ISSN:0003-2654    

DOI:10.1039/AN99217FX001

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

2 ANALYST, JANUARY 1992. VOL. 117 Editorial International Communications The dissemination of information amongst the members of the analytical chemistry community, as is no doubt the case with other communities of researchers and scholars, makes use of the international conference as a key mechanism. Such conferences operate on several levels. They enable the delegates to hear and see formal and/or semi-formal presenta- tions by the researchers who have elected to submit material for inclusion in the conference programme. They allow informal interaction between delegates when detailed discus- sion, which cannot be accommodated in the limited time available immediately after a formal presentation, may ensue. By these means it is possible for both the experienced practitioner in the field and a potential newcomer to obtain information on the state of current research activity.It is possible that by a judicious choice of invited speakers, the representatives of the host country are able to hear, first hand, of the work conducted by the leading figures in the field from other nations, it being assumed that a country with a sufficiently developed scientific infrastructure will already be holding meetings at which its own leading researchers gain national exposure. The benefits of such encounters, particu- larly for young researchers, can be considerable; organizers of such events are encouraged to facilitate the attendance of the younger generation by suitable financial incentives, such as student bursaries. As more nations seek to develop their science base, it is likely that the number of such conferences will increase, especially if the occasion is also used for the running of short courses, specialized satellite symposia and instrument exhibi- tions.It could be argued that in the field of analytical chemistry there are too many such conferences and further proliferation is to be actively discouraged. However, such proliferation is likely to be self regulating, conferences won’t run if delegates don’t go. Many conferences are seeking to link with a publisher in order to produce a more permanent and detailed picture of the current state of affairs than is normally given in the collection of snapshots issued to delegates in the form of the book of abstracts. While such a link may be motivated by financial considerations on both sides, it is clearly an attractive notion that a special issue of a research journal can make available a suite of papers concerning the actively researched areas which formed the basis of the conference, thereby making the conference contents available to a much wider audience.Such an exercise is not without its difficulties. There are many more presentations made at a conference than could be accommodated in a special issue of a journal. Fortunately, the invitation to submit to such a special issue is not answered by every presenter. Material which may be perfectly satisfactory as the basis for a conference presentation may not have reached the stage where it may be submitted in written form for scrutiny by referees.Neither The Analyst nor the Journal of Analytical Atomic Spectrometry (JAAS) makes any conces- sions on the refereeing policy when considering material submitted as the result of an invitation issued to the delegates at a particular conference, and thus there will be some disappointed authors whose material is not considered to have reached the stage of development for inclusion in a primary research journal. However, the exercise of submission and evaluation is by no means futile under such circumstances, as the referees’ comments will almost always be of a constructive nature indicating what needs to be done in order that the work may reach a stage which would be acceptable for publication. Those in the business of supervising research students will know the value of obtaining from the student a summary of the work in the form of a manuscript for publication as a means of identifying areas for further study and, while journals expect senior authors to act as an initial filter, the Royal Society of Chemistry journals consider the provision of constructive feedback through the peer review process to be an important part of the service provided by the scientific publishing arm of a professional society.The Analyst is making some effort to expand its North American referee base for two reasons: firstly, to provide a better service to the authors of an increasing diversity of topic material being submitted and secondly, to avoid overload of the existing referee base. Editors are only too aware of the considerable demand placed on referees, especially as the request to make time to review a submission is based solely on a kind of moral blackmail-‘someone was good enough to referee your paper, so would you be good enough to referee this paper, and this one, and. . .’. Referees and authors alike may be interested to know that both editorial offices of The Analyst are available by electronic mail. The US office in Amherst, MA, can receive Bitnet messages on JFTyson@UMass and Internet messages on Julian. Tyson@chemistry.umass.edu. The UK office is avail- able on Janet at RSCl@gec-b.rutherford.ac.uk. Cross-Atlan- tic communication by this medium is slower than by telephone or fax, but faster than the regular mail and is considerably cheaper than both. Special issues of The Analyst and JAAS containing papers relating to presentations made at the Colloquium Spectro- scopicum Internationale held in Bergen in June 1991 will appear later this year. Julian Tyson US Associate Editor

ISSN:0003-2654    

DOI:10.1039/AN9921700002

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

ANALYST, JANUARY 1992, VOL. 117 3 Robotic Microwave Digestion System for Dissolution of Titanium Dioxide John D. Norris, Brian Preston and Lynn M. Ross Tioxide Group Services Ltd., Central Laboratories, Portrack Lane, Stockton-on- Tees, Cleveland TS182NQ, UK A robotic microwave digestion system for the dissolution of titanium dioxide samples prior to trace element determination is described. The system weighs out samples, adds acids, carries out microwave digestion, dilutes the solutions, transfers the solutions to beakers and cleans the digestion vessels. Sixty samples can be processed in approximately 16.5 h. The robotic microwave digestion system has operated reliably for over 6 months and the accuracy and reproducibility match those which can be achieved manually. Keywords: Robotic; automation; microwave digestion; titanium dioxide With recent advances in analytical atomic spectroscopic instrumentation and associated automated sampling systems, the dissolution procedure has become by far the most time-consuming and operator-intensive stage in the analytical process for the determination of trace elements in titanium dioxide.Microwave digestion has been shown to have considerable advantages over more traditional techniques such as fusions or acid dissolution in open or closed vessels.1 The successful use of microwave digestion for the dissolution of titanium dioxide prompted the investigation and develop- ment of the total automation of the dissolution process using a robotic microwave digestion system. In this laboratory, dissolution of samples of pigmentary or high-purity titanium dioxide can be followed by the determi- nation of any combination of up to nearly 40 trace elements. In order to obtain the maximum sensitivity, these determinations are carried out by a variety of different techniques, e.g., inductively coupled plasma atomic emission, flame atomic absorption, electrothermal atomic absorption or cold vapour mercury atomic fluorescence spectrometry.It was therefore not considered desirable to connect the proposed automated sample preparation system with a single analytical instrument. This paper describes a robotic microwave digestion system capable of dissolving up to 60 samples of titanium dioxide. The system weighs out samples, adds acids, carries out microwave digestion, dilutes the solutions, transfers the solutions to beakers and cleans the digestion vessels. Although other robotic microwave digestion systems have been reported2 that carry out some of these tasks, the system described has several novel features, particularly the handling of titanium dioxide and the washing and re-use of the digestion vessels.Experimental Reagents Analytical-reagent grade hydrofluoric acid (40% m/m) and hydrochloric acid (p = 1.18 g cm-3) were used. Apparatus The components of the system shown in Figs. 1-5 are described below. The system was engineered and supplied by Peerless Systems (Gateshead, Tyne and Wear, UK). Control system Computer control is effected via an electronics interface unit. The input-output (I/O) cards in the computer interface at transistor-transistor logic (TTL) level to the electronics I/O rack mounted in the system bench.This rack converts I/O signals from TTL level to the appropriate levels for the sensors, motors, solenoids and pneumatic devices, in the bench-mounted peripherals. The two balances, the robot and the microwave oven are controlled via a single serial port on the computer, which is achieved by multiplexing the various RS232 devices. The RS232 interface allows the system to tare and read the balances, and allows complete control over the microwave oven cycle. Infrared diffuse proximity sensors are used to detect the presence of a digestion vessel, and safety interlocks prevent acid from being dispensed if the digestion vessel is not present. Computer The computer is a Viglen Vig 1 Plus, with an Epson LX 400 printer, fully IBM AT compatible, 10 MHz 80286 processor, 640K RAM, 30 Mbyte hard disk and monochrome monitor.Software The control software is written in MODULA-2 and consists of a number of discrete modules for data handling, hardware control, error handling, screen formats and process control. 1 I I H I 0 Robot The Peerless Systems robot is a cylindrical motion, track- mounted unit having five axes: waist rotate, 320"; extend, 500 mm; lift (z-axis), 345 mm; track (y-axis), 2 m; wrist, 180"; gripper, pneumatic (with sensors); positional accuracy, -to. 1 mm; and lift capacity, 2 kg. Fig. 1 Layout diagram of robotic microwave digestion system. A, Acid and water dispenser; B, sample bottle rack; C, Mettler balance; D, powder pipette standby and sample bottle holder; E, scraper; F, tip waste bin; G, tip holder; H.robot; I. robot track; J. microwave oven; K, digestive vessel storage rack; L, washing and drying station; M, beaker rack; N, cooling station; P, mixer and extension arm standby; R, capping station; S, torquing station; T. digestion vessel standby; U, watch-glass removal device; and V, watch-glass standby4 ANALYST, JANUARY 1992, VOL. 117 Fig. 2 Sampling system Fig. 3 Acid and water dispenser The multi-tasking facility of MODULA-2 allows the monitor- ing and control of a number of peripheral devices simul- taneously. An error process continuously monitors the system and flags erroneous conditions. The menu-driven software allows control of the system parameters, ie., the microwave digestion programme, acid volume and types, target sample weight, final volume of solution, number of samples and sample identification.Data collected during the run can be displayed on-screen, printed out and saved on disk. The software also provides a comprehensive facility for verifying the correct operation of all of the system peripherals. Sample rack A 60-position rack, constructed from Perspex, to accommo- date standard 2 02, 60 ml glass powder bottles is used. Fig. 4 cooling station Microwave oven, torquing station, capping station, mixer and Fig. 5 Washing and drying station Sampling system Titanium dioxide is a difficult material to handle automatically owing to its adherent properties, which effectively eliminated consideration of many commonly used sampling devices such as vibratory feeders because of cleaning difficulties.In order to avoid contamination problems, it was concluded that any part of the sampling device coming into contact with the sample would need to be disposable. A further disadvantage of the material to be handled is that the bulk density of different types of titanium dioxide can vary considerably. A Peerless Systems powder pipette is employed for sample transfer. It operates on the principle of sample being withdrawn into an evacuated tube and subsequently expelled under positive pressure. Disposable tips, fitted with 10 mm filters, were designed and manufactured for use with the powder pipette. The tip capacity was designed so that it would accommodate about 0.2 g of sample.Hence, depending on the bulk density of the powder, two or three portions are necessary to achieve the target sample mass of 0.5 _+ 0.1 g . The sampling system incorporates the powder pipette, a standby holder with an optical sensor, a Mettler AT400 balance, a sampling bottle position with an optical sensor, a 90-position tip rack, an optical sensor for tip location (ie., to check that the tip has been correctly attached to or removed from the powder pipette), a scraper with bin to remove excess of sample from the tips and a tip extractor with bin for used tips.ANALYST, JANUARY 1992, VOL. 117 5 Acid and water dispenser In view of the use of hydrofluoric acid in the system, it was decided that this (and all other liquids) should not be transferred under pressure.The dispensing system therefore involves gravity feed via poly(tetrafluoroethy1ene) (PTFE)- lined solenoid valves. The system consists of a stand for four acid reservoirs (500 ml) and one water reservoir (3 dm3). These reservoirs are linked, using PTFE tubing to the moveable dispenser head, mounted above a Sauter RE 1622 balance. When dispensing occurs, the head is lowered over the digestion vessel, which has been placed on the balance. The dispense operation is electronically and mechanically interlocked for maximum possible safety. The power to the solenoid valves is interlocked to proximity switches that detect the presence of a digestion vessel on the balance and check that the dispense arm is in the lowered position. Only when these two conditions have been met will there be any supply voltage available to switch on the solenoid valves.Even then, no signals are given from the computer to operate the valves, unless the computer reads a digestion vessel mass on the balance within a specified tolerance. The system can be programmed to select different dis- pensers and to specify volumes taken, although for a complete run the same conditions would be employed throughout. Micro wave digestion vessels Four Milestone SV 140-10 microwave digestion vessels are necessary for the operation of the system. These 140 ml vessels are equipped with a regulating valve designed to give a maximum pressure of 10 bar ( 3 bar = 105 Pa). The vessel caps were modified (Fig. 6) by machining off 3 mm from the body of the cap.A washer was inserted to enable the screw to be reproducibily tightened to the correct position for the valve to operate. The silicon ring seal was changed before each run. Digestion vessel storage rack The rack is fitted with pressure sensors and can store four digestion vessels and four caps. Microwave oven A CEM MDS 81D microwave oven is used. The control system was modified so that all functions could be controlled by the computer. The turntable was removed and a PTFE holder constructed to locate the digestion vessel in the oven. A forked, flat-bladed extension arm was necessary to enable the robot to reach and position the digestion vessel in the holder. A standby position is provided for this extension. Capping station A Peerless Systems capping station, which can accommodate both the sample bottles and the microwave digestion vessels, is used.This includes an optical sensor and a temporary storage Fig. 6 washer; and D , silicon ring Digestion vessel cap. A. Body of cap; B, vent screw; C, position for the sample bottle lids whilst these are detached from the bottles. Torquing station A CEM MDS 810 capping station, modified to accommodate SV 140-10 digestion vessels is used. The torque is set to 9 N m, which is the manufacturer’s recommendation for these diges- tion vessels. Optical sensors are used to determine the exact position of the digestion vessels in the torquing station. Cooling station This incorporates two standby positions for the microwave digestion vessels. One position is for cooling the vessel after it has been in the oven and prior to de-capping.The vessel in this position is cooled by a fan. There is a second position for temporary storage after capping and prior to placing in the oven. Optical sensors are fitted in both positions. Mixer A Stuart Scientific Autovortex SA2 mixer is used. An optical sensor is attached to the mixer. Washing and drying station This is arranged so that the inverted digestion vessel or cap is washed internally by a jet of de-ionized water pumped from a reservoir. In a second position the vessel or cap is dried internally by a jet of air and the digestion vessel is spun at high speed for part of the drying process by a second jet. Optical sensors are fitted to both positions and the entire washing and drying station is located in a poly(propy1ene) bath.After drying, the vessels and caps are allowed to equilibrate to ambient conditions for at least 10 min on the storage rack before further use. Beaker rack A 60-position rack constructed from Perspex and contained in a tray to contain possible spillages, to accommodate 170 ml tapered polythene beakers and PTFE watch-glasses, is used. A pneumatic device to remove the watch-glasses and tempor- ary storage positions for the watch-glass and digestion vessel are incorporated. The holder for the watch-glass handling device and that for the digestion vessel are equipped with optical sensors. Lay-out The system is installed on a 3 x 2 m bench in a room with fume extraction. The computer and printer are located in an adjacent room. Operation The solid samples are presented to the system in standard bottles and the solutions are removed in standard beakers for subsequent analysis, the intermediate stages being carried out automatically.A report is produced giving the sample mass and dilution factor. The operator must ensure that certain procedures are carried out before a run can commence (i.e., the sample bottle rack is filled with the required samples, the beaker rack is filled with the corresponding number of clean beakers covered with watch-glasses, new silicon ring seals are fitted to the digestion vessel caps and the digestion vessels and caps are located on the digestion vessel storage rack, the powder pipette tip rack is filled, the relevant acid and water dispensers are filled and the balances are tared).The system is switched on. Sample identification data and operating parameters (number of samples, target sample mass, acid volumes and types, microwave digestion pro- gramme, final volume of solution) are entered. Normal operating parameters for the dissolution of titanium dioxide6 ANALYST, JANUARY 1992, VOL. 117 Table 1 Operating parameters for robotic microwave digestion of titanium dioxide Target sample mass Volume of HF 7 ml Volume of HC1 3 ml Digestion programme: 0.5 -t 0.1 g Stage 1 Stage 2 5 min at 50% power 10 min at 35% power Final solution mass 5og Table 2 Reproducibility for determination of aluminium in titanium dioxide Parameter Robotic Manual No. of replicates 30 12 Range (% Al) 0.44-0.49 0.46-0.50 Mean (Yo Al) 0.468 0.481 Standard deviation (%) 0.0127 0.0112 are given in Table 1.The robot is calibrated and a status check is run to ensure that all facilities are in position and operational. The run is commenced. The following describes the procedure undergone for the digestion of a single sample. During a run three samples, at different stages in the process, are being handled simul- taneously, and therefore the procedure as shown is not continuous for any given sample. A microwave digestion vessel is transferred from the digestion vessel storage rack to the balance; the vessel is weighed and the balance is tared. A sample bottle is transferred from the sample rack to the capping station. The bottle cap is removed and placed in the bottle cap standby position. The open sample bottle is transferred to the sampling bottle position. The powder pipette is picked up from its standby holder, a tip collected from the tip rack and the pipette passed over the optical sensor to ensure that the tip is correctly located.An aliquot of sample is removed from the sample bottle, the pipette passed over the scraper to remove any excess of sample adhering to the outside of the tip and the sample aliquot transferred to the digestion vessel. The sampling process is repeated until the target sample mass has been achieved and this mass is recorded. The tip is removed using the extractor and the pipette returned to the standby holder. The digestion vessel is transferred from the balance to the acid dispenser. The dispenser head is lowered and the acids are added until the required mass has been achieved.The dispenser head is raised and the digestion vessel is transferred to the mixer, where the contents are mixed, and then to the capping station. A digestion vessel cap is transferred from the digestion vessel storage rack and placed on the digestion vessel in the capping station. The partially capped digestion vessel is transferred to the torquing station for tightening to the correct torque and then to the standby position. The microwave oven door is opened and the extension arm is collected and used to transfer the digestion vessel into its position in the microwave oven. The oven door is closed and microwave digestion is carried out according to the pre-set programme. The sample bottle is transferred to the capping station, the bottle cap replaced and the closed bottle returned to its position in the sample rack.On completion of the digestion programme, the door is opened and the digestion vessel removed and placed in the cooling station (cooling is normally carried out for 15 min). The extension arm is returned to its standby position. The cooled digestion vessel is transferred to the torquing station and partially de-capped, and then to the capping station where the cap is removed. The cap is transferred to the washing and drying station, washed and dried and placed in its position in the digestion vessel storage rack. The open digestion vessel is transferred to the water dispenser. The dispenser head is lowered and water added until the required mass has been achieved. The dispenser head is raised and the digestion vessel is transferred to the mixer and the contents are mixed.The digestion vessel is transferred to the balance and the final solution mass recorded. The watch-glass removal device is collected and used to remove the watch-glass from the appropriate beaker. The watch-glass is placed in a standby position and the removal device returned to its holder. The digestion vessel is collected from the balance, its contents are decanted into the beaker and the empty vessel is placed in the standby position on the beaker rack. The watch-glass removal device is collected, used to replace the watch-glass and returned to its holder. The digestion vessel is transferred to the washing and drying station, washed and dried and placed in its position on the digestion vessel storage rack.At the end of the run, a printout is obtained listing the sample reference, sample mass, final mass of solution, dilution factor and the number of the digestion vessel used. Results and Discussion Microwave Digestion Conditions The microwave digestion conditions were evolved from the manual microwave digestion procedure previously employed, the main differences being that in the manual procedure a turntable is used in the oven and normally several samples are digested simultaneously, whereas in the robotic system one sample is digested in a fixed location in the oven. The conditions employed (see Table 1) were found to be effective for dissolving a wide range of titanium dioxide types without causing the digestion vessel to vent.Contamination The system was designed to minimize possible cross-sample contamination, particularly the sampling process. The main possible source for any contamination is in the washing and re-use of the digestion vessels. Checks for contamination were made over several complete 60-sample runs using alternate batches of six samples of a high-purity titanium dioxide (A1 <lo pg g-1) and of a coated titanium dioxide (A1 = 5 % ) . The solutions of all of the high-purity titanium dioxide samples were analysed for aluminium by inductively coupled plasma atomic emission spectrometry (ICP-AES) and all samples were found to contain <10 pg 8-1 of aluminium. Reproducibility and Accuracy The performance of the system was examined over a large number of sample runs.For most types of titanium dioxide the target sample mass of 0.5 k 0.1 g and a final solution mass between 50.0 and 50.4 g were achieved. In all instances complete dissolution of the sample was obtained. It is difficult to provide quantitative performance data, as robotic sample preparation is only a part of the over-all analytical process, and such data would relate to the whole process and not just of the robotic system. However, the analytical procedure for the determination of aluminium was employed to compare the performance of the robotic micro- wave digestion system with manual microwave digestion. A pigmentary titanium dioxide was selected, which, owing to its physical properties, was expected to provide sample handling difficulties. The solutions of these samples were analysed for aluminium by ICP-AES. The results are given in Table 2 together with those for a batch of 12 replicates of this sample prepared by the manual microwave digestion procedure. The statistical analysis of these data (F-test) shows no significant difference between the variance of the two sets of results. Although further statistical analysis (t-test) shows the means to be different, this is not considered to be analytically significant, and could be caused by limitations in the ICP-AESANALYST, JANUARY 1992, VOL. 117 7 determination. Hence it can be inferred that the accuracy and reproducibility of the robotic microwave digestion system match those which can be achieved by a competent analyst. Reliability The robotic microwave digestion system completes a 60- sample run in about 16.5 h. The first sample is completed approximately 45 min after the start of the run, with subsequent samples being completed about every 16 min. The system has been in operation for over 6 months completing runs of up to 60 samples overnight. References 1 Kingston, H. M., and Jassie, L. B., in Zntroduction to Microwave Sample Preparation, ed. Kingston, H. M., and Jassie, L. B., American Chemical Society, Washington, DC, 1988, ch. 1. 2 Labrecque, J. M., in Introduction to Microwave Sample Preparation, ed. Kingston, H. M., and Jassie, L. B., American Chemical Society, Washington, DC, 1988, ch. 10, pp. 210-229. Paper 1/01492G Received March 27, 1991 Accepted August 26, 1991

