摘要:

FOURTH SYMPOSIUM ON KINETICS IN ANALYTICAL CHEMISTRY(KAC '92)September 27-30,1992Erlangen, GermanyYou are cordially invited to participate in the 4th Meeting on Kinetics in Analytical Chemistry (KAC). Thisconference is intended to continue the series of KAC meetings which started in Cordoba, Spain (1983) andthen Preveza, Greece (1986) and Cavtat, Yugoslavia (1989).The meeting will be particularly important to all scientists involved in kinetic techniques in analyticalchemistry and will provide a forum where interesting and useful applications of kinetics can be reported anddiscussed. In addition to keynote lectures there will be poster sessions and a social programme for delegatesand their guests.The scientific programme will include the following themes:Kinetic determination of substances because of their catalytic or inhibitory effectDifferential rate methodsElectrochemical methods (e.g., electrocatalysis and chemically modified electrodes)Flow methods (e.g., flow injection)Chromatographic methodsChemical and biochemical sensorsApplications of luminescenceKinetics in pharmaceutical analysisInstrumentationEnvironmentalScientific Committee:Professor K.Carnmann (Munster, Germany)Professor U . Nickel (Erlangen, Germany)Professor M. D. Perez-Bendito (Cordoba, Spain)Professor H. A. Mottola (Stillwater, USA)Professor H . L. Pardue (West Lafqette. USA)Professor G. Werner (Leipzig, Germany)Local Organizers:Professor Dr. U. Nickel (Erlangen)Mrs. B. Thormann (Erlangen)Further information can be obtained from the following address:Professor Dr.Ulrich Nickel, Institute of Physical and Theoretical Che.mistry, Egerlandstrasse 3,D-W-8520 Erlangen, Germany.Telephone: +49 9131 857334Telefax: +49 9131 858307The KAC '92 meeting is held in cooperation with the "Fachgruppe 'Analytische Chemie' in der GesellshafrDeutscher Chemiker "The XXVIII Colloquium Spectroscopicurn Internationalewill be held inThe University of York, United KingdomJune 29-July 4,1993This traditional biennial conference in analytical spectroscopy will once again provide a forum for atomic, nuclear and molecularspectroscopists worldwide to encourage personal contact and the exchange of experience.Participants are invited to submit papers for presentation at the XXVm CSI, dealing with the following topics:Basic Theory, Techniques and Instrumentotion of- Applicatwns of Spectroscopy in the Analysis of-Computer Applications and ChemometricsLaser Spectroscopy Environmental SamplesAtomic Spectroscopy (Emission, Absorption, Fluorescence)Electron Spectroscopy Geological MaterialsGamma Spectroscopy Industrial ProductsMass Spectrometry (Inorganic and Organic)Methods of Surface Analysis and Depth ProfilingMolecular Spectroscopy (UV, VIS, IR)Mossbauer SpectroscopyNuclear Magnetic Resonance SpectrometryPhotoacoustic SpectrometryRaman SpectroscopyX-ray SpectroscopyBiological SamplesFood and Agricultural ProductsMetals AlloysPLENARY AND INVITED SPEAKERSThe scientific programme will consist of Plenary and Invited Speakers.To date the following scientists have accepted invitations topresent keynote lectures:Pknaty- Invited-M L Gross, Lincoln, NER E Hester, YorkC L Wilkins, Riverside, CAJ D Winefordner, Gainesville, FLF C Adams, AntwerpF V Bright, Buffdo, NYJ A Caruso, Cincinnati, OHB T Chait, New York, NYR Donovan, EdinburghD E Games, SwanseaD L Glish, Oak Ridge, TNP Hendra, SouthamptonF Hillenkamp, MunsterJ A Holcombe, Austin, TXJ Refher, Stanford, CTB L Sharp, LoughboroughM Sigrist, ZurichM Thompson, LondonJ C Vickerman, ManchesterPRE- and POST-SYMPOSIAIn connection with the XXVIII CSI a number of symposia and workshops will be organized.EXHIBITIONThe conference will feature an exhibition of the latest instrumentation.ACCOMMODATIONAccommodation has been reserved on campus and in the halls of residence, although hotel accomodation in York will be available ifdesired.SOCIAL PROGRAMMEThe scientific programme will be punctuated with memorable social events and excursions of scientific, cultural and tourist interest.The social programme is open to all participants and accompanying persons.For further information contact-THE SECRETARIAT XXVIKI CSIDepartment of Chemistry, Loughborough University of Technology, Loughborough, Leicestershire LEll3TU, UK.Telephone: +44 (0) 509 222575; Fax: +44 (0) 0509 233163; Telex: 34319

ISSN:0003-2654    

DOI:10.1039/AN99217BP017

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

The AnalystThe Analytical Journal of The Royal Society of ChemistryAnalytical Editorial BoardChairman: A. G. Fogg (Loughborough, UK)K. D. Bartle (Leeds, UK)D. Betteridge (Sunbury-on-Tharnes, UK)J. Egan (Cambridge, UK)H. M. Frey (Reading, UK)D. E. Games (Swansea, UK)S. J. Hill (Plymouth, UK)D. L. Miles (Keyworth, UK)J. N. Miller (Loughborough, UK)R. M. Miller (Port Sunlight, UK)B. L. Sharp (Loughborough, UK)Advisory BoardJ. F. Alder (Manchester, UK)A. M. Bond (Victoria, Australia)R. F. Browner (Atlanta, GA, USA)D. T. Burns (Belfast, UK)J. G. Dorsey (Cincinnati, OH, USA)L. Ebdon (Plymouth, UK)A. F. Fell (Bradford, UK)J. P. Foley (Villanova, PA, USA)T. P. Hadjiioannou (Athens, Greece)W. R. Heineman (Cincinnati, OH, USA)A. Hulanicki (Warsaw, Poland)I.Karube (Yokohama, Japan)E. J. Newman (Poole, UK)T. B. Pierce (Harwell, UK)E. Pungor (Budapest, Hungary)J. RSiiCka (Seattle, WA, USA)R. M. Smith (Loughborough, UK)M. Stoeppler (Julich, Germany)J. D. R. Thomas (Cardiff, UK)J. M. Thompson (Birmingham, UK)K. C. Thompson (Sheffield, UK)P. C. Uden (Amherst, MA, USA)A. M. Ure (Aberdeen, UK)P. Vadgama (Manchester, UK)C. M. G. van den Berg (Liverpool, UK)A. Walsh, K.B. (Melbourne, Australia)J. Wang (Las Cruces, NM, USA)T. S . West (Aberdeen, UK)Regional Advisory EditorsFor advice and help to authors outside the UKProfessor Dr. U. A. Th. Brinkman, Free University of Amsterdam, 1083 de Boelelaan, 1081 HVProfessor Dr. sc. K. Dittrich, Institute for Analytical Chemistry, University Leipzig, Linnestr.3,Professor 0. Osibanjo, Federal Environmental Protection Agency, Federal Secretariat, PhaseProfessor K. Saito, Coordination Chemistry Laboratories, Institute for Molecular Science,Professor M. Thompson, Department of Chemistry, University of Toronto, 80 St. GeorgeProfessor Dr. M. Valchrcel, Departamento de Quimica Analitica, Facultad de Ciencias,Professor J. F. van Staden, Department of Chemistry, University of Pretoria, Pretoria 0002,Professor Yu Ru-Qin, Department of Chemistry and Chemical Engineering, Hunan University,Professor Yu. A. Zolotov, Kurnakov Institute of General and Inorganic Chemistry, 31 LeninAmsterdam, THE NETHERLANDS.D-0-7010 Leipzig, GERMANY.II, 1st Floor, IKOYI, Lagos, P.M.B. 12620, Lagos, NIGERIA.Myodaiji, Okazaki 444, JAPAN.Street, Toronto, Ontario M5S 1A1, CANADA.Universidad de Cordoba, 14005 Cordoba, SPAIN.SOUTH AFRICA.Changsha, PEOPLES REPUBLIC OF CHINA.Avenue, 117907, Moscow V-71, RUSSIA.Editorial Manager, Analytical Journals: Judith EganUS Associate Editor, The AnalystDr J. F.TysonDepartment of Chemistry,University of Massachusetts,Am herst MA 01 003, USAFax 41 3 545 4490Editor, The AnalystHarpal S. MinhasThe Royal Society of Chemistry,Thomas Graham House, Science Park,Milton Road, Cambridge CB44WF, UKTelephone 0223 420066. Telephone413 5450195Fax 0223 423623. Telex No. 81 8293 ROYAL.Senior Assistant EditorPaul DelaneyAssistant EditorsBrenda Holliday, Paula O'Riordan, Sheryl WhitewoodEditorial Secretary: Claire HarrisAdvertisements: Advertisement Department, The Royal Society of Chemistry, BurlingtonHouse, Piccadilly, London, W1V OBN.Telephone 071-437 8656. Telex No. 268001.Fax 071 -437 8883.The Analyst (ISSN 0003-2654) is published monthly by The Royal Society of Chemistry,Thomas Graham House, Science Park, Milton Road, Cambridge CB4 4WF, UK. All orders,accompanied with payment by cheque in sterling, payable on a UK clearing bank or in USdollars payable on a US clearing bank, should be sent directly to The Royal Society ofChemistry, Turpin Distribution Services Ltd., Blackhorse Road, Letchworth, Herts SG6 1 HN,United Kingdom. Turpin Distribution Services Ltd., is wholly owned by the Royal Society ofChemistry. 1992 Annual subscription rate EC f276.00, USA $589, Rest of World f310.00.Purchased with Analytical Abstracts EC f604.00, USA $1 299.00, Rest of World f669.00.Purchased with Analytical Abstracts plus Analytical Proceedings EC f712.00, USA $1 527.00,Rest of World f791 .OO.Purchased with Analytical Proceedings EC f351 .OO, USA$749.00, Restof World f395.00. Air freight and mailing in the USA by Publications Expediting Inc., 200Meacham Avenue, Elmont, NY 11003.USA Postmaster: Send address changes to: The Analyst, Publications Expediting Inc., 200Meacham Avenue, Elmont, NY 11003. Second class postage paid at Jamaica, NY 11431. Allother despatches outside the UK by Bulk Airmail within Europe, Accelerated Surface Postoutside Europe. PRINTED IN THE UK.Information for AuthorsFull details of how to submit material forpublication in The Analyst are given in theInstructions to Authors in the January issue.Separate copies are available on request.The Analyst publishes papers on all aspects ofthe theory and practice of analytical chemistry,fundamental and applied, inorganic andorganic, including chemical, physical, biochem-ical, clinical, pharmaceutical, biological,environmental, automatic and computer-basedmethods.Papers on new approaches to existingmethods, new techniques and instrumentation,detectors and sensors, and new areas of appli-cation with due attention to overcoming limita-tions and to underlying principles are a l l equallywelcome. There is no page charge.The following types of papers will be con-sidered:Full research papers.Communications, which must be on anurgent matter and be of obvious scientificimportance.Rapidity of publication is enhancedif diagrams are omitted, but tables and formulaecan be included. Communications receive pri-ority and are usually published within 5-8weeks of receipt. They are intended for briefdescriptions of work that has progressed to astage at which it is likely to be valuable toworkers faced with similar problems. A fullerpaper may be offered subsequently, if justifiedby later work. Although publication is at thediscretion of the Editor, communications will beexamined by at least one referee.Reviews, which must be a critical evaluationof the existing state of knowledge on a par-ticular facet of analytical chemistry.Every paper (except Communications) will besubmitted to at least two referees, by whoseadvice the Editorial Board of The Analystwill beguided as to its acceptance or rejection.Papersthat are accepted must not be published else-where except by permission. Submission of amanuscript will be regarded as an undertakingthat the same material is not being consideredfor publication by another journal.Regional Advisory Editors. For the benefit ofpotential contributors outside the United King-dom and North America, a Group of RegionalAdvisory Editors exists. Requests for help oradvice on any matter related to the preparationof papers and their submission for publicationin The Analyst can be sent to the nearestmember of the Group.Currently servingRegional Advisory Editors are listed in eachissue of The Analyst.Manuscripts (four copies typed in double spac-ing) should be addressed to:Harpal S. Minhas, Editor, The Analyst,Royal Society of Chemistry,Thomas Graham House,Science Park, Milton Road,CAMBRIDGE CB4 4WF, UK or:Dr. J. F. TysonUS Associate Editor, The AnalystDepartment of ChemistryUniversity of MassachusettsAmherst MA 01003, USAParticular attention should be paid to the use ofstandard methods of literature citation, includingthe journal abbreviations defined in ChemicalAbstracts Service Source Index. Wherever pos-sible, the nomenclature employed should fol-low IUPAC recommendations, and units andsymbols should be those associated with SI.All queries relating to the presentation andsubmission of papers, and any correspondenceregarding accepted papers and proofs, shouldbe directed either to the Editor, or AssociateEditor, The Analyst (addresses as above). Mem-bers of the Analytical Editorial Board (who maybe contacted directly or via the Editorial Office)would welcome comments, suggestions andadvice on general policy matters concerningThe Analyst.Fifty reprints are supplied free of charge.@ The Royal Society of Chemistry, 1992. Allrights reserved. No part of this publication maybe reproduced, stored in a retrieval system, ortransmitted in any form, or by any means,electronic, mechanical, photographic, record-ing, or otherwise, without the prior permissionof the publishers

