1,2 Pursuit of this objective is attractive, as significant enhancement factors can be realized at sample throughputs approaching that of conventional ETAAS procedures. Notable examples make use of solution phase chelation with extraction systems based on adsorption onto reversed-phase C18 substrates3 –5 and collection in knotted reactors,6,7 sequestration with immobilized chelating agents8,9 and coprecipitation (organic carrier) with either collection on a membrane filter10 or filterless collection in a knotted reactor.11,12 Precipitation is one of the more commonly used techniques for the enrichment of inorganic analytes,13 trace preconcentration generally being accomplished by co-precipitation of an added carrier, since direct precipitation often yields amounts of precipitate too small to conveniently recover.Both inorganic and organic carriers have been utilized for this purpose in oV- line techniques.14–16 Particularly attractive has been the relatively non-selective multi-element preconcentration technique based on tetrahydroborate reductive precipitation.15,17 This Fig. 1 FIAS manifold. P1 and P2 are pumps; S is a solenoid valve; methodology permits a wide range of analytes to be collected, V is the FIAS valve; W, waste. Note: as illustrated, the manifold is in the sample elution stage (see Table 1, step 5). including transition and noble metals as well as hydride Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 (1281–1285) 1281way solenoid valve (Cole Parmer, Vernon Hills, IL, USA) (Johnson Matthey).National Research Council of Canada Certified Reference Material Coastal Seawater CASS-3 (salinity having zero dead volume with all wetted parts made of PTFE. This valve which, in a normally open configuration, was 30.2‰) was used for method evaluation. plumbed to pass a flow of air, was briefly activated in step 3 of the FIAS program (Table 1), using a remote switch on the Procedure rear of the FIAS-400, to permit uptake of an approximately 60 ml volume of distilled deionized water (DDW).Initial Although all sample and reagent preparation was undertaken in a class-100 clean room environment, all FIAS manipulations optimization of experimental parameters was done by maximizing analyte recovery. Multielement response was achieved by were subsequently accomplished in a regular chemical laboratory.A schematic of the FIAS manifold is shown in Fig. 1 and analysing processed test solutions using a Perkin-Elmer SCIEX ELAN 5000 inductively coupled plasma–mass spectrometer the software controlled sequence of operations is summarized in Table 1. Note that the position of the sampling valve in the (P.-E. Sciex, Thornhill, Canada). Tygon peristaltic pump tubing (Cole Parmer) propelled all reagents (black–black 0.76 mm id manifold illustrated corresponds to the sample elution stage, not loading. Time-based processing of the sample was for sodium borohydride and air, yellow–blue 1.52 mm id for sample and ammonium hydroxide, orange–green 0.38 mm id implemented.Samples were prepared to contain 30 mg ml-1 each of added PdII and FeIII as coprecipitation reagents. During for elution acid). One to 2 m lengths of silicone (Cole Parmer, 1.27 mm id), microbore tygon (Cole Parmer, 1.27 mm id) and the priming step (P, in Table 1), the sample and reagent uptake pump, P1, is on so as to fill the lines with the (next) sample.Teflon (Cole Parmer, 1.27 mm id) tubing were all examined in an eVort to determine the best material for use as a collection In step 1 the FIAS valve is switched (precipitation–collection stage), sample (1.7 ml min-1), ammonium hydroxide (diluted coil. All lines were interconnected using 1/4–28 Tefzel flangeless nuts and ferrules (UpChurch Scientific, Concord, Ontario, to 0.09 M, 1.7 ml min-1) and sodium tetrahydroborate (2% m/v, 0.5 ml min-1) solutions delivered via pump 1 are merged Canada).A 45 cm length of 1 mm id Teflon transfer line separated the collection coil from the last reagent–sample prior to the collection coil in which the precipitate is subsequently collected. One ml sample volumes were processed merge point. The filterless collection coil was wrapped around a (5.7×1.4×1.5 cm) rare earth cobalt ceramic magnet with the program presented in Table 1.During step 2, pump 2 is activated and air (0.25 ml min-1) is used to drive any (Edmund Scientific, Toronto, Canada) of 8.58 kG field strength. This unit was placed in the cavity of a heated aluminium liquid remaining in the collection coil line to waste while at the same time the dissolution acid (2 M HCl and 2 M HNO3) block. The block was maintained at a pre-set temperature of 98 °C using a VICI temperature controller (Model ITCK fills a 20 ml loop. During step 3, a remote switch at the rear of the FIAS unit activates the solenoid for a brief time period 10399; Valco Instrument, Houston, Texas, USA) connected to a 