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ANALYST, JANUARY 1993, VOL. 118 35 Fast-responding, Fibre-optic Based Sensing System for the Volatile Anaesthetic Halothane, Using an Ultraviolet Absorption Technique and a Fluorescent Film Judith A. Barnard Howie and Peter Hawkins* Sensor Research Group, Faculty of Applied Sciences, University of the West of England , Coldharbour Lane, Bristol, UK BS16 IQY An improved version of a fast-responding sensing system for the widely used volatile anaesthetic halothane (2-bromo-2-iodo-I ,I ,I-trifluoroethane) is described. The concentration of halothane is determined using an ultraviolet radiation (UV) absorption technique. Ultraviolet radiation at 230 nm is conveyed by a silica optical fibre to a gas flow-through cell containing the halothane, and the intensity of the UV reaching the other side of the cell is measured using a fluorescent polymer film. This paper describes the development of an efficient fluorescent polymer film for the sensor based on poly(ethy1ene glycol) containing two fluorophores { 2,5-diphenyloxazole and tris[4,4,4-trifluoro-I -(2-thieny1)butane-I ,3-diono]europium(iii)). The film fluoresces strongly with a red line spectrum when excited in the range from about 200 to 380 nm.There is evidence of direct energy transfer between the two fluorophores. A similar effect is observed in films prepared from Carbowax 20M and the europium chelate. An advantage of this approach is that the fluorescent radiation can be transmitted back to a silicon photo-detector using an inexpensive polymer optical fibre bundle. Two experimental sensor systems for halothane are described and the results show that, although the response does not obey the Beer-Lambed law, a reliable system for determining halothane can be constructed, which operates over the medically important range &3%.This paper also describes how the signal-to-noise ratio of the system can be improved by using the long fluorescence lifetime of the film. A system for relaying the fluorescent radiation efficiently to polymer optical fibres is described, which is based on a clear film of poly(methy1 methacrylate) containing the two fluorophores. The sensor head is compact and has a fast response time, thereby making it suitable for end-tidal respiratory gas measurements. The sensing system could also be used with other gases or liquids that absorb in the range 200-380 nm.Keywords: Ultraviolet fibre optic; anaesthetic gas analysis; fluorescence quenching; end-tidal respiratory measurement; europium (3- dike tone chelate Halothane (2-bromo-2-iodo-l,l,l-trifluoroethane) is a com- monly used inhalation anaesthetic. Several different types of anaesthetic gas analysers are available'-2 for measuring inhalation levels of halothane (in the range 0-3%) in oxygen and dinitrogen oxide. Some of these have the disadvantages of being bulky, expensive or unsuitable for routine clinical use. Most are also too slow in their response times to measure fast changes in concentration, particularly the end-tidal value. The end-tidal value is the final measurement of the exhalation prior to the start of inhalation and is related to the gas level in the lungs and the concentration in the arterial blood.3 The aim of this work was to construct a fast-responding measuring system for halothane within the constraints of a number of design criteria.These were that the device should be: fast and reliable enough for end-tidal measurements; safe and simple enough for use in a routine hospital environment; and have a small, remote, sensor head to prevent interference in the surgical procedures. A fibre-optic technique was chosen as the most likcly to fulfil all the above criteria. Many fibre-optic chemical sensors have already been reported.4.5 These include a number with medical uses such as blood gas,6 glucose7 and ion8 analysers and as immunosensors.c) A fibre-optic halothane sensor has also been reported,*O which utilizes the effect of fluorescence quenching by the halothane, but its response time is far too slow for end-tidal measure- ments.A rapid-response halothane sensor based on ultra- violet radiation (UV) absorption spectroscopy has been reported by Tatnall et al.11 This design incorporates a bulky sensor head containing the UV source, sample cell, reference cell, photo-detectors, heaters and associated electronics. The aim of this project could be achieved by improving on this * To whom correspondence should be addrcssed. design by using optical fibres to relay UV, from an external source to the sample cell and to transmit a signal back to an external detector, so reducing the size of the sensor head and making it intrinsically safe.The experimental design described in this paper incorpo- rates a silica optical fibre to transmit UV from a deuterium lamp to a sample cell through which the gas mixture passes. Unabsorbed UV passes out of the cell, through a silica window on the opposite face and strikes a fluorescent film. The amount of UV reaching the film varies with the concentration of halothane in the gas mixture, which in turn affects the intensity of fluorescent radiation produced by the film. There are several potential advantages in using this arrange- ment. An inexpensive non-UV transmitting polymer fibre bundle can now be used to relay the fluorescent radiation to a remote semiconductor detector, thereby reducing errors caused by UV scattered inside the cell. Semiconductor detectors work well in polymer fibre-optic systems and have peak sensitivities in the visible or near-infrared regions that are well matched to fluorescent radiation in the red part of the spectrum. A method of improving the signal-to-noise ratio of the detected signal by using the fluorescent film is also described.This paper discusses the nature of the fluorescent films used and their use in a syster , suitable for the basis of a fast-responding halothane gas analyser. Experimental Reagents The fluorophores used for the sensor films were tris[4,4,- 4-trifluoro-l-(2-thienyl)butane-1,3-diono]europium(111) (Eu- TTA, from Kodak Laboratory Research Products, Liverpool, UK) and 2,5-diphenyloxazole (PPO, Scintran grade from BDH, Poole, Dorset, UK). The polymers used as a solid36 ANALYST, JANUARY 1993, VOL. 118 matrix were Carbowax 20M (Phase Separations, Clwyd, UK), poly(ethylene glycol) with an average relative molecular mass of 10 000, poly(methy1 methacrylate) (PMMA, medium rela- tive molecular mass) and ethylene glycol diglycidyl ether (all from Aldrich, Gillingham, Dorset, UK).The solvents used, chloroform, ethanol, propan-2-01, methanol and acetone, were all of spectroscopic-grade purity (Aldrich). Halothane was used in liquid form (Aldrich). Other fluorophores used were Rhodamine 6G and Coumarin 152 (Kodak). Instrumentation The instruments used for preliminary fluorescence and absorption studies were a Perkin-Elmer LS-5 fluorimeter (Beaconsfield, Buckinghamshire, UK) fitted with a front- surface accessory, and a Perkin-Elmer Lambda 5 spectropho- tometer.Some sensor measurements were performed with a Perkin-Elmer MPF-3 fluorimeter modified to accept a gas- flow cell. In another experimental halothane sensor, UV was produced by a deuterium lamp (Ealing, Watford, UK), and, after passing through a frequency-stabilized light chopper (Model 1000 from Monolight Instruments, Weybridge, UK), was relayed to a 2 m long, 1 mm diameter UV-transmitting silica fibre (manufactured by Mitsubishi and supplied by Oriel, Leatherhead, UK). The fibre was connected to a gas-flow cell containing the sensor film and finally to a large-area (100 mm2) silicon photo-voltaic detector (RS Components, Corby, UK). In the final study, polymer optical fibres used to transmit the fluorescence to a remote detector were also obtained from RS Components.Calibration of the sensor was carried out by a gas-chromato- graphic method using a Pye Unicam 104 (Cambridge, UK) fitted with a 1.5 m Porapak Q column and a flame-ionization detector. Development of the Sensor Film Halothane absorbs UV over a band of approximately 100 nm with a maximum at 208 nm [see Fig. l(A)] and in the range 220-250 nm, and absorbs strongly enough for detection without interference from the other major components in the end-tidal gas mixture, which are oxygen, nitrogen, dinitrogen oxide and water vapour.11 The film is required to fluoresce when excited with UV in the range 220-250 nm and it is important that it emits in the visible region with a high quantum efficiency to avoid any possible problems with low sensitivity and, if possible, to enable an inexpensive photo- diode rather than a photomultiplier tube to be used as the detector. Several fluorophores, polymers and film-making techniques were investigated for preparing films with high fluorescence 200 280 360 440 600 Wavelengthinm Fig.1 A, Absorption spectrum of halothane (hatched area) in propan-2-01. Absorption (solid line) and fluorescence emission (broken line) spectra of B, EuTTA and C, PPO in ethanol quantum efficiencies. Early experiments were performed using the laser dyes, Rhodamine 6G and Coumarin 152, as they seemed promising. However, the work was abandoned as it proved difficult to make reproducible films with these dyes. This is because they are prone to solvent effects and produce shifts in the fluorescence spectrum, depending on the polarity of the polymer matrix.Rhodamine 6G and Coumarin 152 also have relatively small Stokes' shifts (22 and 109 nm, respec- tively, in ethanol) and exhibit self-absorption at high concen- trations, so the thickness of the film and the concentration of the dye in the film are critical. A europium chelate was found to be a more suitable fluorophore. The luminescence characteristics of lanthanide chelates have been extensively studied12 and europium(ii1) (3-diketone chelates, in particular, have interesting fluores- cence properties and high quantum efficiencies. l3 Fig. 1(B) shows that EuTTA has a strong absorption spectrum in the near-UV region, which is characteristic of the ligands, and a strong fluorescence spectrum consisting almost entirely of a number of narrow lines in the red region of the spectrum characteristic of the ion (the strongest being at 614 nm corresponding to the sDo + 7F2 transition in the Eu"' ion).The europium ion is excited by energy transfer from the ligands. The excited europium 4f valence electrons are well screened by the outer filled 5s and 5d shells, and the bulky ligands surrounding the ion also shield it against intermolecular collisions, which encourage non-radiative processes. The result of these factors is that the europium chelate has a narrow-band, ionic-type emission well separated from its excitation spectrum and which is largely unaffected by changes in its chemical environment. The energy transfer process is generally believed to occur through the triplet state and results in the fluorescence having a relatively long lifetime (0.4-0.6 ms for EuTTA).Films prepared from poly(ethy1ene glycol) were found to produce the highest fluorescence quantum yield for a given concentration of europium chelate, and this material was studied in detail. Poly(ethy1ene glycol), however, tends to form spherulites as the solvent evaporates, producing an uneven distribution of the chelate. Good films can be prepared by using methanol-acetone (1 + 1) as solvent to which is added 2.5% v/v ethylene glycol diglycidyl ether, followed by rapid evaporation of the solvent from the film. A commercial product based on poly(ethylene glycol), Carbowax 20M, also produces satisfactory films. These were prepared by dissolving 0.8 g of Carbowax 20M in 8 cm3 of 4 x 10-3 mol dm-3 EuTTA in chloroform.The solution was left for 1 h at 60 "C to ensure that all the polymer had dissolved and was then stirred thoroughly. Films were formed by a simple dipping technique onto either a glass slide for front-surface illumination or a silica-glass slide for rear-surface illumina- tion. The films were dried rapidly in a hot air stream and then stored overnight at 40 "C to ensure complete evaporation of the solvent. Carbowax 20M contains an additive, 2,2'- [ (methylethylidene)-bis(4,1 -phenyleneoxymethylene)]bisoxi- ran, which is itself fluorescent with an absorption band extending to shorter wavelengths than that of EuTTA and an emission spectrum overlapping the absorption band of EuTTA.The additive appears to act as a donor fluorophore to EuTTA and usefully extends the excitation band of the film to shorter wavelengths as the red emission from the EuTTA in the film on its own is relatively weak when excited in the range 220-250 nm (the region required for halothane detection in end-tidal gases). However, as Carbowax 20M is a commercial product, it was not possible to vary the donor concentration to achieve optimum results so this work could not be carried any further, and other possible donor molecules for use in a poly(ethy1ene glycol) and ethylene glycol diglycidyl ether film were investigated instead. The requirements for the donor fluorophore are that it absorbs strongly in the region 220-250 nm and emits in theANALYST, JANUARY 1993, VOL.118 37 range 300-380 nm where the thenoyltrifluoroacetonate ligands in EuTTA absorb strongly. A fluorophore meeting this requirement is PPO, which is commonly used in liquid scintillation counters and has absorption and emission spectra as shown in Fig. l(C). Films containing PPO and EuTTA were prepared from 8 cm3 of 0.6 X 10-3 mol dm-3 EuTTA in methanol-acetone (1 + 1) and included 2.5% of ethylene glycol diglycidyl ether to which was added 0.02 g of PPO and 0.8 g of poly(ethy1ene glycol). A dip coating technique similar to that previously described for Carbowax 20M was used to prepare the films. When a film containing both PPO and EuTTA is excited at 230 nm, the PPO fluorescence is considerably quenched and the europium emission enhanced, as shown in Fig.2. Fig. 2(A) shows the broad absorption spectrum for a film containing only PPO extending to wavelengths below 200 nm and the strong fluorescence spectrum covering a broad band from about 330 to 500 nm with a negligible emission at 614 nm when the film is excited at 230 nm. A film containing just EuTTA has an absorption spectrum, as shown in Fig. 2(C), and when excited at 230 nm produces an emission spectrum consisting almost entirely of the characteristic Eu"' ion emission similar to that shown in Fig. l(B). The corresponding absorption and fluorescence spectra for a film containing both EuTTA and PPO are shown in Fig. 2(B). As expected, the absorption spectrum is dominated by the much stronger absorption produced by the PPO molecules with a small contribution from the EuTTA molecules.When the film is excited with UV at 230 nm, the characteristic emission spectrum in the UV from the PPO is considerably quenched and extends now from about 360 to 500 nm, whereas the characteristic emission from the Eu'" ion at 614 nm is greatly enhanced. Unfortunately, it was not possible to make accurate quantitative measurements as to the extent of enhancement produced by adding PPO to the film containing EuTTA because of difficulties in preparing films of consistent thickness and in placing them in the same place each time in the fluorimeter; however, there is no doubt that a film 1.2 al C m e a 0.8 0.4 1 1 I I 200 280 360 Wavelengthlnm ( b) 440 300 340 380 420 460 500 540 590 630 Emission wavelengthhm Fig. 2 Absorption and emission spectra of three poly(ethy1ene glycol) and ethylene glycol diglycidyl ether films containing A.PPO only; B , PPO and EuTTA; and C, EuTTA only. The emission spectra were recorded with the films excited at 230 nm containing EuTTA and PPO glows much redder when compared visually with a film containing just EuTTA if both are irradiated at 230 nm. The mechanism for the energy transfer between the molecules is being investigated further. Halothane Detection Using the Sensor Film Fluorescence is emitted uniformly in all directions so that, in a practical sensor, the sensing film could be viewed either from the front with the return fibre placed alongside the fibre carrying the exciting radiation or viewed from the rear with the return fibre behind the sensing film. Two experimental arrangements were constructed to investigatc these options.First Experimental Arrangement A gas-flow cell was specially made to fit into a Perkin-Elmer MPF-3 fluorimeter. The fluorescent film on a glass slide was attached to a small adjustable plug and placed inside the flow cell. Ultraviolet radiation at a wavelength of 230 nm (band- width 36 nm) entered the cell through a silica window, passing through the gas sample (effective pathlength about 35 mm) to strike the sensor film. The film angle was adjusted to about 45" to the direction of the UV so that the maximum fluorescence and minimum scattered light entered the emission slit (slit- width 5 nm) of the fluorimeter. The cell also contained a septum fitting to allow gas samples to be removed by syringe for analysis by gas chromatography.The halothane concentra- tion in the gas flow was controlled by using a vaporizer system similar to that of Zbinden et al.1 A stream of nitrogen gas, controlled by a mass flow controller, was passed through liquid halothane in a gas-jar maintained at a constant 0 "C in an ice-bath. The gas stream, saturated with halothane, was then joined by a faster nitrogen flow and thoroughly mixed before passing into the gas-flow cell. Halothane concentrations were measured by gas chromatography and calibration was carried out using liquid halothane standards. The vapour pressure data of Bottomley and Seiflow14 were used to convert the measured molar concentrations of the gas samples, taken from the flow cell, into volume percentages of halothane.Several different fluorescent films were tested and they all produced similar results. Fig. 3 shows a typical plot of the logarithm of initial fluorescence intensity (i.e., with nitrogen flow only) divided by the fluorescence intensity with halo- thane present (log Zo/l) plotted against the concentration of halothane (% v/v) in the cell, obtained using a sensor film containing EuTTA in Carbowax 20M. The graph over this range is non-linear and, therefore, the sensor does not appear to adhere to the Beer-Lambert law of absorbance. 0.4 0.3 c ,a - cn 0.2 -I 0.1 0 2 3 Halothane concentration (% vlv) Fig. 3 Typical response curve for the first experimental halothane gas sensor using a fluorescent film of Carbowax 20M containing EuTTA, showing log(lo/l) versus halothane concentration (% v/v), where lo is the intensity of the fluorescent light with no halothane present and I is the intensity with halothane present38 ANALYST, JANUARY 1993, VOL.118 output G~~ flow i n Summing amplifier I Deuterium [3 I Coupling optics Gas Chopper UV-transmitting fibre flow 1 L- Gas flow out Fig. 4 Arrangement for the second experimental halothane sensor test rig Second Experimental Arrangement The apparatus shown in Fig. 4 was assembled with a silica fibre to transmit a chopped beam of UV to the cell so that a noise reduction technique could be used to improve the signal-to- noise ratio. The halothane gas-flow system was the same as used previously. Ultraviolet radiation, produced by a deuterium source, passed through a chopper maintained at 3 kHz and was then coupled into the fibre.The coupling optics and the large numerical aperture fibre were chosen in order to capture as much incident radiation as possible. The far end of the fibre was connected to a glass flow cell with a pathlength of 35 mm (total internal volume about 1.8 cm3) and with a silica window at the opposite end. The window was coated on the outside with a fluorescent film that was in contact with a large-area photo-diode. In this experimental arrangement, use was made of the relatively long fluorescence lifetime of the europium ion emission (about 0.4 ms) to improve the signal-to-noise ratio of the detecting system. The light chopper was running at a sufficiently high frequency to provide a measurable signal from the photo-detector during the closed periods of the chopper produced by the afterglow of the europium fluores- cence.As the concentration of halothane increases, the amount of UV reaching the film decreases and there is a corresponding decrease in the intensity of the afterglow. The signal at the output of the detector can be analysed electrically into a varying (a.c.) component and a steady (d.c.) com- ponent, which contains the afterglow from the europium ion. The a.c. component can be isolated from the signal by passing it through an a.c. inverting amplifier with unity gain. When this signal is now added electronically, using the summing amplifier, to the original signal coming from the photo- detector the remainder contains only the d.c. component and much of the noise is cancelled out, producing a considerable improvement in the signal-to-noise ratio.Fig. 5 shows a typical plot of the logarithm of the final signal without halothane (Vo) divided by the signal with halothane present (V) versus halothane concentration (% v/v) for a film prepared from poly(ethy1ene glycol) and ethylene glycol diglycidyl ether with EuTTA and PPO. The response curve is very similar to that obtained in Fig. 3, showing that the sensing system again does not obey the Beer-Lambert law. The reproducibility of the response of the sensing system to halothane was investigated by combining the results obtained in the first experimental arrangement with those obtained in the second (see Fig. 6). The results were first normalized at the 1% halothane level and then a polynomial fit was performed to obtain the equation of the best curve through all the points. 0.002 S 2 8 0.001 -I 0 0.5 1 .o 1.5 2.0 Halothane concentration (% v/v) Fig.5 Typical response curve for the sensing system shown in Fig. 4 using a fluorescent film of poly(ethyiene glycol) and ethylene glycol diglycidyl ether containing PPO and EuTTA. showing log(V&’) versus halothane concentration (% v/v), where V, is the output with no halothane present and V is the output with halothane present 0.4 0.3 - Q) C 0 a. * 0.2 . P, -I ? 0 1 2 3 Halothane concentration (% v/v) Fig. 6 with the values normalized to the 1% halothane concentration Data shown in Figs. 3 (El) and 5 ( X) plotted on the same axes The equation of the best-fit curve through all the points is y = 5.34 X 10-3 + 0 .3 1 2 ~ - 0.0315~2 with a correlation coefficient of 0.9984. This suggests that the apparent deviation from the Beer-Lambert law is indepen- dent of the arrangement of the sensing film and that a simple calibration procedure would suffice to allow reproducible results to be obtained. The results published by Tatnall etal.11 show a similar non-linear response, but they do not commentANALYST, JANUARY 1993, VOI,. 118 39 Front view of sensor w Front silvered mirror Bundle of eight plastic fibres Gas flow in , -- I : '\ I output Summing amplifier r y iensorfitm Plastic optical fibres I I I e' Unity gain a.c. inverting amplifier Chopper L- . Gas flow out Fig. 7 Arrangement for the experimental halothane sensor with a return polymer fibre optical bundle to a remote photodiode on it.Diprose et a1.15 observed a similar non-linear response and derived an experimental equation: V , = 129.7 [ l - exp(-l.lx)] + 4860 [l - exp(-O.O32x)] where V, is the reading on the output meter of their instrument. A good agreement is obtained between these two equations (normalized at x = 1.5) with a correlation coeffi- cient of 0.9926. They attribute the non-linearity to the UV used, which consisted mainly of two spectral lines from a mercury lamp as the Beer-Lambert law assumes monochro- matic radiation. A similar explanation could be used to account for the results obtained in this investigation as the UV used has a wide spectral bandwidth. The results show that a reliable measurement system could be assembled with a UV lamp placed outside the sensing head and linked to it through a silica fibre, and with a fluorescent film and a simple silicon photo-detector inside the sensing head, which would be an improvement on the system described by Tatnall et aZ.11 The next stage in the development is to incorporate a return fibre so that the photo-detector can be placed externally.Design of a Sensing System With a Return Optical Fibre A drawback with using fluorescence is that as it is scattered in all directions it makes efficient collection by a return fibre difficult. A means of collecting the fluorescent radiation efficiently, based on some earlier work,l6 was investigated where it was found that, when a clear fluorescent film containing EuTTA and PPO in PMMA is coated directly onto the plastic cover of a semiconductor photo-detector, it forms a waveguide so that a large proportion of the fluorescence emerges from the side of the film.Use was made of this to improve the coupling of the fluorescence into a return fibre bundle (Fig. 7) by positioning eight polymer optical fibres (about 2 m long) radially onto a front-face silvered mirror and then coating the area between the fibres and the ends of the fibres with the fluorescent film. The film was prepared by applying a few drops of a solution containing 0.8 g of PMMA and 0.02 g of PPO in 4 cm3 of chloroform, to which was added 4 cm3 of a 2 x 10-3 mol dm-3 EuTTA solution in chloroform, onto the mirror, spreading the solution and allowing the solvent to evaporate. The other ends of the fibres were bound together tightly to form a bundle and then positioned carefully over the large-area photodiode.The photodiode and the rest of the equipment used were the same as described previously, although additional electronic filtering was added at the output to compensate for a loss in sensitivity that reduced the 0.4 0.3 a, C 0 a * 0.2 2 0 -4 0.1 0 0.5 1 .o 1.5 2.0 2.5 3.0 Halothane concentration (% v/v) Fig. 8 Response of the sensing system shown in Fig. 7 to different concentrations of halothane. The results of four repeat experiments are shown and these have been normalized to obtain the best fit with the curve shown in Fig. 6 signal-to-noise ratio. The loss in sensitivity was probably caused by imperfect coupling between the fluorescent film and the fibres and by the fact that films prepared from PMMA appeared to have a lower fluorescence output than those prepared from poly(ethy1ene glycol).The concentration of halothane in the cell was changed and the response of the measurement system was noted. The results of four repeat experiments (Fig. 8) show a greater scatter in the points on the graph than found previously because of the lower signal level. When the results are normalized there is a reasonable agreement with the earlier results as shown by the solid line which represents the equation determined previously. The signal-to-noise level would be greatly improved by using a more intense UV source and a photomultiplier as the detector. Discussion A major problem with sensors that rely simply on t h e absorption of radiation for detection is drift in the output caused by instabilities in the source and the detector and by the mechanical rigidity of the system.A method often used to correct for drift is to use a reference ce11,11,12 but this adds to the bulk of the detector head. In this instance a proposed method for correcting for drift would be to excite the film alternately at a wavelength where halothane absorbs ( e . g . , between 220 and 250 nm) and at a wavelength where40 ANALYST, JANUARY 1993, VOL. 118 none of the gases in the sample cell absorb (e.g., between 300 and 380 nm) and then subtracting one signal from the other. The response time of the experimental sensing system was not measured because it was not possible to change the halothane concentrations fast enough in the flow system to carry out an accurate measurement. Tatnall et al.*1 reported a response time of about 40 ms for a 90% change in output to a step change in halothane concentration for their system.No information is given about the response time of the optical and electronic measurement system they used, but it is reasonable to assume that this is fast so that the response time is largely controlled by the wash-out time of the sample cell and connecting tube. In our sensing system, the response time of the optical and electronic measurement system is ultimately limited by the decay time of the fluorescence of the film (about 0.4 ms), which is sufficiently fast, so that if a similar sample cell design is used with our system, then a comparable response time would be expected.The only details given in the paper of Tatnall et al. on the dimensions of the sample cells used are the volumes of the adult cell (4.52 cm3) and infant cell (0.56 cm3), and no information is given about the optical pathlength of the cells. As the volume of our experimental cell (1.8 cm3) falls between these two volumes, the wash-out time of the experimental cell should fall between the values for these two cells. Tatnall et al.11 heated the whole of the sensor head to 42 k 0.5 "C to prevent condensation of water on the silica windows of the sample cell and to stabilize the photo-detectors. A small heater would also be required in our system to heat the sample cell to prevent condensation. The temperature stability of the fluorescent films was not investigated, but if this is found to produce a significant drift in the output, then the film too could be heated separately at a constant, but lower, tempera- ture.The over-all size and weight of the sensing head should be substantially smaller in our design than theirs as it does not contain the UV source, filters, iris and photo-detectors. A magnetically shielded casing is also not necessary to reduce interference problems as all of the signals are conveyed by optical fibres. This system was developed originally for use with halo- thane, although it could be adapted to analyse for other gases or liquids that absorb in the range 2W350 nm. 1 2 7 8 9 10 11 12 13 14 15 16 References Zbinden, A., Westenskow, D., Thomson, D., Funk, B., and Macrtens, J . Int. Clin. Monit. Comput., 1986, 2, 151, Hayes. J., Westcnskow, D., and Jordan, W., Anaesthesiology, 1983, 59, 435. Eger, E., and Bahlman, S., Anuesthesiology, 1971, 35, 301. Scitz, W., CRC Crit. Rev. Anal. Chem., 1988, 19, 135. Wolfbeis, O., TrAC, Trends Anal. Chem., 1985, 4, 184. Gerhich, J.. Lubbers, D., Opitz, N., Hansmann, D., Miller, W., Tusa, J., and Yafuso, M., IEEE Trans. Biomed. Eng., Narayanaswamy, R., and Sevilla, Y . , 111, Anal. Lett., 1988,21, 1165. Roe, J. N., Szoka, F. C., and Verkman, A. S., Analyst, 1990, 115,353. Tromberg, B. J., Sepaniak, M. J . , and Vo-Dinh, T., Proc. SPIE-Int. SOC. Opt. Eng.. 1988, 906, 30. Wolfbeis, O., Posch, H., and Kroncis, H., Anal. Chem., 1985, 57, 2556. Tatnall, M. L., West, P. G., and Morris, P., Rr. 1. Anuesth., 1978, 50, 617. Buono-Core, G., Li. H., and Marciniak, M., Coord. Chem. Rev., 1990, 99, 55. Diamandis, E. P., and Christopoulos, T. K., Anal. Chem., 1990. 62, 1149A. Bottomley, G.. and Seiflow, G., J. Appl. Chem., 1963, 13,399. Diprose, K. V., Epstein, H. G., and Redman, L. R., Br. J. Anuesth., 1980, 52, 1155. Howie, J. A. B., Rowles, G. K., and Hawkins, P.. Meas. Sci. Technol., 1991, 2, 1070. Paper 2/01 01 I I Received February 26, 1992 Accepted September 23, I992 1986, BME-33, 117.

