摘要:

ANALYST, FEBRUARY 1986, VOL. 111 245 0.5 0.4 Q, & 0.3 2 :: 0.2 0.1 Selective Spectrophotometric Kinetic Determination of Cobalt with o-Hydroxyp henylt hiourea* - - - - - S. J. Rao, G. S. Reddy,t J. K. Kumari and Y. K. Reddy Department of Chemistry, S. V. University, Tirupati 517 502, India A simple kinetic method has been developed based on the autoxidation of o-hydroxyphenylthiourea to disulphide, which is catalysed by trace amounts of cobalt. The reaction is monitored spectrophotometrically at pH 8.0 and cobalt is determined effectively in the range 5-50 ng ml-l. This method has been used to determine trace amounts of cobalt present in certain plant materials such as carrot roots and corn grains, in which it plays a vital role in different physiological activities. Keywords; Cobalt determination; spectrophotometry; autoxidation; 0- h ydroxyphen ylthiourea A simple and selective catalytic method has been developed for the determination of nanogram amounts of cobalt( 11) based on the autoxidation of o-hydroxyphenylthiourea (OHPTU), a reagent that has been used previously for the determination of trace amounts of copper(I1)l and man- ganese(II).2 The reaction was followed spectrophotometric- ally at 416 nm and the selectivity of the method was improved by the use of masking agents.The method was applied to the determination of cobalt in foodstuffs. Experimental Apparatus A Toshniwal Model RL04/01 spectrophotometer and an Elico Model PH820A pH meter were used. Reagents The reagents used were of analytical-reagent grade unless otherwise specified.o-Hydroxyphenylthiourea (OHPTU). Prepared by the stan- dard procedure.3 Its purity was confirmed from melting-point determinations (161 "C) and elemental analysis, which gave the following results: theoretical, C 50.04, H4.80, N 16.66 and S 19.04; found (CDRI, India), C 50.15, H 5.05, N 16.10 and S Reagent solution, 1 mg ml-1. Prepared freshly in distilled ethanol before use. Cobalt(ZZ) stock solution, 1 mg ml-1. Prepared by dissolving cobalt(I1) chloride (BDH Chemicals) in doubly distilled water. This solution was standardised titrimetri~ally.~ Buffer solution, pH 8.0. Prepared from 0.05 M disodium tetraborohydrate and 0.05 M sodium hydroxide solutions. 18.85 Yo. S NH HN II II HN-C-S-S-C-NH ll HN-C-NH~ I I I This reaction is very slow, but is sharply increased by the addition of trace amounts of cobalt(I1).Increasing the concentration of cobalt increases the rate of the catalysed oxidation and it has been found that the rate of the reaction is proportional to the concentration of cobalt. The A,,,, values of OHPTU and its oxidised product are 280 and 416 nm, respectively, and hence an excess of reagent does not affect the absorbance due to the formation of disulphide. In this study cobalt was determined using the tangent method in which tan 8 values (or slopes) of graphs of absorbance versus time were determined for various amounts of cobalt (Fig. 1) and a final calibration graph was constructed by plotting the value of tan 8 (or slope) against concentration of cobalt (Fig. 2). The calibration graph was linear in the range 5-50 ng ml-1 of cobalt.The fixed-time method was also used for the kinetic determination of cobalt. In this method, the absorbance of the solution was measured after allowing the reaction to proceed for a fixed time and the calibration graph constructed by plotting a graph of absorbance against concen- tration (Fig. 2). The determination of cobalt was possible in the pH range 7.5-8.5. Below pH 7.5, the catalysed autoxidation rate was slow and above pH 8.5, the uncatalysed reaction was sufficiently fast. Hence, a pH of 8.0 was chosen for further studies. A change of 0.5 pH unit was tolerable but an appreciable change of pH during the reaction was prevented Recommended Procedure To a solution containing 125-1400 ng of cobalt, add 7 ml of buffer solution (pH 8.0), 1 ml of 0.025 M pyridine solution (as a promoter) and 4.0 ml of reagent solution and make up to 25 ml with doubly distilled water.Measure the absorbance at 416 nm and plot the absorbance against time. Results and Discussion OHPTU undergoes autoxidation to form a yellow disulphide, N, W-bis(o-hydroxypheny1)-1 , 1 '-dithiobisformamidine: ng ml-' 56.0 A 40.0 24.0 8.0 * Presented in the 3rd Annual Conference of the Indian Council of t Department of Chemistry, S.V.U.P.G. Centre, Cuddapah- Chemists, held in Dharwad, India, 1983. 516 004, India. 5 10 15 20 25 Ti me/mi n Fig. 1. of cobalt Change of absorbance with time for different concentrations246 ANALYST, FEBRUARY 1986, VOL. 111 0.6 a 6 0.4 I- 0.2 0.4 0.2 16 32 48 64 Concentration of Co(ll)/ng ml-1 Fig.2. Calibration graphs for determination of cobalt: A, tangent method; and B, fixed-time method Table 1. Determination of cobalt in plant materials Cobalt found Present method Standard method using using OHPTU*/ nitroso-R salt/ Sample ng 8-l ng g-l Carrot root (Daucus carota var. sativa) 22.0 _t 0.3 22.5 k 0.5 Corn grain (Zea mays) 10.5 5 0.2 10.5 k 0.3 * Average of five determinations. by using a buffer. Increasing the temperature did not have a significant effect on the reaction rate. The addition of 1 ml of 0.025 M pyridine as a promoter increased the sensitivity of the method; an excess of promoter did not affect the reaction rate provided that the pH was kept within the given range. The rate increased with increase in the concentration of the reagent; however, 4.0 mg of the reagent was sufficient to determine up to 50 ng of cobalt.The method has a good reproducibility. Even in the fixed-time method, the relative standard deviation for 40 ng of cobalt was 0.37%. In Fig. 2B, standard deviations of the absorbances are indicated on the points. To study the effect of dissolved oxygen, an alcoholic solution (water - ethanol, 80 + 20 VIV) was heated nearly to boiling and cooled by passing hydrogen through it. To this solution, solid OHPTU was added. A blank experiment was conducted similarly and, after an interval of 30 min, the absorbance was measured at 416 nm. There was no difference in the absorbance values. Interference Studies The effect of foreign ions on the determination of 40 ng of cobalt was studied by the fixed-time method.A change in absorbance of k0.025 was considered to be the tolerance limit for interference. Metal ions such AP+, Cd2+, Ni2+, Mg2+, Se4+, Te4+ , Li+ , Ca2+ , Ba2+, Sr2+ , Sn2+, Be2+ , U022+, Sb2+, TP+, La3+, Zr4+ and Zn*+ did not interfere even when present in up to a 2000-fold excess by mass. Mo6+ and W6+ up to a 65-fold excess and Cr6+ and Ce4+ in up to a 40-fold excess did not interfere in the reaction. Cu2+ and Pt4+ interfered by enhancing the absorbance and Pb2+, Pd2+, Hg2+ and Ag+ interfered by producing turbidity. The interference of iron and manganese was eliminated by adding fluoride and oxalate, respectively. Common anions such as fluoride, chloride, sulphate, nitrate and oxalate had no effect on the reaction rate. Phosphate interfered in the determination when present in a large excess (1000-fold).Determination of Cobalt in Foodstuffs Table 1 shows the results for the determination of cobalt in foodstuffs such as carrot root and corn grain. The results obtained were compared with those obtained with a standard procedure using nitroso-R salt,5 for the same samples. The results show good agreement between the two methods. Conclusion The method described provides a reliable and simple means of determining trace amounts of cobalt by spectrophotometry. The sensitivity of this method is superior to the standard method based on nitroso-R salt and, because it is free from interference , is a useful method for the precise determination of trace amounts of cobalt in biological materials.Further, it is more sensitive than the procedures developed by Prik and Orlova,697 Costache and co-workers,glO Igov et aZ.11 and Alexiev and Angeloval2 and is fairly selective. One of us (S. J. R.) is grateful to UGC, New Delhi, for financial assistance. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. References Rao, S. J., Reddy, G. S., and Reddy, Y. K., Indian J. Chem., in the press. Rao, S . J., Reddy, G. S., and Reddy, Y. K., Proc. Indian Acad. Sci., in the press. “Beilsteins Handbuch der Organischen Chemie ,” Volume XIII, Edwards Brothers, Ann Arbor, MI, 1943, p. 375. Vogel, A. I., “A Text Book of Quantitative Inorganic Analysis,” Third Edition, Longman, London, 1962, p. 443. Vogel, A. I., “A Text Book of Quantitative Inorganic Analysis,” Third Edition, Longman, London, 1962, p. 795. Prik, G. A., and Orlova, M. N., Uch. Zap. Vludimir. Gos. Pedagog. Znst., 1970, 22, 48. Greinke, R. A,, and Mark, H. B . , Jr., Anal. Chem., 1972,44, 295R. Costache, D., and Popa, G., Rev. Roum. Chim., 1971,16,761. Greinke, R. A., and Mark, H. B., Jr., Anal. Chem., 1974,46, 413R. Costache, D., and Junie, P., Rev. Roum. Chim., 1971,16,597. Igov, R. P., Jaredic, M. D., and Pecev, T. G., Bull. SOC. Chim. Beograd., 1980,45, 365. Alexiev, A. A., and Angelova, M. G., Mikrochim. Acta, 1980, 11, 197. Paper A511 67 Received May 7th, 1985 Accepted July lst, 1985