75 W immersion heater placed in a recess in the wall of (2 s), permitting uptake of approximately 60 ml DDW.This plug of DDW is propelled by a flow of air during step 4, the block. thereby washing any residual salt matrix from within the transfer and collection lines with the air serving to leave the Reagents system as free as possible of any moisture that may dilute the minimum volume of acid used in the subsequent dissolution All reagents were purified prior to use.Deionized, distilled step. The position of the valve then changes from the fill mode water (DDW) was produced with a commercial mixed-bed to inject mode in step 5 and 20 ml of acid is driven by air NanoPure ion exchange system (Barnstead/Thermolyne, (0.25 ml min-1) to the collection coil. Both pumps are briefly Boston, MA, USA) fed with distilled water.Concentrated stopped (20 s) during step 6 when the major portion of the HNO3 and HCl were prepared by sub-boiling distillation of precipitate is submerged in acid in order to permit a more feedstocks. A saturated ammonia solution (Environmental complete dissolution of the precipitate. The exit end of the Grade, 28% v/v) was obtained from Anachemia Science collection coil was fixed to the arm of the AS 60 autosampler (Montreal, Quebec, Canada). Stock solutions (1000 mg l-1) of such that in step 7 (elution stage) the eluent is directed into all analytes were obtained by dissolution of high-purity metals the graphite furnace by manually turning the sampling arm or their salts (Spex Industries, Edison, NJ, USA) and working into its injection position and holding it there until transfer of standards were prepared by serial dilution of the stocks with the acid was complete. The entire sequence requires approxi- DDW containing 1% HNO3.Solutions of various concenmately 5 min for a throughput of 12 measurements h-1.This trations of sodium tetrahydroborate (Alfa Aesar, Johnson is fully compatible with the operation of the graphite furnace. Matthey, Ward Hill, MA, USA) were prepared immediately Following dissolution, the furnace program was manually prior to use by dissolution of the material in DDW and initiated and the absorbance transient recorded using in-house stabilized by the addition of sodium hydroxide (0.5% m/v for software which calculated both peak height and area.The 2% NaBH4 concentration). Stabilization was particularly presence of palladium in the furnace served as a matrix important at high concentrations of tetrahydroborate as repromodifier, permitting sample pyrolysis temperatures to be set ducible addition of this reagent to the flowing stream could from 900 (Cd) to 1200 °C for all elements and atomization only be made if bubbles were excluded from the line.Stock with maximum power heating from 1600 (Cd) to 2600 °C. solutions (10 000 mg l-1) of FeIII and PdII were obtained by Quantitation of analyte recovery was implemented by cali- acidic dissolution of the 99.997% Puratronic grade metal wires bration, using integrated absorbance, against a matrix matched standard manually injected into the furnace and subjected to Table 1 FIAS program an identical atomization program. Optimization of such experimental parameters as reagent Step Time P1 P2 Valve Remote concentrations, tubing type and sample pH was achieved by P 30 50 0 Inject OFF processing 1 ml volumes of DDW spiked to contain 1 ng ml-1 1 35 20 0 Fill OFF of all elements of interest.Following elution with acid, the 2 10 0 30 Fill OFF 3 2 0 30 Fill ON solution was diluted to 1.0 ml with DDW and analysed by 4 99 0 30 Fill OFF ICP-MS. Blanks were prepared using DDW in a manner 5 40 0 30 Inject OFF identical to that of the samples. 6 20 0 0 Inject OFF Final optimization of sample and reagent flow rates was 7 99 0 30 Inject OFF done using 1 ml volumes of DDW spiked to contain 1 ng ml-1 1282 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12Table 2 Experimental variables Solution Flow rate/ml min-1 Volume/ml Concentration Tube type Sample 1.7 1.0 — Yellow–Blue NH4OH 1.7 1.0 0.09 M Yellow–Blue NaBH4 0.5 0.3 2% (m/v) Black–Black Acid 0.25 20 ml 2M HCl; 2 M HNO3 Orange–Green DDW 0.25 60 ml — Black–Black Air 0.25 — — Black–Black Cd as the test element followed by analysis by ETAAS.Appropriate blanks were carried through all procedures. Table 2 summarizes all experimental variables relating to solution flow rates, volumes and pump tubing used. NRCC reference material CASS-3 (Coastal Seawater) was analysed by the method of additions using on-line processing of 1 ml volumes of sample and spiked samples. The relatively high concentration of Mn in this material (2.5 ng ml-1) necessitated use of an internal purge gas flow during atomization in order to reduce response to maintain linearity.RESULTS AND DISCUSSION Implementation of the reductive precipitation methodology in Fig. 2 EVect of total mass of FeIII and PdII