ISSN:0003-2654    

DOI:10.1039/AN9931800035

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

ANALYST, JANUARY 1993, VOL. 118 41 Glass pH Electrodes With Improved Temperature Characteristics Part 2.* Systems With Conventional Inner Reference Electrodes Derek Midgleyt National Power pic, Technology and Environmental Centre, Kelvin Avenue, Leatherhead, Surrey, UK KT22 7SE Glass electrodes with internal filling solutions containing zwitterionic buffers are shown to have standard potentials that vary linearly with temperature over a wide range (at [east 5-50 "C). This reduces the errors of temperature compensation compared with the proprietary electrodes currently available. A judicious choice of buffer and inner reference element yields an electrode with almost the ideal characteristics of isopotential pH (=7) and zero-point pH (=7), which enable it to be used with all pH meters.Other electrodes could readily be used with modern microprocessor-based meters. In other aspects of their performance (slope factor, hysteresis, response time) the new electrodes are at least as good as commercial sensors. Keywords: pH; glass electrode; temperature compensation; isopotential; zwitterionic buffer In industrial and environmental analysis, where monitoring may be required over extended periods of time and where control of sample temperature may be impractical, tempera- ture compensation is an important factor. This has long been true in potentiometry,lJ and problems with commercial glass pH and reference electrodes have been identified.3-5 Seven requirements for ideal temperature compensation have been enumerated,h the outstanding problem being for a pH cell that shows a truly linear variation with temperature, so that effective compensation can be applied over a wider tempera- ture range than at present.An inner reference system consisting of a metal-metal oxide electrode enabled this requirement to be met substantially,h but the use of the resulting electrode would have been impractical with most pH meters, which require the isopotential pH (at which the e.m.f. is invariant with temperature) and zero-point pH (at which the e.m.f. is 0 mV) to coincide at pH 7. This paper describes the use of internal filling solutions that show a linear variation of pH with temperature while allowing all other parts of the pH cell to be conventional.7 Theory The derivation of the following eqns., (1)-(6), has been presented in a review of the effect of temperature on pH measurements .3 The e.m.f. of a potentiometric cell for measuring pH is given by a form of the Nernst equation: E = Eocell - kpH (1) where k is the slope factor theoretically equal to RTln(lO)/F, R being the gas constant, T the absolute temperature, and F the Faraday constant.EOcell is the standard potential of the cell, but is only quasi-thermodynamic as it includes a number of factors controlled by the experimental conditions, notably the concentrations of the inner and external reference electro- lytes, and assumes that liquid-junction potentials are constant. The e.m.f. can be represented in a way that allows for convenient temperature compensation by pH meters: E = E, - k(pH - pH,,,) (2) where E, contains all the temperature-independent com- ponents of EOccll, and pH,,, all the temperature-dependent * For Part 1.see rcf. 6. -1 Present address: Hillcroft Research, 41 St Mary's Road, Leatherhead, Surrey, UK KT22 8HB. ones; pHiso is the isopotential pH at which the e.m.f. is, ideally, independent of temperature. It follows that 6 E('ce11/6 T 1 pHiS,6k/6 T (3) Most practical pH cells contain a glass electrode with an Ag-AgCI or, less commonly, an Hg-HgC1 inner reference element; the inner filling solution therefore contains a chloride salt as well as a pH buffer. Reference half-cells contain the same reference elements in concentrated KCI solutions. A detailed analysis of such cells3 shows that +-- 6EO k6logac1 [ 6 T 6 T AEo + EL k pH,,, = pH') - 6AEV6T + 6E,/bT + kbpH'/bT bk/6 T + - ilogac., ( 5 ) where i = 0 or 1 for isothermal and non-isothermal cells, respectively.pH0 = pH' + AEo + EJ + log (U&'(-I) k where EO' and Eo are the standard potential of the internal and external reference electrodes, respectively, a'c, and acl are the activities of CI- inside the glass and reference electrodes, respectively, and pH' is the pH of the solution inside the glass electrode. E, is the liquid-junction potential, AEO = EO' -Eo, and pH0 is the pH at which E = 0 mV (at 25 "C). Non-linearity of the temperature response mainly arises from the term k6pH'lGT in eqn. (4) and this in turn depends on 6pK/6Tfor the buffer component inside the electrode. The values of 6pK/6T required for isothermal and non-isothermal combinations of Ag-AgCI and calomel reference electrodes to yield the desired pH,,, = pH0 = 7.00 can be calculated,3 and Table 1 shows the requirements for external reference electrodes filled with 3 mol dm-3 KCI.The properties of most inorganic and carboxylic acid buffers are such that 6pH/6T is neither zero nor constant. The zwitterionic 'biological' buffers developed by Good and co-workerss-10 contain one or more amino groups, and studies11312 on three of these indicate excellent constancy of 6pK/6T for these groups, which control the pH in the desired buffer range (6-8). Other members of this group have been studied less extensively, but for most of them 6pK/6T lies in42 ANALYST, JANUARY 1993, VOL. 118 Table 1 Characteristics required of solutions inside glass electrodes to yield pH,,, = pH0 = 7.0.(Calibration at 25 "C versus reference electrodes containing 3 mol dm-3 KCI solution) Reference element Buffer. Glass Reference 6p Kl6 T electrode half-cell pH' - log a',-, (isothermal) AgCl AgCl 6.77 0 AgCl Hg2C12 7.54 3.5 x 10-3 HgzC12 Hg2C12 6.77 0 Hg2C12 AgCl 5.99 -3.5 x 10-3 Buffer, 6p Kl6 T (non- isothermal) -2.7 X lo-' -5.3 x 10-3 -8.8 x 10-3 -6.2 X 1 W 3 the range -0.01 to -0.02. Comparison of these data with the results in Table 1 shows that an isothermal cell with the desired characteristics is unattainable with these buffers and the usual reference electrodes (a difference of 0.0033 in 6pK/6T is equivalent to 1 in pHis,). With a non-isothermal cell having calomel inner and external electrodes, the desired value is closely approached by PTPESS (-0.0085).A number of buffers have 6pK/6T = -0.0011, and two with a convenient pH range were chosen for investigation in non-isothermal cells with an external calomel electrode. The values of 6pK/6T required are not strongly influenced by the concentration of chloride in the reference electrode.3 Experimental Apparatus Potentials were measured with a digital pH meter reading to 0.1 mV and were simultaneously displayed on a chart recorder. Electrodes were fixed in B14 ground-glass sockets in the lids of water-jacketed glass cells connected to a C-100 thermo-circulator and a Model 1000 cooler (Techne, Cam- bridge, UK). Reference Electrode A modified Kent 1352 calomel electrode (ABB Kent-Taylor, Stonehouse, Gloucestershire, UK) with a remote ceramic frit junction was used.The main body of the electrode was fitted with a water jacket connected to a Techne C-100 thermo- circulator at 25 "C. The filling solution of the electrode was 3 rnol dm-3 KCI. Experimental Glass Electrodes Bodies from Kent 1070-1 standard glass electrodes were filled with various buffer solutions and fitted with appropriate reference electrodes (see below) driven through soft silicone- rubber bungs fitted into the open end of the glass body. Connection to the exposed ends of the reference electrodes was made by means of a screened cable fitted with a crocodile clip. This arrangement was suitable for experimental pur- poses, enabling solutions and reference electrodes to be changed easily. The signals were surprisingly free of noise.Inner Reference Electrodes Silver-silver chloride electrodes Irradiated polyolefin tubing (Radiospares 399-899, Corby , Northamptonshire, UK) was heat-shrunk onto 1 mm diameter silver wire, leaving about 1 cm exposed at each end. The wire was then driven through a silicone-rubber bung. One end was cleaned in aqueous ammonia, de-greased with acetone and etched in nitric acid; it was then anodized in 0.01 mol dm-3 HCI at 0.01 mA cm-2 for 18 h. Calomel electrodes Irradiated polyolefin tubing (Radiospares 399-899) was heat- shrunk onto 1 mm diameter platinum wire, leaving a 1 cm overlap at one end and 1 cm of wire exposed at the other. The wire was then driven through a silicone-rubber bung and clamped vertically with the overlapping tubing uppermost. A small drop of mercury was injected from a syringe with a fine stainless-steel needle.Electrolytic calomel (BDH, Poole, Dorset, UK) was applied to the top of the mercury, and the tube was plugged with cotton wool soaked in the appropriate glass-electrode filling solution. The mercury and calomel stayed in place when the electrode was turned the right way up and contact with the platinum wire was maintained. Electrode Filling Solutions All materials were obtained from BDH; NaOH and KCl were AnalaR grade. Solutions were prepared in de-ionized water. MES solutions. These were prepared to contain 0.05 rnol dm-3 2-morpholinoethanesulfonic acid (MES) , 0.025 rnol dm-3 NaOH and 0.453 rnol dm-3 KCI. The observed and calculated pH of this solution was 6.10 at 25 "C. ADA solutions. These contained 0.05 rnol dm-3 acet- amidoiminodiacetic acid (ADA), 0.075 mol dm-3 NaOH and 0.140 rnol dm-3 KCI.The pH observed at 25 "C was 6.59 compared with 6.57 calculated. PIPES solutions. These contained 0.05 rnol dm-3 pipera- zine-N, N'-bis(ethanesu1fonic acid) (PIPES), 0.075 rnol dm-3 NaOH and either 0.226 rnol dm-3 KCl (for Ag-AgCI inner reference electrodes) or 1.16 rnol dm-3 KCI (for calomel electrodes). The pH at 25 "C was 6.73 compared with 6.76 calculated. Commercial Glass Electrodes Orion 91-01 (Cambridge, MA, USA) Corning 3111015 (Corning, NY, USA) and Orion 81-02 electrodes were included in the tests as before.6The first two were used versus the same external remote reference electrode as the experimental electrodes, and the last was a combination electrode of the 'Ross' type with Pt-12-I- reference elements.Reagents National Institute of Standards and Technology (NIST) buffers were prepared from BDH AnalaR chemicals: 0.05 rnol kg-1 potassium hydrogen phthalate (pH 4.005 at 25 "C) and 0.025 rnol kg-1 each of potassium dihydrogen phosphate and disodium hydrogen phosphate (pH 6.865 at 25 "C). Procedure As the temperature of the cell was varied, the e.m.f. values were monitored on a chart recorder, and the steady values at each temperature were noted. The steps in temperature was usually 10 "C and both increasing and decreasing trends were followed. The electrodes were first calibrated at 25 "C in pH 4 and 6.86 buffers. Results The results for seven experimental electrodes are shown in Table 2. Results for three commercial electrodes6 are included for comparison.Slope Factor The slope factors for all the electrodes were slightly sub- Nernstian, but quite acceptable.5.13 The temperature cycles did not cause the slope factors to change with time. The standard deviation for the PIPES-calomel electrode was muchANALYST, JANUARY 1993, VOL. 118 43 P C C C Table 2 Results for experimental and commercial pH electrodes Mean slopc factor SD" El) pH (calc.) Mean EOpH (obs.) SD Mean pH,,,, (obs.) SD pH,,, (talc.) ADA/ AgC1-Ag 58.85 0.29(7) 7.0 7.2 0 . 00( 9) 5.5 5.9 0.24( 9) MES/ AgCI-Ag 58.80 0.28(3) 7.0 7.0 5.3 5.8 0.13(4) 0.05( 3 ) PIPES/ AgCI-Ag 58.89 0.4(2) 7.0 7.2 6.3 5.0 0.1(2) 0.1(2) ADA/ Hg2C12 59.08 0.17(2) 7.8 8.0 7.3 7.2 0.46(3) 0. O( 2) MES/ Hg2Cl2 59.051- 58.77 0.19(3) 0.21(2) 7.87 7.0 7.81- 7.1 0.0(4) 0.0(3) 7.11- 6.4 7.51- 6.5 0.15(5) 0.2(3) PIPES/ 58.49 0.07( 9) 7.0 7.3 0.03( 8) 7.4 6.8 0.28(9) Hg2C12 Corning/ AgCl 58.77 0.33(6) 6.98 - 0.03( 7) - 7.9 0.20(8) Ross/ Pt-12-1- 58.31 0.17(5) 7.0 6.8 0.06(5) 7.0 6.6 0.20(5) Orion/ AgCl 58.92 0.20( 3) 0.05 (3) - 6.13 - 6.3 0.98(4) 1 .000 0.999 - - 1.006 - 1.005 1 .001 - 1 .004 (6klb7') (obs.) (6kl67') (theor.) SD 0.005(3) 0.004(2) - - 4 1 ) - -(1) 0.006(2) - -(I) * SD = standard deviation (numbcr of results in parentheses).t With a chloride concentration appropriate for the Ag-AgC1 electrode. I I - C A A iomin Time - Fig. 1 Response to gradual temperature changes. A, PIPES- Hg2C12, B Corning and C ADA-AgCl smaller than those of the others. In a few instances the slope factors were checked at 45 and 9.5 "C, and the experimental ratios of the slope factors were found to be close to the theoretical values.The variations of slope factor were not a function of time, at least over periods of 3 months (MES- calomel) or 6 months (ADA-AgCl). These results indicate that the slope factors were unlikely to be a major cause of uncertainty in the assessment of temperature effects. Zero-point pM Zero-point pH could be predicted fairly accurately by means of eqn. (6), considering the assumptions made about activity coefficients in the moderately complicated and concentrated buffer solutions, the constancy of liquid-junction potentials and the absence of asymmetry potentials. Further empirical adjustment of pHO to the desired value of 7, by changing the chloride concentration, should not be difficult.For instance, the ADA-calomel and the first MES-calomel electrodes had fillings for Ag-AgC1 reference electrodes and so had higher pHO values than desirable. The second MES-calomel elec- trode shows that adjustment was quite straightforward. I n general, the values of pH0 were very reproducible, even after several temperature cycles, and changed by less than 0.1 over a period of up to 6 months (e.g., the ADA-AgCI electrode). Temperature Response The variation of the slope factor with temperature has already been shown to agree with the theoretical value. Time course of e.m.f, change with temperature Figs. 1 and 2 show how the e.m.f. values of a selection of electrodes change in response to gradual and sharp changes in 250c I 50 "C D Time - I I & '10 min' I I Time - Fig.2 (a) Response to sharp changes in temperature. B, Corning; C, ADA-AgCI; and D, MES-Hg2CI2. ( h ) PIPES-Hg2C12 temperature, respectively. When the temperature changed gradually, there was little practical difference between the rates of response of the various electrodes; the experimental electrode with a calomel inner reference was slightly slower to respond than that with the Ag-AgC1 reference, but the commercial electrode was no better. With sharp changes in temperature, the more rapid response of the Ag-AgC1 references was more clearly evident, but electrodes with calomel inner elements came to equilibrium as quickly as the commercial electrode, which was one of the best in previous tests.4 It is noteworthy that the e.m.f. changes for the experimental electrodes were monotonic, without the over- shoot observed for some commercial combination electrodes .5 The experimental electrodes, therefore, are at least equal to most currently available commercial electrodes in this aspect of performance.Hysteresis Electrodes were calibrated at 25 "C and then cycled between solutions maintained at 25 and 49 "C. The changes from the initial e.m.f. in phthalate buffer at 25 "C are shown in Table 3. The changes show no pattern and are generally less than 1 mV (0.017 pH). (The scatter of points around 25 "C in Fig. 5 is another example of the randomness associated with measure- ments at varying temperatures.) The experimental electrodes, including those with calomel elements, were no worse than the44 ANALYST, JANUARY 1993, VOL.118 Table 3 Thermal hysteresis of clectrodes in phthalatc buffer Change in e.m.f.*/mV Electrode 1st cycle-1 2nd cycle? ADA/AgCI MES/Hg?Cl? PIPES/Hg?Cl? Corning Orion 91 -0 1 -0.2 +0.5 + 1 .o +0.3 +0.3 -0.2 -0.9 -0.4 +0.7 +1.1 +0.1 * With respect to initial reading at 25 "C on first cycle. 1 One cycle involves a change from 25 to 49 "C and back again. TPC 0 10 20 30 40 50 440 - 430 - > E I' a 420 - Y + W 410 - 400 - 390 . I 1 I I I I 55 57 59 61 63 Slope factor/mV pH-' Fig. 3 Determination of pH,,, for experimental electrodes with Ag-AgCI inner rcference clemcnts. Filling solutions: A, ADA; B. MES; and C. PIPES commercial designs and were better than some commercial combination electrodes previously tested,s Variation of EO with temperature From eqn.(2), it follows that a plot of E + kpH versus k is (ideally) linear with a slope equal to pH,,,, which is related to 6E/6T by eqn. (3). Plots for a variety of electrodes are shown in Figs. 3 and 4, and the pH,,, values are listed in Table 2. It has been shown that the dependence of the slope factor on temperature is ideal and it is, therefore, permissible to plot slope factor versus temperature if desired, as shown on the upper horizontal axes. The slope is then (2.3R/F)pHI,, = 0. 198pH,,,. Figs. 3 and 4 show that the experimental electrodes with Ag-AgC1 reference elements displayed excellent linearity for 6EBT and that those with calomel elements were little worse. Similar plots for proprietary electrodes4,6 were considerably more curved.Table 2 shows that the experimental electrodes with Ag-AgC1 reference elements had lower than ideal values of pH,,,, whereas those with calomel elements approached the target value of 7 quite closely, and at least as closely as the commercial electrodes. The calculated values of pH,,, were obtained from eqn. ( 5 ) with the observed value of pH0 (thereby correcting for asymmetry potential, liquid-junction potential and inaccurac- ies in activity coefficients). With MES and ADA buffers the agreement between observed and calculated values is very good, but PIPES afforded lower values than expected. As 500 490 480 470 > 460 E 9 a 450 + Y 440 W 430 420 41 0 400 390 TPC 0 10 20 30 40 50 I I 1 I I 55 57 59 61 63 65 Slope factor/mV pH-1 Fig.4 Dctermination of pH,,, for expcrimental electodes with calomel inner reference clcmcnts. Filling solutions: A, ADA; B, MES; and C, PIPES there appears to be nothing significantly wrong with the electrodes in the other examples, it could be that bpH'/6T for PIPES is in error (0.3 in pHiSo corresponds to 0.001 in 6pH'/6T). The observed and calculated pHiSO values would agree if 6pH'/6T = -0.013 or -0.011 for AgCl and calomel inner reference electrodes, respectively, compared with -0.0085 in the literature.8 Errors caused by adopting a fixed pHis(, value Many pH meters lack an adjustment for isopotential and operate at a fixed value of pHibo = 7 (k0.5). The error caused by using an electrode with a different pHiso is T - T (7) where T, is the temperature at which the electrode was calibrated and T is the temperature of measurement.Fig. 5 shows the error for three electrodes when used with a meter set at pH,,, = 7.00. Data from two to four runs with each electrode are included. The solid lines show the errors predicted by eqn. (7). The experimental points show very similar trends to those predicted, but are subject to random and, in some instances, systematic error. The starting tem- perature was always 25 "C, but these data are not shown; the points at 25 "C are for a return to that temperature. The deviations from the predicted line could arise from: ( i ) non-linearity of the variation of EO with temperature; (ii) drift of the standard potential; (iii) experimental errors in the slope and standard potential; ( i v ) changes in room temperature affecting the electrodes, which cannot be in perfect thermal equilibrium with the solution; and ( v ) errors in temperature measurement.Tt can be assumed that effect ( v ) was small, because the thermometers had been checked against a standard. Experimentally, effect (iv) would be included in the drift (ii). The experimental points were obtained with two different pH meters, which exhibited no bias between them. IfANALYST, JANUARY 1993, VOL. 118 4s 0.03 0 I d -0.03 0.02 0 -0.03 0.08 0.04 0: -0.04 -0.08 5 15 25 35 45 T/”C Fig. 5 Error caused by assuming pH,,, = 7 for calibration at 25 “C. B, PIPESXalomel electrode with pH,,,, = 6.8; A, ADA-Ag-AgCl electrode with pH,,, = 5.9; C, Ross clcctrode with pH,,, = 6.6; solid lines are calculated from cqn.(7). Open symbols for decreasing temperatures, closed symbols for increasing temperatures the meters had been set to the mean observed pHiso for each electrode, the errors would not have exceeded 0.01 pH. Discussion Glass pH electrodes having standard potentials with linear temperature characteristics can be prepared by using ‘bio- logical’ (aminosulfonic acid or iminocarboxylic acid) buffers to control the pH of the inner filling solution. This confers linear temperature characteristics on the whole pH cell when conventional inner and external reference electrodes are used. The linearity arises from the constancy of 6pK/6T (and, hence, bpHlb7’) for these buffers. The desired zero-point pH (7.0) can easily be obtained by adjustment of the pH and CI- concentration of the filling solution , for both Ag-AgCI and calomel reference elements.the best approximation to an isopotential pH of 7.0 is obtained for PIPES buffer with a calomel inner reference element and a remote calomel external reference electrode. Calomel ele- ments are not generally favoured for varying temperatures, because of hysteresis and slow response, but the experimental glass electrodes with calomel elements were no worse in these respects than commercial electrodes with Ag-AgCI elements. The temperature coefficients of pK for the buffers of this sort, reported so far, are too large for other permutations of Ag-AgC1 and calomel elements to approach the ideal characteristics; e.g., to use an Ag-AgC1 inner element and a remote calomel reference electrode requires a buffer with 6pK/6T of approximately -0.006, compared with the reported ranges-12 of -0.0085 to -0.027.The linearity of the temperature characteristics iq better than has been observed for commercial pH electrodes44 and for experimental electrodes with Hg-HgO inner reference elements.h The pH0 = pH,,, = 7 condition has been achieved before14 by use of amine buffers inside glass electrodes. As the buffers also contained carboxylic, phosphoric or phenylphosp- honic acids, linearity could have been compromised, although not as much as with conventional electrodes. Other improve- ments in temperature characteristics have been achieved by careful matching of inner and external reference electrodes as to geometry and thermal ~apacity~ls-17 but this does not compensate for non-linear pH changes in the inner reference solution and does not make pH cells independent of temperat- ure, as is sometimes implied.The ‘ideal’ characteristics are set by limitations of pH meters with fixed settings for temperature compensation. Meters with microprocessors can often cope with isopotential and zero-point pH values that differ from each other and from the standard value of 7.0. However, optimal use of such meters probably requires an experienced operator. The literature value8 for the temperature coefficient of PIPES buffer appears to be in error and requires further investigation. No such large discrepancies were observed for ADA or MES buffers and the reported value for PIPES is considerably lower than those reported for all the other ‘biological’ buffers. This PIC. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 paper was published by permission of National Power References Mattock. G., pH Measurement and Titration, Heywood, Lon- don, 1961. Covington, A. K., CRC Crit. Rev. Anal. Chem., 1974, 3, 355. Midgley, D., Anulyst, 1987, 112, 573. Midgley, D., Analyst 1987. 112, 581. Midgley, D., Tulanta, 1988, 35, 447. Midgley, D., Analyst, 1990, 115, 1283. Midgley, D., PCT Int. Appl., WO 92/01220, 1992. Good, N. E., Wingct, G. D., Winter, W., Connolly, T. N., Izawa. S . , and Singh, R. M. M., Biochemistry, 1966, 5, 467. Good, N. E . , and Izawa, S . , Methods Enzymol., 1972, 24, 53. Ferguson, W. J., Braunschweigcr, K. I . , Rraunschweiger, W. R., Smith, J. R., McCormick, J . J., Wasman, C. C., Jarvis, N. P., Bell, D. H . , and Good, N. E., Anal. Biochem.. 1980,104, 300. Batcs, K . G., Vcga, C. A . , and White, D. R., Anal. Chem., 1978, 50, 1295. Sankar, M., and Bates, R. G.. Anal. Chem., 1978, 50, 1922. Method5 for the Examination of Waters and Associated Materials, The Determinution of p H in Low Ionic Strengtlz Water5 1988, HM Stationery Office, London, 198% p. 14. Simon, W.. and Wcgmann, D., U.S. Pat., 3445363, 1969. Ross, J . W., U . S. Par., 4495 050, 19x5. Buhler, H., and Galster, H., Ger. Put., DE 3405401, 1985. Torrance, K., Analyst, 1984, 109, 1555. NorE-Rcf. 6 is to Part 1 of this scries. Paper 2102750J Received May 27, 1992 Accepted June 16, 1992