ISSN:0003-2654    

DOI:10.1039/AN9861100245

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

ANALYST, FEBRUARY 1986, VOL. 111 247 Spectrophotometric Determination of Selenium(lV) with Potassium Butyl Xanthate Nepal Singh* and Arvind K. Garg Department of Chemistry, Hindu College, Moradabad, India A simple and convenient method for the spectrophotometric determination of selenium(lV) has been developed using potassium butyl xanthate as an analytical reagent. The optimum concentration range evaluated by Ringbom's method was found to be 5-12 p.p.m. and the optimum pH range 2.&4.7. The composition of the complex was studied by the molar ratio method and Job's method of continuous variations. The effect of foreign ions on the determination was also studied. Keywords: Selenium(lV) determination: potassium butyl xanthate; visible spectrophotometry The methods recommended for the determination of sel- enium, e.g., iodimetric titration with sodium thiosulphate or ascorbic acid reduction,l have limits of detection of ca. 50 mg and 50 pg, respectively.The American Public Health Associa- tion Standard Methods2 recommend the diaminobenzidine spectrophotometric method with or without distillation. Both are long processes and both diaminochryzin3 and thioacetam- ide4 require reaction at high temperatures. Singh et al.5 separated selenium(1V) and tellurium(1V) morpholine-4- carbodithioates, quantitatively, using TLC and visible spec- trophotometry . In this work potassium butyl xanthate (KBX) was used as a spectrophotometric reagent for the quantitative determina- tion of selenium(1V). The selenium butyl xanthate has a high degree of stability. Experimental Apparatus A Bausch and Lomb Spectronic 20 spectrophotometer was used for the absorbance measurements and a Philips PP 9040 pH meter with glass - calomel electrodes was used for the measurement of pH.Synthesis of the Reagent Potassium butyl xanthate was prepared by dissolving potas- sium hydroxide in distilled water, adding benzene followed by butanol with constant stirring and finally adding carbon disulphide very slowly, maintaining the temperature at ca. 25 "C. (Caution-Benzene is highly toxic and appropriate precautions should be taken.) The proportions of KOH solution, butanol and carbon disulphide were 1 + 1 + 1. The solid product was filtered, washed with diethyl ether and dried under vacuum. Reagents and Solutions Potassium butyl xanthate stock solution, 0.1882% mlV.Prepared by dissolving a known mass of the reagent in distilled water. Selenium dioxide stock solution, 0.02 M. Prepared by dissolving the appropriate amount of selenium dioxide (Cen- tral Drug House, New Delhi), in doubly distilled water and standardising iodimetrically . Determination of Selenium To the selenium dioxide solution containing up to 221 p.p.m. of Se(IV) were added 0.5 ml of acetate buffer (pH 4.0) and 1.5 ml of 0.188% mlV potassium butyl xanthate solution, in a 25-ml glass-stoppered tube. The solutions were mixed and allowed to stand for 40 min for the reaction to go to completion. The selenium butyl xanthate so formed was then * Address for correspondence: 42, Malviya Nagar, Moradabad, Pin 244 001, India.Acetic acid - sodium acetate buffer solution, p H 4.0. shaken with 8 ml of carbon tetrachloride for 2 min to extract the complex. The two phases were separated and the absorbance of the organic phase was measured at 395 nm against a blank obtained by extraction of the reagent containing no selenium. Composition of the Complex When an aqueous solution of KBX and Se(IV) interacted, a yellow oily chelate was formed. The reaction was complete in 35 min and the selenium butyl xanthate formed was com- pletely extractable in carbon tetrachloride. Maximum absor- bance was attained with an eight-fold excess of the reagent. The complex absorbed strongly at 395 nm, the wavelength chosen for further studies. Molar ratio method Equimolar solutions of selenium dioxide and reagent of 0.002 and 0.004 M were used.A series of solutions was prepared, keeping the concentration of selenium ions constant (1 ml of 0.002 and 0.004 M) while varying the concentration of KBX (1-9 ml of 0.002 and 0.004 M) and the pH values of the solutions were adjusted to 4.0 with acetate buffer. The mixtures were allowed to stand for 40 min for completion of the reaction. The selenium butyl xanthate was then extracted with 8 ml of carbon tetrachloride and the absorbance was measured against a blank obtained by extraction of the reagent containing no selenium. The absorbance increased up to an eight-fold excess of the reagent but increased steeply in the presence of up to a four-fold excess of reagent6 (Fig. 1). Jobs method of continuous variations7.g Equimolar solutions of selenium dioxide and KBX were mixed in complementary proportions to a fixed total volume.The procedure adopted for the preparation of the solutions was the same as described earlier. The plot of absorbance 0 2 4 6 8 10 Volume of KBX/ml Fig. 1. Molar-ratio graph for the Se(IV) - KBX system. A, [Se(IV)] = 0.002 M (1 ml) and [KBX] = 0.002 M; and B, [Se(IV)] = 0.004 M (1 ml) and [KBX] = 0.004 M248 ANALYST, FEBRUARY 1986, VOL. 111 0.2 1 a , I I Table 1. Effect of foreign ions on the determination of 15.79 p.p.m. of Se(1V) with KBX I I I I I 2 3 4 5 6 0 ‘ PH Fig. 2. Effect of pH on the absorbance of Se(1V) - KBX versus {[M]4+/([MI4+ + [R])} produced a graph that indicated the formation of a chelate having a selenium to reagent ratio of 1 :4. Eflect of p H The absorption spectra of selenium butyl xanthate at different pH values between 1.8 and 5.5 indicated that complex formation starts at pH 1.8 and produces a maximum at pH 2.0-4.7.Above pH 5.5 the solution becomes red, possibly owing to the separation of selenium from the complex (Fig. 2). Vosburgh and Cooper’s method9 indicated the existence of only one complex at 395 nm, which was studied at pH 4.0. Choice of Solvents For the extraction of yellow oily selenium butyl xanthate, carbon tetrachloride , chloroform , benzene, 1,4-dioxane , pyridine, methanol, ethanol, butanol, dodecylene and hexane solvents were tried. The selenium butyl xanthate was extractable in carbon tetrachloride , benzene and chloroform but the maximum absorbance was obtained in carbon tetrachloride.The colour of the complex became red with methanol, ethanol, butanol and acetone, which may be due to the separation of elemental selenium from the complex. The selenium butyl xanthate was soluble in 1,4-dioxane and pyridine. Linearity and Sensitivity Under experimental conditions it was observed that Beer’s law was obeyed from 5 to 17 p.p.m. of Se(IV), a linear calibration graph being obtained, which passed through the origin. The molar absorptivity calculated from Beer’s law is 6.1 X 102 1 mol-1 cm-1 at 395 nm and Sandell’s sensitivity10 of the colour reaction is 0.1294 pg cm-2. The optimum concentration for the effective spectro- photometric determination of Se(IV), evaluated by Ring- bom’s method11J2 was found to be 5.0-12 p.p.m. Effect of Foreign Ions The interference of foreign ions on the determination of 15.79 p.p.m.of selenium was investigated. Fe(II), Sn(II), Pb(I1) and Zn(I1) form complexes with the reagent, which are not extractable in carbon tetrachloride and therefore do not interfere. The interference of Co was avoided by masking with ethylenediaminetetraacetic acid (EDTA). The effect of foreign ions and their interference limits are shown in Table 1. Results and Discussion Potassium butyl xanthate formed a yellow oily complex with Se(1V) in an acidic medium. The reaction was slow, requiring 30-40 min for full complex formation. The absorbance of the complex in carbon tetrachloride remained stable for at least 24 h, and thereafter the absorbance changed. The maximum absorbance was attained when the reagent was in an eight-fold excess of selenium.Concentration, Se(1V) Foreign ion p.p.m. foundtpg Error, % co2+ . , Cr3+ . . Mg2+ . . Fez+ . . As3+ . . Ba2+ . . Fe3+ . . Sn2+ . . Ti4+ . . Zn2+ I . po43- . . N02- . . s2032- . . C I - . . . . Br- * . . . I- . . . . S042- . . c“ . . CH3 CO 0 - Mo042- . . EDTA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 37 15 34 18 17 34 13 30 8 50 31 50 50 50 59 50 32 50 18 47 15.79 15.49 15.79 15.79 15.79 15.49 15.65 15.28 15.62 15.11 15.79 15.79 15.98 15.49 15.49 15.49 15.49 15.79 15.65 15.28 15.49 Nil Nil Nil Nil -1.9% -1.9% -0.9% -3.2% - 1.1 O/O -4.3% Nil Nil +1.2% -1.9% -1.9% -1.9% -1.9% -0.9% -3.2% -1.9% Nil Table 2. Determination of selenium(1V) Selenium(1V) Selenium(1V) taken, found in complex, p.p.m.p.p,m. Deviation, ‘/o 7.89 7.96 +0.85 9.86 9.88 +o. 12 11.80 11.90 +0.85 15.79 15.84 +0.32 When the complex was subjected to paper chromatography in different organic solvents (light petroleum, carbon tetra- chloride, chloroform) only a single spot developed on the paper, indicating the formation of one complex. The colour of the spot remained unchanged in different solvents but after 12 h the colour of the spot changed from yellow to red, possibly owing to the reduction of the complex to selenium. Determination of Selenium The results are given in Table 2 for the determination of selenium at various levels using the proposed procedure. 1. 2. 3. 4. 5 . 6. 7. 8. 9. 10. 11. 12. References Saren, R. N., and Pandey, S. P., J. Inst. Chem., 1983,55,205. American Public Health Association, American Water Works Association and Water Pollution Control Federation, “Stan- dard Methods for the Examination of Water and Wastewater,” Thirteenth Edition, American Public Health Association, New York, 1971. Brown, R. S., Anal. Chim. Actu, 1975, 74, 441. Futekov, L., and Shocherev, M. G., Tudy Flodiv, Univ., 1972, 10, 81. Singh, N., Rastogi, K . , Kumav, R., and Srivastava, T. N., Analyst, 1981, 106, 599. Yoe, J. H., and Jones, A. L., Ind. Eng. Chem., Anal. Ed., 1944, 16, 111. Job, P., Am. Chem., 1928, 9, 113. Irving, H., and Pierce, I. B., J . Chem. Soc., 1959, 5 , 256. Vosburgh, W. C . , and Cooper, G. R., J . Am. Chem. SOC., 1941, 63, 437. Sandell, E. B., “Colorimetric Determination of Trace Metals,” Interscience, New York, 1959. Ringbom, A., 2. Anal. Chem., 1939, 115, 332. Ayres, G. H., Anal. Chem., 1949, 21, 652. Paper A5127 Received January 28th, 1985 Accepted September 9th, 1985