carrier added to a DDW an on-line FI format proved to be a non-trivial exercise in sample (in equal amounts) on recovery of: A, MnII and B, BiIII present downsizing of the oV-line process. EVorts to scale the procedure at 1 ng ml-1. down by the sample volume ratio (i.e., 900) proved totally inappropriate.A minimum amount of carrier was necessary to eVect recovery. In part, this may arise as a result of the nature ensure any collection of analyte. All variables were thus of the collection technique in that a critical mass of material systematically optimized to achieve maximum analyte recovery is necessary for either sedimentation or magnetic attraction from DDW solutions and then individual parameters were re- against the force of the flowing stream; as well, because no evaluated to determine their overall impact on response.time was permitted for extensive crystal digestion or aging, the Optimization of experimental parameters was done in an eVort initial size of particles is more critical and this is predominantly to achieve the best compromise amongst the various factors influenced by higher solution concentrations of the aVecting throughput (flow rates), element recoveries (pH, acid coprecipitants.composition) and blank (Fe and Pd concentrations) so as to Fig. 3 illustrates the eVect of tetrahydroborate concentration achieve rewarding limits of detection. on the recovery of added CuII. A 2% (m/v) solution was selected for use, based on the lack of further signal enhancement caused by higher concentrations of this reagent. Accounting Parametric optimization for the relative flow rates of sample and reductant (Table 2), Reductive precipitation oV-line is performed at pH 8–9 follow- this solution concentration of BH4- is 10-fold greater than the ing the addition of Fe and Pd coprecipitants.Hydrolysis of oV-line condition, again reflecting the need for higher reagent the iron occurs, creating a colloidal precipitate of large surface concentrations in a kinetically controlled environment. area capable of adsorbing or scavenging trace elements from Collection coils, consisting of 2 m lengths of either Teflon, solution. Addition of the tetrahydroborate reductant serves to microbore tygon or silicone tubing (all of the same 1.27 mm reduce these metals and the presence of Pd inhibits loss of id) were wrapped around the collection magnet and evaluated volatile hydride forming elements such as As, Se and Sb.15 for multielement recovery eYciency.Best performance was Although precipitation can also occur eYciently at pH 6.5, it obtained with the use of silicone tubing. Although silicone was does so more slowly.Since the kinetics of events in a flowing only marginally better than tygon (14% on average), it was system are of significance, a pH of 8.5 was therefore selected. rejected in favour of the latter as the blanks were significantly The pH was conveniently adjusted on-line by merging the higher with this material. The collection eYciency with Teflon sample stream with that of a 0.09 M solution of NH4OH (see tubing was only 60% of that of microbore tygon. Performance Fig. 1). OV-line pH adjustment was not feasible since precipitation reactions resulted in significant losses of material in the sample reservoir itself. With equal flow rates of both sample and base, a 151 dilution of the sample was accomplished online. This had the advantage of reducing the back pressure in the system which arose due to the concurrent precipitation of large amounts of magnesium and calcium hydroxides present when seawater was processed. Fig. 2 illustrates the eVect of the mass of PdII and FeIII added to the sample.Optimum concentrations, varied in unison, were found to be 30 mg ml-1 of each carrier. Manganese and bismuth were selected as representative elements for illustrative purposes since their characteristics are clearly diVerent. Excessive amounts of coprecipitants were avoided in an eVort to both minimize the blank and the mass of material introduced into the furnace. In comparison with the oV-line procedure,15 Fig. 3 EVect of sodium tetrahydroborate concentration on recovery of 1 ng ml-1 CuII from 1.0 ml volumes of seawater.considerably higher concentrations of carriers are required to Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 1283Table 3 Recovery of trace elements from deionized distilled water may be related to the nature of the surface roughness or and seawater ‘stickiness’ of the material, but this is entirely unclear at the moment and beyond the scope of this study.Using microbore Recovery (%)* tygon, the minimum length of tubing comprising the collection Element DDW CASS-3 coil was determined to be 1 m; use of a 0.5 m length resulted in a 55% relative loss of recovery. A 45 cm Teflon transfer Ag 89±2 73±2 AsV 71±1 60±1 line, inserted between the final merge point and the