ISSN:0003-2654    

DOI:10.1039/AN9931800041

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

ANALYST, JANUARY 1993, VOL. 118 47 Simultaneous Determination of Nickel(ii) and Cobalt( 11) by Square-wave Adsorptive Stripping Voltammetry on a Rotating Disc Mercury Film Electrode Anastasios Economou and Peter R. Fielden" Department of Instrumentation and Analytical Science, UMIST, P.O. Box 88, Manchester, UK M60 IQD Adsorptive stripping voltammetry was used for the simultaneous determination of Nil' and Colt; the metal ions were complexed with dimethylglyoxime and the complexes were adsorbed on the surface of a glassy carbon rotating disc electrode, which was pre-plated with mercury. The stripping step was carried out by using a square-wave potential-time excitation signal. An electrochemical cleaning of the mercury film was employed, enabling the same mercury film to be used for a series of measurements.The limits of detection were 12 ng 1-1 for Co" and 14 ng 1-1 for Nil1 (for 120 s preconcentration), the relative standard deviation was typically about 2-3% (at the 1.2 pg I-' level) and linearity held for a concentration range of at least two orders of magnitude. Keywords: Nickel; cobalt; adsorptive stripping electrode voltammetry; square-wave voltammetry; mercury film Adsorptive stripping voltammetry (AdSV) has been estab- lished as a reliable trace analysis technique, especially over the last decade. 1.2 Traditionally the hanging mercury drop electrode (HMDE) has been used as the working electrode together with chemically modified electrodes or solid electrodes in special cases. However, mercury film electrodes (MFEs), which are prepared by electroplating a thin film of mercury on a suitable substrate,3 have a number of advantages compared with the HMDE.(a) More precise and controllable mass transfer to the electrode surface can be achieved by the use of an MFE (by rotating it or by incorporating it in a flow system) than with the stirring used in combination with the HMDE.4 ( b ) Mercury film electrodes are simpler to construct, to maintain and to control than the HMDE.4 (c) Mercury film electrodes are more stable in flow systems than the HMDE (especially at high flow rates)3 and are less seriously affected by vibrations (as with ship onboard operation). (d) Both the HMDE and the MFE have renewable surfaces. While the surface of the HMDE is rapidly and reproducibly renewed by using a new mercury drop for each measurement, the surface of an MFE can be regenerated either electrochemically or by plating a new mercury film.Although renewing the surface of an MFE is more time consuming, precision is high, especially if the operation is automated.5 Linear sweep (d.c.) voltammetry and differential-pulse (DP) voltammetry have so far dominated the stripping process. Square-wave (SW) voltammetry offers a number of advantages over the d.c. and DP excitation signals.6 (a) The background current is well suppressed compared with d.c. voltammetry, resulting in a flat baseline. (b) The sensitivity is high, expecially for reversible reactions. (c) The analysis time is shorter than when DP is used. (d) Oxygen interference is minimized. ( e ) Commercial availability for instruments cap- able of SW voltammetry is now wide, as most modern potentiostats offer an SW choice in their repertoire.In two early applications Eskilsson et aL.5 and Brett et aL.7 made use of flow systems to determine Ni" and CO" using chronopo ten tiome try and differen ti al-pul se vol tamme try, respectively. In this work the suitability of the rotating disc MFE for square-wave adsorptive stripping voltammetry (SWAdSV) is for the first time assessed and finally demon- strated for the simultaneous determination of Nil' and Co". * To whom correspondence should be addressed. Experimental Instrumentation An E G & G Princeton Applied Research (PAR) Model 273 potentiostat/galvanostat controlled by an ARC Proturbo 286 PC through the Model 270 electrochemical software was used for all measurements.The data were collected on the hard disc of the PC and plotted on a Roland plotter connected to the computer. A 10-point moving average filter was used to smooth the data. pH measurements were made with a Kent EIL 7045/46 pH meter calibrated with standard buffer solutions (at pH 7 and 9). The rotating disc electrode (RDE) assembly was a PAR Model 616 RDE with a glassy carbon (4 mm in diameter) working electrode. The analytical cell was a 50 ml vessel, the reference electrode was an Ag-AgC1 (sat. KCl) electrode and the auxiliary electrode was a Pt wire. A Model 303A HMDE (PAR) was also used for comparison purposes. Reagents and Glassware All reagents were of AnalaR grade. The water was doubly distilled. Standard Co" solutions were prepared daily from a 1000 ppm BDH atomic absorption standard solution.Stan- dard Ni" solutions were prepared from a 600 ppm solution [prepared by dissolving the appropriate amount of Ni(N03)2 in water]. The supporting electrolyte was an NH3-NH4CI buffer, pH 9, with 0.1 moll-' total NH3. AO.l moll-1 dimethylglyoxime (DMG) solution was prepared by dissolving the appropriate amount of DMG in 95% ethanol. The mercury plating solution was 1 mmol 1-1 Hg" in 0.1 mol 1-1 KN03-0.01 mol 1 - 1 HN03. The flasks used for the standards were soaked in 2 moll-' HCl for 1 week, thoroughly rinsed with water, filled with the appropriate standard solution, left to equilibrate with it for 1 week and were then ready for use; in this way adsorption on the walls of the flasks was avoided.The cell was kept filled with 6 mol 1-1 HN03 between successive analyses for at least 30 min. For the calculation of the limit of detection, for the determination of Ni and Co in iron and for calculation of accuracy, the water was passed through an Elgastat UHQ water purification system, Aristar grade chemicals were used for the supporting electrolytes and the supporting electrolytes were further purified by equilibration with 1 x 10-4 mol I-' Mn02 solution overnight followed by filtration.848 ANALYST, JANUARY 1993, VOL. 118 Procedure Preparation of the electrode The electrode was polished successively with 600 and 800 grade metaliographic grinding paper, diamond paste and 0.3, 0.075 and 0.015 pm aluminium oxide until a scratch-free and mirror-like surface was achieved.The electrode was rinsed with acetone and water and was then ready for use. When not in use the electrode was kept in the supporting electrolyte. Repolishing of the electrode was carried out when its performance deteriorated or when its surface was deactivated by accidental overpolarization. Results and Discussion Mercury film plating The mercury film was plated from the 1 mmol l-1 solution for 2 min on the glassy carbon working electrode at - 1 .O V (versus Ag-AgCI) and at a rotation speed of 10 Hz. Preparation of the iron sample A 0.250 g amount of high-purity iron was dissolved in 10 ml of 4 mol 1-1 HN03, 0.5 g of potassium tartrate were added (in order to complex Fell') and thc solution was diluted to 1 1. For the voltammetric analysis 5.0 ml of this solution and 45.0 ml of supporting electrolyte were placed in the cell, the pH was adjusted to 9 with concentrated ammonia solution and the sample was ready for the analysis. A blank solution not containing the sample (iron) was prepared in exactly the same way.Determination of Ni" and Co" The blank solution (50 ml) was de-aerated for 10 min, 500 pl of the 0.1 mol 1-1 DMG solution were added (to give a final DMG concentration of 1 X 10-3 mol 1-I), the preconcentra- tion was carried out at -0.7 V (versus Ag-AgC1) at a rotation speed of 10 Hz, the solution was left to equilibrate for 15 s and the analytical current-potential (i-E) response was recorded. The solution was blanketed with Ar during the preconcentra- tion, equilibration and measurement steps.Cleaning of the mercury film The mercury film was cleaned of the remaining adsorbed complexes by keeping the potential of the electrode at -1.2 V (twsus Ag-AgCl) for 60 s; at this potential the complexes of Ni" and Co" with DMG are exhaustively reduced. After a series of measurements the mercury film was removed by wiping the electrode with a wet tissue. 1.400 1.200 1 .ooo Q 0.800 0.400 0.200 Eo'600E -0.200 0.000 -0.600 -0.700 -0.800 -0.900 -1.000 --1.100 -1.200 -1.300 EN versus Ag-AgCI Fig. 1 Two successive cyclic voltammograms for a solution contain- ing 6 pg 1-' of Col' and 10 pg I-' of Nil1 after adsorptivc accumulation of their complexcs with DMG on an MFE. Conditions: preconcentra- tion timc, 60 s; prcconcentration potential, -0.7 V (versus Ag-AgC1); electrode rotation speed.10 Hz; scan rate, 100 mV s-l; supporting clcctrolytc. NI-14Cl-NH3 (pH 9); and DMG concentration, 1 mmoll-1 Cyclic Voltammetry Two successive cyclic voltammograms of 10 pg I-' of NilL and 6 pg 1-1 of Co" in the presence of 1 x 10-3 mol 1-1 DMG are shown in Fig. 1 after preconcentration for 60 s. The peaks at -1.03 and -1.15 V arise from the reduction of Nil1 and Co", respectively, in their complexes with DMG which are adsorbed on the MFE. The absence of a peak in the anodic branch of the cyclic voltammograms indicates that the reduction of the complexes is an irreversible process. Irrever- sibility is also implied by the shift of peak potentials to more negative values on increase of the scan rate (a negative shift of 60 mV was observed for both peaks when the scan rate was increased from 20 to 500 mV s- 1).On the other hand the small peaks for the second scan indicate that the reduction of the metal complexes is exhaustive during the first scan. Comparison Between the MFE and the HMDE Two typical square-wave stripping voltammograms for the same sample containing Nil' and Co" adsorbed on an MFE and on an HMDE are shown in Fig. 2. Despite the fact that d.c. stripping gives better results with the HMDE than with the MFE, for SW stripping both voltammograms exhibit good resolution, excellent background rejection, comparable sensi- tivity (after normalization of the electrode area) and a similar linear range. Linearity and Limit of Detection The calibration graph for the simultaneous determination of Nil1 and Co" at the same concentration is linear with the linear range and sensitivity depending on the preconcentration time.For 60 s preconcentration the calibration graph is linear up to 3.500 i I 3.000 2.500 < 2.000 5 1.500 1.000 a 0.500 - B -0 500 I I I I I I I I I -0.800 -0.900 -1.000 -1.100 -1.200 -0.850 -0.950 -1.050 -1.150 -1.250 EN versus Ag-AgCI Fig. 2 Square-wave stripping voltammograms for a solution contain- ing 1.2 pg 1-1 of both Nil1 and Co" after adsorption of their complexes with DMG on: A, an MFE; and (B). an HMDb. Conditions: preconcentration potential, -0.7 V (versus Ag-AgC1); electrodc rotation speed. 10 Hz; preconcentration timc, 60 s; frequcncy, 40 Hz; scan increment, 2 mV; pulse height, 10 mV; supporting electrolyte.NH4Cl-NH3 (pH 9); DMG concentration, 1 mmol I - ' and HMDE arca, 2.5 mm2 Table 1 Calibration parameters for Ni" and Co" determined by SWAdSV (referring to the linear range of the calibration graphs: sce text). Conditions as in Fig. 3 Preconcentration time ~~~~ ~ 60 S 120 s Parameter Ni Co Ni Co Slope/pA 1 pg- 0.20 0.85 0.46 1.23 Intercept/pA 0.11 0.09 0.006 0.07 Correlation coefficient 0.998 0.997 0.999 0.999ANALYST, JANUARY 1993, VOL. 118 49 12 pg 1-1, while for 120 s preconcentration linearity holds only up to 6 pg 1-1; on the other hand for longer preconcentration times the sensitivity is increased, as shown in Table 1. For concentrations higher than 12 p,g 1-1 (for 60 s preconcentra- tion) or higher than 6 pg 1-1 (for 120 s preconcentration) the graphs are no longer linear, as saturation of the electrode occurs.This behaviour, ie., an initial linear increase of adsorbed analyte with concentration followed by a gradual saturation of the electrode, is typical of Langmuir-type adsorption. At concentrations higher than 24 pg 1-1 double peaks start to appear for Co, indicating multilayer adsorption; this fact may also affect the Nil1-DMG adsorption because the Ni response becomes irreproducible. The limits of detection are 14 ng I- 1 for Ni" and 12 ng 1-1 for Co" at the 30 level (99% confidence level) for 120 s deposition. For Co" the limit of detection can be further decreased by longer deposition times but for NiI1 long deposition times (>I20 s) do not offer any improvement in the limit of detection, however, as even for the blank well defined Nil1 peaks are obtained.The limit of detection can be decreased by isothermal distillation of the ammonia and nitric acid used to prepare the supporting electrolyte and by recrystallization of DMG .9 Simultaneous presence and deter- mination of Nil1 and Co" at the same concentration levels narrows the linear range for both metals compared with the situation where only one metal ion is present, as expected. For instance, for 60 s deposition, the calibration graphs for solutions containing only one metal ion were linear up to the 48 pg 1 - 1 level. Effect of the Deposition Potential As shown in Fig. 3 the adsorption of the NilL-DMG complex is essentially the same from open circuit to -0.8 V. In contrast \ -0.9-0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 Deposition potentialN versus Ag-AgCI Fig.3 Effect of the preconcentration potential on the square-wave stripping response for: A, Nil1; and B, Co", adsorbed on an MFE as their DMG complexes. Ni", 0.6 pg 1-1 and Co", 1.2 pg I-'. Conditions: frequency, 60 Hz; and scan increment, 1 mV; other conditions as in Fig. 2 2.5 1 4 2.0 g . h v c. 1.5 3 m a 1.0 0.5 0 1 2 3 4 5 6 7 8 9 10 (Electrode rotation speed);/(Hz)i Fig. 4 Effect of the electrode rotation speed on the square-wave stripping response for: A, Ni"; and B, Co", adsorbed on an MFE as their DMG complexes. Nil' and Co", 1.2 pg 1-1 each. Other conditions as in Fig. 2 the adsorption of the Co"-DMG complex remains constant between 0 and -0.4 V and then starts to increase rapidly as the potential of the electrode becomes more negative.This behaviour is explained by the fact that the Ni"-DMG complex is adsorbed on the electrode as a neutral species10 while the Coil-DMG complex is adsorbed as a positively charged species;ll as a result the adsorption of Co"-DMG is favoured at potentials more negative than the potential of zero charge for a mercury electrode (about -0.5 V). The Ni"-DMG complex, being neutral, does not show a tendency to be adsorbed strongly at any extreme negative or positive poten- tial. Effect of the Electrode Rotation Speed The effect of the electrode rotation speed on the stripping response is shown in Fig. 4. For a mass-transport controlled adsorption step within the linear part of the adsorption isotherm, the response should increase linearly with the square root of the rotation speed until the equilibrium surface concentration (satisfying the Langmuir adsorption isotherm) is established on the electrode surface.12.13 For the Co"-DMG complex, which adsorbs strongly at -0.7 V, the equilibrium adsorption surface concentration is relatively high and adsorp- tion surface equilibrium is not achieved before a rotation speed as high as 36 Hz.For the Ni"-DMG complex, the adsorption of which is weaker at -0.7 V, the response increases linearly up to 4 Hz and then levels off as soon as the adsorption equilibrium surface concentration is reached. Effect of pH and of DMG Concentration The effect of pH on the stripping responses for Ni and Co has been reported previously14 and similar results were obtained in this work.Resolution between the Ni and Co peaks increases but the sensitivity for the Co peak decreases with increasing pH. For analytical purposes the optimum pH value was about 9. The effect of the DMG concentration has also been studied for the HMDE'S and its effect is the same for the MFE. Comparison Between DP, SW and SC Stripping In Fig. 5 three stripping voltammograms of the same solution are shown, obtained by using typical values of staircase (SC), square-wave (SW) and differential-pulse (DP) stripping. It is clear that DP and SW stripping offer an excellent discrimina- tion against background current. On the other hand SW and SC stripping are typically 4-20 times faster than DP stripping [in this instance the scan rates are (in mV s-1): SC 100, DP 10 8.000 I 1 7.000 6.000 5.000 6 4.000 a 3.000 2.000 1.000 a 7 0.000 L 1 I I I I -0.800 -0.900 -1.000 -1.100 -1.200 -1.300 EN versus Ag-AgCI Fig.5 Comparison between DP. SC and SW modes of stripping for a solution containing 1.2 pg I-' of Nil1 and 0.6 pg 1-I of Co" adsorbed on an MFE as their DMG complexes. Conditions for DP: scan rate, 0.01 V s-1; scan increment 2 mV; drop time, 0.2 s; pulse height, 20 mV; pulse width 0.05 s; and 5-point moving average filter. For SW: frequency, 60 Hz; scan increment, 3 mV; and pulse height, 10 mV. For SC: scan rate, 0.1 V s-l; and scan increment, 2 mV. Other conditions as in Fig. 25 0 ANALYST, JANUARY 1993, VOL. 118 9.000 8.000 (4 - 7.000 6.000 5.000 4.000 3.000 2.000 1.000 0.000 1 I 1 I I I I I A a -0.850 -0.950 -1.050 -1.150 -1.250 m -0.900 - 1 .ooo -1.100 - 1.200 0 10.000 9.000 8.000 7.000 6.000 5.000 4.000 3.000 2.000 1.000 0.000 I I I I 1 -0,800 -0,900 -1.000 -1.100 -1.200 -1.300 EN versus Ag-AgCI Fig.6 ( u ) Effect of the SW frequency on the stripping response for a solution containing 0.6 vg 1 - 1 each of Ni" and Coif adsorbed on an MFE as their DMG complexes. A, 10; B, 20; C , 40; D. 80; and E, 100 Hz. Conditions: pulse height, 1.5 mV; and scan increment, 1 mV; other conditions as in Fig. 2. ( b ) Effect of the scan increment on the SW stripping response for Ni" and Co" adsorbed on an MFE as their DMG complexes. Nili and Co". 1.2 pg1-I each. A. 1; B, 2; C , 5 ; D, 10; and E, 20 mV. [In ( h ) no filtering was used for the voltammograms.] Other conditions as in Fig.2 and SW 801 and give much higher peaks. The SW waveform seems to combine the relative advantages of SC and DP waveforms. Square-wave Parameters The SW parameters that were investigated were the fre- quency, the pulse height and the pulse increment. These parameters are interrelated and have a combined effect on the response but here only the general trends will be examined. Frequency The response for both Ni and Co increases with SW frequency but at frequencies higher than 100 Hz sloping background current renders the measurement difficult, especially for the Ni peak [Fig. 6(a)]. This behaviour may be because, in order to achieve higher frequencies, the pulse width is shortened; as a result the measurement is taken at a time when the capacitive current is still significant and contributes to the measured response.16 Pulse height Increase in the pulse height causes an increase in the Ni peak up to 20 mV and in the Co peak up to 30 mV and a similar behaviour was observed in an earlier study with the HMDE.6 The peak potential shifts to the positive direction with increasing frequency. Scan increment The scan increment, S I , together with the frequency,f, define an effective scan rate, v, according to the relationship: v = S I x f (1) hence increase of the scan increment is expected to result in an increase of the response as the scan rate is increased. However, increase of the scan rate also results in 'aliasing' as fewer points are sampled during the experiment and conse- quently in a less accurate representation of the actual response.This effect is shown in Fig. 6(h) where no smoothing of the data was carried out: at scan increments greater than 10 mV too few points are sampled and the peaks are distorted, whereas at small scan increments (1-2 mV) the response is more accurately recorded but higher frequency noise is also present . An increase of either the frequency or the scan increment results in an increase in the effective scan rate [eqn. (1)); because the reduction of Ni and Co complexes with DMG is an irreversible process, increase in the effective scan rate results in a shift of the peak potentials to the negative direction [Fig. 6(a) and (b)]. Determination of One Ion in Great Excess with Respect to the Other As the relative concentrations of Nil1 and Co" in real samples vary within a wide range it is of importance to be able to determine both metals simultaneously irrespective of the sample.It is possible to control the adsorption process in order to achieve preferential adsorption of a particular DMG complex and thus facilitate the determination of a specific ion; this can be mainly achieved in the following ways. ( a ) By controlling the preconcentration potential it is easy to adsorb the Ni"-DMG complex selectively at potentials more positive than -0.4 V while both the ColI-DMG and Ni"-DMG complexes are adsorbed at potentials more negative than -0.6 V (Fig. 3). ( h ) By varying the preconcentration time it is possible to control the adsorption of the two complexes as long preconcentration times (>lo0 s) result in an enhanced Co"-DMG adsorption.( c ) By increasing the rotation speed of the electrode to more than 4 Hz the adsorption of the Co-DMG complex is promoted, while that of the Ni-DMG complex is not affected (Fig. 4). (d) By using large pulse heights (>20 mV) an enhanced Co response can be achieved. (e) High DMG concentrations (> 1 x mol 1-*) result in an increased Co response. 15 By using the optimum conditions it is possible to determine one metal ion in the presence of more than a 100-fold excess of the other. Reproducibility and Stability of the Mercury Film The reproducibility of the measurements was assessed by carrying out eight successive analyses on the same mercury film for a solution containing 1.2 pg 1-1 each of Nil1 and Co"; the reproducibility in terms of the relative standard deviation was 2.5% for the determination of Ni and 2.1"/0 for the determination of Co.The stability of the mercury film is very important for the reliability of the results. It was found that plating for 2 min produced mercury films that would be stable for at least 2 h if care was taken to exclude oxygen. Moreover, solutions containing chloride ions could be safely analysed if the electrode was polarized only to potentials more negative than 0 V. No deterioration of the film was observed at very high rotation speeds, as a reproducible and well defined response was obtained even at 83 Hz. Both strongly alkaline and acidic solutions could be safely used without deterioration of the mercury film. However, as the plating step can be carried out while the sample is being de-oxygenated the approach taken in this work was to plate a new mercury film after a series of standard additions (4-5 scans).The reproducibility of an alternative method of measure- ment was also assessed: instead of cleaning the mercury film a new film was used for each measurement. This method wasANALYST. JANUARY 1993, VOL. 118 51 0.9 I c 2 0.5 2 0.4 TY2Ll 0.0 0.1 -5 -4 -3 -7 -6 ym 03c \ I 0.211q Log([Triton X-iOO]/g 1-1) Fig. 7 Effect of the surfactant Triton X-100 at different concentra- tions on the square-wave stripping response for: A, Ni"; and B. Co", adsorbed on an MFE as their DMG complexes. Other conditions as in Fig. 3 Table 2 Determination of Ni and Co in a high-purity iron sample and in a laboratory-prepared standard Ni (YO) Co (% ) Sample Certified Found" Certified Found" High Purity Iron 0.0036 0.0040 0.0073 0.0072 BCS-CRM 149/3 +0.0008 f O.o()03 * 0.0005 +0.0006 Ni/pg 1- 1 Co/pg 1 - 1 Added Found-/- Added Found? Laboratory standard 0.48 0.50 * 0.03 0.39 0.42 ? 0.03 * n = 8 .t n = 3 . clearly less satisfactory as far as reproducibility is concerned giving values for the relative standard deviation of 26.8% for Ni and 19.9% for Co. It is assumed that the decrease in reproducibility was mainly due to irreproducible generation of the mercury films rather than irreproducible conditions of adsorption. Interferences Two major sources of interference were investigated. (a) Surfactants present in most real samples are the more serious interference in AdSV. Triton X-100 was used to simulate the effcct of a typical non-ionic surfactant and Fig.7 shows how the stripping peak current is affected for different concentra- tions of Triton X-100. Similar experiments carried out by the authors with an HMDE have demonstrated that the MFE is as tolerant to the presence of Triton X-100 as the HMDE. ( b ) Metal ions can also interfere with the measurement, hence a number of common metal ions were examined: Pb", Hg", Cu", Fe", AIIII, Cd", Tiib, Ca" and Mnll added at a 1000-fold excess over Nil' and Co" did not interfere. Zinc(ii) was found to interfere severely at a 500-fold excess over Ni" and Co" but no interference was observed for excesses lower than 100-fold. For samples with high Zn" concentration the addition of nitrilotriacetic acid (NTA) is recommended.l 4 Accuracy and Applications The accuracy of the method was assessed by determining Ni" and Co" both in a laboratory-prepared standard and in the British Chemical Standard (BCS) Certified Reference n 1.200 I 1 1.000 0.800 5 0.600 0.400 0.200 0.000 z -0.4 0 0.4 Concentration/pg I- -0.200 I I I I I I -0.800 -0.900 -1.000 4 . 1 0 0 -1.200 -1.300 -4.400 EN versus Ag-AgCI Fig. 8 Standard additions (after background subtraction) for a sample containing 0.48 btg I-' of Ni" and 0.39 pg I-' of Co" in 0.5 mol I-' NaCl. Curves: A, sample; B. C and D, standard additions of 0.15, 0.30 and 0.45 pg 1 - 1 , respectively, of Ni" and Co". Other conditions as in Fig. 2 Material (CRM) 149/3 High Purity Iron by the standard additions method. The results are shown in Table 2, indicating good accuracy for both samples. A series of standard additions for a sample containing 0.48 pg 1-1 of Ni" and 0.39 pg I-' of Co" is shown in Fig.8; in this instance a background subtraction was carried out resulting in an extremely flat baseline. The authors express their gratitude to the Royal Society of Chemistry for financial support to A. E., who is in receipt of an SAC studentship. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 References Wang, J . , in Electroanalytical Chemistry, ed. Bard. A. J . , Marcel Dekker, New York, 1988, vol. 16, pp. 1-86. Kalvoda, R., and Kopanica, M., Pure Appl. Chem., 1989, 61, 97. Wang, J . , Stripping Analysis, VCH, Deerfield Beach, FL, 1985, pp. 66-75. Batley, G. E., and Florence. T. M., J. Electroanal. Chem., 1974,55,23. Eskilsson, H.. Haraldson, C., and Jagner, D., Anal. Chim. Acta, 1985, 175, 79. Ostapczuk. P., Valenta, P., and Nurnberg, H., J . Electroanal. Chem., 1986, 214, 51. Brett, C. M. A., Oliveira Brett. A. M. C. F.. and Pereira, J. L. C., Electroanalysis, 1991. 3, 683. Newton, M. P., and van den Berg, C. M. G., Anal. Chim. Actu, 1987, 199, 59. Pihlar, B., Valenta, P., and Nurnberg, H. W.. Fresenius' Z. Anal. Chem., 1981, 307, 337. Pihlar, B., Valenta, P., and Nurnberg, H. W., J . Eleclroanal. Chern., 1986, 214, 157. Jin, W., and Liu, K., J. Electroanal. Chem., 1987, 216, 181. Levich, V. G., Physicoc/zernicul Hydrodynamics, Prentice-Hall, Englewood Cliffs, NJ, 1962. Bard, A. J., and Faulkner, L. R., Electrochemical Methods, Wilcy, Ncw York, 1980, pp. 516-519. Gilbert, M. G. M . , Powell, H. K. J . , and Fardy, J . J . , Anal. Chim. Acta, 1988, 207, 103. Adeloju, S. B., Bond, A. M., and Briggs, M. H., Anal. Chim. Actu, 1984, 164, 181. van den Berg, C. M. G., Anal. Chim. Acta. 1991, 250, 265. Paper 2/02234F Received April 30, 1992 Accepted October 7, 1992