ISSN:0003-2654    

DOI:10.1039/AN9921700003

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

ANALYST, JANUARY 1992, VOL. 117 9 Application of Tryptamine as a Derivatizing Agent for the Determination of Airborne lsocyanates Part 5.* Investigation of Tryptamine-coated XAD-2 Personal Sampler for Airborne lsocyanates in Workplacest Weh S. Wu and Virindar S. Gaind Occupational Health Laboratory, Ontario Ministry of Labour, I0 I Resources Road, Weston, Ontario, Canada M9P 3T1 The development of an efficient solid sorbent personal sampler with increased convenience for sample collection in workplaces is described. Several solid sorbents were coated with tryptamine, and sampling tubes were prepared with the coated sorbents. These tubes were evaluated for the collection of phenyl isocyanate vapour generated in a commercial test atmosphere generation system that permits the simultaneous collection of up t o 12 uniformly loaded samples.Tryptamine-coated XAD-2 resin was shown t o be the most efficient solid sorbent for the collection of airborne phenyl isocyanate. The optimum amount of tryptamine needed for coating XAD-2 resin was investigated. Keywords: Personal sampler; airborne isocyanate; tryptamine-coated XA 0-2 For more than a decade, many occupational health labora- tories have attempted to develop a method capable of determining both monomeric and polymeric isocyanates for isocyanate monitoring programmes in workplaces. Despite the slow progress in this area, the method of Bagon et al.1 provided a significant advance. The later method of Wu and co-workers,2-4 based on the concept of the isolation of a selected x-system in a derivative for specific detection,5 lowered the detection limit and also raised the confidence in the analytical results by using two specific detection systems after separation by high-performance liquid chromatography (HPLC).In our earlier work, the airborne isocyanates were collected in impinger solutions containing tryptamine, result- ing in the formation of the corresponding urea derivatives. Determination via the indolyl groups of tryptamine was carried out on a reversed-phase HPLC system equipped with both fluorescence emission and amperometric oxidation detectors. This method is unique because the determination of tryptamine-derivatized isocyanates can be achieved by calib- rating against a single standard such as tryptamine-derivatized toluene diisocyanate.Such manipulation may be necessary for the determination of polymeric isocyanates as information regarding either the type of polymer or the degree of polymerization is often not available. With efforts being concentrated on a search for methods that are capable of determining total isocyanates, researchers have paid less attention to the development of a convenient personal air sampler. Nearly all of the personal samplers for the collection of airborne isocyanates have been limited to measuring the common monomeric isocyanates such as toluene diisocyanate (TDI) ,fF-8 methylene diphenyl diisocy- anate (MDI)9-11 and hexamethylene diisocyanate (HDI).* A diffusive sampler for personal monitoring of TDI has been described and evaluated in the field, but its utilization for polymeric isocyanates was not investigated.12713. There is only one publication14 on the sampling of polymethylene poly- phenylene isocyanate (PMPPI), but its determination was extremely cumbersome. As the personal sampler is attached to a worker, the sampling medium should ideally be a solid sorbent. Unfortunately, this implies that the derivatization of isocyanate during sampling would occur at the solid phase of * For Part 4 of this series, see reference 19. t Presented in part at the 104th AOAC Annual International Meeting and Exposition, New Orleans, LO, USA, 1990. the coated derivatizing reagent in the sampler, for which the efficiency of derivatization is hindered. Tryptamine has previously been demonstrated to be an isocyanate-derivatizing reagent that gives very reliable results for total isocyanate determination.The feasibility of sampling airborne isocyanate using tryptamine-coated solid sorbents for both monomeric and polymeric isocyanates has been investi- gated's.Various types of common solid sorbents, including a glass-fibre filter, were studied for their efficiency in sampling. In order to validate the use of tryptamine-coated solid sorbents in sampling tubes for the collection of airborne isocyanates, a test atmosphere was generated using a commer- cial test atmosphere generation system (TAGS). It was neither practical nor necessary to conduct the simulation of air sampling for all industrial isocyanates owing to their very low vapour pressure and lack of purity. Therefore, phenyl isocyanate was selected as a model for the generation of test atmospheres.Experimental Chemicals and Solid Sorbents Tryptamine was supplied by Sigma (St. Louis, MO, USA) and phenyl isocyanate by Aldrich (Milwaukee, WI, USA). Tryptamine-derivatized phenyl isocyanate used as the calibra- tion standard was synthesized in the laboratory. Amberlite XAD-2, XAD-4 and XAD-7 were purchased from BDH (Toronto, Ontario, Canada). Molecular sieve 13X (45-60 mesh) was obtained from Supelco (Bellefonte, PA, USA) and Glass Beads (80-100 mesh) from Chromatographic Special- ities (Brockville, Ontario, Canada). Glass-fibre filters (Type N E , 37 mm, Cat. No. 225-7) were purchased from SKC (Eighty Four, PA, USA). Silica gel and charcoal were obtained from the corresponding sampling tubes manufac- tured by SKC (Cat.No. 226-22 for the former and 226-16 for the latter). A nylon-66 filter (0.45 pm) was supplied by Rainin Instrument (Woburn, MA, USA). Apparatus The operating conditions of the HPLC system and the associated fluorescence detector have been described in previous parts of this series.2-410 ANALYST, JANUARY 1992, VOL. 117 I solution Fig. 1 Schematic diagram of the TAGS: A, vapour generator; B, cone-shaped chamber; C, multiple sampling ports; D, critical orifices; E, metal bellows pump; F, filter; P, pressure gauge; and R, pressure gauge and regulator. Connections to 1, 2 and 3 of the ration line are identical Solid sorbent sampling tubes Glass tubing (7 cm x 5 mm i d . ) was filled with various solid sorbents to a height of 3 cm with small glass-wool plugs at both ends.Test atmosphere generation system This system for the simulation of air sampling was manufac- tured by SRI International16 (Menlo Park, CA, USA) and is outlined in Fig. 1. It consisted of the following major sections: (a) vapour generator, where the vapour of phenyl isocyanate was generated (at room temperature) by passing compressed nitrogen through liquid phenyl isocyanate; (b) dilution tower, where the incoming phenyl isocyanate vapour carried by the compressed air was further mixed with a large volume of compressed air; (c) cone-shaped chamber, where the mixture of phenyl isocyanate vapour and compressed air was dynamic- ally homogenized; (d) multiple sampling ports, where a maximum of twelve samples can be collected simultaneously; (e) critical orifices, where the flow rate for air sampling was controlled; and (f) vacuum exhaust system, where a vacuum pump was operated for exhaustion.The amount of phenyl isocyanate collected was adjusted by varying the sampling time andor by adjusting the pressure of nitrogen at the vapour generator. It is practical to allow the system to equilibrate for a minimum of 30 min before sampling, which ensures that the whole system has reached dynamic equilibrium. It should be kept in mind that in most instances, the exact time period for sampling on each set of samplers is not crucial, as all recoveries are with reference to the impinger solutions in the set. Preparation of Sampler Containing Tryptamine-coated Solid Sorbent In order to minimize the variation of packed contents between individual samplers, a batch of tryptamine-coated solid sorbent, sufficient for a minimum of ten samplers was prepared.The coating of solid sorbent was carried out by dissolving tryptamine in acetonitrile, mixing with the solid sorbent and removing the solvent on a rotary evaporator under water aspiration. The evaporation was initially conduc- ted at room temperature for 20 min and the temperature was raised to about 40°C until no visible condensation of acetonit- rile was observed. Excessive amounts of unremoved acetonit- rile caused undesirable sputtering during sampling. All precautions were taken to avoid large variations within a batch of tubes being packed. I Time - Fig. 2 HPLC analysis for phenyl isocyanate sampled by a tryptam- ine-coated XAD-2 tube on the TAGS.Column, CSC-Hypersil-ODS (5 km); flow rate, 0.8 ml min-1; eluent, acetonitriIe-0.6% ammonium acetate (55 + 45); retention time, 8.68 min for PI-TP (11.5 ng) The impregnation of tryptamine on glass-fibre filters was performed in a beaker where the filters were soaked with a minimum amount of acetonitrile solution containing tryptam- ine. The filters were air dried at room temperature before use. Investigation of Sampling Efficiency on Solid Sorbent Sampler The investigation was conducted by using the TAGS, where parallel sampling was used on various types of tryptamine- coated solid sorbent samplers. After sampling, the sorbent from each sampler was emptied into a vial and the tryptamine derivative was desorbed with 5 ml of acetonitrile.A portion of the solution was filtered using a 0.45 pm pore size nylon-66 filter. The filtrate was diluted appropriately with acetonitrile before analysis by HPLC. Recovery of Tryptamine-derivatized Phenyl Isocyanate Spiked on XAD-2 and XAD-4 XAD-2 and XAD-4 sampling tubes were individually spiked with 4.44 pg of tryptamine-derivatized phenyl isocyanate and immediately aerated (Bendix Model 44 portable air sampling pump) for 4 h at an air flow of 0.2 dm3 min-1. Recoveries were evaluated at various time intervals up to 12 d. The main purpose was to assess the stability of the isocyanate derivative on XAD resins during and after sampling. Efficiency of Derivatization and Desorption for Sampling Low Levels of Isocyanate on Tryptamine-coated XAD-2 and XAD-4 The tryptamine-derivatized phenyl isocyanate was found to be relatively stable on XAD-2 and XAD-4 sampling tubes before and after aeration.Subsequently, the efficiency of derivatiza- tion and desorption at various time intervals for microgram levels of isocyanates collected on tryptamine-coated XAD-2 and XAD-4 was evaluated. Acetonitrile was used for all the desorptions. Effective Amount of Tryptamine for Coating XAD-2 For the purpose of establishing the relationship between the maximum amount of airborne isocyanate collected for a known amount of coated tryptamine, experiments were conducted on the TAGS by sampling for various time periods using various amounts of tryptamine for coating (Table 4). The findings would serve as a guideline for an individual laboratory to select the amount of tryptamine to be used for coating according to its specific requirements.Before con- ducting this experiment, the TAGS was adjusted mainly by varying the pressure of the purging nitrogen gas and monitor-ANALYST, JANUARY 1992, VOL. 117 11 ing the amount of phenyl isocyanate reaching the sampling port via impingers containing try ptamine solutions. The adjustments were made so that in a reasonable time period, the amount of phenyl isocyanate vapours reaching each of the samplers was slightly less than the equivalent amount of the tryptamine used for sampler coating. Two identical samplers were connected in series during air sampling. Any break- through of phenyl isocyanate would be collected in the back-up sampler. Recovery Study on Simulated Air Sampling for Phenyl Isocyanate As the generation of an exact amount of phenyl isocyanate through the TAGS was not possible, all recoveries were calculated with reference to the amount of phenyl isocyanate collected simultaneously by the impinger solutions containing tryptamine, which was considered to be 100%.The recoveries of phenyl isocyanate from the tryptamine-coated XAD-2 tubes include the evaluation of the effect of the time lapse before desorption on the formation of the isocyanate deriva- tive. Results and Discussion Methods for the determination of isocyanates are based on the determination of the corresponding derivatives resulting from the instability of the isocyanato groups. Collecting samples with solid sorbent samplers for the assessment of exposure to airborne isocyanates in the workplace is a challenging task, because the derivatization at the solid phase of the derivatiza- tion reagent is much slower than the reaction in the impinger solutions.For a worker, however, the solid sorbent sampler is more convenient to carry around during a work-shift. The results in Tables 1-3 show that tryptamine-coated XAD-2 resin was the most efficient sorbent and compared well with the impinger solutions in the recovery study. Table 2 indicates that the desorption of tryptamine-derivatized phenyl isocyanate from XAD-4 was slightly hindered, with con- sistently lower yields. The efficiency of XAD-2 resins over other solid sorbents is probably due to the relatively non-polar chemical structure (polystyrene) and the appropriate pore size Table 1 Sampling efficiency of various tryptamine-coated solid sorbent samplers Amount of Amount of Type of tryptamine for phenyl isocyanate sorbent coating/pg found/yg Impinger Impinger XAD-2 XAD-2 XAD-4 XAD-4 XAD-7 XAD-7 Molecular seive Molecular seive Glass beads* Glass beads* Charcoal Charcoal Silica gel Silica gel Glass-fibre filter? Glass-fibre filter? 100 100 200 200 200 200 200 200 200 200 200 200 400 400 200 200 200$ 200$ 53.3 53.2 59.1 56.6 56.5 56.9 38.2 38.8 1.3 1 .o 20.5 17.3 44.2 41.6 Trace Trace 32.1§ 45.79 * Height of packing in tube 1.5 cm, as the air flow is substantially reduced for 3 cm of packing.T Using two filters in series. $ Amount for each filter. § Combined yield for both filters. (90 A). The extremely low yield with silica gel may reflect the existence of silanol groups, which are likely to be reactive to the isocyanates.The calculated maximum length17 of the MDI and HDI molecules is approximately 15 8, (the length of the tryptamine molecule is about 8 A), whereas for most of the isocyanate pre-polymers the lengths are well under 30 A. For example, Desmodur N, an HDI pre-polymer, is calculated to be slightly longer than 20 A. All isocyanates are expected to penetrate through the porous surface of the XAD-2 resin. With the molecular seive, the average pore size is only 13 A, which would severely hinder the penetration of the isocya- nates. On the other hand, the excessive pore size (lo4 A) of the glass-fibre filters offers very little retention of isocyanate molecules.The threshold limit values’s for the time-weighed average (TLV-TWA) for all isocyanates are at the level of 0.005 ppm. At present, there are no short-term exposure limits (TLV- STEL)18 for isocyanates, except for TDI which has a TLV-STEL of 0.02 ppm. The corresponding TLV-TWA concentration of phenyl isocyanate would be about 0.025 mg m-3 and the TLV-STEL 0.1 mg m-3. It has been common to operate sampling pumps at an air flow rate of 0.2 dm3 min-1 for solid sorbent samplers. For sampling the TLV-TWA concentration of isocyanate in air during one 8 h work shift, the phenyl isocyanate collected will be about 2.4 pg. For the TLV-STEL concentration, the amount of phenyl isocyanate collected during a 15 min sampling will be 0.3 pg. It is apparent that a sampler containing 100 pg of tryptamine is, in general, sufficient for most sampling (Table 5 ) .In order to have a high degree of confidence, practical sampling can be performed by either using a sampler containing 200 pg of tryptamine or two Table 2 Stability of phenyl isocyanate tryptamine derivative on XAD-2 and XAD-4 resins Time lapse Amount of Type of before derivative Recovery sorbent desorption/d found/pg (Yo 1 XAD-2 XAD-2 XAD-4 XAD-4 XAD-2 XAD-2 XAD-4 XAD-4 XAD-2 XAD-2 XAD-4 XAD-2 XAD-2 XAD-4 XAD-4 2 2 2 2 7 7 7 7 9 9 9 12 12 12 12 3.83 4.13 3.45 3.31 3.85 3.92 3.41 3.31 3.92 3.69 3.13 3.62 3.76 3.26 3.41 86.2 93.1 77.8 74.6 86.8 88.4 76.7 74.6 88.4 83.1 70.4 81.5 84.7 73.5 76.7 Table 3 Efficiency of over-all derivatization and desorption for low levels of phenyl isocyanate in XAD resins Amount of Efficiency Type of tryptamine before cyanate recovery Amount of Time lapse phenyl iso- of sorbent coated/yg desorptiodd foundlyg ( Y o ) Impinger XAD-2 XAD-4 XAD-2 XAD-4 XAD-2 XAD-4 XAD-2 XAD-4 XAD-2 XAD-4 100 300 300 0 0 300 300 300 300 0 0 - 0 0 0 0 2 2 7 7 7 7 3.30 3.01 2.32 1.66 0.85 3.05 1.94 2.92 2.30 0.24 Trace 1 00 91.2 70.3 50.3 25.8 92.4 58.8 88.5 69.7 7.3 -12 ANALYST, JANUARY 1992, VOL.117 tryptamine impingers. It should be noted that the efficiency of sampling isocyanate using tryptamine impinger solutions has been verified previously by comparing a well established impinger method involving 1-(2-methoxyphenyI)piperazine .4 Although both solid sorbent and liquid impinger samplers were comparable in over-all efficiency (from sampling isocya- nates in air to analysis by HPLC), the derivatization step was less efficient with the former. This was observed (Table 6) when the desorption process was delayed for 7 d after sampling.The lack of derivatization efficiency at the solid phase of the reagent should not be viewed as a result of the inferiority of the tryptamine in comparison with other commonly used derivatizing reagents. Previous work4.19 has shown that tryptamine is as efficient as 1-(2-methoxyphenyl)- piperazine and 1-(2-pyridyl)piperazine and superior to N-(p- nitrobenzy1)-N-propylamine for derivatizing isocyanates. Table 4 shows that a 100 pg of tryptamine coated XAD-2 sampler can adsorb and derivatize nearly stoichiometric amounts of phenyl isocyanate when the samples are desorbed with acetonitrile immediately after collection.With derivati- zation in the solid state of the reagent, the total surface area of the coated reagent would be drastically diminished compared with that in the solution. Further, the immobile products from the solid-state reaction would shield the surface layer of the reagent, preventing further reaction of newly approaching isocyanate molecules. It should be emphasized that these causes of inefficient derivatization on solid reagents apply not only to tryptamine but all other commonly used derivatizing reagents. In order to ensure the completion of derivatization, it is advisable that after sampling the tryptamine-coated XAD-2 is immediately emptied from each sampler into 10 ml of acetonitrile.The sampling of airborne isocyanates using tryptamine- coated XAD-2 sorbent in conjunction with the already established analytical procedure provides an elegant method for personal monitoring of both monomeric and polymeric isocyanates in the workplace. References Table 4 Effective amount of phenyl isocyanate derivatized by tryptamine-coated XAD-2 resin Amount of phenyl Amount of Phenyl isocyanate found in tryptamine isocyanate sampler*/yg Sampling for coating/ equivalent/ periodh Pg Pg Front Back 1.5 100 100 100 75 75 75 76.1 75.9 79.1 Av. 77.0 99.9 109.1 104.8 Av. 104.6 172.5 148.9 164.6 Av. 162.0 231.4 240.7 232.1 Av. 234.7 0 0 0 200 200 200 148 148 148 0 0 0 300 300 300 222 222 222 0 0 0 4 400 400 400 296 296 296 0 0 0 * Desorption performed immediately after sampling.Table 5 Recoveries of XAD-2 sampled phenyl isocyanate with no time lapse for desorption Amount of phenyl iso- cyanate Recovery foundlpg (Yo) 16.5 15.8 15.0 17.9 12.5 14.5 16.2 17.3 16.2 13.7 Av. 16.2 100 Av. 15.4 k 1.7 95.1 Amount of Type of tryptamine in sampling sampledpg Impinger 100 200 XAD-2 100 100 200 200 300 300 300 300 1 2 Bagon, D. A., Warwick, C. J., and Brown, R. H., Am. Znd. Hyg. Assoc. J., 1984, 45, 39. Wu, W. S . , Nazar. M. A., Gaind, V. S., and Calovini, L., Analyst, 1987, 112, 863. Wu, W. S . , Szklar. R. S . . and Gaind, V. S . , Analyst, 1988,113, 1209. Wu, W. S . , Stoyanoff. R. E., Szklar, R. S . , Gaind, V. S . , and Rakanovic, M . , Analyst, 1990, 115, 801. Wu, W. S., Stoyanoff, R. E., and Gaind, V. S., J. High Resolut.Lake City, 1983. Tucker, S. P., and Arnold, J. E . , Anal. Chem.. 1982,54, 1137. Anderson, K., Gudehn, A., Sevin, J.-O., and Nilsson, C.-A., Am. Ind. Hyg. Assoc. J . , 1983, 44, 802. Lipski, K., Ann. Occup. Hyg., 1982, 25, 1. OSHA Method No. 47 (Revised), OSHA Analytical Labora- tory, Salt Lake City, 1984. Rando, R. J., Hammad, Y. Y . , and Chang, S.-N., Am. Ind. Hyg. Assoc. J.. 1989, 50, 1. Rando, R. J., Hammad, Y. Y., and Chang, S.-N., Am. Znd. Hyg. Assoc. J . , 1989, 50, 8. Beasley, R. K., and Warner, J. M . , Anal. Chem., 1984, 56, 1604. Wu, W. S., Stoyanoff, R. E., and Gaind, V. S . , paper presented at the 104th AOAC Annual International Meeting and Expo- sition, New Orleans, 1990. Gaertner, R. R. W., Appl. Ind. Hyg., 1988, 3, 258. Ketelaar, J. A. A., Chemical Constitution, Elsevier, Amster- dam, 1958, ch. 3, sect. 28, p. 198. Threshold Limit Values and Biological Indices f o r 1990-1991, American Conference of Governmental Industrial Hygienists, Cincinnati, 1990. Wu, W. S., Stoyanoff, R. E., and Gaind, V. S . , Analyst, 1991, 116. 21. Paper 1101324F Received March 19, I991 Accepted August 12, 1991 Table 6 Recoveries of XAD-2 sampled phenyl isocyanate with time lapse for desorption 3 4 5 Amount of Amount of Time lapse phenyl iso- Type of tryptamine in before cyanate sampling samplerlyg desorptiodd foundlyg 8 9 Impinger 100 100 100 100 10.2 9.9 14.9 14.5 Av. 12.4 * 2.7 12.2 11.3 12.3 14.2 Av. 12.5 k 1.5 8.2 6.9 6.4 6.9 Av. 7.1 kO.8 10 11 12 XAD-2 300 300 300 300 13 14 15 300 300 300 300 XAD-2 16 17 18 samplers in series, each containing 100 pg of tryptamine (Tables 4 and 5 ) . The results in Table 3 also indicate that an XAD-2 tube coated with 300 pg of tryptamine can retain about 3 pg of phenyl isocyanate for 7 d without desorption treatment after sampling. Simulated sampling using various amounts of tryptamine again demonstrated (Tables 5 and 6) that the tryptamine-coated XAD-2 sampler is comparable to the 19