collection Bi 79±2 58±1 coil, permitted some time for the development of the precipi- Cd 86±1 87±1 tation process (while minimizing any losses to the tube wall) CrVI 55±4 33±3 prior to collection. Chemical processing on-line may be kin- Co 89±3 85±5 etically limited if relatively slow reactions are involved.In Cu 92±4 54±3 addition, collection eYciency, whether by magnetic assistance Mn 90±5 67±8 Ni 100±5 83±4 or in a knotted reactor, will be influenced by the hydrodyn- Pb 79±6 65±3 amics of the flowing stream. In the present case, multielement SbV 74±3 57±2 recovery was degraded if sample flow rates in excess of Tl 78±4 52±2 1.8 ml min-1 were used (for a total solution flow rate of sample and ammonium hydroxide of 3.6 ml min-1).At 2.0 ml min-1 * Mean and standard deviation (n=10); 1 ml sample volumes prorecovery decreased by 20% and at 3 ml min-1 by 60%. cessed. All analytes spiked to contain 1 ng ml-1. Although the system can be operated at elevated flow rates in the interest of enhancing sample throughput, the degradation of sensitivity is significant. has been deduced in an earlier study.15 Losses of trace elements from samples of seawater may occur by occlusion in precipi- The relative collection eYciency of a 1 m length of microline tygon tubing was investigated when used in a variety of tated calcium and magnesium hydroxides,21 thereby reducing the eYciency of the system for this sample matrix.In all cases, configurations: in the form of a coil, as a coil wound around the magnet and when configured as a knotted reactor (with recoveries of analytes from seawater are lower with this manifold than that obtained oV-line using a filter for collec- no magnet present).Although knotted reactors have been widely utilized in FI manifolds for the collection of organic tion15 but, with the exception of Cr, never by more than 2- fold. This is remarkable in that samples are processed within based precipitates,2,11 that used in this study exhibited only a 60% collection eYciency relative to a coil wound around a minutes of the precipitation event in the FI manifold, whereas 15 h is typical for precipitate aging before collection in oV-line magnet (magnetic assisted collection) for the elements studied.Magnetic assisted recovery of the precipitate enhanced procedures. Additionally, the highly reproducible nature of the manipulations available with the FI manifold has resulted in eYciency 3-fold over that obtained with the coil alone. In this respect, advantage was taken of the high magnetic permeability good precision of recovery, similar to that which can be obtained oV-line.15 Although not shown in Table 3, recoveries of iron, added as a coprecipitant. Towler et al.20 utilized magnetite, added to seawater samples, to enhance the magnetic of the FeIII and PdII coprecipitants, determined by flame-AAS, were greater than 95% in all cases.recovery of several elements adsorbed onto a manganese dioxide carrier. Use of Co and Ni (in place of iron) as Table 4 summarizes the absolute and relative methodological blanks determined by processing 1.0 ml volumes of DDW coprecipitants for this purpose may also prove attractive as they also have relatively high magnetic permeabilities.through the procedure. Absolute blanks are all in the low pg regime and range from 10–1000-fold lower than their oV-line One of the diYculties experienced with the use of reductive precipitation oV-line is the subsequent acidic dissolution of the counterparts.15 This is a consequence of the closed system used and the significant reduction in reagent volumes.Relative precipitate, this process demanding use of concentrated nitric and hydrochloric acid mixtures.15 Direct transfer of such blanks (ng ml-1), are typically only 10-fold worse than their oV-line counterparts,15 despite the 900-fold smaller sample concentrated acids to the graphite furnace would result in rapid oxidation of the tube and severely shorten its lifetime. volume processed. There is thus an overall enhancement in performance with FI.The major source of the blanks proved Thus, dilute acid mixtures were required and this necessitated raising the reaction temperature to enhance the kinetics of to be contamination from the added PdII in the case of Ag, Pb and Sb, and contamination from the added FeIII for dissolution. A study of various combinations of HCl and HNO3 (in the ratios: 6 M52 M; 3 M52 M; 2 M52 M; 1 M52 M; Mn. No single dominant source of contamination could be identified for the remaining elements. 