ISSN:0003-2654    

DOI:10.1039/AN99217FX019

出版商:RSC    

年代:1992

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN99217BX021

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

ANALYST, MAY 1992, VOL. 117 83 1 Solute-Mobile Phase and Solute-Stationary Phase Interactions in Micellar Liquid Chromatography A Review Maria Jose Medina Hernandez and Maria Celia Garcia Alvarez-Coque” Departamento de Quimica Analitica, Facultad de Quimica, Universidad de Valencia, 46100 Burjassot, Valencia, Spain Summary of Contents I nt rod ucti on Partition Behaviour Electrostatic and Hydrophobic Interactions Binding, Non-binding and Antibinding Solutes Influence of pH Ionic Strength Selectivity With Purely Micellar Eluents Addition of Modifiers to Micellar Eluents Solvent Strength Selectivity With Hybrid Micellar Eluents Use of Reversed Micelles in Liquid Chromatography References Keywords : Re view; reversed-p hase liquid chroma tog raph y; micellar eluen ts; solute interactions; solvent strength; efficiency Introduction Micellar liquid chromatography (MLC) with normal micelles is an alternative to conventional reversed-phase liquid chro- matography (RP-LC), which uses a surfactant solution above the critical micellization concentration (c.m.c.) as the mobile phase, instead of hydro-organic mixtures.1 4 Micelles are not static, but exist in equilibrium with surfactant monomers above the c.m.c. Adsorption of these monomers on alkyl-bonded stationary phases (e.g., C1, C8 and Clx) could occur in at least two ways:s (a) hydrophobic adsorption, where the alkyl tail of the surfactant would be adsorbed and the ionic head group would then be in contact with the polar solution, giving the stationary phase some ion-exchange capacity with charged solutes; and (6) sylano- philic adsorption, where the ionic head group of the surfactant would be adsorbed, the stationary phase becoming more hydrophobic. For most surfactants and stationary phases, the amount of surfactant adsorbed on the stationary phase remains constant after equilibration once the concentration of surfactant is above the c.m.c.6.7 The complexity of MLC is much greater than conventional RP-LC with hydro-organic solvents, owing to the large number of possible interactions (electrostatic, hydrophobic and steric) with the micellar mobile phase and with the modified stationary phase (Fig.1). None of these interactions can occur for a hydro-organic system.8 Almost any compound can be determined by MLC. The separation of inorganic anions9 and of dithiocarbamates10-11 with a micellar mobile phase of hexadecyltrimethylammonium chloride (CTAC) and bromide (CTAB), respectively, has been reported.Complete resolution of the cis and trans isomers of anionic cobalt(iii)-iminodiacetate was achieved with CTAB, and neutral 4,4’-ethylenedinitrilobispentan-2- one complexes of copper(1i) and nickel(r1) were separated with a sodium dodecyl sulfate (SDS) micellar mobile phase.12 A concentrated micellar solution of Brij 35 has been used to extract aldehydes from tobacco samples, which were sep- arated chromatographically with a more dilute solution of the same surfactant .I3 The activity of folylpolyglutamate hydro- lase in crude tissue extracts was determined after denaturation of the enzyme in SDS, which subsequently served as the micellar solvent system for the chromatographic separation of substrate from reaction products.14 Excellent correlations have been found between capacity factors with a tetradecyl- trimethylammonium bromide mobile phase and the bioactiv- ity of 26 para-substituted phenols,lS and between capacity factors with a 0.03 mol dm-3 SDS mobile phase and the site of action of diuretics along the nephron.14 One of the most interesting applications of MLC is the possibility of determining drugs in biological fluids without previous separation of proteins. 17-24 Micellar solutions of SDS or Brij-35 solubilize the proteins in the biological sample and cause them to be eluted at the front of the chromatogram (Fig. 2). Partition Behaviour Armstrong and Nome27 proposed a three-phase model (sta- tionary phase, and bulk aqueous and micellar pseudo-phases) to explain the chromatographic behaviour of a solute eluted with an aqueous micellar mobile phase containing a surfac- tant.The solutes partition not only between water and the stationary phase, but also inside the mobile phase, between water and the micelle. Hence, elution of a solute in MLC depends on three partition coefficients: that between the stationary phase and water ( Psw), between the stationary phase and the micelle (fSM), and between the micelle and water (f MW). First, Armstrong and Nome,27 and later Arunyanart and Cline Love,28 proposed different models to describe the change in retention of solutes at various micelle concentra- tions. The equations can be rewritten as: *- To whom correspondence should be addressed.832 ANALYST, MAY 1992, VOL.117 0- M ice1 le 43 Bulk water / 0- MicelleG Bulk w r r .................. Fig. 1 Solute-micelle and solute-stationary phase hydrophobic (-) and electrostatic interactions (4) with an anionic surfactant: ( a ) apolar solute; ( b ) anionic solute; and ( c ) cationic solute where k' is the capacity factor, [MI is the total concentration of surfactant in the mobile phase minus the c.m.c., Q, is the ratio of the volume of the stationary phase, Vs, to the volume of the mobile phase, VM, in the column, and KAM is the solute- micelle binding constant. By plotting llk' versus [MI one should obtain a straight line. The values of Psw and KAM are given by the slope and intercept of the plot, respectively.The calculation of Psw requires knowledge of V,, which cannot easily be determined. Usually, the difference between the empty column volume and the packed column void volume is taken as V s . This difference gives an overestimation of Vs because it includes the entire volume occupied by the silica solid support particles rather than just the true stationary phase. The use of such a Vs value can be expected to result in a Psw coefficient that is significantly in error. An approach that completely excludes any volume associated with the base silica material should be used.29 Eqn. (1) can be used to describe the retention of apolar, polar, and even ionic solutes, chromatographed with anionic, cationic and non-ionic ~urfactants.8~13 For high relative molecular mass solutes, intercepts are nearly zero in the llk' Time - Fig.2 (a) Chromatogram of urine. (b) Chromatogram of urine spiked with: 1,30 pg ml-1 of amiloride; 2,5 pg ml-1 of spironolactone; 3, 1.2 pg ml-1 of metandienone; 4, 56 pg ml-1 of amino(pheny1)- pro anol; and 5,30 p ml-1 of clostebol. Mobile phase, 0.1 mol dm-3 SD!? solution with 3 2 pentan-1-01; column temperature, 60 "C; flow rate, 1 ml min-1; UV detection, 260 nm. Reprinted, with permission, from ref. 26 versus [MI plot. A zero intercept requires either that the reciprocal of the phase ratio must be zero, which is not physically possible, or that Psw is extremely large. This is not only physically possible but also consistent with solubility data for compounds that show this behaviour (e.g., alkylbenzene homologues beyond butylbenzene are insoluble in water).The direct transfer of these compounds from the micellar pseudo-phase to the surfactant-modified stationary phase, via reversible sorption of the solute-occupied micelle onto the 'hemimicellar' surfactant-modified stationary phase, has been suggested .29 Electrostatic and Hydrophobic Interactions The non-homogeneous nature of micelles creates a unique situation in which different solutes can experience various micro-environment polarities in a given mobile phase. Reten- tion of a solute will depend on the type of interaction with the micelle and the surfactant-modified stationary phase. Non- polar solutes, such as benzene and toluene, should only be affected by hydrophobic interactions [Fig.l(a)], but for solutes that are charged, two distinct situations can be considered: (i) the charge on the solute and surfactant has the same sign [Fig. l(b)]: or (ii) the charge on the solute and surfactant has the opposite sign [Fig. l(c)].3() The first situation is encountered when an anionic solute is chromatographed with an anionic surfactant or a cationic solute with a cationic surfactant [ e . g . , dissociated phenol and 2-naphthol with SDS, and protonated benzylamine with dodecyltrimethylammonium bromide (DTAB) on a C18 column].3* Electrostatic repulsion from the micelle should notANALYST, MAY 1992, VOL. 117 833 affect retention as the solute would still reside in the bulk mobile phase and, therefore, still move down the column.In contrast, repulsion from the surfactant-modified stationary phase should cause a decrease in retention. Solutes may be eluted in the void volume. However, they may also be retained if hydrophobic interaction with the stationary phase exists. Because of the different hydrophobic interaction, dissociated phenol and 2-naphthol are well separated with SDS. The second situation appears when a solute is chromato- graphed with an oppositely charged surfactant, where elec- trostatic attraction occurs between both species. If the electrostatic attraction with the micelle is complemented by a hydrophobic interaction, the solutes will remain in the mobile phase for a longer period of time, and retention will decrease. However, electrostatic and hydrophobic interactions with the stationary phase may be sufficiently large to offset the increase in micellar attraction and would increase retention.Disso- ciated phenol and 2-naphthol are retained to a greater extent with DTAB than with SDS on a Clx column.31 With an appropriate surfactant, mixtures of polar and apolar solutes can be resolved adequately.3’ For example, dissociated phenol and benzene are not well resolved with DTAB, but are completely resolved with SDS. In contrast, p-nitrophenol and p-nitroaniline are not separated with SDS, but are well resolved with DTAB.31 Binding, Non-binding and Antibinding Solutes The function of the micellar pseudo-phase in MLC has been compared with that of the organic modifier in traditional RP-LC, as for many solutes, an increase in the concentration of surfactant in the mobile phase results in a decrease in the retention of the solutes being separated.However, the eluent strength increases with micelle concentration only if the solute interacts with the micelle in the mobile phase. Armstrong and Stine33 proposed a classification of the solutes into three groups according to their chromatographic properties with a micellar mobile phase: (i) solutes binding to micelles; (ii) non-binding solutes; and (iii) antibinding solutes. Compounds that associate or bind to micelles show decreased retention when the concentration of micelles in the mobile phase is increased ( K A M >0).34 For compounds that do not associate with micelles, retention can remain unaltered by the micelle content of the mobile phase (non-binding, K A M = 0) o r their retention can increase with increasing micelle concen- tration (antibinding, K A M <0).Antibinding results from a compound being strongly excluded or repelled from the micelle. High positive values of K A M have been observed with solutes showing electrostatic interactions ( e . g . , benzyl- trimethylammonium bromide with SDS micelles, and benzoic acid with CTAB micelles), o r with co-micellization [ e . g . , sodium octylbenzenesulfonate with SDS, and cetylpyridinium chloride (CPC) with CTABl.8 The similar K A M values for CPC and benzoic acid in CTAB micellar phases explain the similar retention observed for both solutes; however, the location of the solute in the micelle is very different: benzoic acid is bound onto the micelle surface, in the Stern layer, whereas CPC occupies the same site as a CTAB molecule in the micelle, the alkyl tail being in the micellar core and the polar head in the Stern layer.Negative K A M values apparently have no meaning. However, just as compounds that bind to micelles each have a characteristic positive constant, compounds that are excluded from the micelle may have a characteristic negative constant.33 Most non-binding compounds with anionic micelles are negatively charged and with cationic micelles are positively charged. It is apparent that electrostatic repulsion is an important factor in antibinding behaviour. However, there are also many positively charged compounds that bind to cationic micelles in addition to negatively charged compounds that bind to anionic micelles.It is, therefore, sometimes difficult to predict the exact retention behaviour of an organic ion. Antibinding has never been observed between a charged solute and an oppositely charged micelle.33 These effects cannot be observed by using stationary phases that adsorb an appreciable amount of surfactant, i . e . , Cg- or Clg-bonded phases.33 For these phases, when the stationary phase acquires the same charge as the micelle, and no hydrophobic interaction occurs, similarly charged solutes tend to elute in the void volume of the column. When using a C1 or preferably a cyano-bonded phase, however, one can observe increased retention when eluting. The antibinding phenomenon is useful in MLC because it produces unusual selectivities.35 On a cyano column and with SDS in the mobile phase, neutral phenol behaves as a binding compound, whereas the anionic naphthalene-2-sulfonate behaves as an antibinding compound because electrostatic repulsion is stronger than hydrophobic interaction.In con- trast, for pyrene-1-sulfonate, with a binding behaviour, the larger pyrene moiety should produce a counterbalancing of the electrostatic repulsion and associate more strongly with the micelle. By using a Clx column, where negatively charged surfactant monomers are adsorbed, the elution behaviour of phenol and pyrene-1-sulfonate, where hydrophobic effects would dominate, is very similar to that obtained using cyano columns. The less hydrophobic and negatively charged naphthalene-2-sulfonate elutes very rapidly because of repul- sion from both the micelle and the negatively charged modified stationary phase.Influence of pH Retention of weak organic acids and bases is affected by the pH of the micellar mobile phase. Solute-micelle partition coefficients of the dissociated and undissociated forms are