ISSN:0003-2654    

DOI:10.1039/AN9921700009

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

ANALYST, JANUARY 1992, VOL. 117 13 Mutual Separation and Preconcentration of Vanadium(v) and Vanadium(iv) in Natural Waters With Chelating Functional Group Immobilized Silica Gels Followed by Determination of Vanadium by Inductively Coupled Plasma Atomic Emission Spectrometry Kazuo Hirayama, Susumu Kageyama and Nobuyuki Unohara Depa rtm en t o f In d us tria I Ch em is tr y, Co lleg e o f Engineering, Nih o n Un ive rsit y, Ko ri yam a , Fu kus h ima 963, Japan The mutual separation and preconcentration of Vv and VIv using chelating functional group immobilized silica gels (CISs) has been studied, based on inductively coupled plasma atomic emission spectrometric detection. Ethylenediamine-bonded (ED-CIS) and ethylenediaminetriacetate-bonded (ED3A-CIS) silica gels were used.A two-column system consisting of a first column of ED-CIS and a second column of ED3A-CIS was developed for on-line separation and preconcentration of Vv and VIv. In the pH range 2.5-3.0, ED-CIS retains Vv and separates it completely from VIv, whereas ED3A-CIS collects both Vv and VIv. The second column can be used to preconcentrate VIv in the breakthrough solution from the first column. Separation is achieved with recoveries of 91-105%. The recovery of both ions is 98% or better. An enrichment factor of 40 is obtained, with which the detection limit of V with inductively coupled plasma atomic emission spectrometry is improved down to 60 pg ml-l. The method was applied successfully to V speciation in natural waters. Keywords: Vanadium speciation; water analysis; separation and preconcentration of vanadium(v) and vanadium(iv); chelating functional group immobilized silica gel; inductively coupled plasma atomic emission spectrometry In recent years, the chemical speciation of trace metals has been focused on the interpretation of their roles in water surveys and in environmental and biochemical studies.I,* Vanadium has many oxidation states and ionic forms in aqueous solution. It usually exists in two different oxidation states, Vv and VIV, in well aerated natural and industrial waters; the significance of V speciation is that the two oxidation states have different nutritional and toxic proper- ties.3" Therefore, it is essential to be able to determine Vv and VIV in environmental and biological samples. In particular, the accurate determination and knowledge of the distribution of V species in complex matrices such as sea-water is required.The concentration of V in natural waters is very low and usually in the range 0.5-2.5 pg 1-1.7 In order to determine ultratrace amounts of V o r other elements in complex matrices by instrumental analysis, a separation and preconcentration technique is frequently required. The separation procedure eliminates sample matrix components that might interfere with the subsequent instrumental analysis, whereas the preconcentration technique concentrates the analytes of interest from a large volume of sample solution. Many preconcentration techniques for the determination of V have been proposed, including chelation and extraction,8-11 extraction, 12 precipitation, I 3 .l 4 coprecipi tation, 15-18 ion exchangel9-21 and the use of chelating ion-exchange resin+- 2s. These preconcentration techniques have been used with instrumental methods of analysis; viz., spectrometry,8121 densitometry,l9 flow injection,20 neutron activation,24 atomic absorption spectrometry,ll,12,15,1~,2s X-ray fluorescence (XRF)13.14,17.26 and inductively coupled plasma atomic emis- sion spectrometry (ICP-AES).9.10.18,2' Most of the preconcen- tration techniques, however, collect Vv and Vrv together, or one species of V in natural waters; no direct simultaneous separation and subsequent determination of the two oxidation states of V has been reported in the literature. Therefore, a more sensitive and selective separation and preconcentration technique for V speciation would be expected to have widespread application.Chelating functional group immobilized silica gel (CIS) has great potential for the preconcentration of trace metals in water.27-30 Leyden and co-workers27,28 used an ethylene- diamine-immobilized silica gel (ED-CIS) and its derivatives (e.g., dithiocarbamate bonded) for the preconcentration of heavy metals in water. Sturgeon et a1.29 developed a separa- tion procedure that involved adsorption of trace metals onto silica-immobilized 8-hydroxyquinoline. In this laboratory, an ethylenediaminetriacetate-immobilized silica gel (ED3A- CIS)3" has been developed for the enrichment of heavy metals and their subsequent determination by XRF. There are two possible modes for the mutual separation of Vv and V'" with CIS, because the two species have different molecular structures and ionic charges in an acidic solution: (1) an ion-pairing mode, which can retain the negatively charged V species by anion exchange, for example, the ammonium ion (i.e., -NH3+) on the ion-exchange site of ED-CIS; and (2) a chelation mechanism in which ED3A-CIS can act as a chelating ligand in the same way as ethylenediamine- tetraacetic acid (EDTA).In the present paper, the development of a method for the mutual separation and preconcentration of Vv and VIV is described using ED-CIS and ED3A-CIS. The two-column system, consisting of an upper column of ED-CIS and a lower column of ED3A-CIS, permits the complete separation and simultaneous preconcentration of Vv and V'".The method was applied to the quantification of these species by ICP- AES. The application of the method to water analysis is also discussed. The method is simple and rapid, and has adequate sensitivity for practical applications. Experimental Apparatus The inductively coupled plasma atomic emission spectrometer used was an SPS-1100 instrument (Seiko Electronic Co., Tokyo, Japan). The operating conditions are summarized in Table 1 together with the emission lines used throughout this work. Measurements of solution pH were made with a Hitachi-Horiba glass electrode pH meter (Model F-7).14 ANALYST, JANUARY 1992, VOL. 117 Table 1 Operating conditions for ICP-AES measurements* Radiofrequency power 1.2 kW Reflected power 4 . 0 W Plasma gas flow rate Nebulizer gas flow rate Auxiliary gas flow rate Observation height Repetition 5 times Analytical line: 16 I min-l 0.4 1 min-1 0.6 1 min-l 12.0 mm above load coil Integration time 1.0s V 292.40 nm c o 228.62 nm c u 324.77 nm * A concentric glass nebulizer was used.Reagents Distilled, de-ionized water (DDW) was used. Sulfuric acid (97%0), HCI (20%) and CH3C02H (100%) were of Super Special Grade (Nacalai Tesque, Kyoto, Japan). Other chem- icals were of guaranteed-reagent grade. A commercial atomic absorption standard solution (Wako Pure Chemicals, Osaka, Japan) containing 1000 mg 1-1 of Vv (NH4V03 in 0.5 rnol dm-3 H2SO4) was used. A stock solution of VIv (1000 mg 1-1) was prepared by dissolving VOS04 in 0.05 rnol dm-3 H2SO4. Each working solution was prepared by diluting the standard solutions with DDW and adjusting the H2SO4 concentration to 0.01 mol dm-3.Acetate buffer solutions were prepared by mixing 1 rnol dm-3 CH3C02Na and 1 rnol dm-3 CH3C02H appropriately. Silica gel (Wako Pure Chemicals, Column Chromatography Grade C-100,40-100 mesh) was first purified by soaking it in 6 rnol dm-3 HCI for 2 d and then washing repeatedly with DDW. Finally, the silica gel was dried in an oven at 110 "C. Syntheses of ED-CIS and ED3A-CIS A 100 ml volume of 20% v/v N-(2-aminoethyl)-y-amino- propyltrimethoxysilane (Tokyo Chemical, Tokyo, Japan) solution and 10 ml of CH3C02H solution were added to 100 g of the dried silica gel and the mixture was stirred for 24 h at 90 "C. The silylated silica gel was washed with DDW, filtered and then dried at 110 "C.ED-CIS ED3A-CIS A 20 g amount of ClCH2COzH and 100 ml of DDW were added to 20 g of ED-CIS and the mixture was stirred for 1 h at 90 "C; during this time, NaOH solution (6 rnol dm-3) was added to keep the solution pH at 9-10. The mixture was then placed in a drying oven at 90 "C for 24 h. The ED3A-CIS obtained was washed repeatedly with DDW until no Cl- ion was detected in the washing water (AgN03 test). Hot DDW was found to be very effective at removing the CI- ion adsorbed on the silica gel. As the amino group of the ED-CIS reacts easily with salicylaldehyde to give a yellow Schiff's base, the completion of the iminoacetate reaction of ED-CIS was checked by using the above reagent. Finally, the ED3A-CIS obtained was dried at 110 "C. Separation and Preconcentration Procedure A glass column for chromatography (250 X 10 mm i.d.) equipped with a porous glass support at the bottom and a PTFE stopcock was used.The length of the packed column was about 30 mm. A small amount of glass wool was placed on top of the packed silica gel to hold it in place during the passage of the solution. 100 80 60 40 - 20 s - C .- 2 0 3 100 F 80 60 40 20 0 1 2 3 4 5 6 7 PH Fig. 1 Effect of pH on the collection of V species by (a) ED-CIS; and (b) ED3A-CIS: A , V"; and B, V'". Vanadium concentration in both instances, 0.2 pg ml-1; volume of sample solutions used, 50 ml. The sample flow rate was fixed at 10 ml min-1 A two-column system was employed: the first column was packed with 1.5 g of ED-CIS and the second column with 1.5 g of ED3A-CIS. The columns were connected with SPC joints (19/19).The sample solution, after adjustment of the pH to 2.5-3.0 with acetate buffer solution and 1 rnol dm-3 HCl, was percolated through the columns. The columns were then washed with two 20 ml aliquots of DDW adjusted to the same pH. After the columns had been separated, the Vv adsorbed on the first column and the VIv adsorbed on the second column were each eluted with 20 ml of 6 mol dm-3 HCl into separate 25 ml calibrated flasks. Then, 25 pg of Coil were added to each flask to give a concentration of 1 pg ml-1 and the final volume was adjusted to 25 ml with DDW. The eluates were analysed by ICP-AES. Results and Discussion Separation and Collection of Vv and Vrv by CISs The collection of the species on the CISs was examined by using the single column mode.Fig. l ( a ) shows that Vv is collected quantitatively from solution over the pH range 2.5-7; however, VIv is not collected at all at a pH of 3.0 or less using ED-CIS. For ED3A-CIS [Fig. l(b)], both Vv and VIv were retained completely in the pH range 2.5-7. These results indicate that when the sample solution is adjusted to a pH in the range 2.5-3.0, the ED-CIS column separates Vv from ViV and the VIV in the effluent from this column can be collected on the ED3A-CIS column. The collection of the V species with the CISs in the optimum pH range was 98% or better. The two-column system consisting of a first column of ED-CIS and a second column of ED3A-CIS was used to facilitate a simple and rapid on-line separation. Table 2 demonstrates that the two species of V are completely separated and recovered quantitatively using this system, even when Vv is present at ten times the concentration of V'".The proposed system offers the unique advantage of being capableANALYST, JANUARY 1992, VOL. 117 15 Table 2 Mutual separation of Vv and VLv using the two-column system. Sample volume: 100 ml VlPg Recovery Species Added Found ("/.I 10 9.97 99.7 0 0.00 - VV 0 0.12 - V'V 10 9.74 97.4 VV 10 9.48 94.8 V1V 1 1.05 105 VV V'" 1 0.99 99 10 9.32 93.2 VV 1 0.95 95 V1V 0 0.00 - VV 0 0.01 - V1V 1 0.97 97 VV 1 1.02 102 V1V 1 0.94 94 VV* 1 0.91 91 V1V 1 0.95 95 * A 500 ml volume of artificial sea-water was used. Table 3 Capacities of ED-CIS and ED3A-CIS. The capacity was determined by the batch method.A sample solution (50 ml) containing the metal ion at an initial concentration of 0.1 mg ml-l was used. A 100 mg amount of CIS was added to the solution and collection was effected by stirring the mixture for 15 min. The solution was then filtered and the metal ion in the filtrate was determined Capacity/mmol g-1 PH VV- 3 6 9 v1v- 3 6 9 Cull- 3 6 9 ED-CIS ED3A-CIS 0.64 0.05 0.39 0.04 0.10 0.02 0 0.07 0.32 0.09 0.14 0.10 0 0.11 0.04 0.28 0.15 0.32 of in situ separation, as a portable unit can easily be carried into the field. The applicability of the method to field use is very attractive because the two V species might be unstable in the sample solution and the determination of trace amounts of V frequently involves preconcentration of large volumes of samples, which might be troublesome to transport back to the laboratory.Collection Mechanism and V Species Vanadium(v) and VIV have many complex forms in water that change in accordance with the solution pH and their concen- trations. It is known that in the pH range 2-6 the main species of Vv is the orange decavanadate anion VI0O2&, which can exist in several protonated forms, and which changes to the dioxovanadium(v) ion V02+ below pH 2.31 In contrast, VIV exists as the blue oxovanadium(1v) ion VO2+ in acidic solution; this cation readily changes to the anion V1x04212- at about pH 4.31,32 Table 3 summarizes the ion-exchange capacities of Vv, V" and Cu" with ED-CIS and ED3A-CIS at solution pH values of 3, 6 and 9. Copper(I1) was used to determine the chelating ability of the CISs.The ED-CIS has a lower chelating ability at lower pH values as shown by the results for the capacity of Table 4 Preconcentration of Vv in highly concentrated salt solutions with ED-CIS. A 100 pg amount of Vv was added to a 50 ml portion of the salt solutions used Salt concentration Relative Salt ("/.I V foundpg error (% ) Na2S04 1 98.6 -1.4 + MgS04 5 99.1 -0.9 NaCl 1 101.3 +1.3 + KCl 5 97.4 -2.6 NaN03 1 98.8 -1.2 + CaCI2 5 100.4 +0.4 Cu"; however, the capacities of Vv and VIV with ED-CIS increase with a decrease of pH. These results suggest that ED-CIS facilitates collection by an ion-pair interaction between the ammonium ion of ED-CIS and the anionic species of V in acidic solution; the collection curves of Vv and VIV shown in Fig. l(a) correspond to the formation of their respective anionic species.Further, ED-CIS can collect other oxometal anions such as those of MoV1 and Crvl, and the VIV-EDTA complex (i.e., the [VO-EDTA12- anion) from acidic solution. Vanadium(1v) has no affinity for ED-CIS at pH 3 because it is present as VO*+. The ED3A-CIS has lower capacities for Vv and VIV in comparison with those of ED-CIS; however, no significant differences between their capacities were observed at pH 3. The higher capacity values of ED-CIS for V species imply that ED-CIS adsorbs polymeric anionic species of V. Many metal cations passed through the ED-CIS column in the pH range 2.5-3.0; however, the ED3A-CIS column not only collected VIV, but also Cu", Zn", Co", Ni", FeIr1 and Pb" at pH 3, because it acted as a chelating agent in the same way as EDTA and had almost the same stability constants with Vv, VIv and Cu" (i.e., log K = 18-19).The advantages of the use of ED-CIS and ED3A-CIS columns in tandem are that the two-column system permits not only V speciation, but also multi-element analysis. Recovery Efficiency A dilute HCI solution is expected to be an effective eluent for Vv and VIV retained on the columns, because these species are not adsorbed by the CISs below pH 1. The effect of HCI concentration on the elution of Vv and VIV was investigated by using the single-column technique. Both Vv and VIV adsorbed on the columns could be recovered quantitatively by using 20 ml of 6 mol dm-3 HCI at a flow rate of 5 ml min-1. The recoveries were 95% or better. The effects of sample volume and flow rate were also examined. The proposed method could be applied to a 1000 ml sample solution at a maximum flow rate of 15 ml min-1.An enrichment factor of 40 was obtained, and the detection limit of V using ICP-AES could be improved to 60 pg ml-1 with the preconcentration technique. The CISs are stable and can be regenerated by passing 100 ml of water through the columns after elution with acid. The separation procedure was carried out ten times on the same column with no loss of efficiency of the extraction and removal of v . Effect of Foreign Ions In order to evaluate the feasibility of the method for water analysis, the effect of a salt-water matrix was studied. Solutions containing two different salts were prepared and the concentration of each salt was adjusted to 1 or 5% m/v.Table 4 shows that Vv is collected quantitatively from the highly concentrated salt solutions by ED-CIS. Vanadium(1v) was16 ANALYST, JANUARY 1992, VOL. 117 Table 5 Linear regression of calibration data for the determination of V by ICP-AES Slope Intercept Correlation Detection limit/ Calibration (R)/ml pg-1 (R) coefficient ngml-l range/pgml-l Internal standard method*- 6 mol dm-3 HCl 4.467 k 0.031 0.0178 f 0.0176 0.9999 2.4 0-5 DDW 4.469 k 0.050 0.0399 k 0.0266 0.9997 1.6 0-5 6 mol dm-3 HCl 1.080 k 0.029 0.0135 k 0.0164 0.9986 2.6 0-5 DDW 1.133 k 0.017 0.0031 k 0.0092 0.9994 1.7 0-5 Direcr calibration method?- * R is defined as the ratio of the intensity of V to that of Co (Iv : Ice). [CO~~]: 1 pg ml-1. t R x 105 is the emission intensity (counts) of V.100 80 s - 60 .- w 2 fi 40 W 20 0 1 2 3 4 5 6 7 PH Fig. 2 Effect of Fell1 concentration on the collection of V*" by ED-CIS. Fell1 concentration: A, 2; and B, 0.2 pg ml-1. Vanadium(1v) concentration, 0.2 pg ml-1. The broken line shows the original collection curve also recovered from the spiked salt-waters by ED3A-CIS. Chelex-100 resin is commonly used for the enrichment of metals in sea-water.22.33 However, it also preconcentrates Ca and Mg cations in addition to trace metals in sea-water; both Ca and Mg cations cause spectral and physical interferences in ICP-AES mea~urements.22~3~ The advantage of the proposed method is that the CISs have no affinity for alkaline earth metals in acidic solution; in addition, the method is applicable to the direct separation .and preconcentration of V species from sea-water and their subsequent determination by ICP- AES.Fig. 2 shows the effect of the Fell' concentration on the collection of V'" by ED-CIS. The adsorption of V'" in the pH range 2-5 appeared to be enhanced as the Fell' concentration increased. However, when 1 ml of 0.1% m/v ascorbic acid solution was added to the sample solution in order to reduce Fel" or Vv, the collection curve of VIv was not affected by Fe"'. This indicates that V'" is oxidized to Vv by Fellr in weakly acidic solution, and that eventually a redox requilibration between VIV and Vv is reached. Calibration Data In ICP-AES measurements, acids in the solution to be analysed are sometimes prone to spectral and physical interferences.35 In the present study, the internal standard method was utilized to overcome the difficulty caused by the variation of the HCI concentration that was used to elute V ions adsorbed on the CISs.Cobalt was chosen as the internal standard, Fig. 3 shows that the emission intensity of V decreased gradually with increasing HCI concentration; however, the ratio of the intensity of V to that of Co was constant and independent of the HCI concentration over the range 0-8 rnol dm-3. As the concentration of Co" in natural waters is usually very Iow,18,29 the intensity of Co from natural waters can be neglected. 1.3 0 3 0 c.' ul z 1.2 > m al c.' .- c.' - 1.1 Fig. 3 1 5 \- I I I 1 ' 1 1 0 2 4 6 8 [HCl]/rnol dm-3 Effect of HCl concentration on the emission intensitv of V (x), and the correction of emission intensity by the internal stindard method (B).Vanadium and Co concentrations, 1 pg ml-l each. The abbreviations I , and Zco represent the intensities of V and Co, respectively The linear regression calibration data for the determination of V by ICP-AES are presented in Table 5. The slope obtained with the use of 6 rnol dm-3 HCl agreed very closely with that obtained with DDW by using the internal standard method. The calibration data measured on different days were reproducible, and the relative standard deviation of the slope was 1.2% (n = 20). The detection limit (defined as the average of the blank value plus three times its standard deviation) was found to be 2.4 ng ml-1 using 6 rnol dm-3 HCI. Applications to Water Analysis The method was applied to the determiantion of Vv and V'" in natural waters. Sea-water and river water samples were filtered through 0.45 ym pore size membrane filters (Milli- pore, Type HAWP 047XX) immediately after sampling.The filtrate was then acidified to pH 2.5-3.0 with HCI (1 mol dm-3) and acetate buffer solution. The results are given in Table 6. By using the proposed method, sub-ng ml-1 levels of V species could be separated and determined with good reproducibility. In order to calculate the recovery, known amounts of Vv and V" were added to the sample solutions. The recoveries of added V'" were lower than those of added Vv for the sea-water (1) and the river water sample; however, the recoveries of total added V were found to be quantitative.As pointed out by Cole et al.," the oxidation states of V change in aqueous solution; it appears that the redox equilibration, VV-VIv, is not particularly stable in natural waters and is closely related to matrix elements such as Fellr and organic substances. The values of the total concentration of V in the acidified and filtered sea-water samples obtained in this work were similar to the reported values.9~~2~~9 It has been ascertained that natural waters contain Vv and Vlv at similar concentration levels, although many workers tend to deter- mine only Vv.ANALYST, JANUARY 1992, VOL. 117 17 ~~~~ ~ Table 6 Determination of V in natural waters. Sea-waters (1) and (2) were coastal sea-waters and were collected at Toyoma Shore and Usuiso Beach, Iwaki, respectively.River water was collected from the Abukuma River, Koriyama. All locations are in Fukushima Prefec- ture, Japan. All determinations were performed by using 500 ml sample solutions. The reproducibility was calculated from five replicate determinations Sample* Sea-water (1)- VV VIV VV V1V VV V1V VV VIV V t VV V1V VV V1V V t VV V'V VV VIV vt Sea-water (2)- River water- V Added/ pg 0 0 1 2 0 2 2 0 0 0 0 1 1 0 0 0 1 2 0 Foundlpg 0.54 _+ 0.054 0.37 ? 0.053 1.95 1.97 1.20 1.78 1.80 1.03 1.05 0.50 t 0.03 0.28 k 0.03 1.49 1.31 0.77 0.31 k 0.03 0.26 k 0.03 1.52 2.06 0.86 V in sample/ ng ml- 1.08 * 0.11 0.74 k 0.11 1.90 2.40 - - - 2.06 2.10 1.00 4 0.06 0.56 k 0.06 0.98 0.62 1.54 0.62 k 0.06 0.52 k 0.06 1.04 0.12 1.72 Total V concentra- tion/ ng ml-1 1.82 1.84 1.96 1.66 2.10 1.56 1.60 1.54 1.14 1.16 1.72 * Samples were filtered and then adjusted to pH 2.5-3.0 im- 7 Samples were acidified to pH 1 and then filtered.The total mediately after collection. amount of V was determined using ED3A-CIS at pH 3. In general, V exists in soluble, insoluble and organic complex forms in natural waters. The determination of the oxidation state of V is important in water surveys. When the natural water samples were acidified to pH 1 immediately after sampling and then filtered, the total V concentration in the river water was higher than that of the filtered sample. For sea-water, no significant difference was observed between the values obtained with the two processes. These results indicate that V exists mainly in soluble forms in sea-water; however, in river water some of the V is present as insoluble or colloidal forms and is filtered with particulate matter.The proposed method can provide information about soluble Vv and V'" in natural waters, particularly sea-water, whereas direct analysis using other techniques is difficult or not possible. References 1 2 Florence, T. M., and Batley, G. E., CRC Crit. Rev. Anal. Chem., 1980, 9, 219. Nakayama, E.. Suzuki, Y., Fujiwara, K., and Kitano, Y., Anal. Sci.. 1989, 5 , 129. 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 35 Shamberger, R. J., in Toxicity of Heavy Metals in the Environ- ment, ed. Oehme, F. W., Marcel Dekker, New York, 1979, pt. Lagerkvist, B . , Nordberg, G. F., and Vouk, V., in Handbook on the Toxicology of Metals, eds.Friberg, L., Nordberg, G. F., and Vouk, V. B., Elsevier, Amsterdam, 1986, vol. 11, pp. 638-663. Morrison, G. H., CRC Crit. Rev. Anal. Chem., 1979, 8,287. Sadler, P. J . , Higham. D. P., and Nicholson, J . K . , in Environmental Inorganic Chemistry, eds. Irgolic, K . J., and Martell, A. E., VCH, Deerfield Beach, FL, 1985, pp. 255 and 256. Bowen, H. J. M., Environmental Chemistry of the Elements, Academic Press, New York, 1979, pp. 17 and 23. Ramana Murthy, G. V., Sreenivasulu Reddy, T., and Brahmaji Rao, S., Analyst, 1989, 114, 493. Tao, H., Miyazaki, A., Bansho, K . , and Umezaki, Y., Anal. Chim. Acta, 1984. 156, 159. Sugiyama. M., Fujino, O., Kihara, S . , and Matsui, M., Anal. Chim. Acta, 1986, 181, 159. Monien, H., and Stangel, R., Fresenius Z.Anal. Chem., 1982, 311, 209. Shijo, Y., Kimura, Y., Shimizu, T., and Sakai, K., Bunseki Kagaku, 1983,32, E285. Hirayama, K., and Leyden, D. E., Anal. Chim. Acta, 1986, 188, 1. Saitoh, Y., Yoneda, A., Maeda, Y., and Azumi, T., Bunseki Kagaku, 1984. 33, 412. Fujiwara. K., Morikawa, T., and Fuwa, K . , Bunseki Kagaku, 1986, 35, 361. Shimizu, T., Uchida, Y., Shijo, Y., and Sakai, K . , Bunseki Kugaku, 1981,30, 113. Cole, P. C., Eckert, J. M., and Williams, K. L., Anal. Chim. Acta, 1983, 153, 61. Akagi, T . , Fuwa, K., and Haraguchi, H., Anal. Chim. Acta, 1985, 177, 139. Shriadah. M. M. A., and Ohzeki, K., Analyst, 1985, 110, 677. Fukasawa, T., Kawakubo, S . , Okabe, T., and Mizuike, H . , Bunseki Kagaku, 1984, 33, 609. Kiriyama, T., and Kuroda, R., Mikrochim. Acta, Part I , 1985, 405. Cheng, C. J., Akagi, T., and Haraguchi, H., Bull. Chem. SOC. Jpn.. 1985,58, 3229. Rueter, J . , and Schwedt, G., Fresenius Z. Anal. Chem., 1982, 311, 112. Greenberg, R. R.. and Kingston, H. M., Anal. Chem., 1983,55, 1160. Colella, M. B., Siggia, S . , and Barnes, R. M., Anal. Chem., 1980. 52,967. van Grieken, R.. Anal. Chim. Acta, 1982, 143, 3. Leyden, D. E., and Luttrell, G. H., Anal. Chem., 1975, 47, 1612. Leyden, D. E., Luttrell, G. H., Nonidez, W. K., and Werho, D. B., Anal. Chem., 1976,48, 67. Sturgeon, R. E., Berman, S. S., Willie, S. N., and Desaulniers, J . A. H., Anal. Chem., 1981, 53,2337. Hirayama, K., and Unohara, N., Bunseki Kagaku, 1980, 29, 452. Cotton, F. A , , and Wilkinson, G., Advanced inorganic Chemistry, Wiley, New York, 4th edn., 1980, pp. 708-719. Johnson, G. K . , and Schlemper, E. O., J. Am. Chem. SOC., 1978, 100, 3645. Kingston, H. M., Barnes, I. L., Brady, T. J., Rains, T. C., and Champ, M. A., Anal. Chem., 1978, 50,2064. Berman, S. S . , McLaren, J. W., and Willie, S. N., Anal. Chem., 1980, 52,488. Greenfield, S . , McGeachin, H. M., and Smith, P. B., Anal. Chim. Acta, 1976, 84,67. 2, pp. 745-751. Paper 1102685B Received June 5, 1991 Accepted August 19, 1991