ISSN:0003-2654    

DOI:10.1039/AN9861100247

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

ANALYST, FEBRUARY 1986, VOL. 111 249 Oxidative Amperometric Flow Injection Determination of Oxalate at an Electrochemically Pre-treated Glassy Carbon Electrode Arnold G. Fogg Department of Chemistry Loughborough University of Technology, Loughborough, Leicestershire LEI1 3TU, UK Rosa M. Alonso Departamento de Quimica, Universidad del Pais Vasco, Bilbao, Spain and Miguel A. Fernandez-Arciniega Departamento de Quimica Analitica, Universidad Autonoma, 28034 Madrid, Spain An effective electrochemical pre-treatment of a newly polished glassy carbon electrode has been shown to be essential if sensitive and reproducible signals are to be obtained in the oxidative determination of oxalate. Pre-treatment at + 1.75 V for 10 min and then at - 1 .O Vfor 1 min in the 0.05 M phosphate buffer (pH 7.0) used as an eluent for the determination was shown to be effective in producing a well shaped hydrodynamic voltammogram, maximum signal size and improved precision at the measurement potential used (1.5 V).Calibration graphs were rectilinear for up to 100 pg ml-1 of oxalic acid dihydrate. Keywords: Flow injection analysis; oxalate determination; glassy carbon electrode; electrochemical pre-treatment; amperometric detection The electrochemical pre-treatment of glassy carbon electrode surfaces has been shown to improve electrode performance in many analytical applications by increasing the electron trans- fer rate and producing a less irreversible electrode reaction.’-9 Better shaped and more reproducible voltammograms are obtained. Treatment is believed to result in an increase in the number of surface quinone and other groups, which catalyse the oxidation or reduction of the determinand.Despite this previous work, electrochemical pre-treatment of glassy car- bon electrodes that are to be used in electrochemical detector cells in HPLC systems does not appear to be practised widely. This must be partly due to the still limited information on applications where electrochemical pre-treatment has been shown to be advantageous, and further work is required to investigate the wider applications and the durability of such pre-treatments. In many HPLC applications of electro- chemical detection currently in use, seemingly good sensitivity and detection limits are obtained simply by using newly polished glassy carbon electrodes. In examples where irrever- sible electrode reactions are being used, however, it may subsequently be found that sensitivities, detection limits and precision can be improved by electrochemical pre-treatment.The sometimes marked effect of electrochemical pre- treatment of glassy carbon electrodes was appreciated fully in this laboratory when an oxidative method of determining sulphite was being developed.8 In 0.05 M sodium carbonate solution chloride was found to catalyse the oxidation and to increase the size of the signal at a glassy carbon electrode that had been newly polished. More importantly, in the absence of chloride the sulphite signals were observed to become increasingly large when the glassy carbon electrode was used for some time without being repolished. This seemed to indicate that some kind of activation of the electrode surface was occurring.With the use of Engstrom’s pre-treatment proced~re,5~7 which involves subjecting the electrode to a period of time at a high positive potential (e.g. +1.5 V) and then at a negative potential, well shaped FIA hydrodynamic voltammograms were obtained with a seven-fold increase in signal size at the measurement potential. Chloride no longer affected the oxidation signal and even more importantly the signals were now highly reproducible. For systems such as the sulphite one electrochemical pre-treatment appears to be essential if reproducible signals are to be obtained. EDTA was found to interfere in the determination of sulphite and indeed could be determined by a similar method using an electro- chemically pre-treated glassy carbon electrode.9 At this time interest was being shown in these laboratories in the determination of oxalate in urine samples.Clinical methods currently in use are apparently time consuming. A method has been published10 in which oxalic acid in urine is separated as calcium oxalate, which after being redissolved is injected on to an HPLC column containing a strong cation exchanger. The eluent contains a quaternary ammonium salt and an ion pair mechanism is suggested for the retention of the oxalate on the column. An amperometric detector with a wax-impregnated graphite working electrode is used. In this work a study has been made of the determination of oxalate using flow injection analysis with amperometric detection; the effect of electrochemical pre-treatment of a glassy carbon electrode on the signals obtained with oxalate is reported here.Experimental A single-channel flow injection system was used. 11 Eluent (pH 7.0 phosphate buffer in the final recommended procedure) was pumped through the system by means of an Ismatec Mini-S pump. Injections were made with a Rheodyne low-pressure sample injection valve (5020) fitted with a 75-pl sample loop and connected to a laboratory-built ampero- metric detector by means of 1 m of 0.58 mm bore Teflon tubing. The amperometric detector,ll which holds a Metrohm glassy carbon electrode in the wall-jet configuration, is used partly immersed in electrolyte of the same composition as the eluent. The platinum counter and saturated calomel reference electrodes are placed in this electrolyte to complete the three-electrode system.The potential of the glassy carbon electrode was maintained at the required potential by means of a PAR 174A polarographic analyser (Princeton Applied Research). Signals were recorded on a Linseis L650 y - t recorder. Reagents Standard oxalic acid dihydrate solution, 0.01 M. Dissolve 0.315 g of oxalic acid dihydrate in water and dilute to 250 ml. Prepare less concentrated solutions by dilution.250 15 10 f . u C 2 3 5 - 0 0 - ANALYST, FEBRUARY 1986, VOL. 111 - - Phosphate buffer solution, pH 7.0. Dissolve 7.80 g of sodium dihydrogen phosphate in 500 ml of water, add 0.1 M sodium hydroxide solution to bring the pH to 7.0 and dilute to 11 with water.Results Preliminary studies were made with a glassy carbon electrode that had not been electrochemically pre-treated. Linear- sweep voltammograms were obtained for a 10-3 M oxalic acid solution in Britton - Robinson buffer. A well formed peak, which was largely independent of pH, was observed at about 1.2 V for buffer solutions of pH 1.8-7.0. This peak was not present in pH 8.3 Britton - Robinson buffer. Later in these preliminary studies it was observed that signals obtained in a pH 7.0 phosphate buffer were slightly larger and occurred at a slightly less positive potential than those obtained in the Britton - Robinson buffers. For this reason a 0.05 M pH 7 phosphate buffer solution was used as the eluent in the flow injection experiments. At flow-rates above 5.8 ml min-1 the signal obtained at + 1.2 V (the measurement potential initially used) using the same volume of the same standard oxalate solution did not increase markedly and a flow-rate of 6.0 ml min-1 was adopted subsequently.At this stage it was observed that after repolishing the electrode the size of the signal obtained was dependent on the length of time the electrode had been used at 1.2 V in the eluent stream before the particular injection was made. For example, in one set of experiments the signal obtained with an electrode immedi- ately after it was polished was 0.63 pA but the size of the signal grew steadily with the length of time the electrode was used at +1.2 V and the signal reached a value of 2.03 pA after 25 min of use. Clearly, some form of electrochemical activation process is occurring during this time.For satisfactory operation of the electrode for the deter- mination of oxalate it is clear that adequate electrochemical pre-treatment of the electrode must be carried out in advance of making analytical measurements if reproducible signals and good accuracy are to be assured. The effects of 5-min electrochemical pre-treatments at various positive potentials between 1.2 and 2.0 V followed by pre-treatment at -1.0 V for 1 min were studied. The signal was seen to increase with a pre-treatment potential of up to 1.4 V but there was only a small increase in the signal at potentials above this. A potential of 1.75 V was chosen for use subsequently and it was found that a completely steady signal was obtained using a 10-min pre-treatment at this potential: no increase in signal size was obtained for pre-treatment times greater than this.A hydrodynamic voltammogram for the determination of oxalic acid at a glassy carbon electrode pre-treated in this way is shown in Fig. 1; clearly the voltammogram is very well shaped and the use of this electrode was found to give highly reproducible signals. A hydrodynamic voltammogram for the determination of oxalic acid at a newly polished glassy carbon electrode obtained under identical conditions is also given in Fig. 1 for comparison. The improvement in the quality of the signal after electrochemical pre-treatment is apparent. In subsequent work at the pre-treated electrode the detection potential was changed to + 1.5 V so that measurements were made on the plateau of the hydrodynamic voltammogram.Typical signals obtained in the range 1-63 pg ml-1 of oxalic acid are given in Fig. 2. The calibration graphs show good rectilinearity up to 100 pg ml-1 but loss of rectilinearity occurs above this concentration; the coefficient of variation at oxalic acid concentrations from 1 to 100 pg ml-1 was found to be typically <1% for ten injections. Discussion The brief study described here has shown that the exact condition of the surface of a glassy carbon electrode that is to A 0.6 1 .o 1.4 PotentialN Fig. 1. Hydrodynamic voltammogram obtained at a glassy carbon electrode, A, that had been polished and then electrochemically pre-treated using the recommended procedure and B, that had been newly polished but not pre-treated.Oxalic acid dihydrate concen- tration = 63 pg ml-1; flow rate = 6.0 ml min-1 I C D 1 10.5 PA tI I Time - Fig. 2. Typical signals obtained for a calibration graph using a pre-treated electrode. Oxalic acid dihydrate concentration: A, 0; B, 6.3; C , 12.6; and D, 63 pg ml-1. Measurement potential = 1.5 V be used for the determination of oxalate by flow injection analysis with amperometric detection is important if meaning- ful, precise and accurate results are to be obtained. It is likely that this is also the case in applying glassy carbon electrodes in amperometric detectors for use in the determination of oxalate by high-performance liquid chromatography. Glassy carbon electrodes that have been cleaned by polishing should be electrochemically pre-treated using a satisfactory pro- cedure such as the one described here before they are used for making analytical measurements.If this is not carried out then conditioning occurs naturally, inefficiently and indetermi- nately as the electrode is being used. This results in a steady increase in signal size, which is most marked immediately after the electrode has been polished and a fresh carbon surface has been produced. The occurrence of this gradual conditioning process in an HPLC system whilst eluent is flowing would result in a change of base line, as the base line also increases on conditioning the electrode, and this might be taken by chromatographers as a “settling down” period for the detector system. It seems likely that electrochemical pre-treatment of glassy carbon electrodes will be shown to affect markedly the redox signals obtained with many other determinands.The general precision and reliability of amperometric detectors should be improved as a result of adequate electrochemical pre-ANALYST, FEBRUARY 1986, VOL. 111 25 1 treatment of the glassy carbon electrode. It is even possible that the catalytic effect of the pre-treated surface will allow other compounds that are not currently considered to be amenable to amperometric detection to be oxidised or reduced more readily at the electrode and to be rendered amenable to determination by this means. R.M.A. thanks the Patronato de la Universidad del Pais Vasco for the award of a research grant. The authors thank Dr. B. F. Rocks for his interest in this project. References 1. 2. Blaedel, W. J., and Jenkins, R. A., Anal. Chem., 1975, 47, 1337. Chan, H. K., and Fogg, A. G., Anal. Chim. Acta, 1979, 111, 281. 3. 4. 5. 6. 7. 8. 9. 10. 11. Gonon, F. G., Fombarlet, C . M., Buda, M. J., andPujol, J . F . , Anal. Chem., 1981, 53, 1386. Van Rooijen, H. W., and Poppe, H., Anal. Chim. Acta, 1981, 130,9. Engstrom, R. C., Anal. Chem., 1982, 54, 2310. Ravinchandran, K., and Baldwin, R. P., Anal. Chem., 1983, 55, 1782. Engstrom, R. C., and Strasser, V. A , , Anal. Chem., 1984,56, 136. Fogg, A. G., Fernandez-Arciniega, M. A., and Alonso, R. M . , Analyst, 1985, 110, 851. Fogg, A. G., FernAndez-Arciniega, M. A., and Alonso, R. M., Analyst, 1985, 110, 1201. Mayer, W. J., McCarthy, J. P., and Greenberg, M. S., J . Chromatogr. Sci., 1979, 17, 656. Fogg, A. G., and Summan, A. M., Analyst, 1984, 109, 1029. Paper A51276 Received July 24th, 1985 Accepted September 6th, 1985