different. Small changes in pH can significantly alter chromat- ographic retention, particularly when the mobile phase pH is close to the p K , vaIue.23,25,36 Therefore, the pH of the micellar mobile phase must be specified when retention data are reported. With anionic or cationic surfactants in the mobile phase, retention can be modified appropriately by working at a pH value where some compounds are ionized. Cyano-bonded and C18 columns interact very differently with surfactant monomers, resulting in a different elution behaviour of organic acids and bases as a function of the micelle concentration in the mobile phase and pH.35 Ionizable compounds on a cyano packing show a different behaviour depending on pH, that is, the slope of the llk’ versus [MI plot can be positive, negative or zero.Hence, for benzoic acid, at pH <4, k’ values decrease, and at pH >4, k’ values increase with increasing SDS concentration. In the intermediate range of pH values, there is an isoeluting point where k’ is completely independent of SDS concentration. This is the pH value at which two species, acid and base, in equilibrium with each other, have the same k’ value. This is analogous to the isosbestic point in spectroscopy and would be expected to give the acid dissociation constant in the micellar medium.For weak acids, such as Bromocresol Green, using a C18 column and increasing SDS concentration in the mobile phase, k’ values decrease in acidic solution where the neutral form is present and remain constant in more basic solution where the anionic acid form is present, electrostatically repulsed by both the negative micelle and stationary phase.3’ The elution behaviour versus pH of protonated bases on Clx columns will be the opposite of that observed on cyano columns. Adsorption of anionic surfactant monomers on the surface of the CI8 stationary phase causes protonated organic bases, such as aniline, to be retained for a longer period of time than the neutral free-base form because of electrostatic attraction.In contrast, for weak bases using cyano columns, it834 ANALYST, MAY 1992, VOL. 117 is found that the largest k' values occur in more basic solution where the neutral, free-base form is present and are smallest in acidic solution where the protonated positively charged form exists, which has favourable electrostatic attraction to the negatively charged micelles.3~ Dependence of k' on pH at a constant value of [MI is sigmoidal if there is no electrostatic repulsion between any of the two acid-base forms and surfactant molecules.35 For example, for 6-thioguanine, a plot of k' versus pH at various concentrations of SDS on a CI8 column reveals that the largest k' values occur in acidic solution, where the protonated form of the drug is present, and the smallest values occur in more alkaline solutions, where its neutral form is present.The observed increase in retention could be ascribed to the fact that electrostatic attractions of the solute with the surface of the surfactant-modified stationary phase are stronger than those with the micelles.22 Ionic Strength If antibinding is chiefly an electrostatic phenomenon, one would expect to see definite salt effects on the magnitude of K A M . The solute is not only excluded from the micelle but also from the double layer around the micelle. The thickness of the electrical double layer decreases with increasing ionic strength, thus allowing hydrophobic interaction of the solute with the micelle.37 Modification of ionic strength might be sufficient to change completely from an antibinding to a binding type behaviour.In the absence of salt, Bromophenol Blue is an antibinding compound, whereas in the presence of as little as 0.02 mol dm-3 NaCl, it appears to be non-binding. At slightly higher salt concentrations, the compound binds strongly to SDS micelles. The binding to SDS micelles increases substan- tially with the concentration of NaC1.37 For most antibinding solutes the value of K A M becomes less negative with increasing ionic strength, although not all compounds show this type of behaviour. For example, the interaction of Naphthol Green B with SDS micelles appears to be largely unaffected by the addition of salt. Conversely, KAM for thiocyanate ion becomes even more negative with increas- ing NaCl concentration.37 In order for the transition from antibinding to non-binding to binding to occur, the solute ion must have sufficient hydrophobic character to associate with the non-polar portion of the micelle, once electrostatic repulsions have been minimized.The behaviour of thiocyanate is difficult to explain in terms of electrostatic criteria. However, it may be possible to rationalize such behaviour by considering the negligible hydrophobic character of this ion. Selectivity With Purely Micellar Eluents A study of the chromatographic behaviour of a homologous series of compounds provides important information that can be used to distinguish retention and selectivity between conventional RP-LC and MLC.38 With hydro-organic mobile phases the logarithm of k' is linearly related to the number of carbons, nc, in a homologous series in the following form:39 log k' = log a(CH2) nc + log fi where a(CH2) =.k', + l/k',, is the hydrophobic or methylene selectivity, that is, the ratio of the retention factors of two solutes that differ from each other by a methylene group, and log fi reflects the specific interactions between the functional group of the molecule and the mobile and stationary phases. The retention behaviour of a homologous series in MLC is, however, very different and k' is linearly dependent on nc:40 k' = Bnc + A (3) where A and B are the intercept and slope, respectively, of the straight line. A plot of log k' versus nc for these systems has a 1.2 1 1 5 0 Number of carbon 3 atoms 5 Fig. 3 a 0.072 mol dm-3 CTAB micellar mobile phase38 Log k' and k' versus number of carbons for alkylbenzenes with clear curvature (Fig. 3).This is probably due to different solute locations in the micelles for different members of a homologous series, which will experience different polarities. This behaviour has been observed with non-ionic, anionic and cationic surfactants.29 For hydro-organic mobile phases, a(CH2) is independent of the type of homologous series for a given mobile and stationary phase system. In contrast, with micelles the a(CH2) values for alkylphenones are larger than those observed for alkylbenzenes. A methylene group of an alkylphenone undergoes a larger change in its microenvironment polarity as it is transferred from micellar eluents to the stationary phase.38 In conventional RP-LC systems, a(CH2) decreases with an increase in modifier concentration in the aqueous mobile phase. For a purely aqueous mobile phase, a(CH2) -4 and for 100% methanol it is about 1.1-1.2.With micellar eluents, the over-all a(CH2) values are much smaller and the variation with micelle concentration is fairly small. Typical selectivities for alkylphenones range from 1.6 to 1.1 for SDS concentra- tions between 0.06 and 0.5 mol dm-3. The net free energy of transfer of a methylene group from the mobile phase to the stationary phase is the difference in the free energy of transfer from the bulk solvent to the stationary phase and from the bulk solvent to the micelle. Micelles and surfactant-modified phases have a similar molecular organization, which leads to low selectivity.38 Functional group selectivity is defined as the ratio of the value of k' of a compound with a substituent to that of the parent compound [ e .g . , a(R) = k'(Bz-R)/k'(Bz) for substi- tuted benzenes]. For a large group of compounds, particularly non-ionic compounds, hydrophobic interactions play a major role; an alkyl-bonded stationary phase modified with surfac- tant monomers makes the environment of the stationary phase similar to that of the micelles. A decrease in functional group selectivity was observed with increasing micelle concentra- tion .4* In general, the change in selectivity with micellar eluents is evident when log k' for different compounds is plotted against log[surfactant] .31 Frequently, the linear plots are not parallel but intersect each other (Fig.4). Reversal of elution order indicates the occurrence of two competing equilibria: solute- micelle association and solute-stationary phase interaction. The parameters Psw and K A M have opposing effects on retention. As Psw increases, retention increases, whereas as KAM increases, retention decreases. At a low micelle concen- tration, the system resembles conventional RP-LC and Psw controls retention. However, as the concentration of surfac- tant is increased, K A M has an increasing effect owing to the larger number of micelles present in the mobile phase. TheANALYST, MAY 1992, VOL. 117 835 1.2 0.8 L m -I 0.4 0 Log CSDS Fig. 4 Log k’ versus log total SDS concentration in the mobile phase for several diuretics: A, probenecid; B, ethacrynic acid; C.chlorthali- done; D, acetazolamide; and E, hydrochIorothiazide4* difference in KAM values among the solutes is sometimes so large that the elution order is reversed. When comparing the elution of any two solutes, selectivity might increase or decrease with micelle concentration depend- ing on the contribution of electrostatic and hydrophobic interactions, which in turn depend on the structure of the compounds. Selectivity also depends on the type of surfactant. This is true for diverse pairs of zwitterionic amino acids and peptides.41 Addition of Modifiers to Micellar Eluents The predominant factor influencing band broadening in MLC appears to be stationary phase mass transfer. The thickness of the stationary phase layer and its viscosity are significantly increased by surfactant adsorption.13 In conventional RP-LC, for well-designed column packings, this term is considered to be negligible. However, it becomes significant for column packings that have a thick stationary phase or poor stationary- phase diffusion, such as some of the original bonded-phase materials that had thick polymeric layers of stationary phase.43 The surfactant-coated column is analogous to those packing materials. The addition of small percentages of propan-1-01 to micellar mobile phases was recommended by Dorsey et al.44 to enhance chromatographic efficiency and to decrease the asymmetry of the chromatographic peaks. Since then other organic solvents have been studied as modifiers in MLC.45 Of these, short-chain alcohols have usually been demonstrated to be the most suitable.4-8 The term hybrid is used for ternary eluents of water-organic solvent-micelles.The general pore shapes of the parent CI8 material appear to be retained in a surfactant-modified stationary phase, indicating that the surfactant produces a thick film on the interior walls of the capillaries, rather than completely filling the pore. Alcohol modifiers reduce the amount of surfactant sorbed on the stationary phase, and the effect is larger with increasing concentration and hydrophobicity of the modi- fie r .4X I n addition to reducing the carbon loading and film thickness, the addition of an alcohol is also expected to influence the fluidityhigidity of the surfactant-C18-bonded ligand structure on the stationary phase, just as its presence alters the fluidity of the micellar aggregate structure.The solute-stationary phase diffusion coefficient should increase as the microviscosity of the phase decreases.45 At a fixed modifier concentration, the efficiency was observed to decrease as the surfactant concentration in the mobile phase was increased, as in the absence of a modifier. Addition of increasing amounts of alcohol at a fixed surfactant concentration increased the efficiency. Therefore, the addi- tive to surfactant concentration ratio is the dominant factor influencing chromatographic efficiency.45 For example, after addition of 5 or 10% methanol to a 0.02 mol dm-3 SDS mobile phase, the number of theoretical plates, n , increased from 300 to 750 and 1120, respectively, for acetone,44 and after addition of 5% pentan-1-01 to a 0.28 mol dm-3 SDS mobile phase, n increased from 1530 to 3570, and from 50 to 950 for benzene and 2-ethylanthraquinone, respectively,45 even though the relative microviscosity of the micellar mobile phase had increased due to the added alcohol.The low efficiency observed for 2-ethylanthraquinone with SDS and without modifier is due to its low solubility in water. The compound can only partition between the rnicelle in the mobile phase and the surfactant-coated stationary phase. Consequently, in order to desorb/exit the stationary phase or micelle in the mobile phase, for both of which this compound has a great affinity, the ionic micelle should be located close to the surfactant-modified stationary phase. However, both are similarly charged.Hence, an electrostatic repulsive barrier to the direct merger of the micellar entity with the surfactant- coated stationary phase will exist, which will impede solute mass transfer across this interface. The greater the fraction of partitioning that must occur via the direct transfer mode, the poorer will be the observed chromatographic efficiency. It should be noted that another reason why alcohols improve the efficiency in MLC, with ionic surfactant micelles, may be because their presence can reduce the net electrical charge density of the ionic micellar surface.45 This would be expected to diminish the repulsive barrier. The presence of alkane additives does not affect the surface charge density; hence, these types of additive do not improve efficiency for very hydrophobic solutes, even though they reduce the extent of surfactant coverage of the stationary phase.This also explains why alcohol additives do not enhance the efficiency in MLC with non-ionic micellar mobile phases.45 Non-ionic micellar surfactants are long-chain tensioactive alcohols themselves and C1-C5 alcohols are not very effective in desorbing these non-ionic surfactants from the surfactant- modified CI8 stationary phase. Such non-ionic surfactants also have no charge and, therefore, no electrostatic charge barrier is encountered in the direct transfer process envisaged for water-insoluble solutes. In fact, the efficiency achieved in very hydrophobic test solutes with a Brij 35 micellar mobile phase is better than that which can be obtained with any