ISSN:0003-2654    

DOI:10.1039/AN9921700013

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

ANALYST, JANUARY 1992, VOL. 117 19 Instrumental Comparison for the Determination of Cadmium and Lead in Calcium Supplements and Other Calcium-rich Matrices Bernard P. Bourgoin" Environmental and Resource Studies Program, Trent University, Peterborough, Canada K9J 7B8 Dave Boomer and Mark J. Powell Ontario Ministry of the Environment, laboratory Services Branch, Rexdale, Canada M9W 51 1 Scott W i II ie National Research Council Canada, Institute for Environmental Chemistry, Ottawa, Canada K I A OR6 Duart Edgar Nissei Sang yo Canada Inc., Rexdale, Canada M9 W 6A4 Don Evans Ontario Ministry of the Environment, laboratory Services Branch, Dorset, Canada POA 1 EO Three brands of Ca supplement, a laboratory-reagent grade CaC03 and a certified reference material (International Atomic Energy Agency H-5 Animal Bone) were analysed for Cd and Pb by four different analytical techniques, viz., anodic stripping voltammetry, inductively coupled plasma mass spectrometry, flame atomic absorption spectrometry and electrothermal atomic absorption spectrometry.The Pb levels measured by the four techniques in the bone powder were within the certified Pb level in this certified reference material. Similarly, no significant differences [ p <0.05; analysis of variance (ANOVA)] were observed in samples with Pb concentrations greater than 1 pg g-1. However, the Pb levels in the laboratory-reagent grade CaC03 obtained by flame atomic absorption spectrometry (0.79 pg g-1) averaged about three times higher than those measured by the other three techniques (i.e., 0.25 pg g-1).Although no significant differences ( p <0.05; ANOVA) in Cd levels were observed within any of the samples (intra-sample variability), the Cd concentration measured in the different Ca supplements (inter-sample variability) varied by three orders of magnitude (ranging from 0.07 to 3.59 pg 9-1). Keywords: Cadmium; lead; calcium carbonate; interference; supplements The accurate identification of foodstuffs with high Pb concen- trations is becoming increasingly important because of the substantial decrease in the total Pb content in children's diets over the past 15 years.' It has been recognized for some time that Ca supplements, classified as foodstuffs in the US, contain variable amounts of Pb and that the ingestion of some of these products for extended periods of time could be considered a potential health hazard.2.3 Cadmium, another toxic trace metal, behaves similarly to Pb in the environment and is often associated with Ca-rich material such as bivalve shells.4 The various analytical techniques that have been used to determine heavy metal levels in Ca-rich matrices include anodic stripping voltammetry (ASV) ,576 inductively coupled plasma emission spectrometry,7 flame atomic absorption spectrometry (FAAS) and electrothermal atomic absorption spectrometry (ETAAS) ,&lo and proton-induced X-ray emis- sion.1'3'2 There is no standardized method for determining trace levels in these matrices and the extent of sample pre-treatment ranges from simply analysing untreated sample digests to more complex and time-consuming clean-up proce- dures involving the separation and/or coprecipitation of the trace metals from the major ions in solution.Further, there is no certified reference material (CRM) in which a considerable array of heavy metal levels is certified to verify the accuracy of a particular method. In this work, four different analytical techniques, viz., ASV, inductively coupled plasma mass spectrometry (ICP- MS), FAAS and ETAAS, were used to measure Cd and Pb levels in five different Ca-rich matrices. Sample pre-treatment * Present address: National Water Research Institute, Lakes Research Branch, P.O. Box 5050, 867 Lakeshore Road, Burlington, Ontario, Canada L7R 4A6. was purposely kept to a minimum so as to determine under what circumstances interferences were more problematic. Experimental Sample Types Although there are over 400 types of Ca supplement available on the North American market, based on the form in which elemental Ca occurs, these supplements can be grouped into the following three main categories: CaC03, hydroxyapatite or calcium phosphate [Calo(P04)6(OH)2] and Ca bound to various organic chelates (e.g., gluconate and amino acids).Many of these products are also commonly supplemented with other minerals such as Mg or Zn. The five types of sample selected in this work are listed in Table 1, which also summarizes the various forms and concentrations of Ca and other elements contained in the powders. Three different brands of Ca supplement were included (Brands A-C). The elemental Ca in Brands A and B occurred as CaC03 and consisted of ground oyster shells.Group B was further supplemented with Mg and Zn. Brand C also contained Mg and Zn, but all the minerals in this product occurred as chelates. The other two samples included a laboratory-reagent grade CaC03 powder and a CRM, H-5 Animal Bone, produced by the International Atomic Energy Agency (IAEA), Austria. Participants and Analytical Instrumentation Four independent laboratories: a federal and a provincial agency, a university laboratory and a private firm participated in these analyses. These agencies are listed in Table 2 together with the different types of instrument used and the pertinent analytical conditions under which the analyses were per- formed.20 ANALYST, JANUARY 1992, VOL.117 Procedure The laboratory at Trent University was responsible for the selection of the different samples and for soliciting the participation of the other three laboratories. The identifica- tion of the samples and the identity of the participating laboratories were only disclosed to the other participants after all the analyses had been completed. All of the samples were ashed in a muffle furnace at 425 "C for 24 h at the Trent University laboratory. The temperature was gradually increased at a rate of about 100 "C h-l to avoid combustion of the powders. Sub-samples of the ashed samples were then sent to the National Research Council Canada (NRCC) laboratory for subsequent pre-treatment and analy- sis. The sample digests for the other three methods were prepared at the Trent University laboratory.Table 1 Five different Ca-rich samples analysed, corresponding levels and forms of elements specified by the manufacturer. Brands A-C represent various Ca supplements NRCC sample treatment Approximately 0.5 g of sample was weighed into 120 ml digestion vessels (Teflon PFA) to which were added 6 ml of 16 rnol dm-3 HN03. The vessels were capped and heated in a microwave oven (CEM MDS SlD) at high power for 20 min. The internal pressure was maintained at 310 kPa. The vessels were cooled, opened and heated on a hot-plate under a heat lamp to evaporate the remaining acid to a volume of about 1 ml after which the solutions were diluted to 25 ml with distilled, de-ionized water (DDW). At this point the solutions of the laboratory-reagent grade CaC03 and the IAEA CRM samples were clear and could be analysed.The three brands of Ca supplement had undissolved material present. These solutions were filtered through a pre-washed (1.5 rnol dm-3 HCI) Millipore filter (0.45 pm pore size) housed in a Gelman poly(su1fone) filter-funnel, the filtrate collected and analysed. Guaranteed Sample (mineraVelement) Form specified Laboratory-reagent grade CaC03 powder Ca: 400 mg g-I Zn: (0.005% Pb: <0.005% Cd: <0.005% Ca: 330 mg g- CaC03 precipitated As impurities As impurities As impurities CaC03 from oyster shells CaC03 from oyster shells From oxide From gluconate From amino acid From amino acid From amino acid chelate chelate chelate Brand A Brand B Ca: 170mgg-* Mg: 90 mg g-1 Zn: 30 mg g-1 Ca: 160 mg g-l Brand C Mg: 160 mg g-1 Zn: 8 mg g-1 Trent University sample treatmenl For ASV analyses, approximately 0.25 g of the ashed samples was dissolved in 1 ml of 12 rnol dm-3 HCI.Approximately 5 ml of 0.2 rnol dm-3 sodium acetate (NaOAc) buffer were added and the solutions filtered through pre-washed (1.5 rnol dm-3 HCI) Millipore membrane filters (0.45 pm pore size). The filtrate was adjusted to a pH of 1 S O k 0.05 (NaOAc buffer or 12 rnol dm-3 HCl). The typical volume of the solutions after IAEA CRM H-5 Animal Bone Ca: 212 mg g-1(8)* P: 102 mg g-1(8) Mg: 3.55 mgg-'(0.09) Zn: 89 pg g-* (6) Pb: 3.1 pgg-'(0.6) * 95% confidence interval. Ca10(P04)6(0H)2 Ca10(P04)6(0H)2 Ion substitution Not specified Not specified Table 2 Participants, analytical instruments and settings used for the determination of Cd and Pb in Ca-rich matrices ETAAS FAAS Agency Instrument Tube Lamps Background correction Modifier Wavelengths: Cd Pb Char Atomize Measurement Calibration Dry National Research Council Canada Perkin-Elmer 5000, HGA-500 Platform HCL (Cd); EDL (Pb) Zeeman None Agency Instrument Flame Lamps Background correction Modifier Wavelengths: Cd Pb Nissei Sangyo Canada Hitachi 2-8100 Air-acetylene HCL (Cd and Pb) Zeeman None 228.8 nm 283.3 nm 140 "C for 20 s 400 "C for 20 s 2400 "C for 4 s Peak area Standard additions 228.8 nm 283.3 nm Measurement Calibration Peak area Calibration graph ASV ICP-MS Agency Instrument Mode Electrode Electrolyte Trent University Metrohm VA 646, VA 647 Differential-pulse normal Hanging Hg drop Sodium acetate buffer Agency Instrument Ontario Ministry of the Environment Perkin-Elmer-Sciex Elan mass spectrometer Gas flows: Plasma Auxiliary line Nebulizer Ion lens settings: Bessel box barrel Bessel box stop Einzel lens 1 and 3 Einzel lens 2 Measurement* Calibration 14 1 min-1 0.8 I min-1 0.4 1 min-1 Plating time Plating range Sweep rate U step 90 s -900 to -300 mV 2.5 mV s-1 2 mV +3.67VDC -5.78 VDC - 10.60 VDC -12.08 VDC Peak height External calibration graph Measurement Calibration Peak height Standard additions * The minimum acceptable resolution ( i , e ., low resolution) to be within specifications was 1.0 k 0.1 u.ANALYST, JANUARY 1992, VOL. 117 21 Table 3 Levels of Cd and Pb in the five different types of Ca-rich sample measured by the four different analytical techniques.Samples and instruments are detailed in Tables 1 and 2, respectively Cdyg g-I* Laboratory-reagent IAEA CRM H-5 Technique grade CaCOR powder Brand A Brand B Brand C Animal Bone ICP-MS <0.010f <0.010f 0.54 f 0.05$ 3.31 k 0.14 0.14 f 0.04 ASV <0.024? 0.07 f 0.05 0.63 f 0.06 3.49 k 0.02 0.11 f 0.03 ETAAS <O.O06f 0.07 f 0.02 0.71 f 0.10 3.59 k 0.66 0.017 k 0.003 FAAS <0.035? 0.12 f 0.09 0.71 k 0.06 Pbhg g-I§ <0.035? 3.55 k 0.27 3.83 f 0.25 3.50 k 0.86 1.39 k 0.06 2.77 f 0.26 ICP-MS 0.25 t 0.12 ASV 0.25 f 0.07 2.89 k 0.56 3.26 f 0.31 1.42 f 0.25 2.87 5 0.51 ETAAS 0.24 f 0.05 3.25 f 0.44 3.73 f 0.62 1.33 k 0.13 3.09 f 0.22 FAAS 0.79 k 0.17 3.39 f 0.12 3.56 k 0.34 1.97 f 0.29 4.57 f 1.31 * Detection limits for Cd are 0.010, 0.024, 0.006 and 0.035 yg g-* using ICP-MS, ASV, ETAAS and FAAS, respectively.t Below detection limit. 3 95% confidence intervals. 5 Detection limits for Pb are 0.010, 0.120, 0.120 and 0.5 yg g-1 using ICP-MS, ASV, ETAAS and FAAS, respectively. pH adjustment ranged from 25 to 30 ml. Approximately 20 ml of the solution were transferred into the ASV cell and analysed. Samples analysed by ICP-MS and FAAS were prepared as follows. Approximately 0.5 g of the ashed samples was dissolved in 1 ml of 16 mol dm-3 HN03. The solutions were filtered through pre-washed (1.5 mol dm-3 HN03) Millipore membrane filters and diluted to either 25 or 100 ml with DDW and analysed by FAAS and ICP-MS, respectively. Quality control High-purity, certified reagents (Aristar or Suprapur grade) were used for all the analyses. All the sample types were analysed at least in triplicate together with two procedural blanks.No appreciable amounts of Cd and Pb were measured in the blanks by any of the participants. Results and Discussion The identification and subsequent quantification of heavy metals by the analytical techniques employed in this work is essentially based on the following three distinct physical properties: absorption of discrete wavelengths of electromag- netic energy (i.e., FAAS and ETAAS); half-wave potential (Le., ASV); and atomic mass (i.e., ICP-MS). Although all of these methods are prone to interference problems, it is unlikely that the bias (either signal enhancement or sup- pression) apparent in one method would be exactly the same as in another method.The results of the determination of Cd and Pb are summarized in Table 3. The Pb levels in the IAEA CRM measured by all four analytical techniques were within the range of the certified Pb level for this CRM (Table 1). Similarly, no significant difference [p <0.05, one-way analysis of variance (ANOVA)] was observed in the Pb levels among the three brands of Ca supplement as measured by the four analytical techniques (Table 3). Results from FAAS (dis- cussed below) were, however, at variance with those from the other three techniques when comparing Pb levels in the laboratory-reagent grade CaC03 powder (Table 3). A certi- fied Cd level was not listed for the IAEA CRM and hence it is not possible to comment on the accuracy of the various techniques used for the determination of Cd in Ca-rich matrices.Except for the low Cd level measured in the IAEA CRM by ETAAS, the low variability in Cd levels measured by the different techniques in most of the sample types indicates that the Cd measurements were fairly precise (Table 3). No systematic trend based on instrumentation or any significant differences (p <0.05, one-way ANOVA) in Cd levels within any sample types was observed. The results also indicate that there was a substantial degree of variability in heavy metal concentrations among the three different brands of Ca supplement analysed. This variance was greatest for Cd, which displayed a 40-fold difference between the lowest (Brand A) and the highest (Brand C) concentrations. The Pb concentrations in the Ca supplements averaged higher and varied considerably less than those of Cd.Interference ef€ects associated with AAS analyses can arise from reactions between the analyte and matrix components of the sample, which can affect the formation of the analyte vapour. Although gas-specific interference problems have not been reported for the determination of Cd, several anionic interferences can occur during the determination of Pb using an air-acetylene flame. This could explain why the variability associated with the Pb levels measured in the IAEA CRM by FAAS was four times higher than that associated with the ETAAS results (relative standard deviation = 29 and 7%, respectively). The discrepancy between the Pb levels measured in the laboratory-reagent grade CaC03 by FAAS and the other three techniques might have been associated with 'low end deterioration' of the signal due to the use of a hollow cathode lamp (HCL; Table 2).The signal-to-noise ratio of electrodeless discharge lamps (EDLs) is typically 2-3 times better at lower Pb concentrations. High levels of Ca compounds such as CaO or CaCI2 can cause matrix effects during trace metal determinations by ICP-MS.*3.14 Higher levels of Ca (>500 pg g-1) could cause analyte suppression or enhancement and bias the results. In order to reduce the potential for matrix effects the sample was diluted 100 times, bringing the Ca concentration below 500 pg g-1. There was only one isobaric interference in this work. Cadmium-114, which is the most abundant of the eight stable isotopes of Cd, suffers interference from 114Sn.As the isotopic abundance of the latter is relatively low (0.65%), the contribution from this element to the bias of the results is minimal. In addition, a correction factor is applied to the elemental equation for Cd if Sn is detected above the detection limit in the sample. Interference problems in polarographic measurements can be related to matrix effects or to the overlapping of signals. Matrix interferences that are mainly caused by the presence of organics and surface-active substances were not problematic in this work as these compounds were effectively destroyed during the ashing process. Overlapping signals can occur in analytes with potentials that are very close. Generally, a potential difference of about 100 mV is sufficient to allow22 ANALYST, JANUARY 1992, VOL.117 resolution of the species of interest. For example, the half-wave potentials (E4) of TI+ and As3+ are -0.48 and -0.43 V, respectively. These species, when present at high levels, could conceivably interfere with Pb*+ (Eb = -0.045 V). However, when using a hanging Hg drop electrode, As related interferences would be less problematic as this element does not amalgamate well with Hg. Furthermore, various counter- measures based on chemical means can be used to separate overlapping peaks. For example, the separation of T1 and Pb peaks could be achieved either by altering the pH or the electrolyte ( e . g . , by adding ethylenediaminetetraacetic acid). However, T1- and As-related interferences did not pose any problems in this work as the Pb levels obtained by ASV conformed with those obtained with the other techniques.The Cd and Pb levels measured in the Ca supplements by FAAS (Zeeman-effect background correction system) were not significantly different from the levels obtained using the other techniques. This has particular importance for quality control boards. The analyses performed by FAAS were carried out in a fraction of the time required for the other three techniques. Hence quality assurance groups currently using more complex and laborious techniques could greatly increase the number of samples screened/analysed without substan- tially increasing their analytical time or costs. We are currently investigating whether the FAAS analyses can be further ameliorated, particularly at low trace metal concentrations, by using more complex standard solutions ( i .e . , spiked with CaC03) and also EDL sources. The authors thank S. Lingard and L. Bigelow for analytical assistance with the ASV studies, and Professor R. D. Evans and R. J. Cornett for critical comments during the preparation of the manuscript. This work was funded by grants from the Natural Sciences and Engineering Research Council of Canada to R. D. Evans and R. J. Cornett. References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Goyer, R. A., Environ. Health Perspect., 1990, 86, 177. FDA Drug Bulletin, Food and Drug Administration, Rockville, MD, 1982, April, vol. 12(1), pp. 5-6. Health and Welfare Canada, Health Protection Branch, Notes, 1983, No. 8 (April), Vanier, Ontario. Sturesson, U., Ambio. 1978, 7, 122. Rabinowitz, M. B., Leviton, A., and Bellinger, D. C., Bull. Environ. Contam. Toxicol., 1989, 43,485. Dodge, R. E., and Gilbert, T. R., Mar. Biol., 1984, 82, 9. Dermott, R. M., and Lum, K. R., Environ. Pollur., 1986, 12, 131. Denton, G. R. W., and Burdon-Jones, C . , Mar. Pollut. Bull., 1986, 17, 209. Shen, G. T., and Boyle, E. A., Chem. Geol., 1988,67, 47. Bourgoin, B. P., Mar. Ecol. Prog. Ser., 1990, 61, 253. Carriker, M. R., Swann, C. P., and Ewart, J. W., Mar. Biol., 1982, 69, 235. Carell, B., Forberg, S . , Grundelius, E., Henrikson. L., Johnels, A., Lindh, U., Mutvei, H., Olsson, M., Svardstrom, K., and Westermark, T., Ambio, 1987, 16, 2. Olivares, J. A., and Houk, R. S . , Anal. CIzem.. 1986, 58, 20. Gray, A, L., Spectrochim. Acta, Part B , 1986,41, 151. Paper 1 I01 553 B Received April 3, 1991 Accepted August 14, 1991