ISSN:0003-2654    

DOI:10.1039/AN9861100249

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

ANALYST, FEBRUARY 1986, VOL. 111 253 2’-Mercapto-4-propylacetanilide: an Alternative to Thionalide for Precipitating Lead from Weak Acid Solution John C. Burridge, Irene J. Hewitt and Hamish A. Anderson The Macaulay Institute for Soil Research, Craigiebuckler, Aberdeen AB9 ZQJ, UK Six compounds containing the thioglycolic acid group (-NHCOCH,SH) were compared with thionalide for precipitating microgram amounts of lead from acetate-buffered solutions at pH 5. Consistently higher recoveries (ca. 80%) were obtained with 2’-mercapto-4-propylacetanilide than with thionalide (ca. 60%). Keywords: Z’-Mercapto-4-propylacetanilide; thionalide; mercaptoacetanilides; lead precipitation; trace elements Thionalide [2’-mercapto-N-2-naphthylacetamide] has been used for many years to determine those elements which can be precipitated, less conveniently, by hydrogen su1phide.l The procedure developed by Scott and Mitchell,2-5 which uses it in combination with 8-hydroxyquinoline and tannic acid (penta- digalloylglucose), illustrates its application to trace element analysis.Thionalide is now difficult to obtain from commercial sources, however, because of the carcinogenic activity of the starting material, 6-naphthylamine , and we have consequently tested a number of possible substitutes for it in Scott and Mitchell’s procedure. The most effective so far identified is sodium sulphide, and a detailed comparison of that reagent with thionalide for the precipitation of lead and tin has been reported.6 Concurrently with that investigation, several com- pounds containing the thioglycolic acid group (-NHCOCH2SH) were synthesised and tested, in a search for a reagent having the same convenience of use as thionalide. One of these, 2’-mercapto-4-propylacetanilide (I) , was found to be more effective than thionalide for precipitating lead from acetate-buffered solution at pH 5.2.The single-ring structure 0 II H N/CLCH2.SH R3QR1 f32 I R1 = R3 = H; R2 = n-C3H7 I t R, = R3 = H; R2 = 0-n-C3H7 I l l IV R1 = R3 = H; R2 = O-CH2-PI.I R1 = R3 = CH3; R2 = H :: V VI R1 = R3 = H; R2 = O-C-CH2.SH R1 = R2 = R3 = H of 2‘-mercaptoacetanilide (VI) was preferred as a basis for the reagents, on health grounds, to the double-ring structure of thionalide. Although a number of N-aryl mercaptoacetamides have been described,7-10 there appear to be no previous reports of the analytical use (or of the preparation) of I.The main purpose of this paper is to draw attention to it because of its potential value for trace element analysis. Experimen taI Preparation of the Alternative Reagents Avoiding the 2-naphthylamine unit contained in thionalide, a number of reagents were synthesised by a standard methodlo: 2’-mercapto-4-propylacetanilide (I), 2’-mercapto-4-propoxy- acetanilide (11), 2,6-dimethyl-2’-mercaptoacetanilide (111), 2’-mercapto-4-benzyloxyacetanilide (IV) , 4-aminophenol N, O-bis-2’-mercaptoacetate (V) and 2’-mercaptoacetanilide (VI). Their common feature was the basic structural unit VI, containing the thioglycolic acid group. Different aryl substitu- ents were used in an attempt to find a lead complex that was relatively insoluble at pH 5.2.New compounds gave satisfac- tory analytical and infrared spectroscopic data consistent with their assumed structures. Precipitation Procedures The precipitation procedures, using 8-hydroxyquinoline plus tannic acid and either thionalide or sodium sulphide, were detailed previously.6 They utilise 8-hydroxyquinoline as the principal reagent to coprecipitate trace elements from solu- tions buffered to pH 5.2 with acetic acid and ammonium acetate, using aluminium as the collector. Tannic acid is needed to precipitate chromium, and thionalide (or sodium sulphide) to precipitate lead. In this work, the reagents substituted for thionalide were added in the same way, i.e. , as 2 ml of a 1% mlV solution in glacial acetic acid.Lead Determination The precipitates were dried at 80 “C, ignited overnight at 450 “C and then analysed by atomic emission spectrometry using a cathode-layer carbon arc.6 All the materials arced had a common matrix of hydrated alumina. However, in view of the earlier finding that calibration was affected by the specific combination of reagents used for precipitation, all the lead values in this work were determined from a calibration based Table 1. Recovery of lead (15-150 pg) by coprecipitation with aluminium from acetate-buffered solutions at pH 5.2, using various reagent combinations as detailed in the text No. of precipi- Mean Standard Precipitant tations recovery, % error, % 8-Hydroxyquinoline (H) . . 3 H + tannicacid(HT) . . . . 7 HT + sodium sulphide .. . . 22 HT+ reagent1 . . . . . . 17 HT+ thionalide . . . . , . 11 HT+reagentII . . . . . . 6 HT+ reagent111 . . . . . . 2 HT+reagentIV . . . . . . 3 HT+reagentV . . . . . . 3 HT+reagentVI . . . . . . 4 5 31 101 75 55 39 28 30 27 27 0.5 2.6 1.8 1.6 2.1 0.8 1.1 1.5 1 .o 0.9254 ANALYST, FEBRUARY 1986, VOL. 111 Table 2. Distribution of recovered lead (45-150 pg) between precipitate and filtrate for several reagent combinations (see text) Precipitate Filtrate No. of Mean Standard Mean Standard Precipitant precipitations recovery, % error, % recovery, ‘3’0 error, Yo 8-Hydroxyquinoline (H) . . 1 6 0.5 92 3.5 H + tannic acid (HT) . . . . 4 37 2.6 70 1.8 HT + sodium sulphide . , . . 3 97 2.4 1 0.2 HT+reagentI . . . . . . 3 95 2.5 12 0.3 HT+ thionalide .. . . . . 6 62 1.7 38 0.8 HT+ reagent11 . . . . . . 3 39 1.1 68 2.3 on standards precipitated with 8-hydroxyquinoline plus tannic acid and sodium sulphide. Results The recovery of lead, added in amounts ranging from 15 to 150 pg, is shown in Table 1 for precipitations with ten different combinations of reagents. Only a few experiments were made with the ineffective reagents (111-VI). These four reagents gave recoveries that were comparable to those that were obtained with 8-hydroxyquinoline plus tannic acid, which were variable (cf., Table 2). Although reagent I was more effective than thionalide, it did not give a full recovery. A few experiments were carried out in which filtrates were also analysed by a further precipitation with 8-hydroxyquinol- ine plus tannic acid and sodium sulphide.6 The distribution of recovered lead between the filtrate and the initial precipitate, obtained with the different reagents, is shown in Table 2.The lead recoveries in the filtrates are not affected by calibration bias,6 and they confirmed not only that reagent I was better than thionalide, but also that neither of these reagents was as good as sodium sulphide. Discussion When sodium sulphide is substituted for thionalide in the procedure described by Mitchell and Scott4 for coprecipitating trace elements with aluminium, some operations must be carried out in a fume-cupboard, which can be inconvenient. One objective of this study, therefore, was to find a reagent that was at least as effective as thionalide for precipitating lead under the general conditions of their procedure.Apart from the ability to precipitate lead from acetate-buffered solutions at pH 5.2, additional requirements for any suitable substitute for thionalide were ( i ) that it should be easy to prepare and purify in 50-g amounts, using readily available materials, and (ii) that it should be stable at room temperature and convenient to use in the open laboratory. The poor recoveries of lead found with compounds 11,111, V and VI were assumed to be due to the solubility of their lead complexes in the reaction medium. Compound IV was found to be poorly soluble in acetic acid and was discounted as a practical reagent for that reason. It is interesting that the replacement of the 4-propyl substituent in I by a 4-propoxy group in I1 gave a lower recovery of lead, as it was expected that the more electronegative substituent would have enhanced the stability of the lead complex.It is concluded that 2’-mercapto-4-propylacetanilide (I), which has no objectionable odour, could be substituted for thionalide in Scott and Mitchell’s procedure if the disadvan- tages of working with sodium sulphide outweigh the need for the complete recovery of lead that can be obtained with that reagent. The usefulness of reagent I as a more general substitute for thionalide in trace-element analysis merits a fuller investigation, but this is not planned by us in the foreseeable future. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. References Welcher, F. J . , “Organic Analytical Reagents,” Volume IV, Van Nostrand, New York, 1948. Scott, R. O., and Mitchell, R. L., J. SOC. Chem. Znd., 1943,62, 4. Mitchell, R. L., and Scott, R. O., J. SOC. Chem. Znd., 1947,66, 330. Mitchell, R. L., and Scott, R. O., Spectrochim. Actu, 1948, 3, 367. Mitchell, R. L., Mikrochem. Mikrochim. Actu, 1951, 36/37, 1042. Burridge, J. C., and Hewitt, I. J., Analyst, 1985, 110, 795. Weiss, U., J. Am. Chem. SOC., 1947, 69, 2682. Bhandari, C. S . , Mehnot, U. S . , and Sogani, N. C., J . Prukt. Chem., 1971, 313, 849. Bhandari, C. S., and Sogani, N. C . , Bull. Acad. Pol. Sci., Ser. Sci. Chim., 1972, 20, 377. Bateja, S., Verma, S., Bhandari, C. S., and Sogani, N. C., J . Prukt. Chem., 1979, 321, 134. Paper A51252 Received July 15th, 1985 Accepted August 23rd, 1985