ionic micelles.Solvent Strength One of the main disadvantages of purely micellar eluents is their weak solvent strength. The solvent strength can be increased by addition of an alcohol. This significantly alters the equilibrium of the solute away from the micelle towards the bulk aqueous phase, which becomes more non-polar.49 The addition of alcohols to micellar mobile phases would cause changes in certain micellar properties, such as the aggregation number and the c.m.c of the surfactant. However, the observed changes in retention and selectivity in hybrid systems are too large to be explained in terms of changes in micellar properties. The changes might be explained by modification of the micro-environment of the micelles and the stationary phase. Large concentrations of the organic solvent can totally disrupt the micelle structure; hence, the use of alcohols in MLC has been questioned.Some workers have argued that when organic modifiers are used this type of chromatography loses some of its appea1.46836 ANALYST, MAY 1992, VOL. 117 It may seem logical to assume that the separation mechan- ism with a hybrid mobile phase is similar to that with traditional hydro-organic solvents, rather than to that with a purely aqueous surfactant mobile phase. Addition of an organic solvent might reduce the role of micelles and would create a system that is closer to hydro-organic eluents. However, as long as the integrity of the micelles is maintained, addition of an alcohol to a micellar mobile phase will not create a hydro-organic system, even though in hybrid systems interactions are reduced by the presence of an alcohol and the stationary phase is more similar to that in a conventional hydro-organic system.Interestingly, the non-logarithmic behaviour of k’ versus nc for a homologous series is also observed with hybrid micellar systems. This shows that it is micelles that influence the role of an organic co-solvent in the mobile phase.38 With hybrid eluents, solute binding constants to micelles, KAM, and their partitioning into the stationary phase, Psw, both decrease as a result of the addition of an alcohol and, therefore, the eluting power of the mobile phase increases. Binding constants of hydrophobic solutes decrease more than those of hydrophilic solutes with an increasing alcohol concentration.Hence, selectivity is modified. The decrease in Psw may be associated with an alteration of the stationary phase.49.50 In traditional RP-LC with a binary hydro-organic mobile phase, the effect of the organic modifier concentration, 8, on retention is often expressed as log k’ = -SO + log k’0 (4) where S is the solvent strength parameter.51 [The intercept log kro is the logarithm of the capacity factor in a purely aqueous mobile phase (in hydro-organic systems) or at a given micelle concentration without modifier.] This equation is valid only for a small range of concentrations for hydro-organic eluents. In contrast, excellent linearity was observed between log k’ and 8 even for water-rich eluents.40 The over-all S values for micellar hybrid eluents can be ranked as Spentan-1-01 > Sbutan-1-01 > Spropan-1-01 > Srnethanol, which is similar, for the last three solvents, to conventional hydro-organic systems.41.42 The larger S value for pentan-1-01 and butan-1-01 indicates that these solvents interact to a greater extent with micelles.However, the over-all S values for hybrid systems are still smaller than in the absence of a micelle. The S values for a hydro-organic mobile phase change markedly with solute size in a homologous series. Variation in solvent strength with increasing solute size is minimized in the presence of micclles.“) For a hybrid mobile phase, the solvent strength values are almost constant for a group of homologous compounds. The constancy of solvent strength with the variation in solute size is due to localization of solutes in the micelle environments, which reduces the size factor as far as the solvation of the solute by an alcohol is concerned. Selectivity With Hybrid Micellar Eluents In conventional hydro-organic systems, the same ranking of S values for different solutes with butan-1-01, propan-1-01 and methanol is observed, because the three solvents belong to the same selectivity group.In the presence of micelles, this may not be the situation, because these solvents interact differently with micelles. The selectivity of different organic solvents in the presence of micelles might change.41~42 This extends the possibilities for chromatographic separations. In conventional RP-LC a systematic decrease in selectivity occurs as a result of an increase in the volume fraction of the organic modifier, i e ., solvent strength. In the presence of micelles, selectivity may increase, decrease or remain un- changed with solvent strength. In hybrid systems, solvent strength may be enhanced without sacrificing the selectivity. As occurs with purely micellar systems, both the alkyl-bonded phase and the micelles are solvated by an alcohol to a similar extent, so that the methylene groups still find the same difference in their micro-environment polarities on trans- ferrence to the stationary phase. Addition of up to 20% propan-1-01 to a micellar mobile phase of SDS or CTAB has a negligible effect on the hydrophobic selectivity.41 It is not surprising that peculiar behaviour might be observed as different solutes experience different polarities in their immediate vicinities.For example , the carbonyl-group selectivity, cx(CO), defined as k’[C6H5CO(CH2),CH3]/ k’[C6H5(CH2),CH3], changes with the hydrophobicity (or number of carbon atoms in the side-chain) of two homologous pairs for a micellar eluent.41 In this instance, the selectivity decreases with hydrophobicity, approaching unity, whereas it is independent of solute type for a hydro-organic eluent. This can be attributed to the fact that the more hydrophobic pairs are located in the more hydrophobic environment of the micelles and, therefore, experience a smaller change in their micro-environment polarities on being transferred from the mobile to the stationary phase.The a ( C 0 ) selectivity increases on addition of an alcohol to the micellar mobile phase, but decreases with an increase in micelle concentra- tion. The selectivity for a group of peptides and amino acids increases systematically on addition of an organic modifier. Similar behaviour is observed for some of the substituted benzenes, whereas for others, functional group selectivity decreases with increasing solvent strength. For the latter, the largest positive changes of selectivity are observed for compounds that are more hydrophilic o r retained to a lesser extent than benzene (e.g. , benzyl alcohol, acetanilide, phe- nol), whereas for compounds that are more hydrophobic or retained to a greater extent than benzene, selectivity de- creases with increasing solvent strength (e.g., anthracene, naphthalene, butyrophenone) .41 Although the solvent strength can be increased adequately in certain instances by increasing the micelle concentration, chromatographic efficiency in MLC usually deteriorates at higher micelle concentrations. Addition of an organic solvent to the micellar eluent may give an adequate eluent strength; it also improves the chromatographic efficiency, and in many instances can lead to an enhancement of separation selectivity. However, use of organic modifiers is not always appropriate. In certain instances selectivity enhancement might not lead to an improvement in resolution if retention falls below the optimum k’ range as a result of an increase in eluent strength.52 In other instances, the addition of an organic solvent to micellar eluents may have a beneficial effect on retention, but the efficiency may remain low.Use of Reversed Micelles in Liquid Chromatography In conventional normal-phase high-performance liquid chro- matography a problem exists with reproducibility over time because of the variation in the water content of the mobile phase. The use of reversed micelle mobile phases [ e . g . , sodium bis(2-ethylhexyl)sulfosuccinate, known as Aerosol OT, AOT] offers a unique solution to this problem owing to the ability to solubilize water in the interior of the micelle structure.53 However, the presence of a surfactant leads to a loss of efficiency, probably because of the localization of polar solutes in the hydrophilic core with a slow transfer step out of the micelle.The use of reversed micelles in supercritical fluid chromato- graphy (SFC) provides another way of modifying the mobile phase, and is an alternative to polar and modified fluids.54.55 In SFC, solubilization of large polar molecules is possible and the behaviour of the micellar mobile phase may be changed by control of temperature and pressure. Retention times of polar solutes are substantially reduced with a reversed micellarANALYST, MAY 1992, VOL. 117 837 mobile phase and solutes that are more polar can be separated. Reversed micelle chromatography may be better adapted to supercritical fluids owing to the gain in efficiency at higher temperatures. The higher diffusion rates and lower viscosities of supercritical fluids, compared with those of liquids at the same temperature, may enhance micelle diffusion rates, leading to an increased over-all efficiency.This work was supported by the CICYT Project DEP89-0429. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 References Hinze, W. L., in Ordered Media in Chemical Separations, eds. Hinze, W. L.. and Armstrong, D. W., ACS Symposium Series, American Chemical Society, Washington, DC, 1987. vol. 342, Dorsey. J. G., Adv. Chromatogr., 1987. 27, 167. Berthod. A., and Dorsey, J. G.. Analusis, 1988, 16, 75. Khaledi, M. G., BioChromatography, 1988. 3, 20. Berthod, A., Girard, I . , and Gonnet, C., in Ordered Media in Chemical Separations, eds. Hinze. W. L., and Armstrong, D. W.. ACS Symposium Series, American Chemical Society, Washington, DC.1987, vol. 342, p. 130. Dorsey, J. G., Khaledi, M. G.. Landy, J. S . , and Lin, J.-L., J. Chromatogr., 1984, 316, 183. Berthod, A., Girard, I., and Gonnet, C., Anal. Chem., 1986, 5%. 1356. Berthod. A.. Girard, I., and Gonnet, C., Anal. Chem., 1986, 58, 1359. Mullins. F. G. P., and Kirkbright, G. F., Analyst, 1984, 109, 1217. Kirkbright, G. F., and Mullins, F. G. P., Analyst. 1984, 109, 493. Mullins, F. G. P., and Kirkbright, G. F., Analyst, 1986, 111, 1273. Kirkman, Ch. M., Zu-Ben, Ch.. Uden, P. C., Stratton, W. J . , and Henderson, D. E., J. Chromatogr., 1984, 317, 569. Borgerding, M. F., and Hinze, W. L., Anal. Chem., 1985, 57, 2183. Stratton, L. P., Hynes, J. B., Priest. D. G., Doig, M. T., Barron. D. A., and Asleson, G. L., J. Chromatogr., 1986,357, 183. Breyer, E.D., Strasters, J. K . . and Khaledi, M. G., Anal. Chem.. 1991,63, 828. Medina Hernandez, M. J., Bonet Domingo, E., Ramis Ramos. G., and Garcia Alvarez-Coque. M. C., unpublished work. DeLuccia, F. J., Arunyanart, M., and Cline Love, L. J . , Anal. Chem., 1985, 57, 1564. Arunyanart, M., and Cline Love, L. J., J . Chromatogr., 1985, 342, 293. Cline Love. L. J., Zibas. S . . Noroski, J., and Arunyanart, M., J. Phavm. Biomed. Anal.. 1985, 3, 511. Haginaka, J.. Wakai, J., and Yasuda, H., Anal. Chem., 1987, 59, 2732. Kim. Y .-N., and Brown, P. R., J. Chromatogr., 1987,384,209. MenCndez Fraga, P., Blanco Gonzalez, E., and Sanz-Medel. A., Anal. Chim. Acta, 1988, 212, 181. Palmisano, F.. Guerrieri, A.. Zambonin, P. G., and Cataldi, T. R. I., Anal. Chem.. 1989, 61, 946.Sentell, K. B., Clos. J. P., and Dorsey, J. G., BioChromato- graphy. 1989, 4. 35. p. 2. 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 Cline Love, L. J., and Fett, J., J . Pharm. Biomed. Anal., 1991, 9, 323. Carretero, I., Maldonado, M., Laserna, J. J., Bonet, E., and Ramis Ramos, G., Anal. Chim. Acta, in the press. Armstrong, D. W., and Nome, F., Anal. Chem., 1981,53,1662. Arunyanart, M., and Cline Love, L. J., Anal. Chem., 1984,56, 1557. Borgerding, M. F., Quina, F. H., Hinze, W. L., Bowermaster, J., and McNair, H. M.. Anal. Chem., 1988, 60, 2520. Cline Love, L. J., Weinberger, R., and Yarmchuk, P.. in Surfactants in Solution, eds. Mittal, K. L . . and Lindman, B., Plenum Press, New York, 1984, vol. 2, pp. 1139-1158. Yarmchuk, P.. Weinberger, R., Hirsch, R. F., and Cline Love, L. J., Anal. Chem.. 1982, 54, 2233. Landy. J. S., and Dorsey, J. G., Anal. Chim. Acta, 1985, 178, 179. Armstrong, D. W., and Stine. G. Y.. Anal. Chem., 1983. 55, 2317. Pramauro, E., and Pelizzetti, E.. Anal. Chim. Acta. 1983, 154, 153. Arunyanart, M., and Cline Love, L. J., Anal. Chem., 1985,57, 2837. Haginaka, J., Wakai, J., and Yasuda, H., J . Chromatogr., 1989, 488, 341. Armstrong, D. W., and Stine, G. Y., J . Am. Chem. SOC.. 1983, 105, 6220. Khaledi, M. G., Anal. Chem., 1988, 60, 876. Colin, H., Guiochon, G., Yun, Z . , Diez-Masa, J. C., and Jandera, P., J . Chromatogr. Sci., 1983, 21, 179. Khaledi, M. G.. Peuler, E., and Ngeh-Ngwainbi, J., Anal. Chern., 1987, 59, 2738. Khaledi, M. G., Strasters, J. K., Rodgers, A. H., and Breyer, E. D.. Anal. Chem., 1990, 62, 130. Bonet Domingo, E., Medina Hernandez, M. J . , Ramis Ramos, G., and Garcia Alvarez-Coque, M. C., Analyst, 1992,117,843. Snyder, L. R., and Kirkland, J. J., Introduction to Modern Liquid Chromatography. Wiley, New York. 1979, ch. 5. Dorsey, J. G., De Echegaray, M. T., and Landy, J. S., Anal. Chem., 1983, 55. 924. Borgerding, M. F., Williams, R. L., Jr., Hinze, W. L., and Quina, F. H., J. Liq. Chromatogr., 1989, 12, 1367. Yarmchuk, P., Weinberger, R., Hirsch, R. F., and Cline Love, L. J.. J. Chromatogr., 1984, 283,47. Berthod, A., and Roussel, A., J. Chromatogr.. 1988, 449, 349. Borgerding, M. F., Hinze. W. L., Stafford, L. D., Fulp, G. W., Jr., and Hamlin, W. C., Jr., Anal. Chem., 1989, 61, 1353. Tomasella, F. P., Fett, J . , and Cline Love, L. J., Anal. Chem., 1991, 63, 474. Berthod, A.. Girard. I . , and Gonnet, C., Anal. Chem., 1986. 58, 1362. Snyder, L. R.. Dolan, J . W., and Gant, J. R.. J . Chromatogr., 1979, 165, 3. Foley. J. P., Anal. Chim. Acta. 1990, 231, 237. Hernandez Torres, M. A., Landy, J. S . , and Dorsey. J. G., Anal. Chem., 1986, 58, 744. Gale, R. W., Fulton, J. L., and Smith, R. D., Anal. Chem., 1987, 59, 1977. Veuthey. J. L., Caude, M., and Rosset, R., Analusis. 1988,16, 466. Paper 1105258F Received October 16, 1991 Accepted December 13, 1991