ISSN:0003-2654    

DOI:10.1039/AN9921700019

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

ANALYST, JANUARY 1992, VOL. 117 23 Determination of Lead in Plant Tissues: A Pitfall due to Wet Digestion Procedures in the Presence of Sulfuric Acid Erwin J. M. Temminghoff and Ivo Novozamsky Department of Soil Science and Plant Nutrition, Agricultural University, Dreijenplein 10, 6703 HB Wageningen, The Netherlands Two wet digestion procedures for the determination of lead in plant materials by electrothermal atomic absorption spectrometry have been evaluated. The combination of HN03, H202 and HF, though leading to incomplete digestion, yielded values corresponding well with the reference or indicative values of the vegetal tissues used. The mixture of H2S04, HN03 and HC104 gave results that were too low in some instances, owing t o the formation of a mixed precipitate (Pb,Ba)S04.Keywords: Wet digestion of plant tissue; lead determination; electrothermal atomic absorption spec- trometry; lead coprecipitation; formation of (lead, barium) sulfate Wet digestion methods for organic matter in trace element determinations are well documented. 1-3 Often, H2S04 is used in the acid mixtures with the oxidizing agents: nitric acid, perchloric acid, hydrogen peroxide, dichromate and perman- ganate. There are two reasons for this; hot concentrated H2S04 destroys most organic compounds by charring and the oxidizing power of the other agents present is enhanced. The mixture of H2SO4, HC104 and HN03 is known to be one of the most efficient and fast operating reagents, resulting in far better destruction of organic compounds than a mixture of perchloric and nitric acid only.4 In this laboratory, a modification of the tri-acid (H2S04, HCIO4 and HN03) digestion mixture proposed by Schaum- loffel5 has been used for a long time for the digestion of plant tissues.The mixture of H2S04-HC104-HN03 (1 + 4 + 40) contains a rather small amount of sulfuric acid so that for the conditions used (2 g of dried plant material, 20 ml of digestion mixture, 100 ml final volume), the danger of precipitation of CaS04 is minimized. In addition to the 'classical' analysis for micronutrients, determination of Pb and Cd is increasingly important with an increasing interest in the pollution of the environment. The determination of Cd in the above men- tioned tri-acid digest proved successful, but for Pb, although the majority of analyses were correct, in some samples the values were too low as was made clear by the use of laboratory quality control.In the following study, the reasons for the incomplete recovery of Pb are examined and an alternative digestion procedure using a mixture of HN03-H202-HF is proposed. Experimental Instrumentation All measurements were performed using a Varian SpectrAA- 300 atomic absorption spectrometer equipped with a graphite tube atomizer, an automated sampler and a Zeeman-effect background correction system. The operating parameters and temperature programme for the determination of Pb and Ba are given in Table 1. Pyrolytic graphite coated partition tubes were used throughout the experiment. For Pb, 0.2% palladium chloride was used as a chemical modifier (Varian Part No. 63-100012- 00) and for Ba no chemical modifier was necessary.The sample volume that was injected for both the Pb and Ba determination was 10 pl and the volume of the injected chemical modifier was 5 pl. As the Zeeman-effect system was able to correct properly for the background (there was no change in sensitivity when plant digests with standard additions or synthetic standard solutions were measured), all signals were read against the calibration graph. Samples Analyses of plant materials were carried out on two certified reference material samples produced by the Community Bureau of Reference (BCR)6 in Brussels, two reference materials evaluated by the Comitee Inter-Instituts (CII) d'Etudes des Techniques Analytique (Theiller et a1.7) and nine samples with known Pb contents originating from the Inter- national Plant-analytical Exchange Program (IPE) in Wageningen8.9 (a continuous proficiency control scheme, organized by this laboratory).The Pb contents of all the plant materials used, together with the major matrix components, are given in Table 2. Reagents Standard solutions of Pb and Ba were prepared from Titrisol ampoules (Merck, Darmstadt, Germany; Nos. 9969 and 9968, Table 1 Operating parameters and temperature programmes for the determination of Pb and Ba by electrothermal atomic absorption spectrometry using Zeeman-effect background correction Parameters Pb Ra Wavelength/nm 283.3 553.6 Slit- wid th/nm 0.5 0.5 Measurement mode Integrated Integrated Lamp current/mA 5 10 absorbance absorbance Replicates 3 3 Temperature programmes for the graphite furnuce- Pb Ba Tempera- Tempera- t u r d Sheath t u r d Sheath Step "C Time/s gas "C Tim& gas 1 2 3 4 5 6 7 8 9 10 11 95 5 .o 130 35.0 1000 25.0 1000 15.0 1000 5 .0 1000 2.5 2500 0.8 2500 3.0 2700 0.3 2700 5.0 40 13.3 Ar-H2* Ar-H2 Ar-H2 Ar-H2 Ar - - - Ar Ar Ar 95 5.0 Ar 130 35.0 Ar 1000 25.0 Ar 1000 15.0 Ar 40 5.8 Ar 40 2.0 Ar 40 2.5 - 2700 1.4 - 2700 4.0 - 2700 4.0 Ar 40 13.3 Ar * Ar-H2 = 95% argon and 5% hydrogen.24 ANALYST, JANUARY 1992, VOL.117 Table 2 Lead and major matrix components in 13 different plant samples (Pb in mg per kg of dry matter; average k standard deviation and the major matrix components in g per kg of dry matter; average) Sample BCR No. 61 Aquatic Moss (Platihypnidium riparioides) BCR No.62 Olive Leaves (Olea europaea) CII Hay IPE No. 757 Spinach (Spinacia oleracea) 1PE No. 864 Grass (Ruakura) (Poaceae) CII Lettuce (1) (Lactuca sativa) IPE No. 844 Lettuce (2) (Lactuca sativa) IPE No. 846 Carrots (shoot) (Daucus carota L.) IPE No. 838 Onion (Allium cepa L.) IPE No. 857 Cow Parsley (shoot) (A nth viscus sy fvesfris) IPE No. 847 Carrots (root) (Daucus carota L.) IPE No. 761 Pine (needles) (Pinus radiata) IPE No. 813 Sorghem (shoot) (Sorgh urn vulgare) Major matrix components/g kg- * mgkg-l K Na Ca Mg Mn Fe A1 S Si P N Reference Pb/ 64.4k3.5 12.5 2.97 16.9 3.9 3.77 9.30 10.7 2.3 75.3 9.21 33.5 25.0t-0.7 3.1 0.06 17.5 1.2 0.06 0.28 0.45 1.6 1.26 1.05 19.5 4.70k0.40 18.5 0.91 0.5 1.4 0.24 0.19 0.37 2.3 11.6 2.12 16.7 3.24-1-0.72 68.6 4.69 16.0 5.91 0.09 1.09 1.30 4.17 13.4 7.00 44.4 1.36f0.55 25.5 2.94 6.81 1.85 0.07 0.21 0.29 2.60 6.19 3.65 29.8 6.70k 1.20 70.6 4.90 14.0 4.6 0.05 0.49 0.49 2.7 - 7.13 44.4 2.62+-0.41 55.5 1.10 9.14 2.19 0.04 0.55 0.41 3.56 23.4 6.04 33.6 6.19-tO.82 24.4 0.87 24.2 4.59 0.42 0.49 0.33 3.75 7.83 3.78 30.8 1.05k0.28 14.5 0.60 11.8 1.05 0.02 0.80 0.95 2.85 10.3 2.48 18.2 5.8710.77 34.5 0.32 12.4 1.75 0.03 0.26 0.09 1.60 0.77 3.28 18.9 0.76t0.30 48.6 4.02 3.73 1.80 0.04 0.18 0.12 2.21 1.89 5.48 18.8 1.07-CO.25 8.4 0.57 2.44 1.24 0.32 0.08 0.45 0.99 0.22 1.21 22.3 2.5110.71 23.6 0.16 4.09 2.50 0.03 0.16 0.07 1.60 8.85 3.81 17.1 50-52 mm 0.d.8-9 mm 0.d. IS0 19/26 joint Fig. 1 with HN03, HC104 and H2S04 Construction of the digestion apparatus used for the digestion respectively) and diluted appropriately.The Pd chemical modifier solution was prepared by dissolving 0.20 g of PdC12 in 2 ml of concentrated HCl (p = 1.19 g cm-3) transferring into a 100 ml calibrated flask and diluting to the mark with doubly distilled water. Butanol (0.1 ml) was added, to 2 ml of the Pd solution, before use, in order to achieve more reproducible drying conditions in the graphite atomizer. 10 All reagents were of the highest purity and doubly distilled water was used throughout. Digestion Procedures Digestion with H N 0 3 , HC104 and H2W4 A 2 g sample of the plant material (in duplicate) was weighed out precisely into a flat-bottomed 100 ml flask (Fig. l), mixed with 20 ml of the acid mixture [HN03 (p = 1.40 g cm-3) + HC104(p= 1.67gcm-3) +H2S04(p= 1.84gcm-3) = 4 0 + 4 Fig.2 H202 and HF. All dimensions in millimetres Construction of PTFE tubes used for digestion with HN03. + 11 and left to stand overnight (to prevent excessive foaming). The flasks were heated moderately for at least 40 min until most of the nitric acid was distilled off. When the heat was increased, the mixture turned black and a vigorous reaction with HC104 took place. The digestion was complete when the solution cleared and white fumes appeared. There- after the digests were diluted with about 20 ml of doubly distilled water and boiled for about 15 min. After cooling, the solution was transferred into a 100 ml calibrated flask and diluted to the mark with doubly distilled water. The samples were filtered through a fine fluted ash-free filter in a polyethylene bottle.Digestion with H N 0 3 , H202 and HF For the digestion with HN03-H202-HF solution, poly- (tetrafluoroethylene) (PTFE) tubes with a diameter of 36 mm and a height of 102 mm (Fig. 2) and an aluminium heatingANALYST, JANUARY 1992, VOL. 117 25 ~ ~~~~~~ Table 3 Total Pb content (mg per kg of dry matter) in the BCR, CII and IPE plant reference material samples in the two digests studied (average _+ standard deviation) Total Pb/mg kg-1 Digestion Indicative and Plant material reference values HN03-HC104-H2S04 HN03-H202-HF Aquatic Moss Olive Leaves Hay Spinach Grass Lettuce (1) Lettuce(2) Carrots (shoot) Onion Cow Parsley (shoot) Carrots (root) Pine (needles) Sorghem (shoot) 64.4 k 3.5 25.0 k 1.5 4.70 f 0.40 3.46 k 0.37 1.36 k 0.27 6.70 k 1.20 2.74 f 0.31 6.17 k 0.48 1.04 f 0.22 5.78 f 0.42 0.74 k 0.27 1.04 k 0.24 2.51 +.0.48 36.5 k 1.8 19.2 f 0.4 1.59 k 0.20 2.83 k 0.01 0.53 k 0.04 6.00 k 0.07 3.30 k 0.01 6.87 k 0.42 1.40 f 0.06 4.97 k 0.24 0.95 k 0.15 1.43 k 0.02 3.06 k 0.37 Table 4 Equilibrium reactions and constants used for solubility calculations of Pb, Ca and of Ba at a total SO4 concentration of 0.081 mol dm-3, an ionic strength of 0.2 mol dm-3 and pH 1 Theoretical Equilibrium reaction log K" molar solubility H+ + OH- H20 14 H+ + SOj2- % HSOj- 1.98 0.0 Pb2+ + SO42- % PbS04" 2.62 Pb2+ + 2S042- Pb(S04)z2- 3.47 2.0 x (Pb total) 7.79 Ca2+ + SO4*- % CaS04" 2.31 Ca2+ + SO4*- % CaSO,(s) 4.62 18 x (Ca total) Ba*+ + HS04- z Ba(HSO,)+ 1.07 Ba2+ + 2HS04- Ba(HS04)20 1.93 Ba2+ + SO4*- BaS04" 2.3 3.6 x lo-' (Ba total) 2H+ + SO12- z H2SO4" Pb2+ + SO42- % PbS04(s) Ba2+ + SO4'- BaSO,(s) 9.77 block were used.A 2 g sample of the plant material (in duplicate) was weighed into the PTFE tubes, mixed with 10 ml of concentrated HN03 ( p = 1.40 g cm-3) and left to stand overnight. The tubes were covered with lids and placed into a port of the aluminium heating block and the contents were boiled for 4 h at about 120 "C (liquid temperature, just boiling). Then the lids were removed and the liquid was allowed to evaporate until the sample was almost dry. A 1 ml volume of H202 (p = 1.11 g (3117-3) was added three times and the liquid was allowed to evaporate after each addition. Thereafter, 2 ml of concentrated HN03 and 5 ml of concentrated HF ( p = 1.13 g cm-3) were added and the tubes again covered with lids and boiled for 4 h.The lids were removed and the liquid was allowed to evaporate until the sample was almost dry. The residue was taken up in 20 ml of 1 mol dm-3 HN03 and boiled for about 15 min, transferred into a 100 ml calibrated flask and diluted to the mark with doubly distilled water. The samples were filtered through a fine fluted ash-free filter into a polyethylene bottle. Results and Discussion The results of the Pb determination in both digests are given in Table 3. The values given are the means of two completely independent digestions and each digest was measured three times. Clearly, for Aquatic Moss, Olive Leaves, Spinach, Hay and Grass, lower values were found in HN03-HC104-H2S04 digests compared with the HN03-H202-HF digests. All values for the latter compared well with the certified values or reported concentrations in collaborative studies.Examination 66.2 k 2.4 24.4 f 1.3 4.60 k 0.19 4.57 f 0.90 1.80 5 0.24 6.60 f 0.43 3.11 k 0.14 7.53 k 0.21 1.51 k 0.01 4.60 k 0.29 0.91 f 0.03 1.50 k 0.03 2.96 k 0.04 Table 5 Barium content in the BCR, CII and IPE plant samples in the two studied digests. Results given are for total Ba (pmol dm-3; average k standard deviation) Digestion Plant material HN 03-H ClO4-H2S04 H N 03-H*OZ-H F Aquatic Moss 0.41 k 0.06 21.4 k 1.1 Olive Leaves 0.19 k 0.04 5.0 k 0.1 Hay 0.54 k 0.01 4.1 k 0.3 Spinach 0.51 k 0.09 2.8 f 0.2 Grass 0.34 _+ 0.05 3.1 k 0.3 Lettuce (1) 0.52 k 0.03 1.4 f 0.1 Lettuce (2) 0.50 k 0.02 2.6 k 0.1 Carrots (shoot) 0.66 f 0.12 1.8 k 0.2 Onion 0.71 _+ 0.01 1.5 f 0.1 Cow Parsley (shoot) 0.52 k 0.01 1.5 f 0.1 Carrots (root) 0.26 k 0.01 0.57 k 0.02 Pine (needles) 0.09 f 0.02 0.16 f 0.01 Sorghem (shoot) 0.51 k 0.01 0.63 f 0.01 Table 6 Recoveries of Pb, using the HN03-HC104-H2S04 digest after addition of Ba, for three plant tissues Recovery (% ) Added Ba/ Cow Parsley pmol dm-3 (shoot) Carrots (root) Hay 0 100 100 33 3.6 69 96 29 7.3 32 77 25 14.6 20 39 9 of the signals showed no differences and the background signals were not excessive (the sum of the atomic absorption and background signals never exceeded an absorbance value of 1 .2), so that the Zeeman-effect background correction system was able to work correctly.It was concluded that the cause of the differences should be looked for in the sample preparation stage. Looking at the composition of the digestion mixtures, there are two differences that might be responsible for the behav- iour found: the presence of H2SO4 in the tri-acid and the presence of the HF in the proposed digestion.It is known that when the dry ashing without expulsion of silica by HF is used for digestion of plant tissues, often low values for some elements are found." Examination of Table 2 shows that Aquatic Moss and Spinach are high in silica, but not Olive Leaves, Hay and Grass. Therefore, although the influence of the solubilization of silica by HF cannot be excluded as a possible source of differences in Pb concentrations in the two digests, it certainly does not offer a complete explanation.26 ANALYST, JANUARY 1992, VOL.117 Alternatively, the presence of H2S04 can lead to the formation of slightly soluble sulfates and to losses of Pb. The maximal theoretical solubility in the digest in the presence of solid sulfates of Pb itself, of Ca as a major component of the plant tissues and of Ba as the ion forming the most insoluble sulfate known was calculated. For the species taken into consideration, stability constants published by Smith and Martell12 and Sillkn and Martelll3 were used to calculate the molar solubilities of Pb, Ca and Ba for the existing conditions (total sulfate concentration = 0.081 mol dm-3; ionic strength = 0.2 mol dm-3; and pH = 1). The results are given in Table 4. The results of the calculations suggest that direct losses of Pb, due to the precipitation of PbS04 or sorption of Pb by CaS04, are highly improbable in the materials used because the total Pb and Ca content in the matrices is not high enough to allow precipitation of these sulfates. Little is known about the Ba content of plant tissues, Ba being neither a nutrient nor a pollutant.This may be the reason why Ba analysis is almost never performed. In order to discover the Ba status in the plant materials used, the Ba content in both digests (tri-acid and proposed) was determined. The results, shown in Table 5 , strongly suggest that precipitation of BaS04 took place in the digest containing H2S04. The mean concentration of soluble Ba in this digest (0.47 k 0.15 pmol dm-3; Pine Needles excluded) corresponds well with the calculated value from Table 4 (0.36 pmol dm-3).The formation of a mixed precipitate (Pb, Ba)S04 is a possibility; the ionic crystal radii of Pb2+ and Ba2+ are 1.20 and 1.34 A, respectively, which is close enough for the formation of mixed crystals.14 Also, the same crystalline form, rhombic, is possible for both sulfates. Examination of the Ba and Pb concentration in the two digests for samples where losses of Pb took place leads to the conclusion that the precipitate formed has an approximate composition (Pb0.1Ba0,9)S04; about 3 pmol dm-3 of Ba should be present in the digest in order to start coprecipitation. As a further check, two plant samples in which no losses of Pb in the tri-acid digests were observed were chosen and increasing amounts of Ba as BaCI2 added. After digestion with HN03- HC104-H2S04, the Pb content was measured.The results are given in Table 6. It can be seen that in both Cow Parsley, containing a reasonable amount of Pb, and Carrots (root), with very little lead, after addition of about 3-4 pmol dm-3 of Ba the recovery of Pb is not complete any more. Similarly, further addition of Ba to the Hay sample resulted in a further decrease of the recovery of Pb in this sample. From these experiments it can be concluded that when H2S04 is used in the digestion mixture and sufficient Ba is present in the plant tissue itself, coprecipitation of (Pb,Ba)S04 occurs, leading to low results in the determina- tion of Pb. Because the Ba content of plant materials is usually unknown and the danger of coprecipitation cannot be evaluated in time, it is recommended that H2S04 never be used for the digestion of plant samples when Pb is to be determined.1 2 3 4 5 6 7 8 9 10 11 12 13 14 References Gorsuch, T. T., The Destruction of Organic Matter, Pergamon Press, Oxford. 1970. Bock, R., Aufschlussmethoden der Anorganischen und Organ- ischen Chemie, Verlag Chemic, Weinheim. 1972. Sandell, E. B., and Oniski, H., Photometric Determination of Traces of Metals (General Aspects), Wiley, New York, 1978. Martinic, G. D., and Schilt, A. A.. Anal. Chem., 1976, 48, 70. Schaumloffel, E., Landwirtsch. Forsch., 1960, 13, 278. Community Bureau of Reference, Certificates of Analysis Aquatic Moss (BCR No. 61) and Olive Leaves (BCR No. 62), Brussels, 1986. Theiller, G., et Les Membres du Comitke Inter Institut, Rksultats Analytiques sur de Nouveaux Etalons Vigitaux du CII, Proceedings of the VI International Colloquium for the Optimization of Plant Nutrition, AIONP, Montpellier, 1984, vol. 4, p. 1339. Houba, V. J. G., van der Lee, J . J., and Novozamsky, I., Analusis, 1991, 19, 45. Houba, V. J . G., Uittenbogaard, J., and De Lange-Harmse, A.-M., Chemical Composition of Various Plant Species (1980- 1990). Report of the International Plant-analytical Exchange Program, Department of Soil Science and Plant Nutrition, Wageningen Agricultural University, The Netherlands, 1991. Temminghoff, E. J. M., J. Anal. At. Spectrom., 1990, 5 , 273. Ledent. G., dc Borger, R., and van Hentenrijk, S., Analusis. 1984, 12, 393. Smith, R. M., and Martell, A. E . , Critical Stability Constants: Inorganic Complexes, Plenum, New York, vol. 4, 1981. Sillkn, L. G.. and Martell, A. E., Stability Constants of Metal-ion Complexes, The Chemical Society, London, 1964. Kolthoff, I . M., J. Phys. Chem., 1932. 36, 860. Paper 1 I02652 F Received June 4, 1991 Accepted August 19, 1991