ISSN:0003-2654    

DOI:10.1039/AN9931800047

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

ANALYST. JANUARY 1993. VOL. 118 53 Voltammetric Determination of Gold Using a Carbon Paste Electrode Modified With Thiobenzanilide Xiaohua Cai College of Science and Technolog y, Hainan University, Haikou 570028, People's Republic of China Kurt Kalcher,* Christian Neuhold and Wolfgang Diewald lnstitut fur Analytische Chemie, Karl-Franzens Universitat, Universitatsplatz I , A-80 10 Graz, Austria Robert J. Magee La Tro be University, Bundoora, Melbourne, Victoria, Australia Gold(ii1) can be preconcentrated from acidic solution on a carbon paste electrode chemically modified with thiobenzanilide under open-circuit conditions. Using cathodic differential pulse voltammetry, 0.05-6 pg ml-1 of gold can be determined after preconcentration for 1 or 2 min. Except for mercury(ii) and platinum group metals, common ions have little effect on the determination of gold(iii).The preparation and regeneration of the modified electrode as well as the various methodical parameters for the preconcentration and measurement of gold(1li) were investigated. Keywords: Modified carbon paste electrode; thiobenzanilide; voltammetric determination of gold Since the first application of the direct mixing technique to the modification of a carbon paste electrode (CPE) by Ravichan- dran and Baldwin, many publications on chemically modified carbon paste electrodes (CMCPEs) have appeared.2 The advantage of this technique is its simplicity. The disadvantage, however, lies in the difficulty in obtaining homogeneous carbon pastes, when using solid modifiers as additives. For this reason, the 'solvent volatilization' technique coupled with ultrasonic vibration was developed.334 This special form of direct mixing is suitable only for modifiers which can be adsorbed on the carbon powder.This paper presents a new strategy for the modification of CPEs with thiobenzanilide (TBA, CBHS-CS-NH-ChHS) for the preconcentration and subsequent determination of gold. Determination of gold with CPEs can be traced back to the first application of such an electrode to conventional stripping analyskS More recently, determination of gold with CPEs containing various modifiers such as anion exchangers,Gg chelating resins,q Rhodamine BlO and dithizone" has been reported. Thiobenzanilide has a structure similar to that of dithizone. 12 Many of the methods employing CMCPEs for the determi- nation of gold have some disadvantages, mainly poor detec- tion limits of about 1 pg ml-l of gold(iii), or inhomogeneities of the electrode material due to particulate modifiers, or lack of chemical or electrochemical regenerability of the electrode surface.Thus, it seems necessary to design improved modified electrodes which overcome these difficulties. In this work, we have tried to overcome the problem ot inhomogeneity when mixing particdate modifiers directly with the carbon paste. A common procedure for producing a highly homogeneous electrode surface is to dissolve the additive in the pasting liquid of the electrode material. However, this method is only applicable if the modifier possesses a highly lipophilic character, which is not the case for TBA.Thus, it seemed promising to dissolve it in a more polar liquid, i.e., bromoform, which can actually be used as a pasting liquid. 13 Since carbon pastes containing only bromo- form as liquid paste component exhibit poor performance owing to adverse physico-chemical characteristics (low viscos- ity, volatility of CHBr3), a new approach to circumvent this problem by dissolving the modifiers in bromoform and mixing * To whom correspondence should be addressed. this solution with a common pasting liquid, i.e., paraffin oil, was tried. An attempt was also made to regenerate the electrode by a non-mechanical method, which is not possible for most of the CPEs modified with sulfur-containing organic com- pounds. 1 1 I * 4-16 Experimental Apparatus For the voltammetric measurements, a Princeton Applied Research (PAR) 264A polarograph was used in combination with a laboratory-constructed electrode assembly of Plexi- glas.I7 The cell consisted of a titration vessel of glass (Cat.No. 6.1415.220 from Metrohm) with a platinum wire as the counter electrode and a saturated calomel electrode (SCE, Ingold 30W-NS) as reference. The latter was in contact with the solution over a salt bridge with a Vycor frit, filled with 1 mol 1-1 KCl solution. A Teflon tube allowed purging of the solution with nitrogen, which was carried out for at least 5 min each time a fresh solution was put into the vessel. During the measurement nitrogen was passed over the solution. Voltammetric curves were registered on a two-channel recorder PAR Model R E 0089 and evaluated manually by the tangent-fit method.Cyclic voltammograms (CVs) were re- corded using an appropriate interface for A/D-conversion of the data in combination with a personal computer.*s The receptacle for preconcentrating the analyte onto the electrode was a SO ml glass beaker equipped with a poly- (tetrafluoroethylene) (PTFE)-coated stirring bar (length 30 mm, 7 mm c1.d.); the beaker was placed under a suitable holder for the electrode. Working Electrode The body of the working electrode was a Teflon rod ( I 1 mm 0.d.) with a hole (7 mm diameter) bored at one end for the carbon paste filling. Contact was made with a platinum wire through the centre of the rod. The modified paste was prepared by dissolving 0.40 g of TBA in 0.80 g of bromoform.The resulting solution was mixed with spectral carbon powder (2.0 g RWB, Ringsdorff- Werke) and paraffin oil (0.50 g, Uvasol, Merck). The carbon paste was packed into the hole of the electrode and smoothed off with a PTFE spatula.54 ANALYST, JANUARY 1993, VOL. 118 The plain carbon paste electrode was prepared in a similar way, without adding the modifier solution. Reagents De-ionized water was distilled twice in a quartz still and then purified by a cartridge de-ionization system (Nanopure, Barnstead). Hydrochloric acid was of Suprapur grade (Merck). Thiobenzanilide was of analytical-reagent grade (Eastman Kodak). A gold stock solution (1000 pg ml-1) was prepared in 0.01 mol 1- 1 hydrochloric acid from potassium tetrachloro- aurate(m) (pro analysi, Merck).Solutions of lower concen- trations were prepared by dilution of this stock solution just before use. Stock solutions of salts used for investigations on their interference had a concentration of 1 x 103 or 1 x 104 pg ml-' with respect to the ion. Procedure Activation of the electrode The freshly prepared electrode was exposed to a stirred analyte solution (0.01 mol 1-1 HCl) containing 10 pg ml-1 of gold(ii1) for 30 s. After rinsing with water for a short time, the electrode was dipped into dilute hydrochloric acid (7.5 x 10-3 moll-1) and scanned between 0.5 and -0.5 V versus SCE for five complete cycles at a scan rate of 50 mV s-l. This procedure was repeated. Then, the electrode was polarized for 5 min at -0.40 V. Preconcentration After activating the electrode, it was dipped into the stirred (300 rev min-1) analyte solution containing gold(rr1) for the required time.The electrode was removed, rinsed with water for a short time, placed into the voltammetric cell and connected to the polarograph. Voltammetry The supporting electrolyte for the voltammetric measure- ments was 7.5 x 10-3 mol 1-1 HCI. Quantitative determina- tions were performed in the differential pulse voltammetry (DPV) mode. The potential range was set from 0.5 to -0.4 V versus SCE in the cathodic direction. An equilibration period of 1.5 s, with the initial potential applied, was required in order to settle the solution. The pulse height was 50 mV and the scan rate 10 mV s-1 with an increment of 0.2 mV per data point. The current range was set according to the concentration of gold(iIi). Cyclic voltammograms were recorded with a scan rate of 50 mV s-1.The potential range was from 0.5 to -0.5 V. Other parameters were the same as for DPV. Regeneration After recording the voltammogram, the electrode was elec- trolysed at -0.38 V for 2 min. The renewed electrode usually did not show any peak within the potential range but if it did, the regeneration step was repeated. Results and Discussion Composition and Electrochemical Behaviour of the Modified Electrode In preliminary experiments, TBA was mixed directly with the conventional carbon paste. The resulting electrode showed almost no ability to preconcentrate gold(ii1). The solvent volatilization technique was also used with alcohol or ether as a volatile solvent.It was shown that TBA does not mix well with carbon powder after solvent volatilization. The TBA is easily soluble in chloroform and bromoform, which have been used as pasting liquids.5 Bromoform was chosen because it has a lower volatility than chloroform. The bromoform containing dissolved TBA can be easily mixed, as a liquid modifier, with conventional carbon paste. A certain content of paraffin oil in the paste was found to be necessary, because it increases the viscosity of the carbon paste; thus the modified electrode can be used repeatedly for a longer time without any notable change in its electrochemical characteris- tics. The optimum mass ratio between bromoform and paraffin oil was found to be 1.6 : 1. The content of TBA in the carbon paste influences the height of the signal; an optimum concentration was found to be 10% m/m with respect to the analytical performance of the electrode.The CVs of the modified electrode in the absence of gold are shown in Fig. 1. A broad wave appears at about -0.6 V versus SCE when starting the scan at +1 V (curve A). This wave gradually decreases and finally disappears as the initial scan potential becomes less positive (curves B-D). It is known that TBA itself cannot be reduced directly in this potential range. But as the compound contains a C=S group in close vicinity to an NH group, it can be oxidized to its dimeric form via its tautomeric structure -C(SH)=N-. The resulting disul- fide can be easily reduced, as reported previously.19 Thus, the dependence of a voltammetric reduction on the initial potential clearly indicates that the wave must be assigned to the reduction of an oxidation product of TBA, which is produced at the more positive potentials.Therefore, in order to avoid oxidation of the modifier at the electrode surface, an initial scan potential less than +0.6 V should be used for practical applications of the modified electrode. Voltammetric Behaviour of Gold If a plain carbon paste clcctrode is used to preconcentrate gold under open-circuit conditions by interchanging media between accumulation and measurement, no signal response is obtained; this indicates that an unmodified carbon paste electrode does not adsorb gold(ii1). If gold(ii1) is present in the bulk solution of the measurement, the unmodified electrode gives responses due to electrochemical transformations of gold(ii1). Fig.2 shows the CVs. Reduction of gold(iri) to gold(o) occurs at 0.32 V versus SCE and re-oxidation occurs at 1.05 V. Evidently, for the reasons discussed above, only the current response of the reduction of gold(rr1) can be exploited analytically with modified electrodes. Fig. 3 shows the voltammetric behaviour of gold(iii) that had been accumulated externally (open circuit) onto the carbon paste electrode modified with TBA. During precon- centration, gold(iii) is accumulated by TBA onto the surface a E 2? 5 . c 0 1 0.5 0 -0.5 - 1 PotentialN versus reference Fig. 1 Cyclic voltammogram of a modified carbon paste electrode. Supporting electrolyte, HCI (0.0075 mol I-').Initial potentials: A , +1.0; B, +0.8; C, +0.6; and D, +0.5 V versus SCEANALYST, JANUARY 1993, VOL. 118 55 1.2 0.8 0.4 0 -0.4 PotentialN versus reference Fig. 2 Cyclic voltammogram of gold(1ii) on an unmodified carbon paste elcctrode. Supporting electrolyte, HCI (0.0075 mol 1- l ) . A, Blank; and B. 20 pg ml-' gold(iii) f 2 E 3 A I I I I 0.5 0.3 0.1 -0.1 -0.3 -0.5 PotentialN versus reference Fig. 3 Cyclic voltammogram of gold(m) accumulated with a modi- fied carbon paste electrode. Analyte solution: 0.01 moll-1 HC1 and 10 pg ml-1 gold(rr1); accumulation time, 1 min and supporting elec- trolyte, HCl (0.0075 mol 1-1). A, Blank; and B, 10 pg ml-1 gold(m) of the carbon paste electrode, forming a very stable complex Au3+(sol) + 3C6HS-C(SH)=N-C6H5(sur) -+ Au(C6HS-C(S-)=N-C6HS)3(sur) + 3H+(sol) (1) where (sol) refers to the analyte solution and (sur) to the electrode surface.The reactive species actually is the thiol- form of TBA. In the cathodic scan gold(I1i) is reduced to gold(o) at -0.25 V versus. SCE, which cannot be re-oxidized to gold(iii) within the applied potential range. Preconcentra- tion with the modified electrode produces a high concentra- tion of gold(ii1) at the surface of the electrode. As a result of this type of accumulation, the reduction peak with the modified electrode is much more sensitive for gold than an unmodified one in direct voltammetry. A large shift of the peak potential in the cathodic direction, compared with an unmodified electrode, indicates that the reduction of gold(rr1) in its complex with TBA is harder to effect than in its free form.Differential-pulse voltammograms of gold(1ii) accumulated at the modified carbon paste electrode can be seen in Fig. 4. As can be expected, the signals are much broader than for mercury electrodes, but they are well shaped to be exploited for analytical determinations. Therefore, DPV was used throughout this work. [eqn * (1) 1 Activation and Renewal of the Modified Electrode If the freshly prepared, unactivated electrode is used to accumulate gold, the resulting current response is less sensitive to the concentration of gold, and the peak height increases when the procedure is repeated with the same k 0.5 0.2 -0.1 -0.4 PotentialN versus reference Fig. 4 Differential pulse voltammograms of gold(iri) with a modified carbon paste electrode.Supporting electrolyte, HCI (0.0075 moll-*); analytc solution, 0.01 mol 1-1 HCl; accumulation time, 5 min. A, Blank; B, 1 pg ml-I gold(ii1); and C, 2 pg ml-l gold(iii) electrode filling. Therefore, it may be concluded that the complexing groups at the surface of the electrode have not been fully activated. In order to avoid a pre-treatment dependent gold signal and to improve the analytical perfor- mance of the electrode, activation of the electrode is necessary. The optimum method involves exposure of the original electrode to a solution containing gold, because a small amount of gold(ii1) seems to be irreversibly adsorbed on the electrode material. The resulting electrode has good stability, sensitivity and reproducibility. Similar processes have been reported for carbon paste electrodes with other modifiers .2",21 For repetitive use, the regeneration of the reactive func- tional groups on the electrode surface is very important. Although this can be achieved simply by replacing the used carbon paste, it requires a highly reproducible treatment of the electrode, otherwise the physical and chemical properties will not remain constant during repetitive measurements. Therefore, chemical or electrochemical regeneration is prefer- able. Potassium cyanide and thiourea were used to regenerate the electrode surface. Although they are effective in removing gold from the electrode surface by forming complexes with gold(iii), the resulting electrode has poor reproducibility during the ensuing measurements.As can be seen from Fig. 3, the reduction peak for gold(i1i) gradually decreases and finally disappears during repetitive scans. This provides the possi- bility of regenerating the electrode electrochemically. Further experiments showed that after each electrochemical measure- ment, a potential more negative than the peak potential must be applied to the electrode for some time so that gold(ii1) can be completely reduced. As a result, the electrode does not display any DPV peak and can be used again to preconcen- trate gold(1Ii) without further activation. To characterize the reproducibility of the modified elec- trode by regenerating it electrochemically, repetitive precon- centration-measurement-regeneration cycles were carried out. The result of eight ensuing measurements showed a relative standard deviation of 4.9% for 1 pg ml-1 of gold(iii) with a preconcentration time of 1 min.Thus, electrode renewal gives a good reproducible surface. Usually, after eight measurements, the current response for gold begins to decrease gradually, indicating that the preconcentration ability of the electrode decreases. This phenomenon may be due to residues of elemental gold adsorbed at the electrode surface, which decrease the effective area of the electrode and, thus, may block binding sites for tetrachloroaurate. Optimum Conditions for Analysis Acetate buffer, phosphate buffer and hydrochloric acid were investigated as media for the analyte solution. Dilute HCI was56 ANALYST, JANUAKY 1993, VOL. 118 1.80 I 0.90 ' I I I I I 0.005 0.010 0.015 0 020 0.025 Concentration of HCl/mol 1-1 Fig.5 Dependence of the peak current on the acidity of the analyte solution. Supporting electrolyte, HCI (0.0075 rnol 1-1); analyte solution, I pg ml-1 gold(iu); and accumulation time, I min 2.1 a g z 3 1.5 1.2 0 0.004 0.008 0.012 0.016 Concentration of HCVmol I 1 Fig. 6 Dependence of the peak current on the concentration of the supporting electrolyte (HCl). Analyte solution, 0.01 rnol I-' HCI, 1 pg ml -1 gold(iii); and accumulation time, 1 min found to be preferable, because it precipitates, and thus separates, any silver interferent present in the analyte solution. The depcndcnce of the peak height on the acidity of the analyte solution is displayed in Fig. 5 ; a steady signal can be obtained if the concentration of HCl in the analyte solution lies between 5 X 10-3 and 1 X 10-2 rnol I-'.For our investigations, a concentration of 1 X 10-2 mol 1 - 1 HCI in the analyte solution was used. Dilute HCI is also best suited as supporting electrolyte. As can be seen from Fig. ti, the current response in DPV is maximum with a concentration of HCI of 7.5 x 10-3 rnol 1-1. This concentration was used for the subsequent analyses. The dependence of the peak current on the preconcentra- tion time is displayed in Fig. 7. An exponential increase of the peak with increasing preconcentration time is observed for the modified electrode, resulting in a constant value for longer accumulation periods. The overall curve shape of the current- preconcentration time diagram is very typical for this type of modified electrode.1 1 The manifestation of a limiting value for the current at longer periods of time is due to reaching equilibrium conditions for the reaction between complexing reagent groups at the surface and analyte ions in the solution. With suitable preconcentration times, a linear ratio between peak height and concentration of gold(ii1) exists for 0.05-6 pg ml- I as shown in Fig. 8 when a proper accumulation time is chosen, i.e., 2 min for concentrations below 1 pg ml-1 of gold(Ir1) and 1 rnin above this concentration. The detection limit is 0.02 pg ml-1 when the preconcentration time is 5 min. For the analysis of 1 pg ml-1 of gold, the relative standard deviation is 1.9% for five determinations when using an internal standard additions methods.3.5 ' Accumulation time/min Fig. 7 Effect of the accumulation time on the pcak currcnt. Analyte solution, 0.01 mol I - ' HCI, 1 pg m1-I gold(ii1); and supporting electrolyte, HCI (0.075 rnol 1 1) 10 8 5. g 3 =; c 6 4 2 0 2 4 6 8 10 Concentration range Fig. 8 Dependence of the signal current on the concentration of gold(ii1). Supporting electrolyte, HCI (0.075 rnol 1 I ) ; analytc solution, 0.01 tnol I- 1 HCI; and concentration rangcs (0 and 10 of x-axis correspond to minimum and maximum of given range). Accumulation times: A, 0-1.0 pg ml-1 gold(iii), 2 min; and B, 0-10 pg ml-1 gold(iri), 1 min Table 1 Interferences with the determination of gold; analyte solution: 1 pg ml-1 gold, 0.01 mol 1-1 HCl; supporting electrolyte 0.0075 rnol 1-1 HC1; accumulation time: 1 rnin Peak change (%) Concentration of interferent Interferent HglI CU" Ag' ZnlI Cd" In111 Pb" A P As" Fe'II CO" NilI Pd" IF1 IP' Pt" BiIII Added as Hg C12-HC1 CuC12-H35 AgN03-H20 ZnC12-HC1 BiCl,-HCI In(N03)3-HCl AszO3-NaOH FeCI3-HCl NiC12-HCI Na2PdC14-HC1 IrC13-HC1 H2IrCl6-HC1 CdCI2-HC1 Pb(N03)2-H20 As~OS-H~O CoCIz-HCI (NH4)2PtC14 20 pg ml-1 -55 -4.9 -13.3 +11.1 +35.6 -4.6 -11.1 -5.4 -13.2 -11.7 -44.5 -5.0 - 10.0 -6.7 - 13.8 -5.0 -15.2 -9.4 -22.3 -7.5 -11.0 -6.3 -12.0 -7.6 - 17.4 120 pg ml-1 - 90 -26.6 - 100 -33.3 - 100 +4.6 +14.5 Interferences Various common ions were examined with respect to their interference in the determination of gold (Table 1).Most of the species analysed have only a slight effect on the determina- tion of gold(r1i) even up to a 120-fold excess with respect to gold.The influence of weakly interfering components can easily be eliminated by applying the standard additions method for the evaluation of the concentration of gold(ii1). Mercury(i1) and platinum group elements interfere signifi- cantly, as they can also form complexes with TBA. Other thiophilic elements such as silver, bismuth and arsenic(v) interfere severely at higher concentrations.ANALYST, JANUARY 1993, VOL. 118 57 Table 2 Recovery of gold in an artificial mining waste; standard deviations for five determinations Concentrationhg ml- 1 Foundhg ml- 1 50 49.7 k 1.0 100 99.1 * 1.9 500 496 * 7 Mining waste water An artificial sample with representative concentrations of constituents22: 20 mg 1 - 1 manganese(lI), 100 mg 1-1 iron(IIi), 100 pg 1 - 1 chromium(rii), 2.5 mg 1 - 1 cobalt(ii), 5 mg 1-1 nickel(ir), 1.5 mg 1-1 copper(Ii), 5 mg 1-1 zinc(Ii), 10 pg 1-1 cadmium(Ii), 100 pg I--’ lead(ii) was spiked with different amounts of gold.The pH was adjusted to 2.1 with HCI, and the recovery rate of gold was determined by the method presented above (Table 2). Pharmaceutical Gold was analysed in tablets containing Auranofin (Ridaura from Smith Kline Dauelsberg, Gottingen, Germany). One tablet was dissolved in a mixture of 3 ml of concentrated HN03 and 0.2 ml of concentrated HC104, and the liquid was evaporated to dryness. The residue was dissolved in 250 pl of HCI (10 moll-1) and made up to 20 ml. An aliquot of 2 ml was diluted to 20 ml and used for the preconcentration of gold.The gold content was found to be 0.86 k 0.01 mg of gold per tablet (six determinations; reference value 0.87 mg). The method presented here has a much lower detection limit than similar methods with modified carbon paste electrodes. Whereas phosphor-organic compounds or ion exchangers can be used as modifiers for CPEs to determine gold in concentrations higher than 1 pg ml-1 of gold (= 5 x 10-6 mol I-’), TBA is suitable for concentrations down to 50 ng ml-1 of gold (= 2.5 x 10-7 moll-’). Thus, the detection limit is higher than for atomic absorption spectroscopy (AAS) when using acetylene-dinitrogen oxide ( 5 X 10-8 mol 1 - I ) , but comparable when using acetylene-air to avoid some matrix effects. Therefore, the method presented here may be a reasonable alternative to AAS.1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 The authors wish to acknowledge financial support of this work by the Austrian “Fonds zur Fiirderung der Wissenschaft- lichen Forschung” (Project No. P8539-CHE). X.C. acknow- ledges a scholarship from the North-South Dialogue Program of the Austrian government. References Ravichandran, K., and Baldwin, R. P., J . Electroanal. Chem., 1981, 126, 293. Kalcher, K., Electroanalysis, 1990, 2, 419. Ravichandran, K., and Baldwin, R. P., Anal. Chem., 1983,55, 1586. Prabhu, S. V., Baldwin, R. P., and Kryger, L., Electrounalysis, 1989, 1. 13. Jacobs, E. S . , Anal. Chem., 1963,35, 2L13. Kalcher. K., Anal. Chim. Acta, 1985, 177, 175. Kalcher. K., Greschonig. H., and Pietsch, R., Fresenius Z. Anal. Chem., 1987, 327, 513. Peng, T., Shi, Q., and Lu, R . , J. Appf. Sci., 1990, 8, 361. Gao, Z.. Li, P., Dong, S . , and Zhao, Z., Anal. Chim. Acta, 1990, 232, 367. Kolbl, G., Kalcher, K., and Voulgaropoulos, A., Fresenius 2. Anal. Chem., 1992, 342, 83. Kalcher, K., FreseniuJ Z. Anal. Chem.. 1986, 325, 181. Beilstein Handbuch der Organischen Chemie, Frankfurt, 2nd Supplement, 1950, vol. XII, p. 154. Adams, R. N., Anal. Chem., 1958, 30, 1576. Kalcher, K., Fresenius 2. Anal. Chem., 1986, 325, 186. Tanaka, S . , and Yoshida, H., Talanta, 1989, 36, 1044. Sagawara, K.. Tanaka, S . , and Taga, M. ~ J. Electroanal. Chem., 1991, 304. 249. Kalcher, K., Fresenius Z. Anal. Chem.. 1986, 323, 238. Kalcher, K., and Jorde, C., Comput. Chem., 1986, 10, 201. Cai, X., Pei, J., Zhou, X., and Zhao, Z., Anal. Sci., 1991, 7, 109. Amine. A., Kauffmann, J . M., and Patriarche, G . J., Talanta, 1991, 38, 107. Wang, J . , Taha, Z . , and Nasser, N., Talanta, 1991, 308, 81. Fiirstner, U. and Wittmann, G . T. W., Metal Pollution in the Aquatic Environment, Springer, Berlin, 2nd edn., 1981. Paper 2103.5986 Received July 8, 1992 Accepted October 12, 1992