ISSN:0003-2654    

DOI:10.1039/AN9921700831

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

ANALYST, MAY 1992, VOL. 117 839 Determination of Parabens in Cosmetic Products by Supercritical Fluid Extraction and High-performance Liquid Chromatography Santo Scalia" and David E. Games Mass Spectrometry Research Unit, Department of Chemistry, University College of Swansea, Singleton Park, Swansea SA2 8PP, UK A rapid and simple supercritical fluid extraction (SFE) procedure has been developed for the isolation of paraben preservatives from cosmetic matrices. Method optimization indicates that recovery is affected most by extraction temperature and time. The parabens were assayed by high-performance liquid chromatography after extraction of the cosmetic preparations with supercritical carbon dioxide at 60°C and at a density of 0.85 g ml-I. Quantitative recoveries of parabens were obtained with two sequential 7 min extraction steps.Supercritical fluid extraction of cosmetic samples gave better recovery for para bens than conventional liquid extraction techniques within a shorter period of time. Moreover, the automated SFE system used minimized sample manipulation and allowed stand-alone operations. The SFE method is simple t o perform, accurate, reproducible and suitable for routine analyses of commercial cosmetic products. Keywords: Supercritical fluid extraction; high-performance liquid chromatography; paraben preservatives; cosmetic product Preservatives are commonly contained in cosmetic prepara- tions for the primary purpose of inhibiting the development of micro-organisms. However, all the preservatives can be harmful to the consumer by their potency to induce allergic contact dermatitis.1 The European Economic Community (EEC) Directive on cosmetics2 includes a list of preservatives authorized as cosmetic additives and their allowed maximum concentrations. Hence, the assay of these substances in cosmetic products is important for checking compliance with the EEC legislation. The p-hydroxybenzoic acid esters or parabens are the most widely used antimicrobial agents in cosmetics,3 the most important ones being the methyl, ethyl, propyl and butyl esters.' Combinations of two or more parabens are often used to increase the ability of the system to withstand microbial contaminations.4 Published methods for the determination of these preservatives in cosmetic preparations are based on gas chromatography (GC)s and high-performance liquid chroma- tography (HPLC) .fj-g The latter technique offers distinct advantages over GC such as simpler purification procedures and the lack of derivatization steps.However, solid and semi-solid samples which are often encountered in the analysis of cosmetics must first be put into a liquid form before HPLC analyses. This requires several sample manipulations7.8 (e. g., solvent extraction, mixing, sonication, heating, addition of acids and centrifugation), which represent a source of possible errors. Moreover, the organic solvents used must be pure and eventually be disposed of. Supercritical fluid extraction (SFE) is emerging as a valuable techniqueg-10 for the isolation of solutes from solid samples, using supercritical fluids as the extraction media.While supercritical fluids exhibit solvation powers approach- ing those of liquids, they have both lower viscosities and higher diffusivities," which lead to more rapid and efficient extractions of analytes. Moreover, the solvent strength of a supercritical fluid increases with increasing density, allowing modifications of the extraction selectivity simply by changing the pressure or the temperature. Finally, carbon dioxide, the supercritical fluid most frequently used in SFE, is non- flammable, non-toxic and available in a pure form at a * On leave from the Dipartimento di Scienze Farmaceutiche, Universita di Ferrara, via Scandiana 21, 44100 Ferrara, Italy. reasonable cost. Hence, it represents an excellent alternative to the potentially hazardous solvents currently used in sample preparation.This paper describes the development of an SFE procedure, performed with a commercially available system, for the rapid and efficient purification of the complex cosmetic matrices before assay of paraben preservatives by HPLC. The applica- tion of the method to the determination of parabens in commercial cosmetic products is also reported. Experimental Materials Instrument-grade liquid carbon dioxide supplied in cylinders with a dip tube was obtained from BOC (London, UK). Methanol, acetonitrile and water of HPLC-grade were sup- plied by Fisons (Ipswich, UK). Methyl, ethyl, propyl and butyl parabens were purchased from Sigma (St. Louis, MO, USA). Their purity was checked by HPLC prior to use.All other chemicals were of analytical-reagent grade (Sigma). Commer- cial cosmetics were from retail stores. Chromatography The HPLC apparatus consisted of a Hewlett-Packard 1084B high-performance liquid chromatograph (Hewlett-Packard, Avondale, PA, USA) linked to an injection valve with a 10 PI sample loop (Rheodyne, Cotati, CA, USA). The column effluent was monitored by the built-in multiple wavelength ultraviolet/visible detector set at a wavelength of 254 nm and 0.38 a.u.f.s. Separations were performed on a Spherisorb ODS column (particle diameter 5 pm, 100 x 4.6 mm i.d.; Jones Chromatography, Hengoed, Mid-Glamorgan, UK) under gradient conditions at a flow rate of 1.0 ml min-1. Solvent A and solvent B were 20 and 80% v/v acetonitrile in water, respectively.The elution programme was as follows: isocratic elution with 40% solvent B-60% solvent A for 6.5 min, then a 1 min linear gradient to 100% solvent B. The mobile phase was filtered through HVLP-type 0.45 pm filters. Chromatography was carried out at ambient temperature. The identity of the separated compounds was assigned by co-chromatography with the authentic substances. Peak areas were used for calculations.840 ANALYST, MAY 1992, VOL. 117 Table 1 SFE parameters Table 2 Comparison of total concentrations of parabens in cosmetic products purified by SFE or liquid extraction7 Extraction fluid density Extraction fluid flow rate Extraction temperature Equilibration time Extraction time Restrictor temperature Trap temperature Rinse solvent Rinse volume Rinse rate 0.85gml-l 2 ml min-1 60 "C 2 min 7 min 60 "C extractionM0 "C rinse 30 "C extractionh0 "C rinse Methanol 1.2 ml 1.0 ml min-1 I 1 I 5 10 14 20 Fig.1 Influence of the extraction time on the average recovery of a mixture of methyl, ethyl, propyl and butyl parabens from a hand cream. Other SFE conditions as in Table 1. Values plotted are means of triplicate experiments Ti me/m i n Sample Extraction Supercritical fluid extractions were performed with a com- puter-controlled HP 7680A SFE system (Hewlett-Packard). The cosmetic product (0.2-0.3 g) was accurately weighed on a piece of filter-paper which was rolled and inserted into the extraction cell. After initiation of the extraction programme, the supercritical carbon dioxide flows through the extraction cell and then through the restrictor into the analyte trap. The sudden pressure drop at the restrictor causes the supercritical fluid to evaporate depositing the analytes on an internal trap packed with small (diameter 0.36-0.43 mm) stainless-steel balls.Finally, the trap is rinsed with methanol and the rinse solvent collected in sample vials. The contents of the vials were made up to volume (3 ml), filtered if necessary and analysed directly by HPLC. Extraction density and time, cell temperature, supercritical carbon dioxide flow rate, trap temperature and amount of rinse solvent were controlled by the software program in the personal computer. The specific extraction conditions used are reported in Table 1. Recovery and Reproducibility 'Spiked' solutions were obtained by dissolving weighed amounts of parabens in methanol. The test samples were prepared by adding 50 p.1 aliquots of the spiked solutions, corresponding to 0.04% m/m of each single paraben, to the cosmetic products (0.2 g) and mixing them thoroughly.The percentage recovery was determined by comparing the peak areas of the parabens extracted from the samples with those obtained by direct injections of the standard solutions. The intra-assay reproducibility was tested by analysing, on ten different days, 10 p1 of the same stock sample solution from a suncream. The inter-assay variability was evaluated by replicate ( n = 10) extractions of the same suncream product. Results and Discussion A hand cream product, containing no detectable parabens, was spiked with methyl, ethyl, propyl and butyl parabens at 0.04% m/m and extracted for 10 min with supercritical carbon dioxide at 40°C and at a density of 0.95 g ml-1.In order to prevent the matrix from being swept out of the extraction cell, the sample was smeared on filter-paper. In spite of the high Concentration* (% d m ) Sample SFE Liquid extraction Suncream 0.164 0.150 Moisturising lotion 0.179 0.169 Cleansing milk 0.207 0.197 Skin cream 0.237 0.219 Day cream 0.058 0.052 * Mean value of three determinations. t - Q C D 5 1 4 2 3 1 3 0 2 4 6 8 1 0 1 2 0 2 4 6 8 1 0 1 2 Ti me/m i n Fig. 2 HPLC trace of a suncream product purified by (a) SFE or (6) by the method reported in the literature.' 1, Methyl paraben; 2, ethyl paraben; 3, propyl paraben; and 4, butyl paraben. Operating conditions as described under Experimental value of the fluid density used (at the limit of the operating range of the instrument), low recoveries (19.8-22.5%) were observed for all the compounds investigated.Increasing the extraction temperature from 40 to 60°C produced higher recoveries (65.4-67.0%) even though the carbon dioxide density, and hence its solvating power, had to be reduced (0.85 g ml-1 was the maximum density achieved with the SFE system at 60 "C). A second extraction of the same sample was found to recover all the compounds quantitatively. Although the density is generally the most important parameter that influences the extraction efficiency in SFE, for the parabens the temperature has a dominant role. The improved recoveries obtained at higher temperature are due to increased solute solubility and also to sample matrix modifica- tions (such as swelling) and enhanced diffusivity.The influ- ence of the extraction time on the recovery of the parabens from the cosmetic matrix was also investigated. As illustrated in Fig. 1, the extraction is complete after 14 min. The optimized SFE procedure consists of two 7 min extraction steps performed under identical conditions (Table 1) and preceded by a static equilibration of 2 min. Three different cosmetic preparations, containing no detectable parabens, were spiked with each preservative at a concentration of 0.04% m/m and subjected to the SFE method outlined above. The average recoveries f the standard deviations for the four parabens from a hand cream, aANALYST, MAY 1992, VOL.117 84 1 1 I I I I I k 0 2 4 6 8 1 0 0 2 4 6 8 10 Time/min Fig. 3 HPLC trace of a day cream preparation purified by ( a ) SFE or ( b ) by the method reported in the literature.’ Sensitivity, 0.19 a.u.f.s. Other operating conditions as described under Experimental and peak identification as in Fig. 2 sunscreen lotion and a shampoo were 100.3 k 1.7% (n = 6), 96.7 f 1.4% (n = 6) and 99.5 -t 2.3% (n = 6), respectively. Calibration graphs were linear in the range 0.004-0.4% m/m with correlation coefficients greater than 0.998. In none of the graphs was the intercept with the y-axis significantly different from zero at the 95% confidence interval. The minimum quantifiable amounts were at least ten times below the levels normally used in the formulation of cosmetics.6.8 Applying the SFE procedure to a commercial suncream product, the total concentration of parabens (0.164% m/m) was determined with a relative standard deviation of 1.3% ( n = 10) for the intra-assay reproducibility and 1.6% ( n = 10) for the inter-assay reproducibility.The good precision achieved can be traced to the automated extraction process, which minimizes the sample handling steps. As spiked samples do not really simulate real samples, the SFE method developed in this study was further validated by comparison with the previously adopted liquid extraction procedure7 on the same cosmetic preparations known to contain parabens. Five different commercial products were assayed. The levels measured (Table 2) conform to the EEC legislation’ (limiting value, 0.8% m/m for mixtures of parabens) and indicate that the recoveries of these preserva- tives with SFE are higher than those obtained with the liquid extraction technique currently used.7.8 Moreover, the purifi- cation based on SFE is more rapid (taking less than 40 min to perform) and less labour-intensive than others reported in the literature.7.8 Faster sample preparation has been attained by other workers;h however the recoveries of the parabens were not evaluated. Further, rapid column deterioration is a disadvantage of this method as the cosmetic preparation after solubilization is injected directly onto the HPLC column without any sample clean-up.Representative chromatograms of a suncream preparation and of a day cream product, extracted by the SFE procedure described here (a) or by the method reported in the literature7 (b), are shown in Figs.2 and 3, respectively. In addition to improved recoveries (Table 2), the SFE method [Figs. 2(a) and 3(a)] affords a more effective purification of the cosmetic matrices compared with the classical liquid extraction technique [Figs. 2(b) and 3 ( b ) J , as is evident by the absence in the HPLC traces of large peaks close to the void volume. Conclusions An SFE procedure for the rapid isolation of the paraben preservatives from cosmetics has been developed. The pro- posed method is less laborious than others reported in the literature, as sample pre-treatment simply involves weighing the cosmetic product and inserting it into the extraction cell. Moreover, because SFE allows the automation of various processes, method development is faster than with the traditional manual sample preparation.The rapidity, sim- plicity, good accuracy and reproducibility of the SFE proce- dure make it suitable for routine quality control analyses of parabens in cosmetics, particularly to verify their con- formance to the EEC legislation. Work is in progress in this laboratory to investigate the effectiveness of SFE as a generally applicable procedure for extracting a variety of additives from cosmetic matrices. The authors thank the SERC for assistance in the purchase of some of the equipment used in these studies. S. S. thanks the CNR for financial support. Provision of the SFE system by Hewlett-Packard is gratefully acknowledged. 1 2 3 4 5 6 7 8 9 10 I1 References Schopf, E., and Baumgartner, A., J. Appl. Cosmetol., 2990.8, 39. European Economic Community Council Directive 76/768/ EEC, Appendix VI, 1976. Wallhausser, K . H., in Surfactants in Cosmetics, ed. Ricger, M. M., Marcel Dckker, New York, 1985, p.225. Parker, M. S . , in Cosmetic and Drug Preservation, ed. Kabara. J . J . , Marcel Dekkcr, New York, 1984, p. 389. Geahchan, A., Pierson. M., and Chambon, P., J . Chromatogr., 1979, 176, 123. Dong, M. W., and DiCesare. J . L., J . Chromarogr. Sci., 1982, 20,49. Gagliardi, L., Amato, A., Basili, A., Cavazzutti, G., Gattavec- chia, E., and Tonelli, D., J. Chromatogr., 1984, 315, 465. de Kruijf, N., Schouten, A., Rijk, M. A.. and Pranoto- Soetardhi, L. A.. J. Chromatogr.. 1989, 469, 317. Lee. M. L., and Markides, K. E., in Analytical Supercritical Fluid Chromatography and Extraction, eds. Lee, M. L.. and Markides, K. E . , Chromatography Conferences, Provo. UT, Hawthorne, S. B . , Anal. Chem., 1990,62, 633A. Games, D. E., Berry, A. J.. Mylchreest. I. C., Perkins, J. R.. and Pleasance, S . , in Supercritical Fluid Chromatography. ed. Smith. R. M., Royal Society of Chemistry, London, 1988. p. 159. 1990, pp. 313-352. Paper I /05583 F Received November 4, 1991 Accepted December 6, 1991