ISSN:0003-2654    

DOI:10.1039/AN9921700023

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

ANALYST. JANUARY 1992, VOL. 117 27 Determination of Lead, Cadmium, Zinc and Tin in Samples of Poly(viny1 chloride) by Square-wave Voltammetry and Atomic Absorption Spectrometry Rao V. C. Peddy, G. Kalpana and Valsamma J. Koshy Research Centre, Indian Petrochemicals Corp. Ltd., Baroda-397 346, India Many metallic compounds used as stabilizers are incorporated in poly(viny1 chloride) (PVC) to improve its characteristics for various commercial applications. The determination of Pb, Cd, Zn and Sn in PVC samples by square-wave voltammetry (SWV) and atomic absorption spectrometry (AAS) is described. Two different procedures, using H2S04-H202-N H3-ethylenediami netetraacetic acid and H2S04-H202-H NO3 reagents to dissolve lead sulfate, were developed in order to suit the experimental requirements of AAS and SWV methods.The Pb-Cd-Zn mixed metal system was anaiysed by SWV and AAS and good recoveries were found for synthetic samples. The advantage of SWV over AAS is that it is more sensitive and the simultaneous determination of these elements is feasible. A good correlation between the data from SWV and AAS was obtained for all the metals ( r > 0.995). Tin can be readily determined using SWV and AAS methods. The SWV determination of Sn using 1 mot dm-3 HCI-4 mol dm-3 NH4CI buffer was carried out and the recovery was comparable t o that obtained using AAS. The same method was extended to a commercial sample. Statistical evaluation showed no significant bias between the methods used. Keywords: Square-wave voltammetry; atomic absorption spectrometry; lead, cadmium, zinc and tin determination; poly(vin yl chloride) Poly(viny1 chloride) (PVC) is the most important vinyl polymer, and to achieve processability and optimum perfor- mance characteristics, various compounding additives must be added.1 Many of these additives are metallic compounds, which affect the physico-mechanical properties of the end product.2 Lead compounds are used to improve the electrical and heat-stabilizing properties in applications such as cable sheathing and high-temperature extrusions and zinc com- pounds are used as mild stabilizers for non-toxic applications, particularly with epoxy resins.3 Cadmium laurate and stearate are used as light stabilizers and to confer good lubricating properties for obtaining transparent compounds.Mixtures of these metal additives are used for synergistic effects.3 The effectiveness of these metallic stabilizers depends on many factors, such as their concentration and their compatibil- ity with the polymer system. Although it is obviously important to be able to determine these additives in PVC samples, not many methods are available. Usually the most problematic and time-consuming part of the analytical process is the dissolution of the plastic. The classical method of calcination and subsequent dissolution of the resin in acid536 has the disadvantage of the possible loss of volatile com- ponents, and the treatment with concentrated nitric acid proposed by Rombach et af.7 is too long, as the attack by acid takes 8 h. Taubinger and Wilson8 studied the use of 50% hydrogen peroxide together with concentrated sulfuric acid to treat various organic samples, including PVC.Mendiola et af.”lO tested other methods of attack, and concluded that the most satisfactory results were obtained with sulfuric acid and hydrogen peroxide. However, the treatment with sulfuric acid causes difficulties if the PVC contains Pb or alkaline earth metals because of the precipitation of the corresponding sulfates. To overcome this problem, Belarra and co-work- ersl l , I 2 used an NH3-ethylenediaminetetraacetic acid (EDTA) reagent to dissolve the sulfates, but this reagent interfered in analysis by voltammetry. In this paper, an alternative method for the dissolution of PVC samples is suggested. The determination of trace metals, viz., Pb, Cd, Zn and Sn, in different combinations in a PVC matrix using square-wave voltammetry (SWV) and atomic absorption spectrometry (AAS) techniques was carried out and the results are compared.Experimental Instrumentation A GBC Model 902 atomic absorption spectrometer, equipped with a deuterium-arc background corrector, a 10 cm laminar- flow air-acetylene burner, a 5 cm laminar-flow dinitrogen oxide-acetylene burner, an Epson LX-800 printer and hollow cathode lamps, was used under the conditions given in Table 1. A Princeton Applied Research (PAR) Model 384B-4 polarographic analyser system together with a PAR 303-A static mercury drop electrode (SMDE) were used for all SWV measurements and the voltammograms were recorded on a Houston DMP-40 digital plotter.Potentiostatic control of the electrode potential was established by means of a three- electrode system consisting of an SMDE, a platinum-wire counter electrode and an Ag-AgCI-KCI (saturated) electrode as the reference electrode. Triply distilled mercury was used. Reagents and Chemicals All solutions were prepared from analytical-reagent grade chemicals, unless indicated otherwise, and water purified using a Milli-Q system (Millipore). Aldrich AAS standard solutions of Pb, Cd, Zn and Sn of 1000 pg ml-1 were used for preparing calibration standards. AnalaR-grade concentrated sulfuric acid (density 1.84 g cm-3), hydrogen peroxide (30% v/v), concentrated nitric acid (density 1.42 g cm-3) and concentrated ammonia solution (density 0.89 g cm-3) were Table 1 Instrumental parameters used for AAS analysis Distance of Lamp burner below Working Wavelength/ current/ optical range/ Element nm mA Flame* axis/mm pgml-’ Pb 217.0 5 AA 6 0-8 Cd 228.8 3 AA 6 0-2 Zn 213.9 5 AA 6 0-1.4 Sn 235.5 5 NA 6 0-80 * AA = air-acetylene; NA = dinitrogen oxide-acetylene.28 ANALYST, JANUARY 1992, VOL.117 used during different stages of dissolution. The EDTA solution (4% m/v) was prepared by dissolving 4.00 g of EDTA (as the disodium salt) in 100 ml of water together with a few drops of ammonia. Acetate buffer (0.1 rnol dm-3) was prepared by dissolving 8.2 g of sodium acetate in water, adjusting the pH to 4.5 with glacial acetic acid and diluting to 1 dm3 with de-ionized water. The HCI (1 rnol dm-3)-NH4CI (4 rnol dm-3) buffer was prepared by placing 8.25 ml of concentrated HCI and 21.4 g of NH4CI in a 100 ml calibrated flask and diluting to volume with de-ionized water. Sample Preparation Method A For AAS analysis, the dissolution procedure reported by Belarra et al." was followed for preparing sample solutions, the insoluble sulfate being brought into solution by converting it into its EDTA complex. A reagent blank was prepared by a similar procedure.For the determination of Sn in PVC, the sample solutions were clear after the H2S04-H202 digestion step, hence the same solutions were subjected to the analysis. Method B The sample preparation up to the stage of H202 treatment in method A was followed. The solution was then boiled to eliminate the excess of H202, 2 ml of concentrated HN03 were added and the mixture was heated vigorously to dissolve the precipitate. Subsequently the solution was cooled and diluted to 50 ml with water. A reagent blank was prepared by a similar method.Calibration For the AAS determination of all the elements, calibration solutions were prepared from their AAS standard solutions in the range suggested in Table 1. Matrix matching was carried out using the reagent blank, depending on the sample dilution. For obtaining the background current, square-wave voltam- mograms were recorded between an initial potential of -0.2 V and a final potential of - 1.2 V on 10 ml of de-aerated solution containing only the supporting electrolyte. This background current was deducted from the respective peak currents of the analyte. The experimental conditions for the SWV determina- tion of Pb, Cd, Zn and Sn are given in Table 2.Results and Discussion Sample Dissolution The dissolution procedure of Belarra et al.11 for the AAS determination of trace metals in a PVC matrix was followed. For SWV analysis, after the stage of H2S04-H202 digestion, the precipitate of lead sulfate was brought into solution using concentrated HN03 because the NH3-EDTA reagent caused Table 2 Experimental conditions for SWV Calibration Supporting Peak range1 Element electrolyte pH potentiallv yg ml-l Pb 0.1 mol dm--? Cd 0.1 rnol dm--? Zn 0.1 mol dm-3 Sn 1 rnol dm-3 HCI- acetate buffer 2.49 -0.496 1-5 acetate buffer 2.49 -0.670 1-5 acetate buffer 2.49 - 1.080 5-25 4 mol dm-3 NH4CI buffer -* -0.660 1-5 * Highly acidic. serious interference in the voltammetric analysis.It has been found that the presence of EDTA masks the analyte, resulting in a reduction in peak currents in SWV analysis. Pb-Cd-Zn Mixed Metal System As mentioned under Experimental, dissolution of the insol- uble sulfates was effected using the NH3-EDTA reagent for the AAS method and concentrated HN03 for the SWV method. The effect of the reagents was studied for the AAS method. Fig. 1 shows the change in analyte signal with increase in reagent concentration. Although sample dilution was suggested for the interference-free determination of Cd and Pb by Belarra and co-workers, 11.12 interactive matrix matching was found to be more suitable. For SWV analysis, the experimental conditions were optimized (Table 2) with respect to pH and the effect of buffer and its concentration, etc.Fig. 2(a) shows the square-wave voltammograms recorded in 0.1 rnol dm-3 acetate buffer containing Pb2+, Cd2+ and Zn2+ (all at 1 ppm concentration). The voltammetric pattern is characterized by three well defined peaks at -0.438, -0.628 and -1.048 V, which correspond to the two-electron reduction of Pb2+, Cd2+ and 0.130 0.120 0.140 I* It P 0.110 0.30 9 2 4 6 8 Reagent volume/ml Fig. 1 Effect of reagent concentration on atomization of various elements. Reagent composition: 2 ml of concentrated H2S03 + 10 ml of NH3 + 10 ml of 4% EDTA in 100 ml. (a) Pb; (b) Cd; and (c) Zn (cach 1 pprn) t c 2 3 0 -0.4 -0.6 -0.8 -1.0 -1.2 Poten tia IN versus Ag-Ag CI Fig. 2 Effect of dissolution reagent on SWV analysis of Pb-Cd-Zn system. (a) 10 pg ml-1 admixture of Pb, Cd and Zn standards in acetate buffer; ( b ) 10 pg ml-1 admixture of concentrated HN03 digest; and ( c ) 10 yg ml-1 admixture of NH3-EDTA digestANALYST, JANUARY 1992, VOL.117 29 Table 3 Calibration characteristics for the different metal additives in PVC Additive Analyte Method Regression equation* Pb-Cd-Zn Pb SWV y = 2.179~ + 4.800 X AAS AAS AAS AAS y = 3.392 x10-2x + 1.493 X y = 0.353~ + 2.653 X lop2 y = 0.240~ + 3.600 x lop2 y = 4.900 X lO-’x + 6.000 X Cd SWV y = 3.663~ + 0.348 Zn SWV y = 4.681~ + 0.562 Sn Sn SWV y = 3.900~ + 4.348 * x = Concentration (pg ml-1); y = peak current (nA) for SWV and absorbance for AAS. t n = 5 . Correlation coefficient (Y) 0.997 0.999 0.999 0.998 0.999 0.993 0.984 0.999 Relative standard deviation 1 .o 1.2 0.5 1.1 1.0 1 .o 0.8 2.0 (%)t 8.0 $ 6.0 E (3 4.0 2 2.0 Pb Zn n -0.4 -0.6 -0.8 -1.0 -1.2 Potent i a IN versus Ag -Ag C I Fig.3 Square-wave voltammograms of Pb, Cd and Zn with increasing analyte concentrations Zn2+, respectively. It can be seen that the voltammetric peaks of the metal ions are clearly separated under the same experimental conditions using an SMDE. Fig. 2(b) and (c) shows the square-wave voltammograms for a sample solution of the mixed metal salt system using methods A and B for dissolution. It can be seen that the peak potentials of Pb, Cd and Zn were shifted in the cathodic direction with HN03 digestion, probably owing to complexation, and there was no substantial difference in the peak currents. The voltammo- grams for Pb-Cd-Zn solution prepared by the NH3-EDTA route showed a distorted pattern [Fig.2(c)] and the Zn2+ signal was absent. The sensitivity in terms of the peak currents of the metal ions decreased markedly owing to the strong masking nature of EDTA. Hence the solutions prepared by method B were adopted subsequently for all the SWV analyses. Voltammograms for the simultaneous determination of Pb2+, Cd2+ and Zn2+ in a 1 : 1 : 1 molar ratio are shown in Fig. 3. Calibration graphs for Pb, Cd and Zn in the concentration ranges indicated in Tables 1 and 2 were prepared using standard solutions of these metals for analysis by AAS and SWV. The calibration graphs obtained by both methods were linear and the correlation coefficients and relative standard deviations are given in Table 3.The effectiveness of the procedure was tested by analysing admixtures of Pb, Cd and Zn in different proportions by both AAS and SWV. The recoveries were virtually quantitative in all instances (Table 4). Regression analysis was carried out on the data obtained by AAS and SWV and the regression coefficients were greater than 0.995. Interferences from other concomitants in the determination of Pb, Cd and Zn by AAS and SWV were studied and the results are presented in Table 5. Interferences from Sb and Sn were found to be high even at 1 : 1 molar ratios Table 4 Results of recovery assays of Pb, Cd and Zn Total metal Metal found/mg Recovery (%) added/ Element Sample mg SWV AAS SWV AAS Pb s- 1 s-2 s-3 Cd s-1 s-2 s-3 Zn s- 1 s-2 s-3 0.88 0.89 0.88 1.48 1.49 1.49 1.68 1.71 1.68 1.00 0.97 1.00 1.20 1.21 1.21 1.40 1.44 1.39 1.00 1.04 0.98 2.00 1.97 2.00 2.20 2.20 2.22 101.1 100.0 100.7 100.7 101.8 100.0 Mean: 101.2 100.2 97.0 100.0 101.0 101.0 102.8 99.3 Mean: 100.3 100.1 104.0 98.0 98.5 100.0 100.0 100.9 Mean: 100.8 99.6 ~~ Table 5 Results of interference study Tolerated metal to interferent ratio Pb Cd Zn Interferent Sb A1 Pb Cd Zn Ba Ca Sn Mg AAS SWV 1:lOO 1 : l O 1 : l O O 1 : l O O 1:100 1 : l O 1:100 1 : l O 1:100 1 : l O O 1:100 1 : l O 1:loo 1:lO 1:lOO 1:lO - - AAS SWV 1 : l O O 1 : l O 1:lOO 1:100 1:lOO 1:lO 1 : l O 1 : l O 1:100 1:lOO 1:100 1:lO 1:100 1:lOO 1:lO 1 : l - - AAS SWV 1:loo 1:l 1:100 1:lOO 1:100 1:100 1 : l O O 1 : l O O 1:lOO 1:lOO 1:lO 1:100 1:loo 1:lO 1:lO 1:lO - - in the SWV determination of Zn and Cd, respectively.However, the tolerance limits for other metals are good (metal to interferent ratio >1: 100) in both the AAS and SWV methods. The results obtained by the two methods were compared by using standard statistical techniques to assess the systematic errors.13 With the F-test at the 95% confidence level, the calculated Fvalue did not exceed the tabulated value. Hence it was concluded that on a statistical basis there was no significant bias between the methods used. It is clear that the procedure provides an accurate and rapid method for the determination of Pb, Cd and Zn, each with the others present by both AAS and SWV. Sn-PVC System Tin additives can be used with PVC as heat and light stabilizers for obtaining transparent sheeting and flexible compositions.1 In the determination of Sn in this work the NH3-EDTA dissolution step was not necessary as the solution was clear and free from precipitates after the H2S04-H202 digestion30 ANALYST, JANUARY 1992, VOL.117 10.0 8.0 5 6.0 2 2 3 u 4.0 2.0 0 PotentialN versus Ag-AgCI 0.5 0.7 0.9 Fig. 4 centration: A, 2; B, 3; C, 4; and D, 5 ppm Square-wave voltammograms of Sn with increasing con- Table 6 Results of recovery assays of Sn Total metal Metal found/mg Recovery (%) added/ Sample mg SWV AAS SWV AAS s-4 0.500 0.501 0.492 100.2 98.4 s-5 0.750 0.741 0.769 98.8 102.5 S-6 1.OOO 0.989 1.007 98.9 100.7 Mean: 99.3 100.5 s-7* - 0.240 0.246 * S-7 = commercial sample of PVC with Sn stabilizer. step. However, if Sn is present together with additives of alkali or alkaline earth metals or of lead salts, methods A and B described above should be followed for the sample dissolu- tion.In the SWV method, Sn determination was attempted in various buffers, viz., acetate, tartrate, citrate and HCl- NI&CI, but it was found that there is no quantitative response in any of these buffer systems except HCI-NH,CI, probably owing to strong complexation of Sn with the ligand. Calibra- tion graphs for Sn in the concentration ranges indicated in Tables 1 and 2 were prepared using a standard Sn solution for analysis by SWV and AAS. There was a systematic trend observed for the SWV method in 1 mol dm-3 HC1-4 mol dm-3 NH4CI supporting electrolyte and typical square-wave voltam- mograms with increasing concentration of the analyte are shown in Fig.4. The calibration graph equations obtained from a least-squares fit, correlation coefficients and relative standard deviations for Sn using both the AAS and SWV methods are given in Table 3. Recovery assays of Sn in synthetic samples of PVC (S-44-6) were carried out by both the AAS and SWV methods and the results are given in Table 6. An F-test at the 95% confidence level showed no significant error. The analysis was extended to a commercial sample (S-7) and there was reasonable agreement between the SWV and AAS results. The influence of other metals often present as additives in PVC, viz., Cd, Mg, Ba, Zn, Pb, Al, Sb and Ca, was studied and it was found that the tolerance limits were higher in the AAS method (metal to interferent ratio 1 : 100) than in the SWV method (metal to interferent ratio 1 : 10).There was serious interference from Pb, Ba and Ca in both the AAS and SWV determination of Sn owing to the formation of insoluble sulfates even at a 1 : 10 metal to interferent ratio. The SWV method was found to be more sensitive (1-5 pg ml-1 linear calibration range) than the AAS method (10-80 pg ml-1 linear calibration range) for the determination of Sn. Conclusions The results indicate that treatment with H2S04-HN03 solu- tion is suitable for dissolving insoluble sulfates formed when the PVC contains lead compounds. This method serves as an alternative to NH3-EDTA dissolution and its specific advan- tage is that it can be adopted in SWV techniques where the NH3-EDTA reagent gives severe interferences. However, the NH3-EDTA dissolution procedure can be used for all the metals studied when using the AAS method.The reliability of the methods is shown by the results of the recovery studies and by the satisfactory determination of these metals in PVC samples. The procedure is rapid and simple, the effects of the reagents on the AAS analysis are easily compensated for by interactive matrix matching and good accuracy and precision are obtained. The potential of SWV in the determination of trace metals in polymers was explored and its versatility for both qualitative and quantitative analyses was demonstrated. It is possible to determine Pb, Cd and Zn simultaneously by SWV, in contrast to the single-element AAS technique. The tolerance limits were higher for AAS than for the SWV technique.In conclusion, both methods are suitable for the determina- tion of trace metals in PVC but, depending on the combina- tion of metals and their amounts present, appropriate methods have to be chosen. The authors thank N. R. Shah, T. K. Joshi and R. H . Patel for experimental assistance. They are grateful to Dr. I. S. Bhardwaj, Director (Research Centre, IPCL), for his kind permission to publish this work. 1 2 3 4 5 6 7 8 9 10 11 12 13 References Titow, W. V., PVC Technology, Elsevier Applied Science, Barking, 4th edn., 1984. Whelan, A., Developments in PVC Production and Pro- cessing-I, Elsevier Applied Science, Barking, 1977. Nasc, L. I., Encyclopedia of PVC, Marcel Dekker, New York, 1976, vol. 1. Owen, E. D., Degradation and Stabilization of PVC, Elsevier Applied Science, Barking, 1984. Druckmann, D., At. Absorpt. Newsl., 1967, 6, 113. Fassy, H., and Lalet, P., Chim. Anal. (Paris), 1970, 52, 1281. Rombach, N., Ape], R., and Tschochner, G., GZT Fachz. Lab., 1980,24, 1165. Taubinger. R. P., and Wilson, J. R., Analyst, 1965, 90, 429. Mendiola, J. M., Gonzalez, A., and Arribas, S., Afinidad, 1980, 37,39. Mendiola, J. M., Gonzalez, A., and Arribas, S . . Afinidad, 1980, 37,251. Belarra, M. A., Gallarta, F., Anzano, J. M., and Castillo, J. R., J. Anal. At. Spectrom., 1986, 1, 141. Belarra, M. A., Azofra, M. C., Anzano, J. M., and Castillo, J. R., J. Anal. At. Spectrom.. 