ISSN:0003-2654    

DOI:10.1039/AN9931800053

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

ANALYST, JANUARY 1993, VOL. 118 59 Use of a Quadratic Response Surface in the Polarographic Determination of Lead B. Lopez Ruiz Laboratorio de Tecnicas Instrumentales, Facultad de Farmacia, Universidad Complutense, 28040 Madrid, Spain G. Frutos and P. Sanz Pedrero De pa rta m en to de Qu im ica - Fisica Farm aceu tica, Facu ltad d e Farm acia, U n ive rsidad Co m plu tense, 28040 Madrid, Spain J. P. Martin lnstituto de Fermentaciones, CSIT, Madrid, Spain By investigating the influence of the instrumental and chemical factors affecting the differential-pulse polarographic measurements of Pbll and subsequently developing suitable factor and orthogonal designs, a surface response was obtained that allowed the optimization of the response variable, viz., the peak current. In this manner, the polarographic technique was improved in two respects, by increasing its sensitivity (thereby lowering its detection limit) and by shortening the analysis time (the lead concentration was obtained from a single measurement). The results provided by the proposed method are comparable to those afforded by the traditional standard additions method.Keywords: Quadratic response surface; differential-pulse polarograph y; lead determination The determination of meta!s in matrices of various natures is attracting growing interest, particularly as regards biological, environmental and foodstuff samples, on account of the toxic character of some metals even at very low concentrations (of the order of a few pg cm-3 or ng cm-3) and of the essential nature of others which must be ingested in small amounts (usually a few pg ml-'), above which they also become toxic.1 4 Choosing an appropriate technique for the determination of metals is thus no easy task. The technique in question must afford detection limits of a few ng cm-3, be highly sensitive, selective and reproducible, allow small differences in concen- tration to be distinguished and, if possible, be economic to implement. In many instances, particularly in cases of intoxication, the technique of choice must also be rapid. In this work, the determination of lead was examined on account of this metal's toxic character and ubiquity in environmental and foodstuff samples. For this purpose, differential-pulse polarography (DPP) was chosen, as it is a sensitive and reproducible technique that affords low determi- nation limits with low analytical costs.s--") In rapid studies, because of the many instrumental and chemical factors affecting polarographic measurements, determinations required the use of the standard additions method.As this involves at least four measurements on each sample, a significantly decreased sample throughput results in comparison with alternative techniques. Studies were therefore made on the effect on the polaro- graphy of Pb" of instrumental factors such as the scan rate and pulse amplitude and of chemical variables such as the electrolyte pH and concentration. 11712 The influence of these parameters on the sensitivity was investigated in order to determine whether they must be taken into account in applying the DPP technique.The basic aim of this work was to optimize the instrumental and chemical parameters affecting the determination of PbII by DPP. For this purpose, factor designs were used that were in turn employed to develop orthogonal designs for the generation of response surfaces that allow the optimum experimental conditions to be established. The response variable (viz., the peak current) was optimized by selecting these experimental conditions. 13-15 Once optimized, the technique for the DPP determination of lead should be more sensitive and, more important, faster as a result of allowing the metal to be assayed in a single, direct measurement. Experimental Apparatus and Reagents Polarographic measurements were made on a Princeton Applied Research Model 384 analyser that was connected to an RE0082 digital recorder and a Model 303 polarographic vessel module furnished with a static mercury drop (SMD) working electrode, a silver-silver chloride reference electrode and a platinum auxiliary electrode. Lead(") standards were prepared from Pb(N03)Z (Titrisol, Merck).Those of the other metal ions [Co", Cu", Ni", Znl[ and FeIII)] were also prepared from the required volumes of Merck Titrisol solutions. Other reagents used included analytical-reagent grade LiC104 and Suprapur HClO4 (Merck). The mercury used was distilled three times and the water used throughout was distilled twice in Pyrex glass and subsequently de-ionized in a Milli-Q system (Millipore). Nitrogen gas (SEO N-48, 99.98% N,) was also used.Procedure The determination of Pb" involved passing a stream of nitrogen through the polarographic solution for a pre-set time (10 min). A nitrogen atmosphere was maintained throughout each experiment. Potential scans were performed between -0.1 and -0.5 V. The pulse amplitude, drop time and potential increment between pulses used in each experiment were dictated by the instrumental set-up used. The influence of the mercury drop size on the peak current could not be studied because the experimental set-up used could not afford some of the sizes required by the factor designs. A fixed drop size was therefore used throughout. Despite the well known influence of temperature on the peak current and because of the difficulty involved in controlling this parameter accurately, all recordings were obtained at controlled room temperature (between 22 and 24 "C) .60 ANALYST, JANUARY 1993, VOL.118 Statistical Analysis The experimental method developed in this work is based on the execution of a series of factor and fractionated factor designs that allow the influences of the factors studied on the response variable to be determined. The quadratic surface was determined by using a second- order design to obtain a mathematical model descriptive of the measurement procedure. 16 Results and Discussion The factors studied and their levels are listed in Table 1. The first design developed was a Factor 27-2 with resolution IV, i.e. , F = ABCD and G = ABDE (definition relation, I = ABCDF = ABDEF = CEFG). The experimental matrix and the corresponding results of the response variables are summarized in Table 2.The stepwise regression analysis of this experimental design (Table 3) allows us to state that variables R, D and F have no significant influence on the response variable; however, because of the significant influence of the DE interaction, only the drop time ( B ) and pure time ( F ) have been eliminated as Table 1 Levcls of the factors involved in Design 27-2: potential increment between pulses, drop time, pulse amplitude, LiCIOj concentration, pH, purge time and Pb" concentration Lcvels Factors - 1 +1 A = Potential increment bctween pulses (mV) 3 7 B = Drop time (s) 2.5 6.5 C = Pulsc amplitude (mV) 35 75 D = LiCIOl concentration (mol dm-3) 0.01 0.20 E = p H 1 2 F = Purge time (min) 7 20 G = Pbll Concentration (ng cm-3) 60 140 Table 2 Experimental matrix and results of the rcsponse variable for Design 2'p2 A -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 B -1 1 -1 1 -1 1 -1 1 - 1 1 -1 1 -1 1 -1 1 - 1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 C -1 -1 1 1 -1 - 1 1 1 - 1 -1 1 1 -1 - 1 1 1 -1 -1 1 1 -1 -1 1 1 -1 -1 1 1 -1 -1 1 1 D - 1 -1 -1 -1 1 1 1 1 -1 -1 -1 -1 1 1 1 1 -1 - 1 -1 -1 1 1 1 1 -1 -1 -1 - 1 1 1 1 1 E -1 -1 - 1 -1 -1 -1 -1 -1 1 1 1 1 1 1 1 1 -1 -1 -1 -1 -1 -1 -1 -1 1 1 1 1 1 1 1 1 F 1 -1 -1 1 -1 1 1 -1 1 -1 -1 I -1 1 1 -1 -1 1 1 -1 1 -1 -1 1 -1 1 1 -1 1 -I -1 1 G 1 -1 1 -1 -1 1 -1 1 -1 1 -1 1 1 -1 1 -1 -1 1 -1 1 1 -1 1 -1 1 -1 1 -1 -1 1 -1 1 ilnA 15.36 9.50 27.82 16.15 6.66 16.76 13.33 26.08 5.86 12.46 9.37 20.90 14.50 7.24 27.24 12.17 6.08 11.29 12.17 22.03 9.76 5.07 17.68 10.45 8.00 3.62 15.12 7.94 5.50 11.09 11.58 25.60 factors, the LiC104 concentration ( D ) being a dubious factor, the influence of which on the peak current cannot be readily established. Solving this ambiguity calls for supplementary experimentation or a more powerful design. Reducing the overall number of factors LO five allowed a Complete Factor Design 25 to be applied. The features of such a design allow one to obtain non-mixed information on the principal factors A, C, D, E and G, and on their interactions. These five factors were kept at the levels given in Table 1. The drop and purge times were set to 1 s and 15 min, respectively. This short drop time was chosen in order to diminish over-all determination time after checking through parallel experi- ments that it did not influence the response variable of the design.The purge time chosen was an intermediate value suited to the sample volume used (10 cm3). Table 4 shows the experimental matrix and results of the response variable obtained with Design 25, and Table 5 gives the results obtained in the regression analysis by using the Table 3 Results of the stepwise regression analysis applied to Design Z7-2 Increrncnt F No. Variable R R' in R2 to enter 1 7G 0.6458 0.4147 0.4171 24.3248 2 3 c 0.8744 0.7646 0.3475 48.7056 3 1A 0.9155 0.8382 0.0737 14.5724 12.3895 4 22 DE 0.9404 0.8844 0.0462 12.4426 5 21 CG 0.9583 0.9183 0.0339 6 5 E 0.9671 0.9352 0.0169 7.5767 7 14AG 0.9728 0.9463 0.0111 5.7926 Table 4 Experimental matrix and results of the response variable for Design 2' A C D E G i,lnA A C D E C; i,,lnA -1 -1 -1 -1 -1 -1 1 -1 -1 -1 -1 -1 1 -1 -1 -1 1 1 - 1 -1 -1 -1 -1 1 -1 -1 1 -1 1 -1 -1 -1 1 1 -1 -1 1 1 1 -1 -1 -1 -1 -1 1 -1 1 -1 -1 1 -1 -1 1 -1 1 -1 1 1 -1 1 -1 -1 - 1 1 1 -1 1 -1 1 1 -1 -1 1 1 1 - 1 1 1 1 1 5.67 9.00 6.00 11.20 6.00 15.65 6.81 13.91 13.22 23.70 14.10 23.33 12.60 28.40 13.76 26.55 1 -1 -1 - I -1 10.80 1 1 -1 -1 -1 7.25 1 -1 1 -1 -1 3.20 1 1 1 -1 -1 7.53 1 -1 -1 1 -1 6.04 1 1 -1 1 -1 13.48 1 -1 1 1 -1 5.15 1 1 1 1 -1 8.70 1 -1 -1 -1 1 8.50 1 1 -1 -1 1 16.50 1 -1 1 -1 1 8.18 1 1 1 -1 1 16.65 1 -1 -1 1 1 10.14 1 1 -1 1 1 23.91 1 -1 1 1 1 10.86 1 1 1 1 1 19.00 Table 5 Results of the stepwise regression analysis applied to Design 2s Variable Coefficient Student's t Intercept A C D E G AC AD AE AG CD CE CG DE DG EG 12.67 -1.68 8.86 -0.51 1.12 4.15 -0.72 -0.58 0.05 -0.92 -0.20 1.01 1.54 -0.23 0.20 0.18 * Statistically significant value of Student's t-test -5.65* 12.99* -1.71 3.77" 3.96" -2.43" -1.94 0.16 -3.11" -0.67 3.41" 5.19* 0.69 0.59 -0.77ANALYST, JANUARY 1993, VOL.118 61 program BMDPIR. I 7 The effects of the principal variables and their interactions are given by the coefficients of the regression equation. Taking into account the Student's t values given in Table 5 , the following factors and interactions were significant at the a < 0.05 level: G, C, A; CG, E, CE, AG and AC. These results allow one to rule out the initially dubious influence of the LiCI04 concentration on the peak current under the conditions associated with the design.For this reason, the same concentration (0.1 mol dm-3) for the background electrolyte (LiC104) was used in all subsequent experiments. Taking into account that the concentration of lead would be unknown, the response of this factor was used in all subsequent experiments. Taking into account that the concentration of lead would be unknown, the response of this factor should be made independent. For this purpose three designs involving con- stant Pb" concentrations of 20, 140 and 5000 ppb in which the new response variable was the ratio between the measured peak current (ip) and the lead concentration in the sample were investigated. The choice of the new variable was justified by the Parry and Osteryoung equation.18 The peak current in DPP is given by D 0 - 1 i, = nFAC ,/-(-) JCtp O + 1 where o = exp(nF/RT)(AE/2) n being the number of electrons involved in the process, F = 96500 C, A the area of the electrode surface, C the concentration of the electroactive species (Pb"), D its diffusion coefficient, t, the interval between application of two consecutive pulses and AE the pulse amplitude.Eqn. (1) can be rewritten as The choice of the i,: [Pbli] ratio as the response variable allows the design of the Pb" concentration factor to be made independent. This has some advantages, as such a concentra- tion is unknown in the sample. In order to check the behaviour of the factors studied on this new response variable over a wide concentration range, three designs at constant Pb" concentrations of 20, 140 and 5000 ng cm-3 were investigated.The influences of the factors (pulse increment between scans, pulse amplitude and pH) were found to be significant with all three designs. Once the influence of different factors had been deter- mined, the response variable was optimized by developing a second-order orthogonal design. This corresponding matrix was assayed and the results it provided are given in Table 7; Table 6 lists the experimental counterparts of the scaled values given in Table 7. Table 8 shows the results of multiple regression analysis performed by using the program BMDP2R. 17 The quadratic equation obtained by taking the significant terms A , C, CT and El into account resulted in no fitting failure, as can be seen Table 6 Levels of the factors involved in the orthogonal design Variable -1.52 -1 0 1 1.52 A = Potential increment C = Pulseamplitude 25 25 55 75 85 E = pH 0.74 1.0 1.5 2.0 2.26 between pulscs 2 3 5 7 8 from Table 9.Thus, the response surface obtained for a lead concentration of 1000 ng cm-3 was -- I' - 0.1372 - 0.02183A + 0.00649G - 0.00833E2 (2) [Pb"] From eqn. (2) it follows that the optimum value of the response variable is defined by the conditions C = 2.9 and E = 0, the value of A being uncertain. This critical point corresponds to a pulse amplitude of 113 mV and pH 1.5 Fig. 1 shows a three-dimensional representation of the response surface defined by eqn. (2) for the coded value A = 1.52. Fig. 2 shows the contour lines for A = - 1.52,O and 1.52. Within the limits studied, the most favourable experimental conditions would be A = -1.52, C = 1.52 and E = 0 , which Table 7 Experimental matrix and results of thc response variable for thc orthogonal design Experiment No.A 1 -1 2 1 3 -1 4 1 5 -1 6 1 7 -1 8 1 9 1.52 10 -1.52 11 0 12 0 13 0 14 0 15 0 16 0 17 0 18 0 19 0 20 0 C -1 -1 I 1 -1 -1 1 1 0 0 1.52 - I .52 0 0 0 0 0 0 0 0 E -1 -1 -1 -1 1 1 1 1 0 0 0 0 1.52 -1.52 0 0 0 0 0 0 i,/n A 98.60 60.45 174.50 124.45 109.85 69.95 189.00 139.25 11 3.10 177.55 186.20 63.65 118.35 122.95 131.45 138.80 135.25 134.20 137.90 131.20 Table 8 Results of the stcpwise regression analysis applied to the orthogonal design Variable Coefficient Student's t Indepcndent term A C E A A A C A E CC CE EE 0.13576 0.03757 0.00340 0.00228 - 0.0027 1 - 0.0001 9 -0.00649 0.00 107 -0.00833 -0.02 183 * Statistically significant value of Student's r-test.- 11.64* 20.03" 1.81 1.13 -1.15 -0.08 -3.20" 0.46 -4.11" Table 9 Contribution of each of the variables chosen to account for the response variable in the orthogonal design Entering Correlation K2 Step No. variable coefficient, R R2 increment 1 C 0.8318 0.6919 0.6919 2 A 0.9621 0.9256 0.2336 3 E2 0.9770 0.9546 0.0290 4 cz 0.9860 0.9723 0.0176 ANOVA Sum of Degrees of Mean Source squares freedom squares F Regression 0.02509 4 0.006273 131.44 Residuals 0.00072 15 0.000048 Lack of fit 0.00066 10 0.000066 6.60 5 0.000010 Error 0. 0000562 ANALYST, JANUARY 1993, VOL. 118 Table 10 Results of the prediction of the proposed mathematical model for different concentrations of PblI and comparison with the standard additions method Experi- mentNo.A C E 1 -1.52 1.52 0.00 2 -1.00 0.25 1.00 3 0.50 -0.75 0.00 4 1.00 0.75 0.00 5 0.50 -0.75 1.52 6 -0.50 -1.25 -1.52 7 0.00 -0.25 -1.00 ip( exp . )/ nA 217.57 174.75 102.60 141.90 101.35 95.60 120.30 ip( t he or. )/ nA Error(%) 212.49 +2.3 159.69 +8.6 94.95 +7.9 139.90 +1.4 75.20 +25.8 71.77 +24.9 119.07 +2.0 Fig. 1 Three-dimensional representation of the response surface correspond to actual values of 2 mV for the potential increment between pulses, 85 mV for the pulse amplitude and 1.5 for the pH. Under these conditions, the value of the response variable will thus be (3) Table 10 reflects the experiments carried out in order to contrast the predictive capacity of the mathematical model defined by eqn.(2). Every experiment was carried out at the optimum values of the instrumental and chemical variables. Lead(i1) solutions containing the concentrations given in Table 10 and arbitrary concentrations of Co", Cu", Ni", Mg", Zn" and Fell1 between 20 and 5000 ppb in 0.1 mol dm-3 LiC104 of pH 1.5 (adjusted with HC104) were used for this purpose. As can be seen in Table 10, the errors in the predictions were always less than lo%, which testifies to the feasibility of determining lead in a single, direct measurement by applying eqn. (3). The relative errors in the concentrations determined by this method were similar to those obtained by using the standard additions method, which is surpassed by the former in speed by a factor of 4. Finally, the detection and determination limits were deter- mined under the optimum conditions fixed by the experimen- tal design using the Miller method.19 The former was 7.4 ppb and the latter 12.27 ppb.As shown above, the accuracy achieved in the determina- tion of lead by the proposed method is similar to that afforded by the classical standard additions method, which it surpasses in terms of speed of analysis. Therefore, eqn. (3) can be used as a model for analytical purposes such as the determination of lead in complex samples. (a) 6.00 4.30 u 2.00 0 -2.00 (b) 6.00 -4.00 -2.00 0 2.00 4.00 4.00 u 2.00 0 -2.00 (' 6.00 -4.00 -2.00 0 2.00 4.00 4.00 u 2.00 0 -2.00 -4.00 -2.00 0 2.00 4.00 E Fig. 2 Contour lines for: ( a ) A = - 1.52, ( b ) A = 0 and (c) A = 1.52 References 1 2 Bourdon. R., in Les Oligodements en Medicine et Biofogie, ed.Chappuis, P., Lavoisier, Paris, 1991, pp. 11 1-157. Goyer. R. A., in Toxicology. The Basic Science of Poisons, ed. Klaassen, C. D., Arndur, M. O., and Doull, J . , Macmillan, New York, 3rd edn., 1986. pp. 582-635. 3 Boudene, C., in Toxicologie et Securite des Aliments, ed. Derache, R., Lavoisier, Paris, 1986, pp. 159-171. 4 Descotes, J . , Verdier, F., Brouland. J. P., and Puke. C., in Immunotoxicity of Metal5 and Immunotoxicology, eds. Dayan. A. D., Hertel, R. F., Heseltine, E.. Kazantzis, G., Smith, E. M., and Van Der Venne. M. T., Plenum Press. New York, Hoyer, B . , and Florence. T. M., Anal. Chem., 1987, 59,2839. 1990. pp. 209-214. 5ANALYST, JANUARY 1993, VOL. 118 63 6 7 8 9 10 11 12 13 Nedeljkovic, M., Stokadinovic, L., Matovic, V., and Korica- nac, Z., Clin. Chim. Acta, 1987, 168, 169. Ldpez Fonseca, J., Sanz Pedrero, P., and Otero, S., An. Chim., 1981, 77, 171. Constantini, S., Giordano, R., and Rubbiani, M., Microchem. J . , 1987, 35, 70. Onar, A. N., and Temizer, A., Analyst, 1987, 112,227. Oehme, M., Lund, W., and Jonsen, J., Anal. Chim. Acta, 1987, 100,389. Aliakbar, A., and Popl, M., Collect. Czech. Chem. Commun., 1984, 49, 45. Aliakbar, A., and Popl, M., Collect. Czech. Chem. Commun., 1984,49, 1140. Box, G. E. P., Hunter, J. S., and Hunter, W., Statistics for Experimenters, Wiley, New York, 1978. 14 Draper, N. P., and Smith, H., Applied Regression Analysis, Wiley, New York, 1980. 15 Morgan, E., Chemometrics: Experimental Design, Wiley, New York, 1991. 16 Wolters, R., and Kateman, G., J. Chemometr., 1990,4, 171. 17 BMDP Statistical Software Manual, ed. Dixon, W. J., Univer- sity of California Press, Berkeley, 1990. 18 Parry, E. P., and Osteryoung, R. A., Anal. Chem., 1965, 37, 1634. 19 Miller, J. C., and Miller, J. N., Statistics for Analytical Chemistry, Wiley, New York, 1984. Paper 21 03 797A Received July 16, 1992 Accepted September 21, I992

ISSN:0003-2654    

DOI:10.1039/AN9931800059

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

ANALYST, JANUARY 1993, VOL. 118 65 Voltammetric Trace Determination of Uranium and Other Transition Metals in Rock Phosphate Samples Neerja Verma and Krishna S. Pitre Department of Chemistry, Dr. Harisingh Gour Vish wavidyala ya, Sagar-470 003 (M.P.), India Convenient and accurate direct current and differential-pulse polarographic and differential-pulse anodic stripping voltammetric procedures were developed for the simultaneous trace determination of transition metals and uranium in rock phosphate samples. Water leaching followed by acid leaching of the finely powdered rock phosphate sample was used t o prepare the analyte for analysis. The results indicated the presence of Cu, Cr, TI, Te, Zn, Fe and U. Differential complexation with nitrilotriacetic acid was applied for better resolution of the polarographic waves.Uranium gave a well defined polarographic wave with = -0.82 V versus SCE. The method of standard additions was used for quantitative analysis. The voltammetric results indicated that on average the rock phosphate sample contained Cu 215, Cr 363, TI 80, Te 100, Zn 180, Fe 1750 and U 102.0 mg kg-1. The proposed voltammetric methods are capable of giving both qualitative and quantitative analyses of rock phosphate for the reported metals in one run. The accuracy of the methods was verified by comparing the observed results with those observed using atomic absorption and X-ray fluorescence spectrometric techniques. Keywords: Rock phosphate; transition metals; uranium; direct current and differential-pulse polarograph y; differential-pulse anodic stripping voltammetry Phosphorites are a known source of uranium.1-t In these samples uranium is associated with a large number of elements, such as Si, Al, Fe, Mg, Ca and P, which constitute the major part of the sample.However, some other metal ions, such as Ag, As, Ba and Cu, are present in trace amounts .5-s Trace analysis of phosphorite samples has been carried out mainly by using X-ray fluorescence (XRF) and spectrographic methods. In the past decade, electrothermal atomic absorption spectrometry with graphite tubes (ETAAS) has dominated the field of trace analysis of substances of natural origin for metal ions, but the accuracy may be poor.10 In recent years voltammetry and related techniques have been applied increasingly to such analyses. 11-18 Differential-pulse polarography (DPP) and differential-pulse anodic stripping voltammetry (DPASV) have been used for the simultaneous determination of metals.19 These methods are highly sensitive and dependable up to nanogram per gram levels. This present paper deals with the trace analysis of rock phosphate samples using direct current polarography (DCP), DPP and DPASV. Special attention was paid to the uranium content of the samples. The electrochemical results were compared with those obtained using AAS and XRF methods. Experimental Samples Samples were collected from the Hirapur-Bassia area located 0.5 km SW of Hirapur village, Sagar District, M.P., India. A 1 km* area of the Hirapur-Bassia deposit was selected for sampling and 25 samples from different depths were collected.Apparatus Polarographic and vol tammetric measurements were made on an Elico (India) Model CL-90 pulse polarograph coupled with a Model LR-108 X-Y Polarocord. The electrode system consisted of a dropping mercury electrode (DME) as the working electrode, a coiled platinum wire as the auxiliary electrode and a saturated calomel electrode (SCE) as the reference electrode. The electrochemical cell used had pro- vision for inserting a bubbler for deaerating the solutions and for passing nitrogen. A glassy carbon fibre electrode (NF12, Sigti Eletitiogitit, UK) W ~ L S used for DPASV. pH measurements were made on an Elico (India) Model L1-120 digital pH meter. Chemicals and Reagents AnalaR grade chemicals (BDH) were used. Stock solutions of potassium chloride (2 rnol 1 - 1 ) and Cu", C P , TI', Te'", Zn", Fe"' and Uv' (0.01 mol 1 - 1 ) were prepared by dissolving the required amounts of their soluble salts in triply distilled water.Hydrochloric acid (1 mol 1-1) and trisodium nitrilotriacetate (NTA) (0.1 rnol I - [ ) were prepared in distilled water. Gelatin solution (0.1%) was prepared in hot distilled water. The solutions were standardized by known methods and diluted as required .20 Preparation of Sample Solution The rock phosphate solution was prepared by water leaching followed by acid leaching of the sample. A 10 g amount of finely pulverized rock phosphate sample was washed three times with 25 ml of distilled water. The filtrate was treated with dithizone and 2-3 drops of concentrated HCI and then heated at 45-50 "C for 5-10 min.The solution was filtered and the filtrate was treated with concentrated HN03 and evapor- ated to dryness. The residue was dissolved in distilled water. The residue obtained after water leaching was treated in similar manner and the combined filtrate so obtained was diluted to 100 ml with distilled water and subjected to polarographic or voltammetric analysis. Preparation of Analyte and Recording of Voltammograms To 10 ml of the sample solution, 5 ml of KCI (2 mol 1-1) as supporting electrolyte and 5 ml of gelatin (0.1%) as a maximum suppressor were added and the final volume was made up to 100 ml with distilled water. The pH of the test solution was first adjusted to 5.50 k 0.02 and then to 2.20 k 0.02, as the analysis of the sample is carried out at two different pH values.The analyte was placed in a polarographic cell equipped with the electrode assembly specified above. Pure nitrogen was passed through the test solution for 15 min at the beginning of the experiment. The polarograms and voltammograms were then recorded with the instrumental parameters indicated in 'Table 1.66 ANALYST, JANUARY 1993, VOL. 118 Based on the presence of different metal ions in the sample, synthetic sample solutions were also prepared by taking the required amount of the metal ions as indicated in Table 2. Choosing identical experimental conditions as above, the polarograms and voltammograms were recorded and com- pared with those for the rock phosphate sample. The method of standard additions was also employed in order to evaluate the concentrations of the ions present in the solution.After ascertaining the presence of all the transition metals in the sample, special attention was given to the determination of the uranium content. For this purpose, the analyte was prepared by taking 10 ml of the rock phosphate sample solution, 5 ml of HCI (1 moll-1) as supporting electrolyte and 5 ml of gelatin (0.1%) as maximum suppressor, making the final volume up to 100 ml with distilled water. The pH of the test solution was adjusted to 1.32 k 0.02 using dilute HCI or NaOH solution. The analyte was placed in a polarographic cell and pure nitrogen was passed through for 15 min at the Table 1 Instrumental parameters Paramctcr Value Initial applied voltage 0.0 V versus SCE Sensitivity I PA CC compcnsa- ZR compensation 5.0 Height of Hg 140.0 cm tion" 3.0/3.5/2.5 Time constant 5.0 ms Pulse amplitude 50 mV Temperature 25 k 2 "C Parameter Drop timc Scan rate Aquisition O/P zero? PH On Polarocord x-Axis y- Axis Valuc I s 12 mV s-1 Fast 5 .o 1.32; 2.20; 5.50 k 0.02 200 mV cm - 1 200 mV cm-1 * CC compensation = charging currcnt Compensation.Thc valucs t O/P zero = output zero. The valuc was fixcd according to the given arc for DCP, DPP and NPP, respectively. instruction manual. beginning of the experiment. The polarograms and voltammo- grams were then recorded. Results and Discussion In 0.1 mol 1 - 1 potassium chloride solution as supporting electrolyte (pH 5.50 k 0.02), the d.c. and differential pulse polarograms of the sample solution produces five distinct polarographic wavedpeaks [Fig.l ( a ) and (h)] with EL values of -0.04, -0.61, -0.82, -1.01 and -1.30 V versus SCE, indicating the presence of Cull, TelV, UV1, Z,n" and Fc'~', respectively. To investigate the presence of other trace ml;tals in the rock phosphate sample, a d.c. polarogram of the sample solution was recorded in 0.1 mol 1-1 KCI at pH 2.20 ? 0.02. This showed two distinct polarographic waves with E; = -0.50 and -1.50 V versus SCE, indicating the presence of TI and Cr in the sample, together with the other metal ions that were detected earlier with the exception of the disappearance of the polarographic wave due to Te'". The concentration of each metal ion was ascertained by the method of standard additions. Based on the presence of these metal ions in the sample, some synthetic samples with various concentration of the ions were prepared and their polarograms and voltammograms were recorded under identical experimental conditions as indicated earlier (Fig.2). The results are given in Table 2. The results indicated no change in the half-wave potentials of the metal ions under study in each of the synthetic samples, confirming the conclusion regarding the presence of these metal ions in the rock phosphate sample. Once the presence of the metal ions in the sample had been ascertained, quantita- tive analysis was performed using the method of standard additions. There was some confusion whether the polarographic wave with E; = -0.61 V versus SCE is due to Cd or Te, because the addition of either element to the sample solution increased the diffusion current.The problem of ascertaining the presence of Te was solved by differential complexation of the two metal Table 2 Analysis of synthetic samples Compositions of synthetic samples/mg per 100 ml Amount found using DPP/mg per 100 ml* CU 0.06 0.06 0.06 0.06 0.12 0.18 0.24 0.63 0.18 0.24 0.63 0.36 0.18 0.36 0.63 0.24 0.36 0.63 0.24 0.18 0.24 0.63 0.36 0.18 0.24 0.63 0.36 0.28 Cr 0.52 1.04 1.56 2.50 0.52 0.52 0.52 0.52 I .04 1.56 2.50 2.60 1.04 0.54 2.50 1.56 0.54 1.56 2.50 1.04 1 .56 0.54 2.50 1.04 I .56 2.50 1.04 0.54 TI 0.65 1.30 1.95 2.60 0.65 1.30 1.95 2.60 0.65 0.65 0.65 0.65 I .30 1.95 0.65 2.60 2.60 1.95 I .30 0.65 1.95 I .30 0.65 2.60 1.30 1.95 2.60 0.65 Tc 0.12 0.24 0.36 0.48 0.12 0.24 0.36 0.48 0.24 0.36 0.48 0.60 0.12 0.12 0.12 0.12 0.36 0.48 0.12 0.24 0.48 0.12 0.24 0.36 0.24 0.36 0.48 0.12 Zn 0.06 0.12 0.18 0.60 0.06 0.12 0.18 0.60 0.12 0.18 0.60 0.30 0.18 0.20 0.30 0.60 0.06 0.06 0.06 0.06 0.30 0.60 0.18 0.12 0.12 0.60 0.18 0.30 Fe 0.56 0.40 I .58 2.14 0.56 0.40 1.68 2.14 0.40 1.68 2.24 2.80 0.56 I .68 0.04 2.24 2.24 0.04 1.68 0.56 0.56 0.56 0.56 0.56 0.40 1 .68 0.56 2.24 U 0.04 0.10 0.16 0.94 0.04 0.10 0.16 0.94 0.10 0.16 0.94 0.28 0.16 0.10 0.94 0.28 0.28 0.10 0.16 0.94 0.94 0.16 0.10 0.28 0.16 0.16 0.16 0.16 Cu 0.059 0.057 0.058 0.058 0.119 0.178 0.238 0.620 0.190 0.230 0.623 0.355 0.177 0.355 0.625 0.233 0.355 0.626 0.240 0.178 0.238 0.628 0.357 0.175 0.236 0.628 0.356 0.179 Cr 0.515 1.030 1.520 2.480 0.510 0.516 0.515 0.5 18 1.030 1 .555 2.485 2.590 1.032 0.530 2.486 1 .548 0.530 1.550 2.489 1.033 1 .555 0.534 2.480 I .031 1.556 2.488 1.034 0.531 T1 0.630 1.290 1.910 2.550 0.640 1.290 1 .920 2.580 0.650 0.642 0.648 0.639 1.295 1.940 0.640 2.580 2.590 1.940 1.285 0.640 1.946 1.298 0.641 2.593 1.295 1.941 2.592 0.643 Tc 0.118 0.235 0.350 0.460 0.118 0.235 0.350 0.450 0.230 0.350 0.470 0.599 0.112 0.118 0.I19 0.115 0.355 0.476 0.119 0.238 0.474 0.118 0.236 0.356 0.235 0.358 0.475 0.117 Zn 0.058 0.118 0.180 0.590 0.056 0.110 0.175 0.590 0.112 0.175 0.594 0.295 0.177 0.195 0.298 0.599 0.058 0.059 0.059 0.059 0.296 0.598 0.178 0.115 0.116 0.599 0.176 0.299 Fc 0.550 0.395 1.575 2.130 0.550 0.395 1.670 2.120 0.398 1.670 2.238 2.800 0.556 1.676 0.395 2.238 2.230 0.398 I ,670 0.559 0.550 0.555 0.558 0.568 0.398 1.672 0.554 2.236 U 0.038 0.099 0.155 0.930 0.039 0,155 0.155 0.933 0.090 0.158 0.935 0.275 0.157 0.098 0.930 0.279 0.279 0.098 0.159 0.939 0.939 0.1% 0.197 0.278 0.158 0.158 0.159 0. 158 * Average of six dctcrminations.ANALYST, JANUARY 1993, VOL. 118 67 t c 200 mV (a) H Voltage - Fig. 1 ( a ) D.c. polarogram and ( h ) differential-pulse polarogram of the rock phosphate sample in 0. 1 mol I-' KCI + 0.005% gelatin. pH = 5.50 & 0.02 1 200 mV H cu Voltage - Fig. 2 Differential-pulse polarogram of a synthetic sample contain- ing 0.63 mg per 100 ml of Cu", 2.5 mg of Cr"', 0.24 mg of Te'", 0.94 mg of U"', 0.60 mg of Zn" and 0.40 mg per 100 ml of Fell in 0.1 mol 1-1 KCI + 0.005% gelatin. pH = 5.50 k 0.02. Sensitivity = 10 FA V-' ions with NTA (0.1 moll-').On recording the polarogram of the NTA-complexed sample solution, this wave shifted to a more electronegative value with E; = -0.70 V versus SCE. Separate experiments on Cd-NTA and Te-NTA complexes showed that these complexes produced polarographic waves with El = -0.57 and -0.70 V versus SCE, respectively, thus t c C 2 3 u 200 mV - Te-NTA I Voltage - Fig. 3 Differential-pulse polarogram of NTA-complexed Te in 0 . 1 mol I-' KCI + 2 mmol I-' NTA + 0.005% gelatin. pH = 5.50 k 0.02. Scnsitivity = 1 pA V-1 Table 3 Detection limits Detection limit Metal ion Combined Com b i n d Com bi ned Combined Combined Combined Com bined Cu" Individual Cr"l Individual Tll Individual Te'" Individual Zn" Individual Fe"' Individual Uvl Individual DCP/pg 1-' DPP/pg 1-1 DPASV/ng I-' 6.3 6.3 5.2 5.2 10.0 10.0 4.1 4.1 6.4 6.4 5.6 5.6 2.4 2.4 0.063 0.063 0.052 0.052 0.10 0.10 0.08 0.08 0.64 0.64 0.56 0.56 2.4 4.8 6.3 6.3 10.2 10.2 15.0 15.0 4.1 4.I 12.8 12.8 2.8 2.8 0.24 4.8 confirming the presence of Te rather than Cd in the sample. A differential-pulse polarogram of the Te-NTA complex is shown in Fig. 3. Detection Limits of DCP, DPP and DPASV Once the presence of the indicated metal ions in the rock phosphate sample was known, they were determined in the sample by DCP, DPP and DPASV with the method of standard additions. The detection limits of the techniques for measurements of the individual and combined metal ions are given in Table 3. Except for T1' and Cr'll, all the metal ions in the sample could be determined in one run.For T1' and Cr"' the pH had to be adjusted to 2.20 k 0.02. The detection limits were examined by preparing synthetic samples. The data in Table 3 show that the DPASV method using a glassy carbon fibre electrode is highly sensitive in determining the reported metal ions down to the nanogram level. Quantitative Analysis of the Sample by DCP, DPP and DPASV The principal use of DCP, DPP and DPASV was for the simultaneous determination of Cu", Tel", Zn" and Fell' at pH 5.50 k 0.02, TI' and Crlll at pH 2.20 k 0.02 and Uvl at pH 1.32 k 0.02. The results are given in Table 4. The results indicate that the recovery is over 99.5% for most of the metal ions, with high accuracy and precision of determination.68 ANALYST, JANUARY 1993, VOL. 118 10.5 p~ 200 mV Table 4 Results of rock phosphate sample analyses for metal ions (mg kg-I).Results are averages of six determinations H DCP DPP DPASV Metal ion Parameter Added" Cu Amount - 1.32 Recovery (YO) SDt Cr Amount - 2.19 Recovery (YO) SD TI Amount - I .40 Recovcry (YO ) SD Te Amount - 2.55 Recovery (YO) SD Zn Amount - 1.20 Recovery (%) SD Fe Amount - 11.00 Recovery (Y) SD U Amount - 1 .00 Recovery (%) SD * Amount in mg per 100 ml o f analyte. "r Standard deviation (mg kg-1). 99.07 0.27 99.54 0.07 99.06 0.48 99.15 0.45 98.85 0.51 99.68 0.24 99.95 0.47 Found 215.00 343.80 363.00 579.38 80.00 217.95 100.00 352.00 180.00 296.85 1750.00 2840.90 102.00 201.90 Added" 1.32 - - 2.19 - 1.40 - 2.55 - 1.20 - 11 .oo - 1 . 00 99.53 0.28 99.54 0.06 99.16 0.48 99.71 0.50 99.32 0.50 99.82 0.25 99.97 0.48 Found 215.00 345.40 363.00 579.38 80.00 218.16 100.00 354.00 180.00 297.96 1750.00 2845.00 102.00 201.95 Added* 1.32 - - 2.19 - 1.40 - 2.55 - 1.20 - 11 .oo - 1 .oo 99.63 0.29 99.77 0.06 99.37 0.46 99.71 0.5 1 99.52 0.49 99.82 0.25 99.97 0.49 Found 215.00 345.75 363.00 580.70 80.00 2 18.62 100.00 354.00 180.00 298.57 1750.00 2845.00 102.00 201.95 Fe Table 5 Comparison of AAS/XRF and voltammetric trace analysis data for rock phosphate samples Amount found/mg kg- Metal ion CU" Cr"' TI' Te'" Zn" Fe"' U"' * From ref.21. ~ ~~ A AS/X RF* 236.6 320.0 Not reported Not reported 175.0 1 84O.O 93.6 Voltammetry 215.0 363.0 80.0 100.0 18O.O 1750.0 102.0 Voltage - Fig. 4 Differential-pulse anodic stripping voltammogram of a rock phosphate sample in 0.05 mol I - 1 HCl + 0.0005% gelatin.pH = 1.32 31 0.02. Sensitivity = 1 FA V-1 Determination of Uranium The results showed the presence of uranium in the sample, and special attention was paid to its determination by changing the supporting electrolyte, i.e., from KCI to HCl, followed by analysis of the sample solution using polarographic and voltammetric methods. Fig. 4 shows the DPASV trace for the sample in 1 mol 1 - 1 HCI (pH 1.32 k 0.02). There are two peaks, one at Et = -0.22 and the other at -0.82 V versus SCE, the latter being due to uranium reduction. The peak at -0.22 V could not be taken into consideration because it is presumed to be affected by the presence of Cu and Cr in the sample. However, the small peak at -0.82 V did not seem to be affected by the presence of different ions in the sample and could be used to calculate the uranium content in the rock phosphate sample.The concentration of Uvl was ascertained by the method of standard additions and by preparing spiked samples. The result was found to be the same as given in Table 4. Table 5 shows the final results for rock phosphate (average of 25 samples). These results were compared with those obtained using AAS and XRF methods.21 The agreement demonstrates the utility of voltammetric methods for such analyses. In addition, the voltammetric data show the presence of TI' and TeIv in the sample, which have not been reported previously. Further, the voltammetric techniques are simple, fast and economic. The authors are grateful to Professor S. P. Banerjee, Head of the Department of Chemistry and Dean of the Faculty of Science, for providing the necessary laboratory facilities.References 1 2 3 Swanson, V. E., US Geol. Surv. Prof. Pap., 1961, No. 365-C. Ingerson. E., Geochim. Cosmochim. Acta, 1954, 5 , 27. Jeffery. P. G., Chemical Method5 of Rock Analysis, Pergamon Press, Oxford, 1970.ANALYST, JANUARY 1993, VOL. 118 69 4 5 6 7 8 9 10 11 12 13 Adams, J. A. S., Osmond, J. K., and Rogers, J . J. W., Phys. Chem. Earth. 1959,3, 298. Banerjee, D. M., in Geology ofthe Vindhyachal, ed. Valdiya. K. S., Hindustan Publishing, Delhi, 1982, pp. 26-39. Mathur, S. M., Rec. Geol. Surv. India, 1960, 86, 539. Mathur, S. M., and Mani, G., in Proceedings of Symposium on Purana Formations of Peninsular India, ed. West, W. D., University of Sagar, Sagar, 1970, pp. 313-320. Mcdlicott, H. B., Mem. Geol. Surv. India, 1959. 2, 1. Pant, A., in Proceedings of Fertilizers Raw Materials Resources Workshop, eds. Sheldon, R. P . , and Burnett, W. C., East-West Centre, Honolulu, 1980, pp. 331-357. Nurnberg, H. W.. Sci. Total Environ., 1984, 37, 9. Malherbe, P. L., Waudby, J . W., and Towers, D. K., Chem. S.A., 1988, 14, 175. Wang, S.-X., and Li, S.-Y., Yankuang Ceshi, 1988, 7, 31. Khandckar, R. N., Tripathi, R. M., Ragunath, R., and Mishra, U. L., Indian J. Environ. Health, 1988, 30, 98. 14 15 16 17 18 19 20 21 Petak, P., and Vydra, F., Anal. Chim. Acta, 1973, 65, 171. Kutatelodge, G. Sh., Eristavi, V. D., and Tsveniashvii, V. Sh., Soobshch. Akad. Nauk Gruz. SSR, 1972.65,583. Dave, M., and Pitre, K. S . , Zndian J . Chem., 1991, 30A, 198. Pilipenko, A. T., Shpak, E. A., and Samchuck, A. I., Geokhim. Anal. Metody Izuch. Veshchestv. Sostova Osad. Porod. Rud., 1974, 2, 122. Chen, H.-Y., Fang, H.-G., and Guo, S . , Yejin Fenxi, 1987. 7, No. 6, 25. Anger, J. P., Barkat, H., Elenga, F., and Truhaut, R . , Analusis, 1988, 16, 444. Bassett, J., Denney, R. C., Jeffery, G. H . , and Mendham. J.. Vogels Text Book of Quantitative Inorganic Analysis, Long- man, London, 4th edn., 1978, pp. 324-325 and 482. Banerjee, D. M., Khan, M. W. Y., Shrivastava, N., and Saigal, G. C., Miner. Deposita, 1982, 17, 349. Paper 21044896 Accepted August 20, I992