ISSN:0003-2654    

DOI:10.1039/AN9861100253

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

ANALYST, FEBRUARY 1986, VOL. 111 255 Titrimetric Determination of Catecholamines and Related Compounds Via Bromine Oxidation and Substitution Darwish Amin Department of Chemistry, College of Science, University of Mosul, Mosul, Iraq A titrimetric method for the determination of 0.1-10 mg of catecholamines and related compounds as pure substances and in their dosage forms was investigated and found to offer improvements with regard to ease, speed and accuracy. The method is based on bromine oxidation of the catecholamines (adrenaline, noradrenaline, L-dopa, dopamine and methyldopa) to the corresponding benzoquinones; related compounds (octopamine, tyramine and tyrosine) undergo substitution to the dibromoaryl hypobromites. The benzo- quinones and hypobromites oxidise iodide to liberate stoicheiometric amounts of iodine, which can be determined titrimetrically with thiosulphate, using starch as an indicator.Keywords: Catecholamine determination; bromine oxidation; bromination; titrimetry We have recently reported the bromine oxidation of some organic compounds containing two hydroxy groups in ortho or para positions to each other,l and bromine substitution of hydroxy compounds through the formation of hypobromites.2 We have now extended these techniques to the determination of catecholamines and related compounds. Increasing efforts are being directed towards the develop- ment of simple and reliable analytical techniques for the determination of medically and biologically important cate- cholamines and related compounds. Some titrimetric methods have been reported for the determination of adrenaline and noradrenaline,s6 L-dopa,”g dopamine and methyldopa.6 However, to our knowledge, the literature contains no suitable titrimetric procedure for the determination of octop- amine, tyramine or tyrosine.The purposes of the present investigation were to develop a simple assay for catecholamines and related compounds using bromine water and to apply the procedure to various dosage forms. The method is based on the oxidation of catecholam- ines (adrenaline, noradrenaline, dopa, dopamine and methyl- dopa) with an excess of bromine water to the corresponding benzoquinones; octopamine , tyramine and tyrosine undergo substitution to form the dibromoaryl hypobromites. On treatment of the benzoquinones or hypobromites with iodide, equivalent amounts of iodine are liberated and determined titrimetrically with thiosulphate.The excess of bromine is removed with formic acid. Experimental Reagents All chemicals used were of analytical-reagent grade. Catecholamines and related compounds. Sample solutions containing 2 mg ml-1 of each compound were prepared and diluted with distilled water as required. All compounds (Fluka) had a stated purity of not less than 99% of the active ingredient present, except noradrenaline and methyldopa, which were not less than 98% pure. The tablets were weighed and powdered, dissolved in about 20 ml of 0.1 N hydrochloric acid, filtered into a calibrated flask and diluted to volume with distilled water. Sodium thiosulphate solutions, 0.01 and 0.001 N.These were prepared and standardised against potassium iodate solutions of similar concentration. Other solutions. Solutions of bromine water (saturated), formic acid (concentrated) and starch (1%) were used. Procedure Into a 100-ml Erlenmeyer flask, introduce an accurately measured volume of sample solution containing 0.1-10 mg of the sample compound in the pure form or as the dosage form. Dilute with water to make a total volume of about 15 ml. Add 3 ml of bromine water, stopper the flask and shake it for 1 min. Remove the stopper and destroy the excess of bromine with 2 ml of formic acid (the solution becomes colourless). Add about 0.3 g of potassium iodide and titrate the liberated iodine with 0.01 N sodium thiosulphate solution in the usual way, using starch as an indicator. For low concentrations (less than 2 mg of determinant) use 0.001 N sodium thiosulphate solution.Run a blank determination under identical con- ditions but with no active ingredient present. With this procedure, 1 ml of 0.01 N sodium thiosulphate solution is equivalent to 0.9161 mg of adrenaline, 0.8459 mg of noradrenaline, 0.9860 mg of dopa, 0.9482 mg of dopamine hydrochloride, 1.1462 mg of methyldopa, 0.9482 mg of octopamine hydrochloride, 0.9060 mg of tyrosine and 0.8682 mg of tyramine hydrochloride. Results and Discussion Catecholamines are readily and quantitatively oxidised by bromine water to the corresponding benzoquinones. However, the related compounds (containing one hydroxy group) undergo substitution to the corresponding dibromoaryl hypobromit es. Effect of Amount of Bromine The results showed that 3 ml of saturated bromine water were sufficient for the rapid and quantitative reaction of up to 10 mg of the catecholamine or related compound.Greater excesses of bromine increased the blank value. Complete removal of the excess of bromine was achieved with 2 ml of formic acid. Reaction Time Oxidation of catecholamines or substitution of the related compounds by bromine water proceeds to completion within 1 min; a reaction time of up to 20 min had no significant effect on the results. Effect of Volume of Water The method was applied to the determination of 0.1-10 mg of the compounds of interest in a total volume of about 15 ml.256 ANALYST, FEBRUARY 1986, VOL. 111 However, low results were obtained for amounts less than 0.5 mg of the studied compounds in this volume, which may be attributed to the incomplete reaction in dilute solutions.Interferences All compounds that undergo bromine oxidation or substitu- tion will interfere, e.g., amines, aminophenols, catechol, hydroquinone, resorcinol and cresols. Liberation Time of Iodine It was found that on addition of iodide to the benzoquinone or hypobromite formed, iodine was liberated rapidly and quanti- tatively within 1 min; a standing time of up to 10 min had no effect on the results. The end-point, using starch as an indicator, was stable for about 3 min. It should be noted that the average blank value is 0.03 ml of 0.01 N sodium thiosulphate solution. Accuracy and Precision Under the optimised conditions mentioned, the accuracy and precision of the method were checked.The results (five replicate determinations) are given in Table 1, and indicate that the method is reliable. Table 2 shows application of the method to some pharmaceutical preparations containing catecholamines. We always used at least five tablets for each sample preparation. Proposed Reactions When a compound contains two hydroxy groups in ortho positions (as in catecholamines), it will not combine with bromine but undergoes oxidation to the corresponding benzoquinone ,I which oxidises equivalent amounts of iodide to iodine. However, phenols (octopamine, tyramine and tyrosine) undergo bromination in the unoccupied ortho and para positions to form the dibromo compound, and with an excess of bromine the substitution proceeds further to form the corresponding dibromoaryl hypobromites, which react with iodide to liberate equivalent amounts of iodine.Table 1. Accuracy and precision of the method Amount Compound taken/mg Mean recovery,* % 97.9 99.2 98.3 98.2 99.9 100.2 98.0 100.1 100.2 Coefficient of variation,* % 1.5 0.6 0.3 1.3 0.0 0.1 1.7 0.2 0.4 Adrenaline . . . . 0.1 1 .o 10.0 0.1 1 .o 10.0 0.1 1 .o 10.0 Noradrenaline . . L-Dopa . . . . . Dopamine hydrochloride 0.1 1 .o 10.0 0.1 1 .o 10.0 98.7 99.8 99.2 100.3 100.0 100.3 2.1 0.5 0.3 1.8 0.0 0.2 R R Methyldopa . . . . Octopamine hydrochloride . . OH 0 0.1 1 .o 10.0 98.6 99.8 99.0 2.3 0.7 0.4 R- I R - I Tyramine hydrochloride . . 0.1 1 .o 10.0 0.1 1 .o 10.0 98.5 100.0 100.1 98.5 99.9 98.9 0.6 0.0 0.0 0.6 0.1 0.1 Tyrosine ..... . OH OBr * Five determinations. R R Table 2. Determination of catecholamines in some pharmaceutical preparations Amount R- R- Stated by the Found by the Sample Catecholamine manufacturer present method Adrenaline* Adrenaline 1 mg ml-1 1 f 0.05 mg ml-1 ArterenolS Noradrenaline 1 mg ml-1 1 f 0.05 mg ml-l AldometO Methyldopa 250 mg 250 f 0.7 mg * Adrenaline ampoules, labelled to contain 1 mg ml-1 of adrenal- ine. Gedeon Richter, Budapest, Hungary. j. Epifrin ophthalmic solution, labelled to contain 2.0% of adrenaline. Allergan Pharmaceuticals, Irvine, CA, USA. $ Arterenol ampoules, labelled to contain 1 mg ml-1 of noradrenal- h e . Hoechst Laboratories, Frankfurt, FRG. 0 Aldomet tablets, labelled to contain 250 mg of methyldopa per tablet. Merck, Sharp and Dohme, Rahway, NJ, USA. Epifrin? Adrenaline 2 % 2 f 0.2 Yo OBr 0- R = - CH(OH)CH2NHCH3 (adrenaline),-CH(OH)CHzNH2 ( no rad re na I i n e ) ,- C H ZC H ( N H 2) COO H ( do pa 1 ,-C H zC H N H (dopaminelor CH2C -COCH (methyldopa). R ' = CHzCHzNHz (tyramine), -CH2CH(NHz)COOH (tyrosine) or-CH(OH)CHzNH2 (octopamine) / A H 3 \ N H ~ANALYST, FEBRUARY 1986, VOL. 111 References 1. Amin, D., Analyst, 1985, 110, 211. 2. Amin, D., and Bashir, W. A., Talanta, 1984, 31, 283. 3. Abou Ouf, A., Walash, M. I., and Salem, F. B., Analyst, 1981, 106, 949. 4. “United States Pharmacopeia, Nineteenth Revision,” Mack, Easton, PA, 1975, pp. 274 and 320. 5. Subert, J., Bachrata, M., Knazko, L., and Stemborova, A., Cesk. Farm., 1975, 24, 68. 257 6. 7. 8. Greenhow, E. J., and Spencer, L. E., Analyst, 1973, 98, 485. “British Pharmacopeia 1973,” HM Stationery Office, London, 1973, pp. 263 and 264. Jouin, J., and Sains, E., A n n . Pharm. Fr., 1972, 30, 139. Paper ASJ213 Received June 17th, 1985 Accepted August 19th, 1985