ISSN:0003-2654    

DOI:10.1039/AN9921700839

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

ANALYST. MAY 1992, VOL. 117 843 Evaluation of Diuretics in Pharmaceuticals by High-performance Liquid Chromatography With a 0.05 mol dm-3 Sodium Dodecyl SuIfate-3% Propanol Mobile Phase Emilio Bonet Domingo, Maria Jose Medina Hernandez, Guillermo Ramis Ramos and Maria Celia Garcia Alvarez-Coque* Departamento de Quimica Analitica, Facultad de Quimica, Universidad de Valencia, Burjassot, Valencia, Spain The use of micellar liquid chromatography for the determination of diuretics in pharmaceutical preparations is studied. Micellar mobile phases containing sodium dodecyl sulfate (SDS), with and without different alcohols, are considered in order to determine the most appropriate combination. The elution behaviour of each diuretic in an unmodified micellar mobile phase is related to the solute-micelle association constants and the stationary phase-water partition coefficients.Diuretics of high, intermediate and low efficacy, contained in several pharmaceutical preparations, are determined using a 0.05 mol dm-3 SDS-3% propanol micellar mobile phase and a CI8 column. Keywords: Diuretic; pharmaceutical analysis; sodium dodecyl sulfate; high-performance liquid chromatography; micellar mobile phase Diuretics enhance renal excretion of water and electrolytes and are among the most extensively used drugs. The action of diuretics is based on interference with the mechanism of ionic transport along the complete length of the nephron. Accord- ing to their action diuretics are classified as having high, intermediate or low efficacy. Numerous procedures have been described for the control of the content of diuretics in pharmaceutical formulations using reversed-phase liquid chromatography with hydro- organic mobile phases.1-12 Most of the reports consider the determination of one or two diuretics. For these analyses, a C18 stationary phase, with methanol-water and acetonitrile- water mobile phases and acetate and phosphate buffers were usually used. Detection was performed in the ultraviolet (UV) region and recoveries and reproducibility were usually high. In the last 5-10 years, reported applications of micellar liquid chromatography have increased. Micellar liquid chro- matography, which employs solutions of surfactants as the mobile phases, is a mode of liquid chromatography that can be considered as an alternative to classical partition chromato- graphy.Some advantages of the technique are the low cost and the non-flammability , non-toxicity and easy disposal of the solvent. Difficult separations of hydrophobic and hydrophilic compounds have been achieved as a result of the large number of interactions of the solutes with the stationary and mobile phases. First, micellar mobile phases without the addition of modifiers were used, however, it was demonstrated that the presence in the mobile phase of a small amount of alcohol, giving rise to the so-called ‘hybrid’ mobile phases, enhances the efficiency of the separation and improves the retention control. 13-15 Khaledi et al. 16.17 demonstrated that the mechan- ism of separation with hybrid micellar eluents more closely resembles that in a purely micellar phase than that in conventional hydro-organic phases, as long as the integrity of the micelles is maintained.In this work, the use of micellar liquid chromatography for the determination of diuretics in pharmaceutical preparations was studied. Different micellar mobile phases were con- sidered in order to determine the most appropriate. A sodium dodecyl sulfate (SDS) micellar solution of increasing concen- * To whom correspondence should be addressed. tration, without any modifier, was first used. Next, the addition of several alcohols to the micellar mobile phase was studied. Nine diuretics, present in several different phar- maceutical preparations commercially available in Spain, were determined using a 0.05 mol dm-3 SDS-3% propanol micellar mobile phase and a C18 column.Experimental Reagents Aqueous SDS (9970, Merck, Darmstadt, Germany) solutions were used as the mobile phases. Micellar mobile phases with modifier were prepared by mixing the surfactant solution with a small amount of the alcohol to obtain the working concentration (v/v). Methanol [for high-performance liquid chromatography (HPLC)] and propanol (analytical-reagent grade) were from Panreac (Barcelona, Spain), and pentan-l- 01 (analytical-reagent grade) was from Merck. Nanopure de-ionized water (Barnstead Sybron, MA, USA) was used throughout. The mobile phases were vacuum-filtered through 0.47 pm nylon membranes from Micron-Scharlau (Barcelona, Spain). Stock solutions of 100 pgml-1 of the diuretics were prepared.Most of the compounds were soluble in 0.1 mol dm-3 SDS, but for some it was necessary to dissolve them in a small volume of methanol prior to the addition of the SDS solution. Most of the compounds were kindly donated by several Spanish pharmaceutical laboratories: acetazolamide (Lederle, Madrid, Spain), amiloride and atenolol (ICI Farma, Madrid, Spain), bendrofluazide (Davur, Madrid, Spain), bumetanide (Boehringer Ingelheim, Barcelona, Spain), chlor- thalidone (Ciba-Geigy, Barcelona, Spain), ethacrynic acid (Merck Sharp and Dohme, Madrid, Spain), frusemide (Lasa, Barcelona, Spain), hydrochlorothiazide (Galloso Wellcome, Madrid, Spain), spironolactone (Searle, Madrid, Spain), and xipamide (Lacer, Barcelona, Spain). Probenecid and triam- terene were purchased from Sigma (Buchs, Switzerland). No decomposition was observed in the diuretic stock solutions for between 15 d and 1 month; except for bendroflu- azide, which should be prepared every 2-3 d.The decomposi- tion was evident by the appearance of a peak during the dead volume of the chromatographic column and another with a retention time shorter than that of bendrofluazide, which increased with the age of the solution.844 ANALYST, MAY 1992, VOL. 117 Table 1 Influence of the concentration of SDS in the mobile phase on the values of capacity factor, k ' , efficiency, N . and asymmetry, BIA, of the peaks* [SDS]/mol dm-3 Diuretic k' Ace tazolamide 1.9 Amiloride 67.6 Bendrofluazide 28.3 Chlorthalidone 18.1 Ethacrynic acid 4.1 Frusemide 2.0 Hydrochlorothiazide 2.2 Probenecid 5.2 Triamterene 149 Xipamide 38.0 Spironolactone >150 * Asymmetry factors.18 N 1315 1780 1688 204 600 632 42 96 - - - BIA 1.10 1.09 1.21 3.00 0.92 1.89 4.70 2.14 - - - 0.03 k' 1.8 33.7 15.4 10.7 6.2 3.0 1.8 6.9 78.0 40.6 > 80 0.05 0.075 N 920 111 978 1287 65 147 1100 52 677 - - BIA 1.43 3.69 1.36 1 .oo 4.43 2.17 1.20 3.90 1 .oo - - k' 1.6 20.8 9.9 7.1 5.6 2.9 1.5 7.1 44.9 12.5 >50 N BIA 995 1.50 131 3.77 798 1.40 920 1.27 115 3.67 460 0.67 1019 1.31 59 3.80 96 3.47 148 2.93 - - k' 1.5 14.1 7.3 5.3 4.6 2.5 1.3 6.0 30.4 20.7 >35 0.1 N 838 201 844 812 102 739 865 41 123 879 - BIA 1.56 3.25 0.95 1.30 2.90 0.43 1.22 3.30 2.41 0.71 - k' 1.3 9.6 4.9 3.7 3.9 2.5 1.0 6.2 21.2 9.6 >25 0.15 N 772 147 605 555 139 494 792 85 50 273 - BIA 1 S O 3.20 1.21 1.17 2.82 1.22 1.45 2.90 3.29 2.00 - Apparatus Absorption spectra were obtained using a Hewlett-Packard 8452A diode-array spectrophotometer (Avondale, PA, USA).The HPLC system consisted of a Hewlett-Packard HP 1050 chromatograph, with an isocratic pump, a programmable UV visible detector and an HP 3396A integrator. The sample was injected through a Rheodyne valve (Cotati, CA, USA), with a 20 p1 loop. A Spherisorb octadecyl-silane (ODs)-2 CI8 analytical column (5 pm particle size, 12.5 cm x 4 mm) and a C18 pre-column of similar characteristics (2 cm X 4 mm) both from Hewlett-Packard were used. The dead volume (tM = 0.77 min) was determined from 10 replicate injections of an aqueous solution of potassium iodide and measurement of the absorbance at 254 nm.Efficiencies were calculated as theoretical plates, according to the equa- tion of Foley and Dorseylg for skewed peaks. Analysis of Pharmaceutical Formulations The pharmaceuticals analysed in this work were presented as tablets, except one containing probenecid, which was a powder for oral suspensions. In order to perform the determinations, the tablets were pulverized and an adequate amount weighed out. A 0.05 rnol dm-3 SDS solution was added and the sample immersed for 5 min in an ultrasonic bath to facilitate dissolution. Dilutions were made with 0.05 rnol dm-3 SDS. It was unnecessary to add methanol for solution. In order to eliminate any solid particles the sample was filtered through sintered glass and vacuum-filtered through the 0.47 pm membrane. Results and Discussion Mobile Phases Without Modifier Table 1 shows capacity factors, k ' , and efficiencies, N , for various SDS concentrations, together with the asymmetry factors of the chromatographic peaks.Detection was carried out at 254nm. As observed, the capacity factors usually decreased at increasing SDS concentrations. In 0.03 and 0.05 rnol dm-3 SDS, retention of acetazolamide, ethacrynic acid, frusemide, hydrochlorothiazide and probenecid was in the 1 < k' < 10 range. For the other diuretics, k' > 10. At larger SDS concentrations, the k' value of more diuretics is in the 1 < k' < 10 range. However, the value of k' for triamterene was still too large even in 0.15 rnol dm-3 SDS; retention of spironolactone was even longer. At increasing SDS concentrations, a decrease in efficiency was observed for acetazolamide, bendrofluazide and chlor- thalidone.For the other compounds, efficiency did not show a clear trend, although for hydrochlorothiazide and xipamide an important increase in efficiency was observed in 0.05 rnol dm-3 SDS compared with the 0.03 rnol dm-3 mobile phase. Efficiencies were extremely low in all the unmodified SDS mobile phases for amiloride, ethacrynic acid, probenecid and triamterene. There was no clear trend in the values of the asymmetry factors either. Very asymmetric peaks were obtained for amiloride , ethacrynic acid, probenecid, triam- terene and xipamide. These results indicated that an aqueous micellar mobile phase of SDS is not appropriate for the chromatographic determination of diuretics, partially due to the poor ef- ficiencies obtained.Partitioning Behaviour of the Diuretics Armstrong and Nome19 and Arunyanart and Cline Love20 proposed equivalent equations to describe the behaviour of a solute in micellar liquid chromatography as the micelle concentration is changed. The equations can be re-written as: where K A M is the solute-micelle association constant; [MI is the micelle concentration, i. e . , the surfactant concentration minus the critical micellization concentration (c.m.c.); and +fsw is the stationary phase-water partition coefficient multiplied by the phase ratio, @, where @ = (Vs/VM) and Vs is the volume of the stationary phase and VM the volume of the mobile phase. The value of Psw was not calculated because of the difficulty in determining the volume of the stationary phase.Table 2 shows the values of +fsw and K A M , calculated from eqn. (l), for several diuretics. For the most retained of the diuretics, the intercepts (1/@Psw) were almost zero (ami- loride, -1.2 x 10-3; bendrofluazide, 5.7 x 10-3; and triamterene, -7.2 x 10-4). Borgerding etal.21 indicated that a zero intercept requires the reciprocal of the phase ratio to be zero, which is not physically possible, or that Psw must be very large. This behaviour has been observed with sparingly soluble solutes. Bendrofluazide and triamterene were not soluble in water, but amiloride was soluble. On the other hand, as expected, for diuretics with low values of +fsw (acetazolamide, frusemide and hydrochloro- thiazide), retention times were very short.Retention of probenecid was not appreciably modifed by varying the SDS concentration and should be considered as a non-bindingANALYST, MAY 1992. VOL. 117 845 Table 2 Values of QPsw and KAM for some diuretics obtained from the llk' versus cSDS - c.m.c. linear plots [eqn. (l)] Diuretic w s w KAM Acetazolamide 2.1 4.5 Bendrofluazide 177 249 Chlorthalidone 56 101 Frusemide 3.9 5.8 Ethacrynic acid 8.2 7.9 Hydrochlorothiazide 2.6 10.8 solute22 ( KAM being close to zero). Xipamide had a variable behaviour when any condition was changed. Order of Elution It is of interest to relate the elution behaviour of each diuretic to the parameters obtained from eqn. ( l ) , KAM and @Psw. At a low micelle concentration, the system resembles conven- tional reversed-phase chromatography.If the compounds are ordered according to their elution with a 0.03 rnol dm-3 SDS mobile phase, it is observed that Psw controls the retention: the less retained solutes had lower +Psw values and the most retained had the largest ( e . g . , acetazolamide, 2.1; hydro- chlorothiazide, 2.6; frusemide, 3.9; ethacrynic acid, 8.2; chlorthalidone, 56; and bendrofluazide, 177). The values of @Psw for amiloride, triamterene and xipamide were also large. In micellar liquid chromatography, when log k' is plotted versus log cSDS (surfactant concentration), for solutes of different character, the linear plots intersect one another, which leads to a reversal in the elution order.23 This occurs as a result of the concurrence of two competitive equilibria: solute-micelle association and solute-stationary phase inter- action.An increasing micellar concentration brings the solute into the micellar phase, whereas it has no effect, or a small effect on the stationary phase equilibria. For solutes with