ISSN:0003-2654    

DOI:10.1039/AN9931800065

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

ANALYST, JANUARY 1993, VOL. 118 50 71 [M - Br - COl+ 281 - \ 173 0.11 ' I I I 1 Ill I Identification of Trihaloacetaldehydes in Ozonated and Chlorinated Fulvic Acid Solutions Yuefeng Xie and David A. Reckhow Environmental Engineering Program, Department of Civil Engineering, University of Massachusetts, Amherst, MA 01003, USA Three brominated trihaloacetaldehydes (THAs), bromodichloroacetaldehyde, dibromochloroacetaldehyde and tribromoacetaldehyde, were identified, by gas chromatography-mass spectrometry, in ozonated and chlorinated aqueous fulvic acid solutions containing inorganic bromide. Mass spectra of these THAs were obtained in the electron-impact and positive chemical-ionization modes. The significance of the formation of brominated THAs on water quality and future disinfection by-product regulation is discussed.Keywords: Mass spectra; chlorination; ozonation; disinfection by-products; trihaloacetaldeh ydes Since brominated trihalomethanes (THMs) were first re- ported in the 1970~~1 a number of other brominated disinfec- tion by-products (DBPs) have been found in chlorinated drinking waters high in inorganic bromide.2.3 Owing to the widespread occurrence of bromide in raw waters and signifi- cant health effects associated with bromine substitution, the US Environmental Protection Agency (EPA) is likely to establish strict maximum contamination levels (MCLs) for some of these compounds4 in the coming disinfectants and disinfection by-products (D-DBP) rule. Trichloroacetaldehyde (chloral hydrate) was first reported in chlorinated waters by Uden and Miller.5 Since then several studies have been conducted to investigate the formation and toxicity of chloral hydrate in drinking water.2.6 Because of the carcinogenic properties of chloral hydrate, the US EPA is currently considering the establishment of an MCL in the new D-DBP rule.4 As with the brominated THMs, brominated trihaloacetaldehydes (THAs) can be formed in chlorinated waters high in inorganic bromide.However, the formation of brominated THAs in chlorinated water has not been investi- gated. The objective of the present study was to investigate the presence of brominated THAs in chlorinated and ozonated aqueous fulvic acid solutions containing bromide, and to characterize their mass spectra. Experimental Sample Preparation Solutions of fulvic acid were prepared from a stock concen- trated fulvic acid extract7 (from Thousand Acre reservoir, Athol, MA, USA) with Super-Q water (Millipore, Bedford, MA, USA) to yield a dissolved organic carbon (DOC) concentration of 4 mg 1-1.Following the addition of potassium bromide (0.5, 1.5 and 4 mg I-* as bromide), the aqueous ozone was applied to the fulvic acid solution, buffered at pH 7 with phosphate, at a dose ratio of ozone to DOC of 1 mg mg-1. After storing the ozonated samples in the dark at 20 "C for 3 h, chlorine (in the form of NaOCI) was added to samples containing 0.5 and 1.5 mg 1-1 bromide at a concentration of 20 mg 1-1, and samples were set to react for 24 h in the dark at 20 "C. A 500 ml ozonated or chlorinated sample was extracted with 2 X 50 ml of methyl tert-butyl ether (MtBE).Before injection, the combined MtBE extracts were concentrated to 10-50 yl in a slow nitrogen flow at 45 "C. GC-MS Analysis Gas chromatography-mass spectrometry (GC-MS) analyses were performed on an HP5899 gas chromatograph coupled with an HP5988 quadrupole mass spectrometer (Hewlett- Packard, Avondale, PA, USA). Extracts were introduced by splitless injection and separated on a PTE-5 capillary column (30 m x 0.32 mm i.d., 0.25 pm film thickness, Supelco, Bellefonte, PA, USA). Helium was used as the carrier gas at a flow rate of 30 cm s-1. The oven temperature was kept at 30 "C for 10 min, then ramped to 200 "C at a rate of 25 "C min-1 and kept at 200 "C for 5 min. The injector temperature was 200 "C and the transfer line was kept at 280 "C.The mass spectrometer was tuned immediately before analysis, with use of perfluorotributylamine as the calibrating compound (mlz 69, 219 and 502). In the electron-impact (EI) mode, the ion-source temperature was 200 "C, the election energy was 70 eV, and the mass scan range was 30-400 u. In the positive chemical-ionization (PCI) mode, the ion-source temperature was 100 "C, the electron energy was 240 eV, and the mass scan range was 100-400 u. Methane was used as the reagent gas for PCI. Results and Discussion Identification of Brominated Trihaloacetaldehydes Mass spectra for major constituents in the extracts from ozonated fulvic acid solutions were obtained in the EI and PCI 1 (a) Cluster A loo t II .- [M - Brl+ [M + HI+ 100 150 200 250 300 mlz Fig.1 Mass spectra of tribromoacetaldeh de in ozonated fulvic acid solution ( H = 7, DOC = 4 mg I - l , [Br-i = 4 mg I-', O3 dose = 4 mg 1 - 1 ) . 6) In El mode. ( b ) in PCI mode72 ANALYST, JANUARY 1993, VOL. 118 Table 1 Proposed fragment clusters ion assignments for tribromoacetaldehyde Sub-cluster A1 A2 B1 B2 (170 : 172 : 174) (171 : 173 : 175) (249 : 251 : 253 : 255) (250 : 252 : 254 : 256) Fragments [ M-Br-CHO] + [ M-Br-CO] + [ M-CHO] + [M-CO] + Isotope ratios- Observed 51:100:52 50 : 100 : 50 32 : 100: 102 : 38 34 : 200 : 104 : 34 Theoretical" 51 : 100 : 49 51 : 100 : 49 34: 100:97:32 34: 100:97:32 * Theoretical isotope ratio, calculated by using 7"Br:SlBr (50.69 : 49.31)." Two Br atoms in cluster A and three Br atoms in cluster B. modes to acquire information on both structure and relative molecular mass.One mass spectrum resembling that of bromoform and tribromoacetic acid methyl ester8 was obtained at a retention time (tR) of 13.0 min and this is reproduced in Fig. 1. On careful inspection it appears that cluster A of 170 : 171 : 172 : 173 : 174 : 175 is composed of two clusters, viz., 170 : 172 : 174 and 171 : 173 : 175. Based on the relative abundances, both clusters appear to contain two bromine atoms, as shown in Table 1. The cluster of 249 : 250 : 251 : 252 : 253 : 254 : 255 : 256 also comprised two similar clusters. Each of these apparently contains three bromine atoms. Based on the PCI mass spectra, the relative molecular mass of this compound is 278. After logical assignment of fragments this mass spectrum was concluded to be that of tribromoacetaldehyde.Therefore, the four sub-clusters result from the following losses from the present molecule: [M-CO]+, [M-CHO]+, [M-Br-CO]+ and [M-Br-CHO]+, as shown in Table 1. The experimentally determined isotope ratio matches the theoretical ratio fairly well. Small differences could be partly due to 13C in the apposing sub-cluster, which is not considered in calculating the theoretical isotope ratios. Three additional EI mass spectra resembling THMs and trihaloacetic acid methyl esters8 were obtained in pre-ozo- nated fulvic acid solutions following chlorination. Based on relative molecular masses determined from the PCI mass spectra and structural information provided by El mass spectra, they were identified as bromodichloroacetaldehyde (tR = 5.1 min), dibromochloroacetaldehyde (tK = 10.8 min) and trichloroacetaldehyde (tR = 2.6 min).Similar double clusters were found in these EI mass spectra. For bromodi- chloroacetaldehyde and dibromochloroacetaldehyde, these clusters were attributed to [M-CO]+ and [M-CHO]+, [M-Br- CO]+ and [M-Br-CHO]+. All THAs except bromodichlo- roacetaldehyde were confirmed by comparison of tK and mass spectra with those of commercial or synthesized chemicals. Significance of THAs in Finished Drinking Water There is limited information available on the toxicity of brominated THAs and the effect of THAs on general water quality. Owing to the carcinogenicity of trichloracetal- dehyde,6 brominated THAs are of potential human health concern.In the anticipated D-DBP rule, trichloroacetal- dehyde is likely to be regulated in finished waters. Formation of brominated THAs in water high in bromide is also likely to result in lower concentrations of trichloroacetaldehyde. This is based on similar effects reported for brominated THMs,' haloacetic acids*() and cyanogen halide.3 Future Research Studies are underway in this laboratory aimed at ( a ) synthesiz- ing bromodichloroacetaldehyde, (b) developing a GC-based method to determine the four THAs in drinking waters, and ( c ) investigating the effects of pH, pre-ozonation and bromide concentration on the formation of THAs. Pilot-scale studies are also planned to investigate the formation and removal of THAs during water treatment. Conclusions Three brominated THAs were identified in ozonated and chlorinated fulvic acid solutions containing inorganic bro- mide.Mass spectra of the THAs are similar to those of the THMs and trihaloacetic acid methyl esters, except for the unique double clusters in the mass spectra of THAs. The authors thank the US National Science Foundation and Dr. Edward H . Bryan for the financial support of this research under grant number BCS-8958392. Thanks also go to the Hewlett-Packard company for their generous financial assis- tance. References 1 2 3 4 5 6 7 8 9 10 Rook, J . J . , Gras, A. A.. van der Hcijdcn, B. G., and de Wee, J., J. Environ. Sci. Health, 1978, A13, 91. Krasncr, S. W., McGuire, M., Jacangclo, J. C., Patania. N. L.. Reagan, K. M., and Aieta. M., J. Am. Water Works Assoc., 1989, 81, 41. Xie, Y . , and Reckhow, D. A.. Water Res., in the press. US EPA, Status Report on Development of Regulations for Disinfectants and Disinfection By-products. US Environmental Protection Agency, Washington, DC, 1991. Uden, P. C., and Miller. J . W., J . Am. Wuter Works Assoc., 1983, 75, 524. Daniel, F. B . , DeAngelo, A. B.. Stober, J. A., and Page, N. P., Fundam. Appl. Toxicol., 1992, 19, 159. Thurman, E. M., and Malcolm, R. L . , Environ. Sci. Technof., 1981, 15, 463. Xie, Y., Rajan, R. V.. and Reckhow, D. A., Org. Mass Spectrom., 1992. 27, 807. CRC Handbook of Chemistry and Physics,ed. Lide. D. R., CRC Press, Boca Raton, FL, 71st edn., 1991. Pourmoghaddas, H.. Doctoral Dissertation, IJniversity of Cincinnati, OH, 1990. Paper 2102458F Received May 12, 1992 Accepted September 24, I992

ISSN:0003-2654    

DOI:10.1039/AN9931800071

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

ANALYST, JANUARY 1993, VOL. 118 73 Use of Rapid Scan Correlation Nuclear Magnetic Resonance Spectroscopy as a Quantitative Analytical Method Herve Barjat, Peter S. Belton and Brian J. Goodfellow* AFRC Institute of Food Research, Norwich Research Park, Colney, Norwich, Norfolk, UK NR4 7UA Rapid scan correlation (RSC) nuclear magnetic resonance (NMR) spectroscopy combines the advantages of rapidity of Fourier transform methods with advantages in dynamic range of continuous wave methods. A systematic investigation of the technique for quantitative analysis has been carried out. It is concluded, provided suitable precautions are taken, that RSCNMR can be useful as a quantitative method. Application is illustrated by the determination of ethanol in aqueous media and wines over the range 0.01-15% v/v.Keywords: Rapid scan correlation nuclear magnetic resonance spectroscopy; ethanol; quantitative analysis Although there are a wide range of analytical applications of high resolution nuclear magnetic resonance (NMR) spectro- scopy,l its use for quantitative analysis in a quality control environment, as opposed to a research environment, has been limited. There are two main reasons for this: the first is the high cost of modern Fourier transform (FT) spectrometers and the second is related to the way in which an FT system collects data. In FTNMR all the frequencies of the spectrum contribute to each point of the time domain signal. This results in a multiplex advantage,Z which gives higher signal-to-noise ratio (S/N) spectra but also means that the intensity of one very large signal may dominate all the others.Hence small signals are insufficiently digitized to appear in the spectrum. This problem is generally referred to as the dynamic range problem. It is often encountered with a dilute solute in the presence of a solvent. The problem can be solved by isotropic dilution of the solvent, which may not be practical, or by a solvent suppression technique. Although the latter approach is often used it is not without its own problems.3 The cost and dynamic range problems of FTNMR can be overcome by using continuous wave (CW) NMR. Continuous wave NMR spectrometers are relatively inexpensive and by scanning only the region of the spectrum of interest, intense peaks can be avoided. Major problems with CWNMR, however, are the poor signal-to-noise ratio and speed of data acquisition.If these drawbacks could be overcome, CWNMR would be an attractive technique for routine quantitative use. Rapid scan correlation (RSC) NMR is a technique that combines the speed advantage of FTNMR with the low cost and partial scanning capability of CWNMR. The principles of RSCNMR were first outlined in 1974.4.5 In normal CWNMR the spectrum is acquired by sweeping the magnetic field while irradiating the sample at a constant radiofrequency (r.f.). If the field is swept too fast, distortion of the peaks in the spectrum occurs. This distortion, or ringing, can only be avoided if the scan rate is substantially smaller than l/T1 (spin-lattice relaxation time) or l/T2 (spin-spin relaxation time) of the sample under study.This means that 1 scan at high resolution can take about 10 min to acquire. In order to co-add scans and improve signal-to-noise ratios, the scan rate must be fast. A fast scan, however, results in the distorted spectrum shown in Fig. 1 (A). In order to interpret and analyse this spectrum the distortion needs to be removed. The origins of effects that give rise to ringing are well understood and an expression for it can be obtained which is of the forrn"35 ~ ( t ) = e-Iht* (1) where h is the sweep rate in rad s-1 and t is the time after peak maximum. The ringing is removed from the spectrum by * To whom correspondence should be addressed. cross-correlating eqn. (1) with the inverse FT of the rapidly scanned spectrum. Cross-correlation involves multiplying the complex conjugate of one function by another.After cross- correlation the time domain function is Fourier transformed to produce a spectrum in the frequency domain with the ringing remo:red [Fig. l(B)]. Hence, by co-adding a number of rapidly scanned spectra and then cross-correlating it is possible to obtain undistorted spectra with excellent S/N. The above process, i.e., rapid scan followed by correlation, takes place on a timescale comparable to that of FTNMR. Rapid scan correlation NMR, therefore, has the speed advantage of FTNMR and partial scanning advantage (avoiding dynamic range problems) of CWNMR. It does, however, lack the full multiplex advantage of FT techniques. In order to test the potential of this method as a quantitative analytical technique, the proton NMR spectra of ethanol in water over a wide concentration range were examined under different acquisition conditions.This system was chosen as being typical of a real analytical problem in the food industry and as typifying the problems of large solvent and small solute signals. Experimental A Hitachi (Nissei Sangyo) R-1200 rapid scan NMR spec- trometer operating at 60 MHz for protons was used for all B 1 - b Fig. 1 correlation Spcctrum of 5% ethylbenzene in CCI?: A, before and B, after74 ANALYST, JANUARY 1993, VOL. 118 experiments. The permanent magnet operated at 35 "C. All samples were equilibrated at 35 "C in a heated chamber, provided with the spectrometer, before measurement. Resol- ution was adjusted using a tetramethylsilane (TMS) sample.The ethanol calibration solutions (2.85-14.25% v/v) were prepared using 95% ethanol (Hayman) by pipetting the relevant amounts of ethanol into calibrated flasks. Solutions of concentration 2.85, 4.75, 7.60, 11.40 and 14.25% v/v were obtained. The lower concentration standards were prepared by dilution of the above stock solutions to give solutions of 9.5 X 10-3, 2.85 x 10-2, 4.75 x 10-2, 6.65 x 10-2, 9.50 x 10-2 and 19.0 x 10-2% v/v concentration. The wine and beer samples were obtained from a local retail source. The samples were stored at + 1 "C and shaken before being pipetted directly into a 5 mm NMR tube. Independent determinations of the ethanol contents of the wine and beer samples were carried out by Lincolne, Sutton and Wood, Analytical Consulting Chemists, Norwich. In order to adjust acquisition parameters for a whole spectrum, a sweep width of 10 pprn (600 Hz) was selected with a scan rate of 10 s per scan and a filter width of 200 Hz.After correlation to produce the undistorted spectrum the phase was adjusted and r.f. power and receiver gain were optimized to produce the best signal. In order to determine the best conditions for partial spectral acquisition, a variety of experimental conditions were tried. Details of these are given under Results and Discussion. When a number of scans are co-added to improve the S/N, it is important that any field drift is eliminated or accounted for as this could produce unacceptable line broadening. This can be effected by adding a deuteriated solvent to the sample and using a deuterium lock. However, with the R-1200 spec- trometer it is possible to use a technique known as 'field cure' in which a reference peak is selected and each subsequent spectrum is shifted by the computer software so that the reference peaks are in alignment.In samples with suitably large signals one of the spectral peaks was chosen. With dilute ethanol samples there was insufficient signal and 0.12% v/v acetone was added to act as a reference for these samples. Results and Discussion The spectrum of ethanol consists of a triplet due to the CH3 group at 1.7 pprn and a quartet due to the CH2 group at 3.7 ppm. The signal from the hydroxyl proton is merged with the water peak. The peak at 1.7 pprn was chosen for detailed study as it is the most remote from water and has contributions from the largest number of protons.In order to test the quantitative response of the spectrometer, the effects of a number of different experimental variables were examined. The first of these was the effects of the irradiating r.f. power. The acquisition conditions for this experiment were: sweep width , 100 Hz; sweep rate, 15 s per scan; and filter, 200 Hz. In general, higher powers result in increased signal intensity , but excessive power can lead to line broadening and loss of signal intensity owing to saturation effects; hence the highest r.f. power which avoids saturation should be chosen. This consideration is complicated, however, by the interfering effects of the water signal, because, although the water peak maximum was not irradiated, the weak signal in the wings of the Lorenztian line was inevitably irradiated and at high power gave rise to a very uneven baseline, making quantification difficult.At the other extreme of r.f. power level it was found that the lowest settings gave insufficient signal for detection in samples with less than 0.475% v/v ethanol. The choice of irradiating power must therefore be a compromise between over-all signal intensity and baseline quality. However, the latter problem may be improved by use of baseline fitting routines, which were not available to us in these experiments. In order to obtain the best S/N it is desirable to adjust the amplification of the signal so that the analogue-to-digital (ND) converter is filled. On the other hand it is desirable for calibration purposes to leave instrumental parameters unc- hanged.It was found that a single amplification factor was inadequate to cover the whole range of ethanol concentrations investigated (0.01-15% v/v) and hence the samples were examined in the ranges 0.01-3 and 1-15% v/v. These represent two ranges of practical interest, for example, 'alcohol-free' drinks and the normal range of ethanol in wines and beers. The sweep rate and filter width are interdependent parameters. The fastest acquisition requires the maximum sweep rate, but if the spectrum is to be undistorted then an increase in sweep rate must be accompanied by a correspond- ing increase in filter width, which will degrade the S/N. In Fig. 2 the effects of filter width from 1 Hz [Fig.2 (F)] to 200 Hz [Fig. 2 (A)] are shown for a spectrum of 1% v/v ethanol with a sweep rate of 5 s per scan (this is equivalent to the 5 s taken to scan a 600 Hz spectrum and is equivalent to a scan rate of 8.3 ms Hz-1). There is little difference between the 100 and 200 Hz filter widths but clear broadening at 5 Hz width. Below this value further signal distortion is evident, but signals are still measurable. It may, therefore, be possible to improve S/N in the spectrum at the cost of signal distortion while still retaining sufficiently recognizable data to be useful. Probably a better strategy would be t o capture the data using fairly broad filter settings and using digital filtering methods in the software. When the filter width is kept constant and the scan rate is increased there appears to be some spectral distortion at the highest scan rates (Fig.3). In principle this should not occur as even in Fig. 3 (D) the scan rate is only equivalent to 200 Hz s-1, which is similar to the 200 Hz filter width chosen. Filter effects are not apparently the only difficulty as at the higher scan rates there also appears to be baseline distortion. The origins of these effects are not clear but the results clearly indicate that lower scan rates are likely to give better signals. In practice the choice of low scan rates over a limited spectral range is not such a severe problem because the time taken for a 60 Hz (1 ppm) scan for the fastest rate is 0.3 s and for the slowest 1.5 s. Even for a 256 scan acquisition, therefore, the time difference is only a few minutes.An important feature of the RSCNMR method is that many signals can be co-added to improve signal-to-noise ratios. Fig. 2 of A, 200; B, 100; C , 5 ; D, 4; E, 3; and F, 1 Hz Methyl triplct of cthanol in watcr (1% v/v) with filter settingsANALYST, JANUARY 1993, VOL. 118 75 - 6 Fig. 3 A , 15; B, 10; C. 5 ; and D, 3 s per scan Mcthyl triplct of ethanol in water (1% v/v) with sweep rates of 8 z, in 0 0 0 10 20 30 40 N: Fig. 4 field drift compensation Plot of S/N versus square root of thc number of scans without This is only useful, however, if there is no magnetic field drift between scans. In order to test this, a series of spectra were obtained on a 0.95% v/v ethanol solution at a high scan rate over the complete 600 Hz range. The plot of SIN versus the number of scans is shown in Fig.4. Clearly the expected N112 dependence is not observed. This is due to field drift effects causing the spectral lines to widen as the number of scans increases. However, when the field cure system is used a good linear relationship is obtained (Fig. 5 ) even though the spectral range has been reduced to 300 Hz to exaggerate any field drift effect. Rapid scan correlation NMR is, therefore, able to deliver the expected S/N enhancement on co-adding scans. Good quantitative results depend on precise peak area measurements. Ten integrations were carried out for the 14.25 100 150 i- I $ 1 50 0 0 0 0 0 I . 0 I I I 0 2 4 6 8 N: Plot of S/N versus square root of the number of scans with field Fig.5 drift compensation 500 Gi 400 4d .- t 3 > 5 300 c .- e m 200 2? - Y (0 a" 100 0 2 4 6 8 10 12 14 16 Ethanol in water (% v/v) Fig. 6 Calibration graph for ethanol in water over the concentration rangc 2.85-14.25% v/v. The gradient of the line is 32.95 with an intercept of 13.02. The correlation coefficient is 0.999 and 7.6% v/v ethanol standards to check the variation. It should be noted at this point that the phasing of the spectrum is critical for good integration; hence the phase of the signal was carefully adjusted before data acquisition. For the 14.25% v/v sample the mean was 480.9 with a standard deviation of k2.92 and for the 7.6% v/v sample the values were 269 and k2.4, respectively. The 15-3% v/v calibration was carried out with the power level set so that the lowest concentration sample gave a suitable integration value (~100).The amplitude was set so that the signal from the most concentrated sample just filled the A/D converter. Sixteen scans were co-added with a sweep rate of 15 s per scan. Fig. 6 shows the calibration obtained. Each spectrum in the calibration took about 60 s to acquire, resulting in the whole calibration taking less than 15 min. After the calibration had been obtained, the wine and lager samples were run. The results are presented in Table 1. The calculated results show good agreement with those given by the manufacturers and demonstrate that the tech- nique is quantitative in real systems of interest. The reproducibility of the calibration graph was tested by running the calibration over a number of days.Seven calibrations were carried out giving mean values of 33.64 k 1.32 for the gradient, 8.35 k 11.33 for the intercept and 0.9992 ? 0.0005 for the correlation coefficient of the line. Although the variations from day to day are fairly small, the fluctuation of the intercept means the calibration needs to be run each day. However, as the whole calibration takes less than 15 min to acquire, this is practical. By using the calibrations obtained over the 11 d period, the commercial samples (see Table 1) gave mean values of 12.0 k 0.20 (C6tes76 ANALYST. JANUARY 1993, VOL. 118 Table 1 Results for ethanol content in wine and becr samples Calculated Peak area concentration of Concentration (arbitrary ethanol given on label Sample units) (Yo v/v) (Yo v/v) C6tes du RhBne 41 1 12.1 12.0 Muscadet 413 12.1 12.0 Nierstcincr 31s 9.2 9.0 Lager 192 5.4 5.0 Table 2 Comparison of results obtained with different techniques for ethanol content in wine and beer samples.All values in % v/v Distillation/ specific Label Sample RSCNMR GC gravity value CBtes du RhBne 12.4 12.0 12.1 12.5 Muscadet 12.5 12.1 12.1 12.0 Niersteiner 9.3 9.2 9.2 9.5 Lager 5.2 5.1 5.1 5 .O 6.0 5.0 4.0 3.0 2.0 1.0 0 -1.0 6 (PPm) Fig. 7 shows result of increased amplification and partial scanning Spectrum of 0.475% v/v ethanol in water. Expanded region du RhGne), 11.7 k 0.28 (Muscadet), 8.9 k 0.20 (Niersteiner) and 5.1 k 0.20 (lager). In order to obtain a comparison between RSCNMR and two standard chemical methods, new wine and beer samples were analysed by RSCNMR, distillation/specific gravity and gas chromatography (GC); the last two analyses were carried out in a commercial analytical laboratory under standard condi- tions.The results are shown in Table 2.1- It can be seen from Table 2 that the various methods give results that are in good agreement with the amount of ethanol givcn by the manufacturers. It should be noted here that the ethanol content given by the manufacturers in Table 2 differs from that given in Table 1. The brands of drinks purchased were identical for the two sets of experiments; however, they were purchased at different times. The different ethanol contents must, therefore, result from different batches being purchased. The errors involved for each method are: RSCNM R, rt0.22% v/v; distillation/specific gravity (relative density), +0.35% v/v for the wine samples and rf-0.14% v/v for the beer.The different errors obtained for the distillation/ -1 The reference method (Commission regulation, EEC. number: 2676/90) is distillation followed by density measurement at 20 "C using a pycnometer. Specific gravity measurements may also be used and the results converted to YO v/v using tables given. The repeatability of this method is 0.10% v/v and the rcproducibility 0.19% v/v (using trained staff). I I 2.0 1 .o (PPm) Fig. 8 D, 0.0665; and E , 0.095% v/v Spectra of ethanol in water at A, 0.0095; B, 0.0285; C, 0.0475; specific gravity method for the wine and beer samples reflected the different number of repeat measurements taken (14 for the beer and 6 for the wine).The error for the GC method was not given. These results indicate that even at this early stage in the development of RSCNMR the results obtained are at least as good, within experimental error, as those obtained by standard methods used for ethanol determi- nation for a range of wines and beers. The low level (0.01-0.3% v/v) ethanol calibration required more careful adjustment and set up. Fig. 7 shows the single scan spectrum of 0.475% v/v ethanol in water, with the power and amplification adjusted to optimize for the water signal. The methyl signal at 1.2 ppm cannot be seen. However, by partially scanning the methyl region and increasing the irradiation power and the amplification the signal can clearly be observed (see expansion in Fig.7). This clearly illustrates the advantages of partial scanning. The low concentration ethanol solutions gave, even under optimized conditions, very weak signals. The spectra for five of these solutions are shown in Fig. 8. In order to obtain a signal the irradiation power level was near the maximum. The problem with these high power levels is that the tail of the water peak extends into the methyl region and gives a distorted baseline. This also gives problems with peak height and peak area measurements. Shimming, for low concentra- tion samples, is also important to reduce spinning side bands, which can interfere with peak measurement. For these low level ethanol concentrations the signals were too small to be used as a reference for the field cure function. Acetone was added (0.12%) and used as a reference peak.In order to obtain sufficient S/N, 64 scans were co-added with high power and a 45 s per scan sweep rate. The intensity of the ethanol triplet at these concentrations was too weak for adequate integrations; hence peak heights (central peak of triplet) were measured. The resulting calibration graph gave a line (grad-ANALYST, JANUARY 1993, VOL. 118 77 ient 280.0, intercept -0.45 and correlation coefficient 0.999) with excellent linear correlation. Each spectrum in this set took 17 min to acquire, hence this is not a rapid calibration. However, it does demonstrate the sensitivity of the instrument. A 0.0095% v/v ethanol sample with 256 scans co-added gave an S/N of 10 for the central peak. Conclusions The results obtained suggest that RSCNMR has considerable potential as a quantitative analytical method. The spec- trometry is similar to that in conventional NMR techniques but has the advantages of high speed and relatively low cost. A wide range of concentrations can be determined and further developments in data handling arc possible. There arc problems with spectrometer drift, and the origins of these are not yet fully understood. However, even within the existing limitations the accuracy and speed of the instrumentation make it suitable for a wide range of analytical applications. The authors thank Nissei Sangyo Co. Ltd. for the loan of the R-1200 spectrometer and Dr. P. Gadsby for advice and assistance. References Analytical NMR, eds. Field, L. O., and Sternhell, S., Wiley, Chichester, UK, 1989. Marshall, A. G., and Verdun, F. R.. Fourier Transforms in NMR, Optical and Mass Spectrometry, Elsevier, Amsterdam, 1990, p. 98. Hore, P. J . , Methods Enzymol., 1989, 176, 64. Gupta, R. K., Ferretti, J. A., and Becker, E. D., J . Magn. Reson., 1974, 13, 275. Dadok, J., and Sprecher, R. F., J . Mugn. Reson., 1974,13,243. Paper 2103688F Received July 13, 1992 Accepted September 30, 1992