ISSN:0003-2654    

DOI:10.1039/AN9861100255

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

ANALYST, FEBRUARY 1986, VOL. 111 COMMUNICATION 259 Material for publication as a Communication must be on an urgent matter and be of obvious scientific importance. Rapidity of publication is enhanced if diagrams are omitted, but tables and formulae can be included. Communications should not be simple claims for priority: this facility for rapid publication is intended for brief descriptions of work that has progressed to a stage at which it is likely to be valuable to workers faced with similar problems. A fuller paper may be offered subsequently, if justified by later work. Manuscripts are usually examined by one referee and inclusion of a Communication is at the Editor's discre tion. Supported Chemoreceptive Lipid Membrane Transduction by Fluorescence Modulation: the Basis of an Intrinsic Fibre-optic Biosensor Ulrich J.Krull," Chrisula Bloore and Gareth Gumbs Chemical Sensors Group, Department of Chemistry, Erindale College, University of Toronto, Mississauga, Ontario, Canada, L5L I C6 Keywords: Lipid membrane; biosensor; chemoreceptor; fluorescence; evanescent wave Dedicated selective chemical analysis has become one of the forefronts of analytical research, and numerous new technolo- gies and devices have been reported.1J This group previously introduced the manipulation of the properties of ordered bilayer lipid membranes (BLM) for implementation of a transduction mechanism suitable for electrochemical sensing of membrane-embedded selective receptor binding events.s5 Organised lipid structures have tremendous analytical potential as transducers owing to the sensitivity of the membrane structure to interactions with proteins, and as the membrane environment can often optimise the functional binding characteristics of embedded receptors.The recent identification of the lipid membrane parameters that can be readily perturbed by selective receptor complexation4 makes possible their analytical exploitation in non-electrochemical modes. One useful transduction strategy could employ fluorescence variations of membrane-associated fluorophores, which are sensitive to surface charge, dipolar potential and lipid mobility.6 Practical limitations in the exploitation of this membrane technology originate with instability of the lipid membrane and the lack of convenient and reliable instrumen- tation for probing membrane structure.Previous work by McConnell and co-workers7f8 and Andrade et aL9 provides evidence that membranes can be stabilised on to surfaces suitable for direct optical analysis. This work demonstrates that phospholipid membranes can be interfaced to borosilicate glass surfaces in a stable and reproducible manner. The fluorophore 1-anilinonaphthalene-8-sulphonate (ANS) was shown to be sensitive to supported membrane structures, and reported membrane perturbations caused by the membrane probes phloretin and valinomycin. These results demonstrate that a fluorescence-based chemoreceptive transduction strategy is possible. A membrane-coated fibre-optic biosensor that maximises membrane stability and transduction sensitivity is described.* To whom correspondence should be addressed. Experimental Reagents Phosphatidylcholine from egg yolk (Avanti Biochemicals, Birmingham, AL, USA) and cholesterol (Sigma, St. Louis, MO, USA) were used for lipid membrane formation. The fluorescent agent 1-anilinonaphthalene-8-sulphonate (ANS) (Eastman Kodak, Rochester, NY, USA) and membrane probes phloretin and valinomycin (Sigma) were all used as received. All solvents were of analytical-reagent grade, and all water used was purified to remove trace organics and was doubly distilled. Apparatus A Lauda Model 1974 thin-film balance (Brinkman, Toronto, Canada) was used in association with an in-house film liftlo for deposition of lipid monolayers on to glass wafer surfaces. Glass wafers were cut to dimensions of 0.5 X 3 cm from plain borosilicate glass microscope slides of thickness 1.0 mm (J.B. EM Services, Dorval, Canada). Surface analysis was accomplished with an Auto EL I1 ellipsometer (Rudolph Research, Flanders, NJ, USA) and an electron spectrometer, using an Mg Ka source. Contact angle estimates were obtained from an in-house assembly consisting of a horizontally mounted low-power optical microscope with variable stage height, background illuminator and photo- graphic attachment. All measurements were derived from photographic records. Fluorescence studies were made with a Model 204-A spectrofluorimeter (Perkin-Elmer, Norwalk, CT, USA), where the sample wafers were inserted into standard quartz cuvettes. Procedures The glass wafers were scored with a diamond knife and then cleaved to the required size. These wafers were washed with sodium dodecysulphate detergent, treated with chromic acid260 ANALYST, FEBRUARY 1986, VOL.111 for periods of up to 1 h and then rinsed extensively with copious amounts of water. Equal mass ratios of phospholipid and cholesterol were used in deposition experiments. These surfactants were prepared as solutions containing approxi- mately 4 mg of total lipid in 5 ml of hexane. Approximately 70 pl of this solution were deposited slowly by syringe on to the aqueous sub-phase in the Lauda trough. After a period of at least 10 min, the monolayer was compressed to a pre-selected value, which was then maintained by operation in an automatic constant film pressure mode. The sub-phase was varied between experiments, and was chosen to contain distilled water or 0.1 M KCl solution or an aqueous solution of M ANS.Monolayer transfer to a wafer was accomplished by immersion and withdrawal of the substrate through the air - water interface at linear casting rates of approximately 0.5 cm min-l. The wafers were investigated for fluorescence by mounting the latter in conventional quartz cuvettes filled with water. Variability was observed in the fluorescence signal as a function of the orientation angle of the wafers to the source, and experimental results are reported for a constant angle of 45". Phloretin and valinomycin membrane probes were prepared as concentrated methanolic solutions, and were then added to sample cuvettes at concentrations of 10-5 and 10-6 M, respectively.Investigation included the use of blank samples where only methanol was added to cuvettes containing coated wafers. Results and Discussion Lipid Membrane Deposition Of fundamental importance to the proposed sensing strategy is the character of the organised lipid membrane. The primary advantage of this matrix rests in its ability to provide an interior environment for fluorescent agents that is sensitive to surface perturbation, so that membrane surface interactions with analytes of interest can potentially be transduced into fluorescence variations.6 Deposition of conventional BLM-forming phospholipid - steroid mixtures was attempted on prepared glass wafers. Deposition involved transfer of lipid monolayers from an air - water interface to the substrate by substrate penetration through the organised lipid assemblies while maintaining constant membrane surface pressure to ensure membrane structural reproducibility.Transferred membranes remained on substrate surfaces by means of weak physical and chemisorptive processes rather than by covalent linkage. The character of deposited membranes was investigated to establish criteria for the evaluation of film quality, i. e . , surface hydrophobicity, thickness, homogeneity of coverage and lipid packing density. The use of simple visual probes such as a fine dispersion of the mineral talc dusted on to the surface of a compressed floating monolayer readily demonstrated whether monolayer transfer to a substrate actually occurred. Visual results indicated that the maximum surface area of phospholipid monolayer transferred on to the substrates occurred on the first withdrawal from the sub-phase.Ellipsometric investigation determined that subsequent complete cycles of deposition did not result in multilayer formation. Contact angle approximations were obtained and clearly showed that the substrates employed in this work could physically retain a monolayer of lipid with head-groups interfacing to the glass and hydrophobic chains oriented away from the interface. Contact angle estimates range from 10 k 3" for cleaned glass surfaces, to 55 _+ 5" for glass wafers coated with lipid at monolayer pressures maintained at 35 mN m-l. Comparison of coated glass wafer contact angle with monolayer surface pressure in the trough also produced a correlation ranging from approximately 40 to 55" over compression pressures of 2&35 mN m-1.The compression curve for these phospholipid membranes has been carefully studied and showed no discontinuities or inflections before collapse pressures of 40 mN m-1 were attained.5 This implies that the structure of the monolayer on the trough was directly reflected in the character of the supported film, and provides a means of engineering a deposited monolayer structure. Preliminary results of the time dependence of the deposited phospholipid monolayer structure as estimated from contact angle measurements indicated no significant alteration over a period of 24 h. The formation of bilayers or multilayers is more difficult to