1989, 4, 101. 1985 Annual Book of ASTM Standards, American Society for Testing and Materials, Philadelphia, 1985, vol. 03.06, Standard E 876 82, p. 346. Paper 1f03505C Received July 11, 1991 Accepted August 22, 1991

ISSN:0003-2654    

DOI:10.1039/AN9921700027

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

ANALYST, JANUARY 1992, VOL. 117 31 Determination of Lead in Wines by Hydride Generation Atomic Absorption Spectrometry Juan Cacho, Vicente Ferreira and Cristina Nerin Department of Analytical Chemistry, Faculty of Sciences, University of Zaragoza, Zaragoza, Spain The optimization of lead hydride generation in aqueous ethanolic media and the influence on its generation of the wine components, both white and red, have been studied. These interferences were overcome by careful control of the parameters affecting hydride generation and the procedure was applied to the determination of Pb in wines. The method is fast, accurate and sensitive and can be used to quantify 24 ppb of Pb in wines. Keywords: Lead determination; atomic absorption spectrometry; wines; h ydride generation; atomization Lead is one of the elements that has received considerable attention in recent years, because of its high toxicity, and the broad range of organs and systems that are affected in man and animals through many routes. Lead is present in the atmosphere and in foodstuffs, either naturally or as a contaminant, although usually at very low concentrations.Therefore, the determination of Pb requires procedures that are sufficiently sensitive for detection at the ppb level. Atomic spectrometry is one such procedure. Lead hydride generation and its measurement by atomic absorption spec- trometry has been shown to meet the sensitivity and selectivity requirements for the determination of Pb. However, the efficiency of the lead hydride generation is critically depen- dent on the generation conditions.For this reason, much work has been carried out to optimize the acidity, and on the use of an oxidant,' the nature of the acid and oxidant'-10 and their role in the generation.3.11-12 Whereas the generation of lead hydride in water is well established, the same is not true for aqueous ethanolic media, where the generation efficiency is very low. This is a great disadvantage, as it prevents this method from being directly applied to the analysis of alcoholic beverages, and conse- quently tedious procedures must be used. The official method for determining Pb in wine involves digestion with mineral acids followed by spectrophotometric determination, 13 which is clearly more time-consuming and also less sensitive. In order to apply the lead hydride generation method to such an important beverage as wine, the generation of the gas in the presence of the characteristic components of wine was optimized, and a procedure for determining Pb in wine with this technique was developed.This paper describes the results of the study. Experimental Apparatus A Perkin-Elmer 2280 atomic absorption spectrometer and a Perkin-Elmer MHS- 10 hydride generator were used. The argon head pressure was 250 kPa. The pH measurements were made using a Crison 2002 pH meter. Reagents Aqueous solutions of Pb(N03)2, 6 g 1-1 and 6 ppm. Identical solutions of Pb(N03)2 in wine and in aqueous ethanolic-tartaric acid media (12.5% ethanol, 0.4% tartaric acid). Sodium tetrahydroborate solutions in 1% NaOH, 5 , 7 , 10, 12, 15, 17, 19, 21, 23 and 25%.Hydrochloric acid, 0.8, 1.2 and 1.4 mol dm-3. Ethanol solution in water, 12.5% with 0.4% mlv tartaric acid. Aqueous HCl solution, p H 0.86. Hydrogen peroxide solutions, 4, 5 , 6 , 7, 7.5, 8 and 10%. Study of the Influence of Different Components of Wine In the reaction flask place 5 ml of Pb solution in wine, or Pb in a 12.5% ethanol-0.4% aqueous tartaric acid solution. Add 3 ml of HCl solution and 2 ml of H202. Turn on the argon flow and inject the NaBH4 solution in 1% NaOH, to obtain a signal maximum at 283.3 nm. One parameter was varied while keeping the others fixed. The optimum values were adopted for subsequent studies. Recommended Procedure Take 30 ml of wine, adjust the pH to 0.86 by adding 1.2 mol dm-3 HCl and dilute to 50 ml with aqueous HCl solution (pH = 0.86).Transfer 7.5 ml of this solution into the hydride generator reaction vessel, add 2.5 ml of H202 (7.5%) and leave the argon gas flowing for 3 min. Then inject the 21% NaBH4 solution until a maximum signal is obtained at 283.3 nm (after approximately 10 s). Results and Discussion Ethanol, tartaric acid and SO2 were chosen as the characteris- tic components of wine. Their individual and combined influence on the generation of lead hydride was studied. Influence of Ethanol As expected, on changing from aqueous to aqueous ethanolic media, the hydride generation efficiency decreased markedly , as can be seen in Fig. 1. However, above a concentration of 5% ethanol, the efficiency varies very little, which may be owing to the low variation in pH and oxidation-reduction potentials of the substances present.This slight variation is an 0 5 10 15 20 25 Ethanol (%) Effect of the presence of ethanol on PbH4 generation Fig. 132 ANALYST, JANUARY 1992, VOL. 117 0.2 I I I I I 1 0 2 4 6 8 10 Tartaric acid/g I-' 1 I I 1 1 I 1 0 0.2 0.4 0.6 0.8 1 .o Sulfur dioxide/g I-' Fig. 2 Effect of the presence of tartaric acid and SO2 on PbH4 generation. A, Tartaric acid; and B, SO2 V J I I 1 I I I 4 5 6 7 8 9 10 Hydrogen peroxide (%) Optimization of the concentration of H202. The signal value is Fig. 3 expressed in relation to the optimum signal obtained in water advantage when preparing a calibration graph for the determi- nation of Pb in alcoholic beverages, as the ethanol content in different samples is generally similar, although not identical. Influence of Tartaric Acid The presence of tartaric acid also affects the lead hydride generation efficiency, although to a lesser extent than ethanol, as shown in Fig.2. The decrease is due to a modification of the solution pH. Influence of Sulfur Dioxide Sulfur dioxide at the concentrations normally found in wines does not interfere in lead hydride generation even if the SO2 concentration is more than twice the highest SO2 concentra- tion founded in the wines (1 g 1-1). Higher concentrations have not been studied, as they are unlikely to be encountered in real samples. Fig. 2 shows the influence of SO2. Optimization of Lead Hydride Generation in Aqueous Ethanolic Media Once the influence of ethanol and tartaric acid was known, the generation of the lead hydride in aqueous solutions containing proportions of these compounds similar to those found in wine was studied.The concentrations of the reducing agent, HCl and oxidizing agent were examined. The lead hydride absorption signals obtained in this way were compared with those obtained in water under optimum conditions. The influence of acidity reveals that the HCI concentration has a strong influence on the generation of lead hydride, and that small variations in the acid concentration produce large variations in the signal. This marked effect is not observed in waters nor in organic solvents such as dimethylformamide (DMF).14 Less HCl is required to produce the maximum signal in this medium than in water. The optimum pH was found to be 0.86.I I I 1 I 1 0 5 10 15 20 25 Tetrahydroborate (%) Fig. 4 Optimization of the concentration of NaBH4. The signal value is expressed in relation to the optimum signal obtained in water The peroxide concentration is also important, although much less so than that of HCI. Working with optimum HCl concentrations, the maximum signal is obtained at an H202 concentration of 7.5% (Fig. 3 ) , i . e . , the same concentration which gives the maximum signal in water. However, the signal varies much less with variations in H202 concentration in an aqueous ethanolic medium than in water, unlike HCI. The variation of the atomic absorption signal with the NaBH4 concentration is completely different from that found in water. Using optimum conditions for HCl and H202, the signal maximum is found when the concentration of added NaBH4 is 21% (Fig.4), whereas in water the maximum is obtained with 7%. However, even with the different optimum conditions for lead hydride generation found in water and in aqueous ethanolic media, the signals are equal for the two media if the amount of Pb is the same and in both instances the work is performed under the respective optimum conditions; Le., the interferences due to ethanol and tartaric acid are eliminated. Other experimental parameters were studied in order to optimize the working conditions. The argon flow rate was varied slightly and it was found that small variations in the argon head pressure did not affect the generation and detection conditions and that 250 kPa was the optimum argon head pressure.The two main absorption lines for Pb were tested and it was found that better blanks were obtained by working at 283.3 nm. Lead Hydride Generation in Wine It is difficult to study the influence on lead hydride generation of other compounds present in wine because of their large number, and the fact that there are no standards for most of them. Therefore, for this study, different wines were taken and the lead hydride signal variation was measured when the generation conditions varied slightly around the optimum conditions found above for aqueous ethanolic media. The characteristics of the wines are given in Table 1, and as there is a broad range, the results can be extrapolated for any type of wine. A study of the signal variation with the pH shows that the other components in wine do not alter the optimum pH, which is still 0.86 in all instances.However, they do produce substantial modifications in the dependence curves (Fig. 5). Therefore, this parameter must be measured with a pH meter to guarantee reproducibility and accuracy. The signal variation with NaBH4 and H202 concentrations is similar to that found in aqueous ethanolic media, and the maximum is obtained at the same concentration, 7.5% H202 and 21% NaBH4. This last study showed that the lead hydride signal depends on the reaction time between the oxidizing agent and the wine components. A study of the influence of reaction time showed that white and red wines behave differently, as shown in Fig. 6. White wines take about 50 s to reach the maximum signal, whereas red wines need 2.5 min.After this time, the signal isANALYST. JANUARY 1992, VOL. 117 33 0.6 I 1 Table 1 Physico-chemical characteristics of the wines selected for the study Acidity/ Ethanol mequiv 1-1 Relative Total SO2 Extract/ Sample (%) H2S04 density (ppm) gl-I 1 Cava-brut 2 White-dry 3 White-dry 4 White-dry 5 White-dry 6 White-semi 7 Rose 8 Rose 9 Rose 10 Red 11 Red 12 Red 13 Red 14 Red 10.97 11.94 12.92 12.40 13.10 12.27 12.73 11.30 10.45 13.16 13.29 13.13 13.14 12.94 85 79 77 66 72 93 69 72 81 76 69 70 84 74 0.99230 0.99175 0.99090 0.99 140 0.99095 0.99465 0.99070 0.99165 0.99670 0.99290 0.99227 0.99660 0.99435 0.99330 157 15.4 155 19.3 188 20.1 162 14.8 212 20.9 232 27.9 113 19.0 70 23.7 129 21.4 115 26.1 106 27.2 0 19.8 248 22.6 104 26.3 20 0.7 I 0.8 0.9 1 .o PH Fig.5 Variation of the signal with pH in two different types of wine the same for both white and red wines and is stable for at least a further 3 min. For this reason, in other studies and in the recommended procedure, the reaction time between the H202 and Pb in the wine was fixed at 3 min. After this time, the NaBH4 solution is added and the lead hydride generation takes place rapidly, reaching the maximum value in 10 s . Having found the optimum generation conditions, the parameters of the analytical method were studied. The following were found: Beer’s law holds for all wines up to at least 800 ppb of Pb; the calibration graph is described by the equation A = 0.005 + 0.75c, where c is expressed in ppm and the correlation coefficient, r = 0.998.The value of the blank varies considerably with the origin of the NaBH4 used (this study found extreme absorbance values of 0.015 and 0.062). For the same quality of NaBH4, the standard deviation of the measurement was 0.002 A. Under these conditions, the detection limit, for a value of K = 10, is 24 ppb. In order to test the repeatability of the method six different wines were used. Each wine was analysed following the proposed method with five replicates. The reproducibility was tested by repeating exactly these operations on five different working days. The mean relative standard deviation (RSD) found for the repeatability was 2.2% with extreme values of 1.3 and 3.2% for each of the five working sessions and the mean RSD found for the reproducibility was 2.4% with extreme values of 1.9 and 3.7%.This procedure was tested on wines with different charac- teristics and origins. The Pb content was determined both by interpolation of the calibration graph and by the standard additions method. Table 2 shows the results obtained, together with the characteristics of the wines analysed. 0.5 0.4 a C 4 0.3 0 Ll a 0.2 O.’ tlf 14 I I I 0 100 200 300 Tirne/s Fig. 6 Variation of the Pb signal with time of reaction between the oxidizing agent and white and red wines. A, Red wine; and B, white wine Table 2 Comparative study between the official method, proposed method and standard additions method (mean of three replicates) Pb content (ppb) Sample 1 Cava-brut 2 White-dry 3 White-dry 4 White-dry 5 White-dry 6 White-semi 7 Rose 8 RosC 9 RosC 10 Red 11 Red 12 Red 13 Red 14 Red * Ref.13. Proposed method 63 89 65 0 123 91 280 72 17 162 95 77 49 43 Standard additions method 69 94 68 1 116 86 275 70 14 159 95 76 51 45 Official method* 64 92 68 0 119 88 274 74 4 167 98 79 49 41 simple, fast, sensitive and accurate, and is therefore recom- mended for determining the Pb content of wine. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 References Fleming, H. D., and Ide, R. G., Anal. Chirn. A d a , 1976,83,67. Thompson, K. C., and Thomerson, D. R., Analyst, 1974, 99, 595. Vijan, P. N., and Wood, G. R., Analyst, 1976, 101,966. Jin, K., and Taga, M., Bunseki Kagaku, 1978, 27, 759. Jin, K., and Taga, M., Bunseki Kagaku, 1980, 29, 522. Vijan. P. N., and Sadana, R. S . , Talanta, 1980, 27, 321. Smith, R., At. Spectrosc., 1981, 2, 155. D’Ulivo, A., and Pappof, P., Talanta, 1985, 32, 383. Ikeda. M., Jiro, N., and Nakahara, T., Bunseki Kagaku, 1981, 30, 368. Kumamaru, T., Nakata, F., and Hara, S . , Bunseki Kagaku, 1984. 33, 624. Jin. K., and Taga, M., Anal. Chim. Acta, 1982, 143,229. Nerin, C., Olavide, S . , Cacho, J . , and Garnica, A . , Water, Air, Soil Pollut., 1989, 44, 339. Metodos Oficiales de Analisis, Ministerio de Agricultura, Madrid, 1980. Aznarez, J . , Palacios, F., Ortega, M. S., and Vidal, J . C., Analyst, 1984, 109, 123. Conclusion Lead can easily be determined in wine by generating its hydride and atomizing it in a silica tube. The procedure is Paper 0101 166 E Received March 16, 1990 Accepted July 26, 1991