ISSN:0003-2654    

DOI:10.1039/AN9931800073

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

ANALYST, JANUARY 1993, VOL. 118 79 Microanalysis of Bismuth Indium Selenide Thermoelectronic Materials by X-ray Fluorescence Spectrometry With Reference Assays of Indium Stanislav Kotrly, Jitka Sramkova and Radko Chadima Department of Analytical Chemistry, University of Chemical Technology, 532 10 Pardubice, Czechoslovakia Josef Cerma k Computing Centre, University of Chemical Technology, 532 10 Pardubice, Czechoslovakia Screening of a small area on the natural cleavage face of a layered single crystal was used for X-ray fluorescence spectrometric (XRF) assays. A precise determination of indium (within the range I-8% m/m) in Bi2-,ln,Se3 single crystals was based on the calibration of the net intensity of the In Ka1,2 line with the use of the results of extraction-photometric analysis after decomposition of the crystal chips exposed to X-rays.In acetate buffer medium (pH = 4.0, V,, = 40 cm3) and with an adequate excess of quinolin-8-01 and thiosulfate (masking of bismuth), the tris(quinolin-8-olato)indium(i1i) chelate was completely transferred into 10 cm3 of chloroform. The absorbance of the chloroform extract was measured at 400 nm against a blank extract. For calibration of the X-ray spectrometer a computer program was written using a simplified least-squares method of linear regression for both variables subject t o experimental error. The XRF method allows the selection of a piece of crystal suitable for measurements of physical parameters. Keywords: Bismuth indium selenide; X-ray fluorescence analysis; single crystal reference samples; indium assay; quinolin-8-01 extraction A growing need for the microanalysis of semiconducting materials with regard to their physical parameters has presented the analyst with challenging new problems.Some interesting approaches to the microanalysis of semiconducting crystals have been explored mainly with the use of atomic absorption spectrometry. 1-3 Bismuth and indium have been determined in 100 mg samples of Bi-In-Te thermoelectronic material by visual ethylenediaminetetraacetic acid (EDTA) titrations.4 Controlled-potential coulometry has been applied for the precise and accurate determination of the components in thin films of copper indium selenide.5 In studies of semiconducting materials, considerable atten- tion has been devoted to the AyBY' compounds of tetradymite structure (where the main components A = Bi, Sb and B = Se, Te), which find promising applications in thermoelectronic devices.Single crystals doped with atoms of another element (e.g., In) can be grown by a modified Bridgman method to obtain a crystal cone of layered structure;6 however, there is always a certain concentration gradient of the added com- ponent within the crystal. For the interpretation of the nature of point defects due to the substitution of indium atoms for bismuth in the lattice of an AZBY' crystal, it is necessary to compare the changes in free carrier concentration, as deter- mined from the transport coefficient and reflectivity measure- ments, with the real content of indium. Thus an exact assay of the built-in component should be carried out on actual samples taken for the measurement of physical properties. X-ray fluorescence spectrometry (XRF), as a non-destruc- tive analytical method, is useful in this field.Previous experience with quantitative XRF microanalysis of semicon- ducting materials has been summarized in a critical review,7 but no further major contributions to this field have been published recently. In this paper, statistical analysis of numerous experimental data is used to assess the applicability of XRF measurements on a small area of a natural crystal face. The problems of XRF calibration based on an accurate chemical determination of indium8 are also discussed in detail. Experimental Apparatus For the X-ray measurements an ARL 8420+ XRF sequen- tional spectrometer equipped with a DEC PDP-11/73 com- puter was used.The In Kal,2, Se Kal.2 and background intensities were measured with a rhodium-target X-ray tube operated at 60 kV and 40 mA, with a lithium fluoride (200) crystal, a fine collimator and a scintillation detector. The counting time was 10 s unless stated otherwise. A screening mask was made of phosphor bronze with a 2 x 12 mm cut-out in the centre to define a selected area on the crystal face for the XRF measurement (Fig. 1). The crystal chip was placed on the rear side of the mask and fixed with adhesive tape. Along the circumference of the opening the chosen area of the crystal face was marked with a steel needle. The mask with the fixed crystal was then placed on a poly(tetrafluoroethy1ene) (PTFE) spacer and fastened with a spring to the sample holder of the spectrometer cassette.The effective layer thickness, tef, for the crystal chips was calculated for an intensity ratio RJR, = 0.999 of the In Kal,z line (51.4 pm). Interpolation with the use of tabulated mass absorption coefficients for the elements Se, In and Bi at Ag K a and Mo K a lines was carried out with the Bragg-Pierce law; hence using relevant mass coefficients and assuming additivity, the mass absorption coefficients p/p for the pure matrix of Bi2Se3 and Bil,7Tno.3Se3 and Bil ~51no.5Se3 crystals were calculated as 47.1, 44.1 and 42.0 cmz g-1, respectively. Then, with the use of our experimental density values of 7.69, 7.47 and 7.33 g cm-3 and using an approximate expression (e.g., ref.9), t,f = 460 (p/p)-l p-1 pm, the effective layer Sample for physical measurements Fig. 1 Selection of a sample from a single crystal chip80 ANALYST, JANUARY 1993, VOL. 118 thickness was calculated to be 127, 140 and 149 pm, respectively. The spectrophotometer arrangement and the equipment used for the extraction-photometric determination of indium were described previously.8 Reagents and Solutions All reagents were of analytical-reagent or semiconductor grade. The solutions necessary for the extraction-photometric procedure were described previously.8 Standard indium(1ii) solution, 5 x 10-4 rnol dm-3 in approximately 10-3 mol dm-3 HN03, was prepared by exact dilution of a 0.01 mol dm-3 stock solution made by dissolving pure indium metal in dilute nitric acid.The concentration of indium in the two standard solutions was determined by photometric EDTA microtitration using Xylenol Orange as indicator (pH = 2.9, 540 nm), which proved to be an exact standardization procedure. Decomposition of Samples Single crystals of bismuth selenide are difficult to dissolve in mineral acids. The following procedure allows indium(ir1) nitrate solution to be obtained in the presence of an excess of bismuth(iii) and selenic acid in a medium of approximately 0.3 mol dm-3 nitric acid. As the decomposition procedure is to be modified with respect to the amount of indium present in the final solution (1-20 pg cm-3), the following procedure is given here for a sample mass of up to 30 mg and a final volume of 100 cm3. Add 5 cm3 of reversed (Lcfort) aqua regia [nitric acid- hydrochloric acid (3 + l ) ] to a sample (1-8% In) in an evaporating dish and heat on a steam-bath. If oxidation of the sample is not complete, add another 5 cm3 portion of the Lefort mixture and evaporate to dryness.A white residue should be obtained. Rinse the covering watch-glass with 5 x 10-3 rnol dm-3 nitric acid and evaporate again. Use 10 cm3 of dilute nitric acid (1 + 4) to dissolve the residue and then use 20 cm3 of 0.2 mol dm-3 nitric acid to transfer the solution into a 100 cm3 calibrated flask and finally 5 X 10-3 mol dm-3 nitric acid for washings and dilution to the mark. The resulting concentration of nitric acid should be about 0.3 mol dm-3. Extraction-Photometric Determination of Indium Transfer the following reagents into a 100 cm3 separating funnel: 10 cm3 of 0.22 rnol dm-3 sodium acetate, 5 cm3 of a 0.25% solution of quinolin-8-01 in redistilled water acidified with acetic acid and 5 cm3 of 2% sodium thiosulfate solution. Mix well, add a 5 cm3 aliquot of the sample solution and dilute the aqueous phase to about 40 cm3.Extract with 10 cm3 of chloroform by shaking for 4 min. Discard first a small portion of the extract and then transfer the chloroform phase through a filter-tube fitted with small dried filter-paper into a 10 mm glass cell. Use a cell with a longer inner length if the content of indium is below 1%. Cover the cell with a well fitting PTFE cap and measure the absorbance at 400 nm against a blank extract as reference, obtained with the use of a solution containing a corresponding amount of dissolved pure Bi2Se3.For washing, the cells and glassware were also rinsed with dilute nitric acid (1 + 10). Burettes and pipettes were calibrated by weighing measured volumes of distilled water. Regression Analysis of the XRF Calibration Data The calibration experiments (cf., Table 1) showed that the errors of the extraction-photometric method were more or less comparable to those of the X-ray intensity measurements. Therefore, it was not possible to assume, as is commonly done in applications of simple regressions, that the independent variable, i.e., the analyte content, is virtually free from errors. This was confirmed by computations using both a linear and a quadratic fit to the calibration XRF data and assuming that the mean values for the reference assays were sufficiently precise.The results were not satisfactory and similar conclusions were obtained for inverse approaches. A relatively simple least-squares method for a linear relationship if both variables are subject to experimental errorl03l1 was modified for our situation of asymmetric sets of data and used to write a program for a personal computer. For each sample of the monocrystal the number of repeated intensity measurements [corrected for the background, R(In), y l ] was ny = 12, whereas the number of parallel assays of indium (xl, In %) was rz, = 4. Thus, for six calibration samples the total number of points was 6nynx = 288. The computation procedure is briefly outlined below. First, a regression y ( x ) , i. e., line 1 y = a l + hlx (1) is calculated under the assumption that only the dependent variable y is subject to experimental error.Then, for an inverse regression x(y), error-free values of y are assumed to yield a regression line x = a; + b h , which is rearranged as line 2 y = a2 + b2x (2) This inverse regression need not be carried out at all, because the correlation coefficient rxy for the whole set of data can be expressed in terms of the slope values of both regression lines: (3) where the common symbols denote the sample values of the covariance sxy. and the standard deviations s,, sy. The regression lines 1 and 2 have a common intercept at x = X and y = j i . The angle a between these lines can be calculated from the values of the line slopes bl and b2: (4) The line representing the best fit lies between the two regression lines and its position is given by an angle LO with respect to line 1.The method under consideration10 is based on the idea that this angle is affected by the statistical weights p , and p y , Their ratio can be expressed by the inverse ratio of particular variances: pJpx = s$/s;. The angle o is then defined as P y P x + PJ o = a x If the errors of the two variables are approximately compar- able, as here, then px- = p y and o = a/2. The slope for the best fit is then defined as b1 + t a n o 1 - bl t a n o 63 = and this value is then used to calculate the intercept a3 using the sums of all experimental y and x data. Computation of the calibration set presented in Table 1 gave the following equation: y = -0.03215 + 0.22945~ (7) where y is the net intensity in counts s-1 x 103 andx represents In (% m/m).For computation of a quadratic fit to the calibration data on consideration of both variables subject to experimental error, the minimum for weighted sums of the squares of deviations for both variables was found with the use of theANALYST, JANUARY 1993, VOL. 118 81 Brent optimization method. The following regression equa- tion was obtained: y = -9.225 X 10-3 + 0.22313 x + 7.4925 x 1 0 - 4 ~ 2 (8) For statistical tests and the calculation of confidence intervals, a probability level of 95% was assumed. Results and Discussion Samples of Layered Monocrystals The single crystal of Bi2-,In,Se3 prepared by a modified Bridgman method was obtained in the form of an elongated cone (length 50-60 mm and diameter about 9 mm), which could easily be cleaved into thin slices parallel to the cone axis.The cleavage faces had a smooth and mirror-bright surface. For physical measurements a small rectangle (approximately 2 X 12 mm or less) was cut out from a chosen area of the crystal slice (Fig. 1). As there is always a certain concentration gradient of the doped element in the directions parallel and perpendicular to the crystal axis, each crystal segment taken for the measure- ments represents an individual sample. As found by electron microprobe analysis, the indium content does not change significantly within such a crystal chip (cf., ref. 6). The total mass of the crystal sample may vary from less than 1 to about 40 mg depending on its size and thickness (0.1-0.5 mm).The advantage of non-destructive XRF analysis is obvious, as the whole crystal chip has to be decomposed if chemical analysis is required. The reliability of the chosen method must be adequate to provide safe results that can be correlated with the physical parameters. Extraction-Photometric Determination of Indium As a chemical reference method, the extraction-photometric determination of indium with quinolin-8-ols was chosen, because its reliability was well tested in previous assays of telluride monocrystals. It was realized, however, that decom- position of selenide crystals was difficult. It was necessary to use Lefort aqua regia to achieve a complete dissolution. As bismuth is easily hydrolysed in the presence of chloride ions, the sample solution was evaporated to dryness and the residue dissolved in nitric acid (cf., Experimental). In this way selenium was oxidized to selenic acid and a sample solution of indium(i1i) with an excess of bismuth(rr1) and a defined hydrogen ion concentration was obtained.A 5 cm3 aliquot was then taken for the extraction-photo- metric analysis. From the aqueous phase (Vaq = 40 cm3) buffered with acetate to pH 3.6-4.2, indium(ii1) was trans- ferred to the chloroform phase (Vorg = 10 cm3) in a single quantitative extraction step as the tris(quino1in-8-olato)- indium(ii1) chelate. Bismuth was masked effectively with thiosulfate. The absorbance of the chloroform extract was measured at 400 nm against a blank extract which was carried out simultaneously.For calibration of the procedure a standard 5 x 10-4 mol dm-3 solution of indium nitrate was prepared and checked with the use of a precise photometric EDTA microtitration with Xylenol Orange as indicator. To a solution obtained by dissolving pure Bi2Se3 (20-60 mg in 100 cm3) a certain volume of the standard indium(ri1) solution was added to obtain, after appropriate acidification and dilution to 100 cm3, the same nitric acid concentration as in a sample solution. Thus, for a lower concentration range of indium(ii1) (3-9 pg in a 5 cm3 aliquot), the chloroform extracts were measured with SO mm glass cells against corresponding blanks and the following regression line ( n = 14, sxv = 0.0054) was obtained: A X lo3 = -12.3 k 7.7 + (33.31 k 1.31) ml, (9) where mI, i s the amount of indium(m) in micrograms per aliquot of the sample solution.For higher concentrations of indium(ii1) (up to 110 pg per aliquot), it was necessary to measure the absorbance of chloroform extracts in 10 mm cells. The calibration measure- Table 1 Calibration data for the assay of indium in samples of Bi2-,In,Se3 layered monocrystals X-ray fluorescence- X-ray intensity, R(1n Kcx,.~) In found? Sample Coefficient x Calculated* Average/counts s-l In (YO) (ny = 12) RSD (Yo) In (YO) A h (YO) No. for In I 0.01 2 0.1 3 0.2 4 0.3 5 0.4 6 0.5 Extraction spectrophotometry- Samplc No. 1 2 3 4 5 6 0.176 33.8 7.45 0.287 f 0.005" 0.193 k 0.007b 1.78 322,6 1.14 1.546 k 0.007" 3.61 651.1 0.87 2.98 f 0.012a 5.50 1169.4 0.48 5.24 f 0.011" 7.44 1626.7 0.40 7.23 k 0.013" 9.45 2248.3 0.31 9.94 ?c 0.014" 9.80 f 0.019b +0.111 +0.017 -0.23 -0.63 -0.26 -0.21 +0.49 +0.35 In found Absorbance9 Takent/mg (n, = 4) RSD (Yo) mI"/Pg Assay (%) 69.762' 0.160' 5.43 5.55 0.160 t 0.014 27.72d 0.27 If 0.83 43.15 I .57 * 0.02 41.374" 0.401t 2.15 63.87 3.11 k 0.11 25.569" 0.426f 1.22 67.92 5.34 k 0.11 22.15ZC 0.490" 0.17 78.05 7.09 * 0.02 9.92 2 0.04 17.699' 0.548f 0.26 87.29 * From masses of the reactants if homogeneity of the crystal is assumed. t With the use of regression equations: a cqn. (7) or (1l);b eqn. (8). AIn = difference with respect to the calculated composition. $ Total volume of thc sample solution: 5 Average of four parallel extractions; measured against the relevant blank extract: e 50 mm and f 10 mm cell. Confidence intervals arc givcn with the means.100 and 50 cm3.82 ANALYST, JANUARY 1993, VOL. 118 1400 Table 2 Assay values for some samples of layered monocrystals 1120 I v) v) Extraction In found spectrophotometry 4- 5 840 Coefficient x by XRF* Sample for In (Yo) mass t/mg y1 In found (YO) AIn (%)$ 0 5.11b -0.15 4- C > . 4- 0.1 1.522" 28.262d 6 1.534 k 0.007 -0.011 0.3 5.15" 31.03% 8 5.26k0.04 -0.11 .gj 560 0.4 7.63" 21.727C 8 7.79 k 0.07 -0.16 - 280 0.5 9.23" 17.155~ 6 8.90 k-0.07 +0.33 9.11b +0.21 0 In Ka1.2 I - ,..'I?, ,I*. 1 'i\ /I I ': - ; I 'Xi / I *%.I. I !? Background : I j I I I - , I i "%. ------ I 1 ---..' I I [ I $ AIn = In(XRF) - In(spectrophotometry). - Fig. 2 Scan of the In Kal.2 line (counting time, 10 s) ments were characterized by a regression line (n = 18, syx = 0.0075) passing through the origin: A x 103 = (6.276 k 0.041) mi, (10) The results of several series of parallel extraction-photometric determinations of indium in actual samples of layered monocrystals are summarized in Tables 1 and 2.As can be seen, the method yields precise data on which standardization of the XRF analysis can be based. Quantitative X-ray Fluorescence Analysis In preliminary experiments with solid solutions of tellurides and selenides, difficulties arose with sample preparation for XRF. The materials that were synthesized to yield a poly- crystalline bulk solid were apt to give a more or less homogeneous powder after prolonged grinding in an agate mortar, but in the grey-black powder so obtained there were always some microscopic silvery scales.Nevertheless, the surface of a briquette made from such powder was found to be adequate even for electron microprobe analysis. However, a large sample of the material was needed to make a briquette of adequate diameter. Effective grinding of layered selenide crystals was found to be even more difficult, so for milligram samples this procedure was not applicable. A fusion technique was also found to be impracticable [Caution: selenide and telluride materials affect gold alloy crucibles]. Layered monocrystals can easily be cleaved into thin slices. As their trigonal axis is always perpendicular to the pulling direction, i.e., to the axis of the crystal cone, the cleavage face obtained coincided with the (0001) crystal plane. This orientation was confirmed using the Laue back-reflection method.6 The natural face so obtained had in most instances the lustre of a metallic mirror and was found to be adequate for XRF measurements, This allowed a certain area on the crystal chip to be selected for the XRF analysis.In order to define such a chosen piece of the crystal, a screening mask with a rectangular opening was used. Hence it was possible to obtain the necessary analytical information before that piece of the crystal was cut out and taken for physical measure- ments. A relatively simple matrix of the Bi-In-Se crystal made the choice of the analytical lines easy. The channel Se Kal,z was measured for the main component of approximately constant content. For indium the channel In K c Y ~ , ~ was selected (Fig.2). The vicinity of these lines was scanned using a fine collimator and a 60 s count and so it was found that there were no interferences caused either by the matrix or the components of the bronze screening mask (Cu, Sn, P). In order to achieve a high sensitivity, the effect of the excitation voltage on the counts at the In Kal,Z channel was examined within the range 40-60 kV. The maximum voltage of 60 kV was then used for further measurements. The effect of the thickness of a crystal chip (see above) on the intensity measurements was tested experimentally. First, a chip from a Bil.51no.5Se3 crystal of thickness 0.60 mm was measured; then a layer 0.11 +_ 0.02 mm thick was splintered off and taken for a repeated measurement. The difference between the net counts at the indium channel was statistically insignificant.On consideration of the calculated effective layer thickness and the manipulation risks with thin, friable crystals, the samples for the XRF assays should be at least about 0.2 mm thick. The stability of the background close to the In K C Y ~ , ~ line was confirmed by scanning (60 s count) the crystal samples 1 and 6, which represent the minimum and maximum content of indium, respectively. For 12 measurements the standard deviation for the background was 1.0 counts s-1. 'The reproducibility of the net count rate corrected for the background is shown by the calibration data in Table 1. Each sample was measured 12 times. The pooled standard deviation for all 72 measurements If(degrees of freedom) = 661 was 5.4 counts s-1.Even for sample No. 1 with the lowest percentage of indium, the intensity at the channel In Ka1,2 was signifi- cantly above the background. Various approaches to quantitative evaluation of the XRF measurements of indium were investigated with respect to the results of extraction-photometric assays of monocrystal sam- ples that were cut out exactly from the area of the crystal exposed to the primary X-radiation. The radiation scattered and emitted from the screening mask is superimposed on the analytical lines of the sample and this fact has to be taken into account in considerations about appropriate calibration models. A calibration plot of log [R(In)/R(Se)] versus log [ C(In)/C(Se)] applicable for such a pseudo-binary system was found to be less reliable than a simple calibration of the In Kal,* intensity [corrected for the background in counts s- I , R(In)] against the indium content.As errors of the reference assays had to be considered, regression methods for both variables subject to experimental errors were applied. For the main concentration range (about 1-8% m/m In) a straight-line fit was found to be adequate. The computation with the data in Table 1 (rxy = 0.9995, n = 288) gave a regression line that was used to calculate the content of indium from the corrected intensity R(In), in counts s--1 x 103, of the indium channel Ka1,2 in the following reverse form: In (YO) = 0.1401 + 4.358 R(1n) (11) The results are given in Table 1. Regression eqn. (8) obtained for a quadratic model provided a closer fit to the two outer standard samples 1 and 6; however, the results for the main region deviated on the same level or even more than those of the linear model.However, it was possible to use eqn. (8) for estimating the limit of determination (LD) based on a conventional assumption of a permissible 10% relative error. If only the counting errors for the peak and backgroundANALYST. JANUARY 1993, VOL. 118 83 intensities were considered, the net count R(In)LD was calculated with the use of well known expressions as 31 counts s-1, which corresponded to 0.18% In. With the use of a pooled standard deviation of experimental intensities a more realistic estimation of R(In)LD was 54 counts s-1 and hence an estimate of the LD of 0.28% In was obtained, which corresponded well with the statistical analysis of the actual XRF assays.For recalibration of the spectrometer, one crystal sample with composition Bi, ,51no.5Se3 (0.5 mm thick) was preserved for further measurements. For example, with one series of assays the correction factor for the average In Kal,Z net counts was R(calc.)lR(meas.) = 2248/2219 = 1.013. Applications in Routine Assays of Single Crystals After recalibration of the X-ray spectrometer, a series of samples were measured on their natural cleavage faces, marked crystal pieces were cut out, dissolved and taken for a reference extraction-photometric determination of indium. Table 2 gives results for this series of assays, which were evaluated with the use of the above-given calibration proce- dures. It is interesting that virtually the same results of the extraction-photometric assays were obtained if the average values of absorbance were taken for calculation with the use of the regression line given in previous work for 10 mm cells [cf., eqn.(1) in ref. 81. Thus, for the sample with x = 0.1 the result was 1.515% In, for x = 0.3 5.26% In, for x = 0.4 7.78% In and for x = 0.5 8.88% In. If it is taken into consideration that the extraction-photometric method was newly developed with all necessary calibrations, these results provide interesting evidence of the reproducibility and repeatability of this method. Thc main advantage of the XRF method is that it is able to provide sufficiently precise (about 1 % relative) analytical information prior to the preparation of actual samples for physical measurements, thus allowing screening and selection of the crystal chips. The results presented in this paper have also shown that despite all the problems involved in XRF measurements on a limited area of a natural cleavage face of a single crystal, it is possible to obtain data that are representa- tive of the actual sample of the crystal within the error of an independent reference assay. In this respect the approach explored in this paper may be useful for further applications. 1 2 3 4 5 6 7 8 9 10 1 1 References Yudclcvich, I. G., and Beizcl, N . F., Zzv. Sib. Otd. Akud. Nuuk SSSR, Ser. Khim. Nuuk, 1978, 94. Dittrich, K . , and Vogel, H., Tuluntu, 1979, 26, 737. Shelpakova. I. R., Shcherbakova, 0. I . , Yudelevich. I. G., Bcizel. N. F., Dittrich, K., and Mothes, W., Tuluntu, 1982, 29, 577. Danzaki, Y., Shoji, T., Sasc, M . , and Takeyama, S . , Runseki Kagaku, 1983,32, 89. Yang, M. H., Lee, M. L., Lin, Y. M., and Hwang, H. L., Thin Solid Films, 1987, 155, 317. LoStak, P., BcncS, L., CiviS, S . , and Sussmann, H., J . Muter. Sci., 1990. 25, 277. Herrington, C. R., J. Electron Microsc. Tech., 1985, 2, 471. Sramkova, J., Kotrly, S . , and Kalischova, Y., Collect. Czech. Chem. Commun., 1988. 53, 3029. Rontgen~uoreszenzunulyse, ed. Ehrhardt, H.. Deutscher Vcr- lag fur Grundstoffindustrie, Leipzig, 1981, p. 94. Nyvlt, J . , Chem. Prim., 1959, 9. 468. Mdritz, P., in Wilson and Wilson’s Comprehensive Analytical Chemistry, ed. Svehla, G., Elsevier, Amsterdam, 1981, vol. XI, p. 109. Puper 21037001 Received July 13, 1992 Accepted September 21, 1092