achieve. The presence of a strong electrolyte sub-phase such as 0.1 M KCI or 10-4 M concentrations of divalent species such as Ba*+ and Ca2+, useful for repetitive stearic acid deposition, 11 does not significantly influence phospholipid multilayer formation.Supported Lipid Membrane Fluorescence A common hydrophobic fluorescent probe of membrane interior hydrophobicity and order is ANS.I2 This probe is believed to reside within the non-polar region of a lipid membrane, although its exact location has not been conclu- sively proved.6 The quantum yield of this particular fluoro- phore is sensitive to molecular mobility within membranes, where increased mobility causes greater loss of excitation energy through non-radiative collisionally based processes. This probe has been previously reported in vesicular phos- pholipid suspensions in aqueous solutions,l2 and here we extended the investigation of its properties to supported phospholipid systems.The fluorescent emission of ANS, which can be intense even in dilute aqueous solution, is centred at approximately 510 nm, and excitation is optimised by sample irradiation at 365 nm. Clean glass wafers placed in cuvettes containing water and irradiated at the latter excitation wavelength in the spectroflu- orimeter indicated no significant emission signal in the 100 nm range centred on 510 nm. Similar results were obtained for monolayer-coated glass wafers. Spectroscopic investigation of glass wafers dipped into an aqueous solution of 10-4 M ANS indicated that fluorescence was present at very low intensity, leading to the conclusion that ANS does not selectively bind to the hydrophilic glass surface.Glass wafer deposition of a lipid monolayer floated over a similar ANS solution produced supported lipid membranes that could generate measurable fluorescence intensities when placed in cuvettes containing water, even though only monolayer coverage was present. The small fluorescence signal profile necessitated measure- ment by integration above an established base line. Widely variable fluorescence intensity could be measured for depo- sition on to different glass wafers at a fixed high monolayer compression pressure of approximately 35 mN m-1. Analysis of the relative fluorescence values clearly indicated a bimodal distribution where approximately 25% of coatings produced fluorescence comparable to blank reference standards, while the other 75% demonstrated consistent and reproducible fluorescence signals.This indicated that even though mono- layers could be transferred to glass wafers by casting from a trough, they could subsequently be floated from the glass when the wafers were immersed in the water in the sampling cuvettes. The physical feature responsible for adhesion or lack of adherence to the glass surface has not yet been identified. It was possible to observe fluorescence from monolayer-coated wafers that were not placed in water, but the emphasis of this work was directed towards solution-based interfacing of supported lipid membranes for sampling purposes. Table 1 represents fluorescence observed from monolayers maintained at significantly different compression pressures during transfer to the glass wafer.These results clearly demonstrate that fluorescence was related to the supported membrane structural characteristics, and that these charac- teristics could be defined on a trough and maintained after monolayer transfer to the support. Time studies indicated thatANALYST, FEBRUARY 1986, VOL. 111 261 Table 1. Fluorescence measurements for lipid monolayers supported on glass Sample description Relative fluorescence signal Clean glass wafer . . . . . . . . . . . . 0-1 Clean wafer after immersion in M ANS . . . . 2 k 1 Monolayer coated wafer cast in for compression pressure: M ANS sub-phase 20 m~ m-1 . . . . . . . . . . . . . . . 1 + 1 25 mN m-l . . . . . . . . . . . . . . 2 + 1 30mNm-l .. . . . . . . . . . . . . 4 + 1 35 mN 14?3 . . . . . . . . . . . . . . Table 2. Fluorescence response of supported lipid monolayers to phloretin and valinomycin Relative fluorescence signal Membrane perturbant Initial After 5 min After 1 h Phloretin M) . . . . 14 f 3 7 2 2 5 5 2 Valinomycin (10-8 M) . . 10 k 1 2 f l 2 i l only a slight decrease in fluorescence signal occurred after periods up to 24 h, and this was most prevalent in membranes cast at high compression pressures. Even though this may be related to loss of structural order in such membranes, further experimentation is required to determine if ANS is slowly extracted from the supported membrane into the aqueous cuvette environment, resulting in a fluorescence decrease. Spectroscopic investigation of the water in the sample cuvettes after fluorescence analysis of wafers showed no ANS fluor- escence; however, the fluorophore must have been in very low concentration in a medium unsuitable for an efficient quantum yield.We have proposed that receptors act primarily on ordered lipid structures by local dipolar potential and packing/fluidity perturbation.5 In order to probe fluorescence alteration of supported lipid monolayers as a function of these latter parameters, the experimentation included membrane interac- tion with the dipolar potential probe phloretin and the structural perturbant valinomycin. Phloretin (relative mole- cular mass 275) is known to partition into the lipid membrane polar zone, and has a large dipole moment (p = 5.6 D) known to align against the inherent transmembrane potential. 13 Valinomycin (relative molecular mass 1111) is a cyclic polypeptide capable of complexing Group I cations and dissolving into the hydrocarbon phase of lipid membranes where it causes structural disorder.14 These two agents at concentrations of 10-5 M and 10-8 M, respectively, provided no fluorescence signal in the required analytical range when they were added as methanolic solutions to the water of a sample cuvette.Table 2 summarises the substantial effect that these membrane probes had on the fluorescence of supported lipid membranes coated at 35 mN m-1 presure. The effect of both probes was to cause a net decrease in fluorescence, implying they caused substantial alteration of membrane structure and/or electrostatics.Fluorescent Membrane-based Chemical Sensor measurement limits. This work has demonstrated that lipid membranes can reproducibly be supported and stabilised on conventional substrates for the purpose of chemical trans- duction by fluorescence. One problem faced by the described fluorescence strategy is the difficulty in achieving a suitable device configuration that would eliminate complications of optical system interfacing to the proposed chemically selective surface. We are currently investigating an intrinsic method based on fibre-optic technology. The chemical - instrumental interface consists of a receptor and fluorophore-modified lipid membrane, coated directly on to the surface of an optical fibre. Monochromatic light chosen to stimulate the fluoro- phore is transmitted by total internal reflection through the optical fibre.The evanescent wave experienced at each reflection acts as the optical stimulant penetrating completely through the membrane coating. The resulting fluorescence, controlled by selective receptor binding of stimulant, is transmitted by the fibre to a detector. Appropriate fluoro- phores can be embedded in the lipid matrix or conjugated to the selective receptor. This provides a practical sensor device strategy optimising the use of stabilised lipid membrane technology. We are indebted to the Department of Chemistry, University of Toronto, for providing a fellowship to C. B., and to Allied Canada Inc. for providing access to the equipment used for monolayer casting and characterisation. 1.2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. References Thompson, M., and Krull, U. J., Trends Anal. Chem., 1984,3, 173. Seiyama, T., Fueki, K., Shiokawa, J., and Suzuki, S . , Editors, “Proceedings of International Meeting on Chemical Sensors, Fukuoka, Japan,” Elsevier-Kodansha, New York, Tokyo, 1983. Thompson, M., and Krull, U. J., Anal. Chim. Actu, 1983,147, 1. Krull, U. J., and Thompson, M., Trends Anal. Chem., 1985,4, 90. Krull, U. J., Thompson, M., Vandenberg, E. T., and Wong, H. E., Anal. Chim. Actu, 1985, 174, 83. Yguerabide, J., and Foster, M. C., in Grell, E., Editor, “Molecular Biology, Biochemistry and Biophysics,” Vol. 31, Springer-Verlag, New York, 1981, p. 200. Weis, R . M., Balakrishnan, K., Smith, B. A., and McConnell, H. M., J. Biol. Chem., 1982, 257, 6440. Tamm, L . K., and McConnell, H. M., Biophys. J., 1985,47, 105. Andrade, J. D., Van Wagenen, R. A., Gregonis, D. E., Newby, K., and Lin, J.-N., IEEE Trans. Electron Devices, 1985, 32, 1175. Krull, U. J . , Thompson, M., and Wong, H. E., Analyst, 1985, 110, 1299. Blodgett, K. B., and Langmuir, I . , Phys. Rev., 1937, 51, 964. Radda, G . K., and Vanderkooi, J., Biochim. Biophys. Acta, 1972, 265, 509. Reyes, J., Greco, F., Motais, R., and Latorre, R., J. Membr. Biol., 1983, 72, 93. Thompson, M., and Krull, U. J., Anal. Chim. Acta, 1982,141, 33. The chemistry of the sensing strategy is the crucial factor determining the selectivity and sensitivity of the technique, although instrumental configuration will also impose physical Paper A5/351 Received October 2nd, I985