high KAM values, the modification in surfactant concentration leads to important changes in retention, and the elution order is altered. Such elution order reversals were also observed for diuret- ics. The following compounds altered their elution order at the SDS concentration indicated: probenecid-ethacrynic acid, 0.03; acetazolamide-hydrochlorothiazide, 0.06; probenecid- chlorthalidone, 0.08; xipamide-amiloride, 0.09; bendrofluaz- ide-probenecid, 0.1; and chlorthalidone-ethacrynic acid, 0.13 rnol dm-3.Table 3 shows that the diuretics with the largest changes in capacity factors (the slope of the log k' versus log cSDS plot was larger) were triamterene, amiloride, bendrofluazide, and chlorthalidone. The latter three reversed their elution order. Triamterene, because of its long retention, although being affected, did not produce any reversal. If the diuretics are ordered according to the value of KAM, the order in Table 3 is observed: bendrofluazide, 249; chlorthalidone, 101; hydrochlorothiazide, 10.8; ethacrynic acid, 7.9; frusemide, 5.8; and acetazolamide, 4.5. Elution order reversals occurred between diuretics with sufficiently different K A M values, such as bendrofluazide and probenecid, chlorthalidone and probenecid, and ethacrynic acid and chlorthalidone.Addition of Modifiers to the Micellar Mobile Phase The addition of short-chained alcohols to the micellar mobile phase reduces the thickness of the film of surfactant molecules covering the stationary phase and thus, produces an enhance- ment in efficiency.21 The presence of the alcohol in the micellar mobile phase also alters the retention mechanism by shifting the equilibria of the solutes from the stationary phase Table 3 Regression line for the log k' Diuretic Triamterene Amiloride Bendrofluazide Chlorthalidone Hydrochlorothiazide Ethacrynic acid Acetazolamide Frusemide Slope -1.2 -1.2 -1.1 -1.0 -0.45 -0.43 -0.25 -0.22 versus log cSDS plot Intercept 0.29 -0.05 -0.22 -0.26 -0.34 0.24 -0.09 0.20 Coefficient of regression 0.997 0.998 0.999 0.9995 0.994 0.986 0.981 0.913 (4 Table 4 Influence of the modifier on the values of capacity factor, k ' , efficiency, N , and asymmetry, BIA, of the peaks 5% methanol 3% propanol 1% pentanol Diuretic k' N BIA k' N BIA k' N BIA Acetazolamide Amiloride Bendrofluazide Bumetanide Chlorthalidone Ethacrynic acid Frusemide Hydrochlorothiaz Probenecid Spironolactone Triamterene Xipamide 1.3 1302 1.46 1.1 - - 0.8 1654 1.13 29.5 237 3.74 22.4 241 4.76 5.7 207 3.40 12.5 780 2.57 9.9 2552 0.87 4.0 983 0.68 - - - 1.4 939 1.10 2.0 1080 1.00 8.7 2145 1.07 6.0 2240 1.00 2.2 1345 1.13 2.9 270 2.92 1.4 1240 1.12 1.2 593 1.54 1.5 1165 1.83 0.6 475 0.75 0.4 1740 0.67 idel.3 1136 1.67 1.1 380 4.83 0.8 1093 1.28 3.4 59 - 0.4 896 1.75 1.1 320 1.46 - 55.5 - - 10.8 - - - - 60.0 - - 37.1 336 2.05 9.9 452 0.94 18.9 94 3.52 1.1 36 3.25 3.8 220 3.00 and the micelle toward the bulk aqueous phase.This leads to a reduction in the capacity factors.16,17 A comparative study was performed to observe the effect of different alcohols added to the SDS micellar mobile phase, on the retention of the diuretics, and on the efficiency and asymmetry of the chromatographic peaks. For the preparation of these mobile phases a 0.05 rnol dm-3 SDS solution was selected. This concentration is not high and the values for efficiency were large compared with other SDS concentra- tions. The alcohols used were 5% methanol, 3% propanol and 1% pentanol. Table 4 gives the chromatographic parameters. Addition of 5% methanol produced the smallest modifications of the capacity factors; all were lower than with a 0.05 rnol dm-3 SDS mobile phase without modifier. However, only acetazol- amide, ethacrynic acid, frusemide, hydrochlorothiazide and probenecid shifted to shorter retention times as compared with a 0.075 rnol dm-3 SDS phase.When compared with a 0.15 rnol dm-3 SDS phase, only ethacrynic acid, frusemide and probenecid were less retained in a 0.05 rnol dm-3 + 5% methanol mobile phase. The addition of 3% propanol to a 0.05 rnol dm-3 SDS mobile phase had a similar effect. Compared with a 0.15 rnol dm-3 SDS mobile phase, acetazolamide, ethacrynic acid, frusemide, probenecid and xipamide were less retained. A 0.05 rnol dm-3 SDS + 1% pentanol mobile phase gave the largest eluent strength, and with the elution being largely enhanced compared with the 0.15 rnol dm-3 SDS mobile phase.The behaviour of bumetanide, probenecid and xipamide was different, as retention was decreased when propanol was used as modifier compared with pentanol as modifier. Spironolactone underwent important changes in retention: with a 0.05 rnol dm-3 SDS mobile phase without an alcohol modifier, and with a 5% methanol modifier, its retention time was >60 min, whereas with a 3% propanol modifier it was reduced to 43.5 min and with 1% pentanol it was 9 min. Other diuretics for which the retention times suffered an important diminution when using a 1% pentanol modifier in the mobile846 ANALYST, MAY 1992, VOL. 117 Table 5 Detection wavelength and limits of detection (LOD] for use with a 0.05 rnol dm-3 SDS-3% propanol mobile phase Compound Acetazolamide Amiloride Atenolol Bendrofluazide Bumetanide Chlorthalidone Frusemide Hydrochlorothiazide Probenecid Spironolactone Triamterene Xipamide Unm 224 220 220 274 224 274 224 224 224 242 242 224 LOD/pg ml-1 0.037 0.057 0.59 0.019 0.0032 0.0052 0.0029 0.0036 0.0051 0.55 0.10 0.0040 Table 6 Nominal contents, recoveries and reproducibility for the drugs in the pharmaceuticals Recovery Pharmaceutical Content (”/I Aldactone-A (Searle) 25 mg spironolactone 102.5 BlCnox (Farma) 1 g probenecid 96.1 2.5 g amoxycillin 40 mg sodic saccharin Diamox (Cyanamid I bCrica) 250 mg acetazolamide 117.0 Diurex (Lacer) 20 mg xipamide 98.3 Fordiuran (Boehringer lactose Ingelheim) 1 mg bumetanide 103.7 lactose Hidrosaluretil (Gayoso Wellcome) 50 mg hydrochlorothiazide 104.4 100.2 Triniagar (Farmasines) 50 mg triamterene Aldoleo (Leo) Ameride (Merck Sharp & Dohme) Neatenol (Fides) Normopresil (Semar) Spirometon (Davur) 50 mg mebuticine lactose 50 mg spironolactone 99.8 50 mg chlorthalidone 97.5 50 mg hydrochlorothiazide 100.7 5 mg amiloride chlorhydrate lactose 5 mg bendrofluazide 101.4 100 mg atenolol 107.5 25 mg chlorthalidone 102.9 100 mg atenolol 108.0 2.5 mg bendrofluazide 103.2 50 mg spironolactone 99.4 RSD 4.2 2.8 (Yo 1 2.6 0.8 0.5 0.5 3.6 4.3 6.8 0.3 1.5 1.4 4.5 2.1 0.4 2.8 phase were amiloride, bendrofluazide, chlorthalidone, triam- terene and xipamide.When elution was performed with 1% pentanol in the mobile phase, k’ < 10 for all diuretics, except spironolactone. With respect to the efficiencies, the behaviour was variable, but frequently the best efficiencies corresponded to a micellar mobile phase with 3% propanol, as has been indicated by other workers.24.25 The improvement was most marked for bendrofluazide, ethacrynic acid and probenecid.However, with the 3% propanol in the mobile phase, efficiency was decreased compared with the other two mobile phases for frusemide, hydrochlorothiazide and xipamide. When reten- tion times were either extremely short (acetazolamide) or extremely long (triamterene and spironolactone), background noise was excessive and sometimes the efficiencies could not be calculated, as the peak width at 1/10 peak height, Wo.l, was needed. For many diuretics, the most symmetrical peaks were obtained with a ‘hybrid’ 1% pentanol mobile phase. In conventional liquid chromatography with hydro-organic mobile phases, as the elution strength of the solvent increases there is a systematic decrease in selectivity, expressed as a = (k’2/kli) (V1 and k’2 are the capacity factors of two solutes, with kI2 > k‘,).17 In contrast, in micellar liquid chromato- graphy, selectivity might increase or decrease with micelle concentration depending on the nature of the compounds, that is, on the electrostatic and hydrophobic interactions with micelles. Selectivity modifications were observed with different pairs of diuretics eluting close together.Elution order reversals occur when the mobile phase is changed. Thus, in order to compare selectivity among different mobile phases, a set order of elution should be taken to obtain the selectivity factors, such as the order of elution in 0.05 mol dm-3 SDS solution, in the absence of modifier.With the values of k‘ given in Tables 1 and 4, it can be calculated whether the selectivity increases or decreases for different pairs of compounds. Analysis of Pharmaceutical Formulations Retention of diuretics in a purely SDS micellar mobile phase should be decreased in order to perform the analyses. With methanol as modifier, elution was still too slow, however, when using pentanol, elution of the diuretics appearing at the beginning of the chromatogram was markedly accelerated and resolution deteriorated. Propanol showed an intermediate behaviour and sometimes gave the best efficiencies. There- fore, a 0.05 mol dm-3 SDS-3% propanol mobile phase was chosen.Table 5 indicates the wavelengths of detection used for each diuretic, which was close to a maximum wavelength. A calibration curve was obtained for each diuretic with five 0.05 mol dm-3 SDS solutions at different concentrations. Usually, the coefficients of regression were >0.99. The highest sensitivities corresponded to bumetanide, frusemide, hydrochlorothiazide, probenecid and xipamide. The limits of detection were calculated from the background noise in the nearest of the chromatographic peaks (30 criterion, 10 replicates). The lowest limits of detection corresponded to bumetanide, frusemide, hydrochlorothiazide and xipamide (Table 5 ) , i.e., the diuretics showing the highest sensitivity. The poorest limits of detection corresponded to spironolac- tone and triamterene, owing to their long retention times.Table 6 shows the pharmaceuticals analysed that contained one or two diuretics; when two diuretics were together, one was of high or intermediate efficacy and the other of low efficacy. The peaks were always well resolved. Some prepara- tions also contained another drug, such as a &blocker or a stimulant, which did not interfere with the analyses. The determination of atenolol was also considered. When the solutions of the preparations were injected into the column, a peak was always observed in the dead volume of the system, which probably corresponded to the excipient . Amiloride, triamterene, spironolactone and atenolol had long retention times. The determination of these compounds is more appropriate with a mobile phase of high eluent strength, e.g., with an SDS-pentanol mobile phase.The determination of some of these compounds with the SDS- propanol mobile phase was also examined. Table 6 also shows the recoveries with respect to the composition given by the manufacturers. These were usually in the 96104% range. Relative standard deviations (RSDs) of five replicate injections were usually in the 0.3-4.270 range. The results indicate that micellar liquid chromatography is adequate for the determination of diuretics in pharmaceutical preparations. This work was supported by the Comisi6n Interministerial de Ciencia y Technologia (CICyT), Project DEP89-0429. References 1 2 Honigberg, I . L.. Stewart. J . T.. Smith. A. P.. and Hester. D. W., J . Pharm.Sci., 1975, 64, 1201. Menon, G. N.. and White. L. B., J. Pharm. Sci.. 1981.70,1083.ANALYST, MAY 1992, VOL. 117 847 3 4 5 6 7 8 9 1 0 11 12 13 14 Roth, J . , Rapaka, R. S., and Prasad, V. K., Anal. Lett., 1981, 14, 1013. Walters, S. M., and Stonys, D. B., J . Chromatogr. Sci., 1983, 21,43. de Croo, F., van den Bossche. W., and de Moerloose, P., Chromatographia, 1985. 20, 477. Fogel, J., Sisco, J . , and Hess, F., J . Assoc. Of5 Anal. Chem.. 1985, 68, 96. de Croo. F., van den Bossche, W., and de Moerloose, P., J . Chromatogr.. 1985, 329, 422. Yarwood, R. J., Moore, W. D., and Collett, J. H., J . Pharm. Sci., 1985. 74, 220. Sane, R. T.. Sadana, G. S . , Bhounsule, G. J . , Gaonkar, M. V., Nadkarni, A. D., and Nayak, G.. J . Chromatogr., 1986, 356, 468. de Croo, F., van den Bossche, W., and de Moerloose. P., J . Chromatogr., 1986,354, 367. Hitscherich, M. E., Rydberg, E. M., Tsilifonis, D. C., and Daly. R . E., J . Liq. Chromatogr., 1987, 10, 1011. Bachman. W. J., and Stewart, J . T., J . Chromatogr. Sci.. 1990, 28, 123. Dorsey. J., Khaledi, M. G., Landy. J . S . , and Lin, J.-L.. J. Chromatogr., 1984, 316, 183. Borgerding, M. F., Hinze, W. L., Stafford, L. D., Fulp, G. W., Jr., and Hamlin, W. C., Jr., Anal. Chem.. 1989, 61, 1353. 15 16 17 18 19 20 21 22 23 24 25 Berthod, A., Girard. I.. and Gonnet, C., Anal. Chem.. 1986, 58, 1362. Khaledi, M. G., Anal. Chem., 1988, 60, 876. Khaledi, M. G., Strasters, J . K., Rodgers, A. H., and Breyer, E. D., Anal. Chem.. 1990, 62, 130. Foley, J . P., and Dorsey, J. G., Anal. Chem., 1983, 55. 730. Armstrong, D. W., and Nome, F., Anal. Chem., 1981,53,1662. Arunyanart, M., and Cline Love, L. J., Anal. Chem., 1984,56, 1557. Borgerding, M. F., Quina, F. H . , Hinze, W. L., Bowermaster, J . , and McNair, H. M., Anal. Chem.. 1988, 60,2520. Armstrong, D. W., and Stine, G. Y., Anal. Chem., 1983, 55, 2317. Yarmchuk, P., Weinberger, R., Hirsch. R. F., and Cline Love, L. J., Anal. Chem., 1982, 54, 2233. Dorsey, J . G., DeEchegaray, M. T., and Landy, J . S., Anal. Chem., 1983,55, 924. Berthod, A., and Roussel, A., J . Chromatogr., 1988,449, 349. Paper 1f04774D Received September 16, 1991 Accepted November 11, 1991