ISSN:0003-2654    

DOI:10.1039/AN9921700031

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

ANALYST, JANUARY 1992, VOL. 117 35 Catalytic-Adsorptive Stripping Voltammetric Determination of Ultratrace Levels of Molybdenum in the Presence of Organic Hydroxy Acids Joseph Wang, Jianmin Lu and Ziad Taha Department of Chemistry, New Mexico State University, Las Cruces, NM 88003, USA The adsorptive collection of the molybdenum complex with 3-methoxy-4-hydroxymandelic acid was coupled with the catalytic current of the adsorbed complex to yield an ultrasensitive voltammetric procedure for measuring picomolar levels of molybdenum. Optimum solution conditions (particularly the solution composition) were established to give a detection limit of 4 ng dm-3 (4 x 10-11 rnol dm-1) of molybdenum (following a 2 min preconcentration). The relative standard deviation (at 0.5 pg dm-3) was 3.7%.Possible interferences were investigated and the applicability to assays of tap water is illustrated. Such coupling of catalytic and adsorptive collection processes holds great promise for the development of ultratrace voltammetric procedures for other analytes. Keywords: Molybdenum determination; adsorptive stripping voltammetry; catalytic current; 3-methoxy-4-h ydrox ymandelic acid Catalytic and adsorptive processes have greatly enhanced the sensitivity of voltammetric procedures. For example, catalytic polarographic waves 100-1000-fold larger than ordinary diffu- sion-controlled currents can offer convenient pulse polaro- graphic measurements down to the nanomolar level.' Even lower detection limits (usually down to 10-1(' rnol dm-3) can be achieved through adsorptive stripping voltammetric pro- cedures (based on the interfacial accumulation of analytes').It has been shown recently that the coupling of catalytic and adsorption processes, via controlled adsorptive accumulation of a catalyst, yields remarkable sensitivity and detectability down to the picomolar (ppt) The determination of platinum and titanium, in the presence of formazone and mandelic acid, respectively, has thus been reported. This paper describes an extremely sensitive catalytic- adsorptive stripping voltammetric procedure for the determi- nation of ultratrace amounts of molybdenum. Because of the importance of molybdenum and its extremely low levels in various matrices, an ultrasensitive method is required for its determination. Voltammetry has been shown to be useful for the measurement of trace amounts of molybdenum.Catalytic currents of molybdenum (e.g., in nitrate of perchlorate media) have permitted convenient polarographic determinations of nanomolar concentrations.s.6 The adsorptive collection of molybdenum complexes with quinolin-8-ol7 or phosphates has been exploited for stripping measurements down to the 1 x 10-l0 rnol dm-? level (10 min accumulation). The catalytic- adsorptive stripping procedure reported here lowers the detectability further to the picomolar level (i.e., 4 x 10-11 rnol dm-3 with 2 min collection). The procedure is based on the catalytic response of the accumulated molybdenum complex with 3-methoxy-4-hydroxymandelic acid (VMA). Hasebe et al.9 reported o n the catalytic polarographic determi- nation of VMA in urine in the presence of molybdenum and bromate ions. By reversing this scheme and controlling the accumulation of the complex, an ultrasensitive procedure for monitoring molybdenum is obtained, the characteristics and advantages of which are discussed below.Experimental Apparatus and Reagents The equipment used to obtain the voltammogram, a PAR 264 A voltammetric analyser with a PAR 303 static mercury drop electrode, has been described in detail elsewhere. A medium-sized hanging mercury drop electrode (HMDE) with a 0.016 cm2 surface area was employed. All solutions were prepared with doubly distilled water. A 1000 ppm stock molybdenum solution (atomic absorption standard, Aldrich) was diluted as required. Stock solutions ( 5 X 10-2 rnol dm-3) of VMA and homovanillic acid (HVA) (Aldrich) were prepared in 0.1 rnol dm-3 formic acid.The blank solution consisted of 2 x 10-6 mol dm-3 VMA, 5 x 10-4 mol dm-3 KBr03, 5 X 10-3 rnol dm-3 K2S04 and 2 X 10-3 rnol dm-3 H2SO4. Drinking water samples were collected at this laboratory. Procedure The blank solution (10 ml) was pipetted into the cell and purged with nitrogen for 8 min. The preconcentration potentizl (usually 0.0 V) was applied to a fresh mercury drop while the solution was stirred. Following the accumulation period, the stirring was stopped and after 15 s the voltammo- gram was recorded by applying a differential-pulse scan (at 10 mV s-1) in the negative direction; the scan was terminated at -0.7 V. After background voltammograms had been re- corded, an aliquot of the diluted molybdenum standard solution was introduced.Throughout this operation, a stream of nitrogen was passed over the surface. All data were obtained at room temperature. Results and Discussion Fig. 1 shows cyclic voltammograms for ( a ) 1 and (b) 5 vg dm-3 (ppb) molybdenum in the presence of 2 x 10-6 rnol dm-3 VMA and HVA, respectively (and also 5 x 10-3 rnol dm-3 K2S04, 2 x 10-3 rnol dm-3 H$04 and 5 x 10-3 rnol dm-3 KBr03), obtained after (A) 0 and (B) 60 s accumulation. A cathodic peak is observed (at about -0.48 V) during the scan in the negative direction. Scanning in the reverse direction also produces a cathodic peak, indicative of a catalytic process. The response increased dramatically when an ac- cumulation period preceded the potential scan (B versus A), indicating an interfacial accumulation of the molybdenum- hydroxy acid complex.Subsequent scans resulted in smaller, but stable, cathodic peaks (not shown) that indicate desorp- tion of the complex from the surface. Higher sensitivity was observed in the presence of VMA, which was used in all subsequent work. The fact that a defined and intense (microamps) response is obtained in cyclic voltammetry for pg dm-3 concentrations indicates the remarkable sensitivity associated with the coupling of the catalytic and interfacialANALYST, JANUARY 1992, VOL. 117 36 t A - I I I I I I I 1 I 0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 PotentialN Fig. 1 Cyclic voltammograms for ( a ) 1 and (6) 5 pg dm--3 of Mo in the presence of 2 x mol dm-3 VMA and HVA, respectively.after A. 0 and B, 60 s accumulation at 0.0 V. The solutions also contained 2 X lo-' rnol dm-3 H2SO4 and 5 x 10-3 rnol dm--3 K2S0,; scan rate, 10 mV s-I t c.' 2 3 0 0.5 0.3 L 3 u ' 0.1 -0.3 -0.4 -0.5 -0.6 Poten ti a IN 0 1 2 3 4 5 6 Time/m in Fig. 2 ( a ) Differential pulse voltammograms for 0.1 yg dm-3 of Mo after preconcentration periods of A. 0; B , 30; C, 60; and D. 120 s (at 0.0 V and 400 rev min-l stirring) and (b) the resulting current versus time plot. Cell solution. 5 x 10-3 rnol dm--3 K2S04. 2 x 10-3 rnol dm--3 H2S04, 2 X mol dm-3 VMA and 5 x 10-4 mol dm--3 KBr03; scan rate, 10 mV s-l; and amplitude. 25 mV accumulation processes. Additional gains in sensitivity can be obtained in a differential-pulse stripping operation, as illus- trated below.Fig. 2(a) shows differential-pulse voltammograms for 0.1 pg dm-3 (1.05 x 10-9 rnol dm-3) molybdenum after different preconcentration periods (0-120 s, A-D). Despite the extremely low (sub-ppb) concentration and the short accumu- lation times, well defined peaks are observed. The peak I , 2 4 6 10 20 [VMA]/pmol dm-3 2 4 6 8 tH7SOal/rnmol dmP3 20 - I I I I 4 8 12 16 [KBr031/10-4 rnol dm-3 on the catalytic adsorptive stripping current of = 2 pg dm-3 and preconcentration time = 30 s); = 1 yg dm-3 and preconcentration time = 15 s); and = 5 pg dm-3 preconcentration time = 15 s). Other conditions as in Fig. 2 increases rapidly with increasing preconcentration time, indicating (again) an enhancement of the concentration of the complex on the mercury surface.For example, a 1 min accumulation yielded about a six-fold enhancement of the peak (relative to that obtained without preconcentration; compare A and C). In Fig. 2(b) the resulting current versus preconcentration time plot is shown. The rapid increase of the current observed for preconcentration times shorter than 3 min is followed by a levelling off for longer periods. A detection limit of 4 ng dm-3 (4 X 10-11 mol dm-3) can be calculated based on the signal-to-background characteristics of the response shown in Fig. 2(a), D. Such an extremely low detection limit, obtained with a 2 min accumulation, compares favourably with the 5 x 10-9 and 1 x 10-10 rnol dm-3 detection limits reported for conventional adsorptive stripping voltammetry, following 2 and 10 min preconcentration, respectively.7 3 The catalytic-adsorptive stripping response is strongly dependent on the solution conditions (Fig. 3). The peak increases with increasing VMA concentration until it levels off at 4 X 10-6 rnol dm-3 [Fig. 3(a)]. In addition to the complexing hydroxy acid, it is essential to have a bromate ion oxidant and acidic medium to produce the catalytic response (through regeneration of the MoV1-VMA species from the MoV speciesg). The largest catalytic-adsorptive stripping peakANALYST, JANUARY 1992, VOL. 117 37 & A -0.3 -0.4 -0.5 -0.6 PotentialN 2 L 1.5 u 1 .o 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Concentration (ppb) Fig. 4 (a) Catalytic-adsorptive stripping voltammograms for solu- tions of increasing Mo concentrations; A, 0; B.0.2; C, 0.4; D, 0.6; E, 0.8; F. 1.0; G, 1.25; and H, 1.5 pg dm-3; preconcentration time, 15 s. Other conditions as in Fig. 2. ( h ) Calibration graphs for A , 15 and B 30 s preconcentration times was obtained in the presence of 1.5 X mol dm-3 H?SO4 [Fig. 3(6)]. The response increased linearly with increasing bromate ion concentration up to 8 x mol dm-3 and then more slowly [Fig. 3(c)]. The optimum solution conditions (with 2 x 10-6 mol dm-3 VMA and 5 x 10-4 mol dm-3 Br03-) were selected also for minimizing the background peak (due to the bromate ion reduction) which appeared at the same potential as that of the target Mo-VMA species. The effect of the accumulation potential was evaluated over the range from 0.2 to -0.3 V (2 pg dm-3 molybdenum with 10 s preconcentration; not shown).Only slight variations in the response were observed, with 0.0 V yielding the largest peak. Some enhancement (about 40%) of the response to 0.5 pg dm-3 molybdenum was observed by stirring the solution (during the 30 s preconcentration), compared with analogous measurements with a quiescent solution. The differential- pulse stripping mode yielded slightly better signal-to-back- ground characteristics than linear scan measurements and was used throughout. Fig. 4(a) shows voltammograms obtained after increasing the molybdenum concentration in 200 (B-F) and 250 (G, H) ng dm-3 steps. Well defined peaks are obtained following the very short (15 s) preconcentration period. These measure- ments are part of a calibration experiment up to 1.8 pg dm-3. The resulting calibration graphs (at 15 and 30 s preconcentra- tion) are shown in Fig.4(h). High linearity (Y = 0.999) prevailed up to 0.8 pg dm-3, with slower current increases at higher levels (slopes of the initial linear portions = 1.83 and 1.55 pA dm-3 pg-I). The exact nature of the discontinuity at about 0.8 pg dm-3 is not clear. I t does not appear to relate solely to surface saturation, but rather to the multitude of processes involved in the catalytic-adsorption process. A prolonged series of 30 repetitive measurements of 0.5 pg dm-3 molybdenum was used to establish the reproducibility of the data (30 s accumulation). The mean peak current found was 1.02 pA, with a range of 0.91-1.06 PA and relative standard deviation of 3.7%. In addition to high sensitivity and -0.3 -0.4 -0.5 -0.6 PotentialN Fig.5 Voltammograms for a drinking water sample (A), as well as for subsequent concentration increments of 0.4 pg dm-3 of Mo (B-D): preconcentration, 30 s; and sample, 1 ml of drinking water plus 9 ml of the blank/electrolyte solution. Other conditions as in Fig. 2 precision, the catalytic-adsorptive stripping procedure offers high selectivity toward molybdenum. Ions tested at the 5 pg dm-3 level and found not to interfere with the determina- tion of 0.5 pg dm-3 molybdenum were Cd", Pb", Bill1, Cull, Ni", V", Zn", TI', Crlll, Mn" and Co" (30 s preconcentration). Surface-active materials, in contrast, could compete with the surface adsorption sites of the catalyst. For example, albumin at 1 and 2 pg dm-3 resulted in 9 and 27% depressions, respectively, of the 0.5 pg dm-3 molybdenum response (30 s preconcentration). Depending on the complexity of the sample, destruction of organic surfactants, e .g . , through ultraviolet irradiation, may therefore be required. The applicability of the method to the analysis of drinking water is demonstrated in Fig. 5 . With a short preconcentration time of 30 s and ten-fold sample dilution, the method yields a well defined molybdenum peak for the sample (A), and subsequent standard additions of 0.4 pg dm-3 (B-D). A molybdenum level of 4.1 pg dm-3 was therefore calculated for the sample. In conclusion, this study demonstrates again that the coupling of catalytic and adsorption processes can constitute the basis for an ultrasensitive voltammetric procedure. Anal- ogous catalytic systems involving other metals may be utilized for their detection at picomolar levels. The power and utility of catalytic-adsorptive stripping voltammetry will continue to expand in the near future. This work was supported by grants from SANDIA NL and Battelle PNL. 1 2 3 4 5 6 7 8 9 10 References Mairanovski, S. G., Catalytic and Kinetic Waves in Polar- ography, Plenum Press, New York, 1968. Wang. J., Am. Lab (Fairfield CT), 1985. 17, No. 5 , 41. Wang. J., Zadeii, J.. and Lin, M. S . , J. Electroanal. Chem., 1987. 237, 281. Yokoi. K . , and van den Berg, C. M. C., Anal. Chim. Acta, 1991, 245. 167. Hidalgo, J . L., Caballero. G. M., Ccla, R., and Pcrez-Busta- mente, A. P.. Talanta, 1988, 35, 301. Navratilova. Z . , and Kopanica. M.. Anal. Chim. Acta, 1991. 244, 193. van den Berg. C. M. G., Anal. Chem., 1985, 57, 1532. Fogg, A. G., and Alonso, R. M., Analyst, 1988, 113, 361. Hasebe. K., Kakizaki, T., and Yoshida, H., Anal. Chem., 1987, 59, 373. Wang. J . . Farias. P. A. M., and Mahmoud, J . S . , Anal. Chim. Acta, 1985, 172. 57. Paper 1 I039376 Received July 30, 1991 Accepted August 28, 1991

ISSN:0003-2654    

DOI:10.1039/AN9921700035

出版商:RSC    

年代:1992

数据来源: RSC