ISSN:0003-2654    

DOI:10.1039/AN9861100259

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

ANALYST, FEBRUARY 1986, VOL. 111 263 BOOK REVIEWS Gas und Flussigkeits-Adsorptionschromatographie A. V. Kiselev and Ja. I. JaSin. Pp. 391. VEB Deutscher Verlag der Wissenschaften. 1985. ISSN 0079 1997. The present book is a German translation (published in East Germany) of a work that was issued originally in Moscow in 1979. It contains ten chapters-an Introduction to the subject, followed first by chapters on gas (including capillary) chrom- atography, and then by chapters covering liquid adsorption chromatography. There are sections dealing with apparatus, methods, selectivity for both techniques and a chapter about the theory and use of the Henry constant and Kovats retention index, and their relationship with different types of column packings. The efficiency and applications of each of the techniques are covered.The book is described as being on the theory and practice of adsorption chromatography and it is fair to say that the theoretical aspect is covered particularly well. Practical details are given where necessary and are illustrated by good, clear chromatograms. The applications given are drawn from the fields of biochemistry, food, pharmaceuticals and related areas, and include a considerable number of helpful examples of the analysis of gases and solvents. Other types of compound to which reference is made include glycosides, phenols, vitamins, antibiotics and diazepam and its derivatives, with many others for which tables of references are provided. The volume contains 184 illustrations and 35 tables. It has between 1500 and 2000 references, which are arranged conveniently by chapter at the end of the book, with a list provided at the start of the symbols used in the text and a reasonable index.Essentially it is a review volume with theory rather than a practical handbook, but it should find a place on the library shelf of both academic and industrial research establishments and be a very useful addition to the reference libraries of German-speaking chromatographers. D. Simpson Stripping Analysis. Principles, Instrumentation, and Applications Joseph Wang. Pp. viii + 160. VCH Publishers, Deerfield Beach. 1985. Price DM120. ISBN 0 89573 143 6 (VCH Publishers); 3 527 261 92 3 (VCH Verlagsgesellschaft, Weinheim). This is the first monograph on electrochemical stripping techniques to appear for well over a decade and the author has been very successful in his aim of summarising recent advances in the field.He reports extensively on the development of both commercial and laboratory-constructed instrumentation, with emphasis on rapid stripping techniques, such as square- wave voltammetry, fast linear scan voltammetry and poten- tiometric stripping analysis, and also instrumental com- puterisation. A comprehensive account is given of the increasing importance of flow systems as opposed to batch systems. The design of the various flow cells used in electrochemical stripping measurements, such as wall-jet cells, thin-layer cells and dual electrode cells, is described in detail and the advantages and drawbacks of the different designs are discussed briefly. Reduction of stripping potential overlap by medium exchange in flow systems is illustrated by means of several examples.Two of the most recent trends in stripping analysis, namely the determination of electroactive organic compounds or metal chelates by means of cathodic stripping voltammetry, subsequent to adsorptive accumula- tion, and the use of chemically modified solid electrodes, are also illustrated. In the final chapters a complete survey of all applications, prior to 1985, of stripping analysis is given, focusing on trace metal determinations in body fluids, foodstuffs and environmental samples. The book can be highly recommended to those who are interested in recent developments in stripping analysis and to those considering the commencement of stripping analysis in their laboratories.Those who are interested in a detailed theoretical treatment of the theory of stripping analysis will need to look elsewhere. Daniel Jagner Maximum Concentrations at the Workplace and Bio- logical Tolerance Values for Working Materials 1984 Commission for the Investigation of Health Hazards of Chemical Compounds in the Work Area. Report No. XX. Pp. 80. Verlag Chemie. 1985. Price DM23. ISBN 3 527 27332 8 (Verlag Chemie); 0 89573 379 X (Verlag Chemie International). Practitioners in the field of health and safety at work will be aware of the benefits of examining the policies and guidelines developed in other countries. This is especially true for air pollution, where too often the real situation can be compared with the idealised guidelines only with difficulty and a great deal of interpretive skill.As a practitioner in Britain who for official purposes must work with the Occupational Exposure Limits of the Health and Safety Executive, I wholeheartedly recommend this, the German equivalent, as a valuable adjunct and aid. The essential list of compounds and their maximum concentrations are well presented, with good spacing, struc- tural formulae and even line drawings for the more complex molecules. Where appropriate, compounds are flagged with symbols to indicate the dangers from sensitisation or cutaneous absorption. Also, the saturated vapour pressures for highly volatile compounds at 20 “C are quoted to indicate the potential hazard due to vaporisation from open vessels.Five categories of excursions above the time-weighted average concentrations are defined, and allocated as appropriate within the tables. Nine pages are given to a concise description and discussion of mutagens and carcinogens and how to assess them. For substances that cannot be regarded as safe at any level of exposure, this section has a table of “Technical Guiding Concentrations,” i.e. , concentrations at which the degree of risk may be regarded as acceptable. Special discussion is also devoted to the assessment and interpretation of hazards from dusts (7 pages), mixtures of compounds such as gasoline, turpentine and pyrolysis products, and metal working fluids. Consideration is given to the problem of dealing with compounds that promote a feeling of nausea without at the same time constituting a toxic hazard.One could wish for more guidance on this subject, but it is at least advantageous that it has been mentioned. The section on “Biological Tolerance Values” has 6 pages. It deals with blood, plasma, urine and alveolar air, and includes advice on sampling technique. In summary, this report is notable for its clear presentation, the concise and comprehensive treatment of its subject and its willingness to treat those difficult areas of assessment where the real situations cannot be fitted neatly into any idealised guidelines. B. I. Brookes264 ANALYST, FEBRUARY 1986, VOL. 111 trates on regulatory aspects. Each chapter is written by experts in the relevant disciplines and the examples, based on their own experiences and expertise, are drawn from throughout Europe.The effect of this multi-national approach is to produce some sections where the translation into English makes hard reading. With such a diversity of topics contained in one book, individual chapters could easily be criticised by specialists in each subject. Thus, for example, it is surprising that with the considerable interest in acid deposition, a review of sampling and analysis techniques for atmospheric acids occupies just over five pages of text, including the reference list. The reviewer’s criticisms are relatively minor, however, and there is no doubt that the book offers a well presented and valuable overview of an important interdisciplinary subject. Libraries serving environmental researchers will surely find a demand for this text, which it is pleasing to note has many of the lists of literature references citing recent work. R. S. Barratt Pollutants and Their Ecotoxicological Significance Edited by H. W. Nurnberg. Pp. xiv + 515. Wiley. 1985. Price f49.50. ISBN 0 471 90509 7. Increasingly there is concern about the proliferation of new chemical substances and their potential risks to humans and to the environment. One reaction to this concern is the develop- ment of legislation requiring new chemical products to be investigated thoroughly before manufacture, importation or marketing. The implications for the testing of these chemicals are considerable in view of the complex ecological chemistry involved. The 30 chapters in this book attempt to give a comprehensive review of the problems associated with this field of applied trace chemistry. The book is divided into four sections, of which the first three deal with the atmosphere, the aquatic environment and the terrestrial environment. The final and smallest concen-

ISSN:0003-2654    

DOI:10.1039/AN9861100263

出版商:RSC    

年代:1986

数据来源: RSC

摘要:

264 ERRATUM ANALYST, FEBRUARY 1986, VOL. 111 Automatic Two-stage Thermal Desorption Gas Chromatography for Low-volatility Organic Vapour Determination J. F. Alder, E. A. Hilderbrand and J. A. W. Sykes Analyst, 1985, 110, 769-773 Page 773, Tables 5 and 6: the final sentence of the title should read “Cold trap and ATD 50 tubes packed with silanised glass-wool’’ in both instances.

ISSN:0003-2654    

DOI:10.1039/AN9861100264

出版商:RSC    

年代:1986

数据来源: RSC