ISSN:0003-2654    

DOI:10.1039/AN9931800079

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

ANALYST, JANUARY 1993, VOL. 118 85 Column Preconcentration of Cobalt in Alloys and Pepperbush Using 2-(5-Bromo-2-pyridylazo)-5-diethylaminophenol and Ammonium Tetraphenylborate Adsorbent Supported-on Naphthalene With Subsequent Determination Using Atomic Absorption Spectrometry Masatada Satake Faculty of Engineering, Fukui University, Fukui 910, Japan Tohru Nagahiro Department of Chemistry, Himeji Institute of Technolog y, Himeji 671-22, Japan Bal Krishan Puri Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi- 11007, India A method has been established for column preconcentration and determination of cobalt using an ion-pair produced from an ammonium cation and a tetraphenylborate (TPB) anion supported on naphthalene in a simple funnel-tipped glass tube.Cobalt forms a water-soh ble chelate cation with 2-(5-bromo-2-pyridylazo)-5- diethylaminophenol (5-Br-PADAP). The chelate cation is retained as a water-insolu ble Co-5-Br-PADAP-TPB complex on the surface of the naphthalene, which is packed in a column. Cobalt is quantitatively retained on the ammonium tetraphenylborate adsorbent supported on the naphthalene in the pH range 3.0-8.0 at a flow rate of 1 ml min-1. The solid mass is stripped from the column with 5 ml of dimethylformamide (DMF) and the cobalt determined by flame atomic absorption spectrometry at 240.7 nm. The calibration graph is linear over the concentration range from 1 to 20 pg of cobalt in 5 ml of final DMF solution. Seven replicate determinations of 12 pg of cobalt gave a mean absorbance of 0.124 with a relative standard deviation of 1 .O%.The sensitivity for a 1 % absorption is 0.085 pg ml-1. The effect of foreign ions was studied and the proposed method applied to the determination of cobalt in certified reference material samples of alloy, steel and pepperbush. Keywords: Cobalt determination; ammonium tetraphen ylborate adsorbent; alloy, steel and pepperbush; nap h t halen e column p reconcen tra ti0 n ; 2- (5- b rom 0-2-p yrid ylazo)-5-dieth yla mino p hen o I; flame a to mic absorption spectrometry Sodium tetraphenylborate (TPB) or its derivative has been utilized in the gravimetric determination of some alkali and univalent metal ions.’-3 Recently it has been utilized as the counter ion in the extraction and adsorption of iron, cobalt, copper complexes of 2,2’-dipyridyl, 1,lO-phenanthroline (1,lO-phen) , 3-(4-phenyl-2-pyridyl)-5,6-diphenyl- 1,2,4-tri- azine, 2,6-bis(2-pyridyl)-4-(4-methoxyphenyl)pyridine, 2,4,6- tris(2-pyridyl)-1,3,5-triazine and 3-(2-pyridyl)-5,6-diphenyl- 1,2,4-triazine (PDT) into molten naphthalene4-9 or on microcrystalline naphthalenel(L15 as these ternary complexes cannot be extracted into the usual organic solvents owing to their poor solubility.The various advantages of these tech- niques have been discussed previously .4-15 A survey of the literature reveals that column methods have been reported for the separation and preconcentration of metal ions using various adsorbents such as activated car- bon,16 green tea leaves,l7 a chelating resin,’* thiol cotton19 and polythioether foam.”) Although some of these methods are fairly effective, once prepared they are reusable, unlike the material used in this procedure.Furthermore, introduc- tion of Chelex-100” and an ion-exchange resin22 minicolumn into the flow injection system increases the sensitivity to between ppb and ppt level, thus allowing natural waters to be analysed. Column methods have already been reported for the preconcentration of trace metals using various adsorbents, e.g., 2-mercaptobenzothiazole,*~ 2,4,6-tris(2-pyridyl)-1,3,5- triazine tetraphenylborate,24 mixed ligands of dimethylgly- oxime and acenaphthenequinone dioxime ,25 benzyldimethyl- tetradecylammonium perchlorate26 and tetradecyldimethyl- benzylammonium-l,2-dihydroxybenzene-3,5-disulfonate .27 In this paper, a highly selective and sensitive preconcentra- tion method has been developed, that uses 2-(5-bromo-2- pyridylazo)-5-diethylaminophenol (5-Br-PADAP) as the complexing agent and in addition to the (NH4+) (TPB-) ion-pair supported on naphthalene.Numerous reagents have been used for the spectrophotometric determination of cobalt. Among them, 5-Br-PADAP and its derivatives are highly selective for cobalt.28 Once the cobalt complex cation is formed, it does not decompose even in strongly acidic medium (as opposed to other metal complexes), this results in a high selectivity for cobalt. The 5-Br-PADAP reacts with cobalt to form a water-soluble complex cation, but in the presence of the TPB anion it forms a water-insoluble complex (Co-5-Br- PADAP-TPB). Preliminary observations revealed that the Co-5-Br-PADAP complex cation could be quantitatively retained on an ammonium tetraphenylborate adsorbent sup- ported on naphthalene.The method developed is very simple as only the cationic cobalt complex of 5-Br-PADAP is passed through the naphthalene adsorbent, which is packed in a funnel-tipped glass column. The metal complex along with the naphthalenc is dissolved out from the column with a small volume of solvent [2-5 ml of dimethylformamide (DMF)] and can be directly aspirated into the flame of an atomic absorption spectrometer. The operating parameters have been evaluated and the proposed method applied to the determination of cobalt in various certified reference material samples. It can also be employed for various other biological and environmental samples.Experimental Apparatus A Perkin-Elmer Model 403 atomic absorption spectrometer and a Toa-Dempa HM-SA pH meter were used. All absorp- tion measurements were performed under the following qperating conditions: wavelength, 240.7 nm; slit setting, 3(7 A); current, 10 mA; acetylene flow setting, 15 (pressure, 0.386 ANALYST, JANUARY 1993, VOL. 118 kg cm-2); and air flow setting, 55 (pressure, 2.1 kg cm-2). A funnel-tipped glass tube (SO X 6 mm i.d.) was used as the column, which was plugged with poly(propy1ene) fibres and then slurry-packed with the naphthalene material. The column loaded with the naphthalene was lightly compressed with the flat end of a glass rod so that its height was 1 .O-1.2 em. Reagents All the reagents were of analytical-reagent grade.Standard cobalt solution (3 ppm) was prepared by diluting 1000 ppm atomic absorption standard cobalt chloride solution with doubly distilled water. A 0.01% solution of 5-Br-PADAP in ethanol was prepared. Buffer solutions of pH 3-6, 6-8 and 8-11 were prepared by mixing appropriate ratios of a 0.5 rnol 1-1 acetic acid and 0.5 mol 1-1 ammonium acetate solution, 0.1 mol 1-1 sodium dihydrogen phosphate solution and 0.1 rnol 1- dipotassium hydrogen phosphate solution and 0.5 mol 1-1 ammonia solution and 0.5 mol 1-1 ammonium acetate solution. Doubly distilled water was used throughout. Preparation of Naphthalene-NH4-TPB Adsorbent A solution of naphthalene was prepared by dissolving 20 g of naphthalene in 40 ml of acetone on a hot-plate stirrer at 35 "C.This solution was transferred into 1500 ml of distilled water, containing 100 ml of 1 mol 1-1 ammonium acetate-1 rnol 1-1 ammonia solution (pH 9 . 9 , in a fast stream with continuous stirring at room temperature, and to it was added 500 ml of an aqueous solution of 1.7 g of TPB in the same manner. The naphthalene material coprecipitated with NH4+ and TPB-, the mixture was stirred for about 2 h and then allowed to stand for 2 h. The supernatant solution was decanted off and the naphthalene washed twice with distilled water. The slurry of naphthalene in water was stored in a bottle until required. General Procedure An aliquot of solution containing 1-20 pg of cobalt was made up to 13 ml with distilled water in a 20 ml beaker to which were added 1.0 ml of the 0.01% 5-Br-PADAP in ethanol solution and 0.5 ml of 0.1 moll-' phosphate buffer (pH 6.0-6.5).This solution was passed through a column loaded with naph- thalene-NH4-TPB at a flow rate of 1 ml min-1. The packing was washed with a small volume of distilled water and then aspirated strongly for 5 min, pushing down the naphthalene material with a flat glass rod to eliminate the excess water attached to the naphthalene. The metal complex along with the naphthalene was dissolved out from the column with 5 ml of DMF. The solution was aspirated into an air-acetylene flame and the absorbance measured at 240.7 nm against a reagent blank. Results and Discussion Retention Characteristics of NH4-TPB Sodium tetraphenylborate is soluble in water, it forms water-insoluble precipitates with some alkali metal ions such as K+, Rbi , Cs+ (but not Li+ and Na+), and univalent metal ions such as Ag+, TI+ and Cu+, but does not form precipitates with multivalent metal ions.It has been used as a gravimetric and volumetric reagent. Furthermore, it also reacts with onium salts such as NH4+ and its derivatives, e.g., ammines, quaternary ammonium salts, alkaloids and onium compounds, to form water-insoluble precipitates. The TPB- forms a weakly bonded ion-pair with NH4+ in aqueous solution and coprecipitates with microcrystalline naphthalene, as follows: From the experimental observation, the NH4-TPB ion- pair, produced from TPB and ammonium acetate in aqueous solution, when supported on naphthalene was unstable and NH4+ + TPB- $ (NHd+) (TPH-) (s).partly desorbed from the surface of the naphthalene in the column on passage of the buffer. Thus the NH,-TPB ion-pair was prepared in acetate buffer of pH 9.5. The adsorbent shows excellent adsorption characteristics for various metal complex cations such as Fe( 1 ,10-phen)32+ or CU(PDT)~+. In this work, TPB- is selected as the counter ion because of its high purity and moderate price. Reaction Conditions Keeping the other variables constant, experiments were carried out using 12 pg of cobalt. The adsorption of cobalt starts at pH 1.2 and is constant and maximum over the pH range 3.0-8.0; above pH 8.0, the adsorption is decreased. Addition of 0.3-2.0 ml of the phosphate buffer did not affect the adsorption of cobalt and use of 0.5 ml at pH 6.5 is recommended Various amounts of 0.01Y0 5-Br-PADAP in ethanol were added to the solution containing 12 pg of cobalt.Cobalt was quantitatively adsorbed onto the naphthalene over the con- centration range of 0.5-2.0 ml of reagent solution. Thus 1 .0 ml was used in all subsequent experiments. The flow rate was varied from 0.5 to 8 ml min-1. The adsorption of cobalt was not affected over this range. A flow rate of 1 mi min-1 was recommended. The effect of the volume of the aqueous phase on adsorption of cobalt was investigated by the general procedure. The adsorption was constant and maximum when the volume of the aqueous phase did not exceed 700 ml. In this case, cobalt is preconcentrated by about 140-fold for 5 ml of DMF solution. In the subsequent work, 15 ml of the aqueous phase were used for convenience.Choice of Solvent An attempt was made to dissolve out the naphthalene-Co-5- Br-PADAP-TPB complex from the column. As the solid mass was dissolved out with a small volume ( 5 mi) o f solvent it was essential to select a solvent in which the chelate was highly soluble and could be determined sensitively. The solid material was soluble in methanol, ethanol, butan-1-01, aceto- nitrile, dimethyl sulfoxide and DMF. The DMF was preferred because of the high solubility and sensitivity attained. Methanol and ethanol are better solvents, but the flame on the burner-head was unstable owing to their volatility. It was found that 2-3 mi of DMF were sufficient to dissolve the mixture, it also enhanced the sensitivity of the method by a further 2-fold.The surplus water attached to the naphthalene caused the absorbance to decrease by 10'3'0 and created an error in the determination, it was, therefore, necessary to eliminate the water by aspirating. Linearity, Sensitivity and Precision Based on the optimum conditions described above, the calibration graph was linear over the concentration range 1-20 pg of cobalt in 5 ml of DMF solution. Seven replicate determinations of 12 pg of cobalt gave a mean absorbance of 0.124 with a relative standard deviation of 1.0%. The sensitivity for 1% absorption was 0.085 pg ml-l (0.24 pg ml-1 for direct flame atomic absorption spectrometric measure- ment on the aqueous solution). Effect of Diverse Ions Sample solutions containing 12 pg of cobalt and alkali salts or metal ions were prepared individually and the General Procedure applied.The tolerance limit was set as the amount of diverse ion required to cause +3% error in the determina- tion of cobalt. The results obtained are given in Table 1. Of the salts examined, most could be tolerated up to gram or milligram levels. Disodium ethylenediaminetetraacetic acid (EDTA) (10 pg) could be tolerated. Of the metal ions studied,ANALYST. JANUARY 1993, VOL. 118 87 most metal ions could be tolerated up to a level of 1 mg; 100 mg of Ag+, Pt4+ and V-5+ could be tolerated. Copper was masked by the addition of thiourea. Thus, the proposed method is selective and can be applied to the determination of cobalt in various certified reference material (CRM) samples without any preliminary separation and can be employed for other complex materials.Determination of Cobalt in CRM Samples of Aluminium Alloy, Stainless Steels and Pepperbush The proposed method has been applied to the determination of cobalt in Nippon Keikinzoku Kogyo (NKK) CRM 920 Aluminium Alloy, Japanese Standards of Iron and Steel (JSS) CRM 651-7 Stainless Steel and National Institute for Environ- mental Studies (NIES) CRM No. 1 Pepperbush. A 0.054.1 g Table 1 Effect of diverse salt and ions Salt or ion CH3COONa.3H20, NaC1O4.Hz0, KN03 NaCl KI, thiourea Na2S04 NH4CI, KSCN, Na, K-tartrate Sodium citrate L-Ascorbic acid KCN Disodium EDTA Call, Mg", Al"', Mo"', Mn", Fe"' Pb". Bill', Cd", W"', Crvl, Ru"l Zn" . Cr"l , Pd" Ag', Ptlv, Vv Cu" KH2PO4 NHJF. GEDTAt K?C?OI. H20 Tolerance limit 1 g* Ig 500 mg* 500 mg 100 mg 10 mg* 10 mg 5 mg" 5 mg 3 mg 500 P8 10 I-18 500 I % 10 pg, 500 pgj 5 mg* 1 mg" 100 * Maximum value tested: Co, 12 pg, pH, 6.5; and 0.01% t GEDTA = glycol ether diaminetetraacetic acid.$ Masked with 500 mg of thiourea. 5-Br-PADAP, 1 .O ml. sample of CRM 920 and CRM 651-7 was completely dissolved in a minimum volume (about 8 ml) of hydrochloric acid (1 + 1) by heating on a water-bath. To this was added 1 ml of 30% hydrogen peroxide. The excess hydrogen peroxide was decomposed by heating the solution on a water-bath. The solution was cooled, filtered if necessary and diluted to 100 ml in a calibrated flask. A 2 g amount of NIES CRM No. 1 Pepperbush was completely dissolved by heating in a minimum volume of concentrated nitric acid (about 25 ml) and 1 ml of concen- trated perchloric acid.The solution was evaporated to very small volume. To this a small volume of water was added. The solution was cooled, filtered and diluted to 50 ml in a calibrated flask. An aliquot (1-2 ml) of each sample was transferred into a 20 ml beaker, and to this were added: suitable amounts of masking agents, 1.0 ml of 0.01% 5-Br-PADAP in ethanol solution and 0.5 ml of the buffer. After being left to stand for 20-30 min, the sample solution was adjusted to pH 1.0 with concentrated nitric acid in order to decompose any interfering metal complexes and then the General Procedure was applied. The results are given in Table 2. These results are in agreement with the certified values. Conclusion A solid ion-pair compound produced from NH4+ and TPB- on naphthalene provides a simple and economical method for column preconcentration of cobalt in alloys and biological samples.The proposed method is the first attempt to separate and concentrate cobalt using the rcaction of Co-5-Hr-PADAP complex cation with NH4-TPB supported on naphthalene. As the proposed method requires only simple glassware, such as a funnel-tipped glass tube and small volume beakers, and as only a small volume of organic solvent is used for the dissolution of the complex, it is vcry economical. The sensitivity and selectivity of the method might be further improved by using alternative optical analytical techniques of analysis such as flameless atomic absorption spectrometry. Table 2 Analysis of samples for cobalt Concentration of cobalt (%) Sample Stainless Steel JSS 651-7 NKK920 Al u m i n i u in Alloy NIESNo.1 Pepperbush11 Composition (%) C, 0.047; Si, 0.72; Mn. 1.72; P, 0.028; Cr, 18.60; S, 0.0063; Mo, 0.84; Cu, 0.082; Al, 0.002; N , 0.0312; and Ni, 9.20 Si, 0.78: Fe, 0.72; Cu, 0.71; Mn, 0.20; Mg, 0.46; Cr, 0.27; Zn, 0.80; Ti, 0 . 15; Sn, 0.20; Pb. 0.10; V. 0.15; Sb, 0.01; Bi. 0.06; Ga, 0.05; Ni, 0.29; Sb, 0.01; and Ca, 0.03; K, 1 .S I -t 0.06; Mg 0.4O8 t 0.020; Ca, 1.38 k 0.07; and Mn, 0.203 k 0.017. Fe, 205 f 17; Zn, 340 t 20; Ba, 165 f 10; Na, 106 2 13; Rb, 75 f 4; Sr, 36 t 4; Co, 23 t 3; Cu, 12 2 1; Ni, 8.7 t 0.6; Cd, 6.7 t 0.5; Pb, 5.5 f 0.8; As, 2.3 f 0.3; P, (1100);Cr.(1.3);Cs. (1.2); TI. (0.13); and Hg, (0.056) v g g - * Certified value Found* 0.22 0.240 t 0.003-t 0.10 0.099 k 0.003$ 23 k 3 pgg-1 25 -t 29 pgg-1 * Mean of four determinations t 100 mg o f thiourea and 1 0 mg of ammonium citrate were added as masking agents at pH 4.04.5.$ 2 ml of 20% triethanolamine solution were added as masking agents at pH 8.0-8.2. 10 mg of ammonium citratc, 2.0 ml of 0.1 mol I- sodium pyrophosphate solution and 1 .O ml of 0.01 mol I-' GEDTA solution were added as masking agents at pH 4.0-4.5. Before passing the sample solution to the column, the sample solution was readjusted to pH 1 with conccntrated hydrochloric acid in order to decompow interfering metal complexes. 11 Values in parentheses are tor reference and are not certified.88 ANALYST, JANUARY 1993, VOL. 118 1 2 3 4 5 6 7 8 9 10 11 12 13 14 References Barnard. Jr., A.J.. Chemist-Analyst, 1955,44, 104. Barnard, Jr., A. J., Chemist-Analyst, 1958, 47, 46. Advances in Analytical Chemistry and Znstrumentation, ed, Reilley, C. No, Interscience, New York. 1960, vol. 1. Satake, M., Nagahiro, T., and Puri, B. K., Analyst, 1984, 109, 31. Chang, L. F., Satakc, M., Kuwamoto, T., and Puri, B. K., Microchem. J . , 1986. 33, 46. Nagahiro, T., Uesugi. K., Satake, M., and Puri, B. K., Bull. Chem. Soc. Jpn., 1985, 85, 1115. Puri, B. K., Mehra, M. C., and Satake, M., Chem. Anal. (Warsaw), 1986, 31, 185. Mehra, M. C., Satake. M., Chang, L. F., and Kuwamoto. T., Acta Cient. Venez., 1984,35, 232. Nagahiro, T., Satake. M., and Puri, B. K., Zndian J . Chem., 1986, 25A. 99. Satakc, M., Mehra, M. C., Singh, H. B., and Fujinaga, T., Bunseki Kagaku, 1983, 32, E165. Chang, L. F., Satake, M., Puri, B. K., and Bag, S. P., Bull. Chem. Soc. Jpn.. 1983, 56, 200. Lin, J. L., Chang. L. F., Katyal, M., and Satake, M., Fresenius’ Z. Anal. Chem., 1984,319, 308. Nagahiro, T., Satakc, M., Lin, J . L., and Puri, B. K., Analyst, 1984, 109, 163. Lin, J . L., Satake, M., and Puri, B. K., Analyst, 1985,110,1351. 15 16 17 18 19 20 21 22 23 24 25 26 27 28 Mehra, M. C., Nagahiro, T., and Satake, M., Microchem. J., 1986, 33, 198. Smith, S., Nelissen, J . , and van Grieken, R. E., Anal. Chim. Acta, 1979, 111, 215. Kimura, M., Yamashita, H . , and Komada, J.. Bunseki Kaguku. 1986, 35, 400. Florence, T. M., and Batley, G. E., Talanta, 1976, 23, 179. Yu, Mu-Qing. and Liu, Gui-Qin, Talanta, 1983. 30, 265. Khan, A. S., and Chow. A.. Talanta, 1986, 33, 182. Olsen, S., Pesscnda, L. C. R., RiiiCka, J . , and Hansen, E. H., Analyst, 1983, 106, 905. Kamson, 0. F., and Townshend, A., Anal. Chim. Acta, 1983, 155, 253. Satake, M., Ishida, K., Puri, B. K., and Usami. S., Anal. Chem., 1986,58, 2502. Usami, S., Yamada, S., Puri, B. K., and Satake, M., Mikro- chim. Acta, Part I , 1989, 263. Usami, S., Fukami, T., Kinosita, E., Puri, B. K., and Satake, M., Anal. Chim. Acta, 1990, 230, 17. Miura, J., Arima, S., and Satake, M., Analyst, 1990,115, 1191. Satake, M., Nagahiro, T., and Puri, B. K., Tulanta, 1992, 39, 1349. Taguchi, S., and Goto, K., Tulanta, 1980, 27,819. Paper 2f02284B Received May I , 1992 Accepted October 2, 1992

ISSN:0003-2654    

DOI:10.1039/AN9931800085

出版商:RSC    

年代:1993

数据来源: RSC