0 M52 M) revealed that a 20 ml volume of a 2 M52 M mixture of HCl5HNO3 at a temperature of 98 °C was as eVective as the Estimated absolute limits of detection (LODs) based on a 3s(blank) criterion are also summarized in Table 4. Con- more concentrated acid mixtures. A ratio of 1 M52 M produced a precipitous drop in recovery for all elements, suggesting that centration LODs (based on a 1 ml sample volume) are comparable to their oV-line counterparts,15 despite the fact that sample Cl- is a key element in the solubilization process.Operation of the system at elevated temperatures for extended periods of volumes processed are 900-fold smaller; in the worst cases, these data are degraded by factors of 20 (for Cr), 10 (for Pb), time did not visibly deteriorate the collection coil. The system was cycled several hundred times without degradation. 6 (for Ag and Ni) and 5 (for Co and Sb). Detection limits for As, Bi, Co, Mn and Sb are likely all limited by instrument noise. However, it is important that the magnet be isolated from the tygon coil by a polyethylene sheet in order to minimize Throughput is approximately 12 h-1, which is fully compatible with the operation of the graphite furnace.However, since corrosion by acidic vapours diVusing through the tygon line. the method of additions is required, actual sample throughput is somewhat less, typically 9 h-1 when a blank and two spikes Figures of Merit are processed.Linear calibration curves could be constructed for all Table 3 summarizes the average recoveries of analytes from both spiked DDW and coastal seawater (CASS-3) matrices. elements by varying the volume processed at constant concentration and also by varying the concentration at constant Typically,15 recoveries are somewhat higher from DDW and reflect the need for analysis of samples by the method of volume. Linearity was limited by the upper range of instrument response in that curvature in the absorbance–mass relationship additions.No attempt was made to investigate the influence of oxidation state of elements such as As, Sb and Cr, as this set in. In a similar manner, a linear relationship was observed 1284 Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12Table 4 Absolute and relative methodological blanks and absolute limit of detection* Blank Element Relative/ng ml-1 Absolute/pg Absolute detection limit†/pg Ag 0.012±0.001 12±1 4.2 As 0.031±0.007 32±7 21 Bi 0.029±0.004 29.0±0.5 1.5 Cd 0.012±0.001 12.0±0.9 2.8 Cr 0.071±0.007 70±7 21 Co 0.017±0.007 17±7 21 Cu 0.020±0.005 20±5 15 Mn 0.002±0.001 2±1 3 Ni 0.07±0.01 65±13 38 Pb 0.11±0.01 110±11 32 Sb 0.030±0.007 30±7 22 * Mean and standard deviation (n=10); 1 ml sample volume.† Defined as the mass of analyte which gives a response equivalent to 3 times the standard deviation of the methodological blank, based on the processing of a 1 ml sample volume.Table 5 Analysis of CASS-3 Coastal Seawater reference material achieved. Complete automation of the system (including control of the autosampler arm during acid elution into the Concentration/ng ml-1 furnace) is feasible with current FIFU software technology (Perkin-Elmer). The methodology should also prove useful for Element Determined* Certified value† the preparation of samples for analysis by other atomic spectro- As 1.06±0.02 (±0.05) 1.09±0.07 metric means, including ICP-AES and ICP-MS.For the latter, Cd 0.032±0.003 (±0.007) 0.030±0.005 avoidance of hydrochloric acid for minimization of isobaric Cr 0.087±0.013 (±0.032) 0.092±0.006 Cu 0.48±0.04 (±0.10) 0.517±0.062 interferences may be necessary. Mn 2.7±0.3 (±0.7) 2.51±0.36 Ni 0.37±0.02 (±0.05) 0.386±0.062 The authors thank Perkin-Elmer Bodenseewerk for loan of the FIAS-400 used in this study. Dr. J. Lam of this laboratory * Mean and standard deviation (n=3); 1 ml sample volume.Values is thanked for his invaluable help in the ICP-MS analysis of in parentheses are corresponding 95% confidence limits. samples as is Dr. N. Panichev for helpful and stimulating † Precision expressed as 95% confidence limits. discussion. S.S. thanks the CNPq of Brazil for a fellowship which made this study possible. between the blank response and the volume of sample processed, as expected. Although sample throughput is degraded, REFERENCES it is possible to process larger sample volumes without diY- 1 Fang, Z., Spectrochim.Acta Rev., 1991, 14, 235. culty. Experiments to determine the upper limit to the volume 2 Fang, Z., Xu, S., and Tao, G., J. Anal. At. Spectrom., 1996, 11, 1. of sample which could be processed were constrained by the 3 Fang, Z., Sperling, M., and Welz, B., J. Anal. At. Spectrom., 1990, maximum pumping time available with the current FIAS 5, 639. software. 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A FI manifold for reductive precipitation concentration of 21 Lin, C.-R., Analyst, 1993, 118, 189. samples has been successfully implemented based on the filterless magnetically assisted collection of precipitate. Despite Paper 7/02608K the substantial decrease in sample volume processed (900- Received April 16, 1997 Accepted July 22, 1997 fold), performance comparable to oV-line methodology can be Journal of Analytical Atomic Spectrometry, November 1997, Vol. 12 12