ISSN:0003-2654    

DOI:10.1039/AN9921700843

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

ANALYST, MAY 1992, VOL. 117 849 Determination of Neutral Sizing Agents in Paper by Pyrolysis-Gas Chromatography Tatsuya Yano,* Hajime Ohtani and Shin Tsuget Department of Applied Chemistry, Faculty of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya, 464-0 7, Japan Takao Obokata DIC-Hercules Chemicals lnc., lchihara 290, Japan Neutral sizing agents, viz., alkylketene dimers (AKDs) and alkenylsuccinic anhydride (ASA), in paper were determined by pyrolysis-gas chromatography, including those that reacted chemically with the paper. The peaks of intact AKDs and related ketones in the pyrograms were used as the key peaksfor the determination of AKDs in paper samples containing between 0.025 and 1 .O% of AKDs. The results obtained suggest that almost 75% of the AKDs added are retained in all the paper samples.Further, the relationship between the AKD content and the degree of sizing is interpreted in terms of the possible paper sizing mechanisms. The ASA content in paper was also determined in essentially the same way as for the AKDs, and is discussed in relation to the degree of sizing. Keywords: Pyrolysis-gas chromatography; neutral sizing agent; paper; alkylketene dimer; alken ylsuccinic anhydride As paper is composed of hydrophilic cellulose fibres, it tends to absorb aqueous liquids by capillary action of the inter- and intra-fibre voids. Although this property is useful for filter and blotting paper, printing and writing paper has to be resistant to the blotting of ink to some extent. Therefore, sizing agents are often added to the pulp slurry or applied to the surface of the paper in order to improve the printing and writing qualities by developing resistance to penetration by aqueous liquids.Rosin-alum is one of the most popular and traditional sizing agents. However, it is difficult to preserve paper sized with rosin-alum for long periods of time because of the acidic nature of alum (aluminium sulfate). On the other hand, sizing agents such as alkylketene dimers (AKDs) and alkenylsuccinic anhydride (ASA) react covalently with the hydroxy groups of cellulose to form ester linkages under neutral or alkaline conditions.1.2 Therefore, small amounts of AKDs and ASA can provide strong sizing effects for prolonged periods without destroying the paper matrix. These sizing agents are generally added to the pulp slurry and are partly lost with wasted white water.Moreover, the AKDs retained in the paper are known to have at least three different forms: I ( a ) intact AKDs physically adsorbed with cellulose R-CH=C-CH-R’ ( 6 ) ketones formed by hydrolysis R-CH~-CC=O R-CH=C -CH-R’ I I I + H20 heat R’-CH-C=O 0-c=o I * Present address: DIC-Hercules Chemicals Inc.. lchihara 290, I- To whom correspondence should be addressed. Japan. (c) (3-keto esters formed by reaction with the hydroxy groups of cellulose R-CH=C-CH-R‘ I I 0-c=o By analogy with (a) intact ASA possible forms: heat OH Cellulose + I ____) R’ 0 I II R-CH2-C-CH -C-0-Cellulose II 0 AKDs, ASA in paper could have three 0 II R-HC-C\ I ’0 H2C-C’ C16H33 II I 0 (R = -CH=C-C16H33, etc.) (b) di-acids formed by hydrolysis 0 0 II II R-HC-C\ R-HC-C-OH H2C-C H 2C-C -0 H I ,o + H20 - I II II 0 0 (c) The ester acids formed by reaction with the hydroxy groups of cellulose 0 0 II II I R-HC-C -0-Cellulose - R-HC-C, OH I , o + t H2C-C Cellulose H 2C- C-0 H I1 II 0 0 By examining ASA in paper with infrared spectroscopy, McCarthy and Stratton2 reported that the esterification of ASA with the hydroxy groups of cellulose and the resulting sizing effect were strongly influenced by the drying conditions.Therefore, the discriminative determination of each form of AKD or ASA would be highly desirable in the field of paper manufacture. In order to study the mechanism of neutral850 ANALYST, MAY 1992, VOL. 117 sizing, Roberts and Garner1 determined the content of AKDs in papers by using 14C-labelled reagents.The radioactivity of the paper samples before and after extraction with chloroform was measured in order to distinguish between the reagents that had reacted with cellulose and those that had not. Pan et al.3 reported that ultrasonic attenuation by paper in water corresponded to the amount of AKDs in a paper sheet. Recently, Dart and McCalley4 determined the content of AKDs in paper by capillary gas chromatography-mass spec- trometry (GC-MS) based on hydrolytic extraction followed by quantification of the resulting long-chain ketones. However, the recovery of the AKDs was not quantitative because they reacted with the hydroxy groups of cellulose still present in the paper even after the extraction. In recent work, pyrolysis-gas chromatography (GC) was successfully applied to the determination of small amounts of a polyamide-epichlorohydrin wet-strength resin added to paper.5 In the present work, AKDs and ASA in paper, including those that cannot be extracted with a solvent, were determined by using essentially the same technique.The results obtained are interpreted in terms of the possible mechanisms of sizing. Experimental Materials The AKD emulsion (Aqapel 12), which contains a solid fraction (20%) including AKDs (86%), produced by DIC- Hercules Chemicals, was added to the pulp slurry at pH 8.0 in order to prepare the paper samples. The samples were then dried with a drum dryer at 100 "C for 50 s. The eight paper samples containing various amounts of AKDs are listed in Table 1 together with their Stoeckigt degrees of sizing6 All the samples contained 0.5% of polyamide-epichlorohydrin resin. The ASA (hexadecenylsuccinic anhydride containing 16% octadecenylsuccinic anhydride) produced by the Dixie Chem- ical Co.was also added to the pulp slurry at pH 7.5 to prepare the other two paper samples containing 0.12% of ASA. These samples also contained 20% of CaC03, 0.75% of cationic starch, 0.5% of A12(S04)3-14H20 and 0.02% of poly- acrylamide. In addition, standard paper samples containing known amounts of AKDs or ASA were also prepared for calibration. A prescribed amount of the AKD emulsion was added to two sheets of filter-paper (Advantec Toyo No. 6,90 mm diameter) and then dried at 105 "C. Seven types of standard paper samples with AKD concentrations ranging from 0.004 to 2.3% were prepared.The standard paper samples containing ASA were also prepared in essentially the same manner. All the paper samples were cryo-milled into a fine powder by a freeze/mill (Spex 6700) prior to pyrolysis-GC measure- ments in order to homogenize the samples. Conditions for Pyrolysis-GC The high-resolution pyrolysis-GC system utilized in this work was essentially the same as that described previously.7 A Table 1 Paper samples containing AKDs Sample No. 1 2 3 4 5 6 7 8 AKD dosage (% ) 0 0.025 0.05 0.075 0.1 0.2 0.5 1 .0 Stoeckigt degree of sizing/s 0.1 4.6 21.9 32.0 44.7 91.8 125.0 162.0 vertical microfurnace-type pyrolyser (Yanagimoto GP-1018) was directly attached to a gas chromatograph (Shimadzu GC-9A) equipped with a flame-ionization detector.About 0.5 mg of the milled paper sample, 0.1 mg of the AKD sample or 0.02 mg of the ASA sample was placed in a platinum sample cup and then pyrolysed under a flow of nitrogen carrier gas. The pyrolysis temperature was set empirically at 500 "C, which was sufficiently high to attain almost complete thermal degradation of the main cellulose matrix of the paper and to achieve thermal desorption of the various reagents added to the paper sample. A fused silica capillary column (30 m x 0.25 mm i.d.) coated with dimethylsiloxane or 5% phenylmethyl- siloxane (0.25 pm thick), immobilized by chemical cross- linking, was used. The 50 ml min-' carrier gas flow rate at the pyrolyser was reduced to 1.0 ml min-1 at the capillary column by means of a splitter.The column temperature was initially set at 50 "C and then programmed to increase to 300 "C at a rate of 5 "C min-1. Identification of the peaks on the pyrograms was mainly carried out using a GC-MS system (Shimadzu QP-1000) with an electron impact ionization source to which the pyrolyser was also directly attached. Results and Discussion Determination of Retained AKDs The pyrogram of the AKD sample at 500 "C is shown in Fig. l(a). The peaks in the pyrogram are summarized in Table 2. The main products are intact AKDs and their hydrolysis products (ketones), which are observed in the latter part of the pyrogram. As the AKD sample utilized consists of a mixture of various ketene dimers mainly with C12- and C14-, C14- and C14-, C14- and C16-, and C16, and C16-alkyl chains, the main peaks in the latter part of the pyrogram are assigned to the corresponding four AKDs and the four associated ketones.0 ~ ~~~ 6 : : : : : . . . . . i ; ; : : . . . . . . . . . . . . . . . . . . . . . . . _ . . . . . . . . . . . . . . . . . . . Levoglucosan \ J I I I I 20 40 60 80 Retention time/min Fig. 1 Pyrograms of a paper sample containing AKDs and pure AKDs at 500 "C. ( a ) Pure AKDs: ( b ) paper with 1.0% of AKDs added; and ( c ) control paper. Peak numbers correspond to those in Table 2ANALYST, MAY 1992, VOL. 117 Table 2 Assignment of the main peaks derived from AKDs Structure of alkyl chains, R,R’* Total number of Peak No. Compound carbon atoms Combination Ketone AKD Ketone AKD Ketone AKD Ketone AKD 26 26 28 28 30 30 32 32 0 R-CH=C -CHAR’ II I I * Kctone, R-CH~CCHZ-R’ AKD.0 - C C O 1 I 0 0.5 1 .o AKDs in feed (%) Fig. 2 Relationship between feed of AKDs into the pulp slurry and AKDs retained in a paper sample, estimated by pyrolysis-GC. Solid line: observed relationship. Broken line: hypothetical relationship for complete retention (100%) About 70% of the weighed AKD sample was recovered as the intact AKDs and the associated ketones in the pyrogram under the pyrolysis conditions used, whereas the remaining 30% was mostly degraded to a series of hydrocarbons (up to C,,), which were observed in the early part of the pyrogram, each of which mainly consisted of a doublet corresponding to an alk-1-ene and an n-alkane formed from the alkyl groups in the AKDs. The pyrogram of the paper prepared by feeding 1.0% of AKD into the pulp slurry is shown in Fig.l(b). Although the region for the peaks of the hydrocarbons from AKD overlaps that for the pyrolysates from cellulose, the eight major peaks of the ketones and AKDs are clearly observed in the pyrogram after a large number of polar pyrolysates such as levoglucosan, formed from cellulose, have been eluted. On the other hand, no significant peaks are observed in the latter part of the pyrogram of the paper containing no additives [Fig. l(c)]. Therefore, the AKDs and the ketones can be used as the key peaks for the determination of AKDs. Here, the AKD content in a given paper sample was calculated on the basis of the total intensities of the eight major peaks (1-8) observed in the pyrogram.Firstly, the relationship between the AKD content and the total inten- sities of the eight peaks relative to the sample mass was established by using the seven standard samples with AKD concentrations ranging from 0.004 to 2.3%. The correlation coefficient for the seven calibration points was 0.099. Then, the peak intensities in the pyrogram of a weighed unknown sample were correlated to the calibration graph thus prepared. The contents determined in this manner must be the total 150 rn 0 . .- .N 100 rn + 0, 0 E 50 85 1 ) 0 0.3 0.6 AKDs retained (%) Relationship between AKD content determined by pyrolysis- Fig. 3 GC and degree of sizing of paper samples. For details, see text ASA ( a ) n 0 10 20 30 40 50 Retention ti me/m i n Fig. 4 at 500 “C. (u) Pure ASA and (6) paper with 1.2% of ASA added Pyrograms of a paper sample containing ASA and pure ASA Table 3 ASA contents determined by pyrolysis-GC and degree of sizing of paper samples prepared in the presence of 0.12% of ASA Sample ASA in paper ASA retained ’ Stoeckigt degree No.(Yo) (Yo 1 of sizing/s 1 0.056 47 27.8 2 0.077 64 38.5 amounts of AKDs in the paper, including those AKDs that have reacted chemically with the hydroxy groups of cellulose and those that have been physically adsorbed, because the cellulose matrix is completely degraded to volatile products under the pyrolysis conditions used. This method was success- fully applied to the analysis of a sample containing only 0.001% of AKDs. The observed reproducibility was within 4% of the relative standard deviation for five repetitive runs with the same sample (sample No.3). As shown in Fig. 2, an almost linear relationship holds between the amounts of AKDs fed into the pulp slurry and the amounts retained in the paper as calculated by pyrolysis-GC. The broken line refers to the hypothetical relationship for complete retention (100%). The results obtained suggest that852 ANALYST, MAY 1992, VOL. 117 almost the same fraction (about 75%) of AKDs added is retained in the paper samples prepared by adding AKD at concentrations of between 0.025 and 1.0%. On the other hand, as shown in Fig. 3, the relationship between the AKD contents determined by pyrolysis-GC and the degree of sizing of the paper samples does not exhibit a simple linear tendency. The degree of sizing is approximately proportional to the AKD content up to about 0.15%, whereas the slope of the graph rapidly deviates from linearity at higher contents.Although initially (<0.15% AKDs retained), both the chem- ical reaction to the AKDs with the hydroxy groups of cellulose (I) and their physical adsorption onto cellulose (11) might lead to an almost linear increase in the degree of sizing, the chemical reaction would eventually be fully completed at a certain amount of retained AKDs (about 0.15%) when the available hydroxy groups of cellulose on the surface of the paper matrix had been consumed. On the other hand, physical adsorption can still proceed at the higher concentrations of AKDs added, so that the degree of sizing might then be increased mostly by the physically adsorbed AKDs.The ratio of the increase in the degree of sizing in the first stage to that in the second stage (see Fig. 3) is about 1 : 5. This result suggests that the chemically reacted AKDs might be about four times as effective as the physically adsorbed AKDs for the develop- ment of sizing. Determination of Retained AS A The pyrograms at 500 "C for ASA and the paper to which 0.12% of ASA has been added are shown in Fig. 4. The main peaks observed in Fig. 4(a) are due to intact ASA together with minor pyrolysis products such as alkanes and alkenes formed from the alkyl groups of the ASA. As the ASA generally consists of a complex mixture of various homologues and isomers with alkyl groups of different chain length and/or structure, the resulting chromatogram is very complex.As the small peaks of vaporized ASA are also clearly observed in Fig. 4(6), the ASA content in the paper samples can also be calculated from the intensity in a similar manner to that for the AKDs. The ASA contents determined by pyrolysis-GC, and the Stoeckigt degrees of sizing for the two paper samples prepared from the same pulp slurry containing 0.12% of ASA, are shown in Table 3. However, the degree of sizing of the two paper samples is different, in spite of the fact that the initial ASA dose was the same in both instances. The amounts of ASA retained in the paper, as determined by pyrolysis-GC, are consistent with the degree of sizing of the paper. These results demonstrate that pyrolysis-GC can be used to deter- mine trace amounts of neutral sizing agents in paper. References 1 Roberts, J. C., and Garner, D. N.. Tappi J., 1985, 68, 118. 2 McCarthy, W. R., and Stratton, R. A., Tappi J., 1987,70, 117. 3 Pan, Y. I., Kuga, S., Usuda. M., and Kadoya, T., Tappi J.. 1985, 68, 98. 4 Dart, P. J.. and McCalley, D. V., Analysr. 1990, 115, 13. 5 Yano, T.. Ohtani, H., Tsuge, S., and Obokata, T.. Tappi J.. 1991, 74, 197. 6 JIS P 8122, Testing Method for Stoeckigt Sizing Degree of Paper, Japanese Industrial Standards Committee, Tokyo, 1976. 7 Ohtani, H., Kimura, T., andTsuge, S . , Anal. Sci., 1986,2.179. Paper 1/03409J Received July 8, 1991 Accepted November 18, 1991

ISSN:0003-2654    

DOI:10.1039/AN9921700849

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

ANALYST. MAY 1992, VOL. 117 853 trans-Cyclohexano Crown Ethers as Ion Sensors in Cation-selective Electrodes R. D. Tsingarelli, L. K. Shpigun,” V. V. Samoshin, 0. A. Zelyonkina, M. E. Zapolsky, N. S. Zefirov and Yu. A. Zolotov N.S. Kurnakov Institute of General and Inorganic Chemistry, USSR Academy of Sciences, Leninsky Prospect 3 I , Moscow I I 7907, Russia Several trans-cyclohexano crown ethers have been synthesized and studied as sensor materials for different metal cations in PVC-matrix membranes of ion-selective electrodes. The potentiometric selectivity of the membranes has been found t o correlate with the chemical structure peculiarities of the incorporated compound and can be changed by variation of the substituent attached t o the cyclohexane fragment. Some of the compounds show superior selectivity for potassium over many other metal cations.Keywords: Ion-selective electrode; macroc yclic crown ether; sensor material Ion-selective electrodes (ISEs) with liquid membranes con- taining electrically neutral lipophilic macrocyclic or non-cyclic ion carriers are widely used for the determination of different metal ions in various samples.’-3 The significance of macro- cyclic compounds as cation-sensor materials in ISEs has been recognized, and increasing interest has been focused on the molecular design of these structures. This has resulted in the development of ISEs based on synthesized ionophores.4 The function mechanism of such sensors is based on a cation- transfer reaction at the membrane interface (or at the aqueous solution-organic phase boundary) by means of reversible complexation of the cation by the neutral carrier introduced into the membrane.5 In fact, the potentiometric selectivity of these compounds for various equally charged metal cations is related to the ratio of their corresponding complex stability constants,6 and has been found to correlate with their extraction constants.7 Although a number of compounds have been synthesized, the molecular design of a sensor with given analytically relevant ion selectivity still remains empirical.”.” Hence, some specifically complexing macro-heterocyclic com- pounds, e .g . , some crown ethers,5-10 either do not act as ionophores in membranes or do not reveal an electrode activity 1 1 -12 (q Hd ’;- OH The present paper describes a comparative study on the potentiometric selectivity of a series of solvent polymeric membranes, containing a number of trans-cyclohexano crown ethers (trans-CCEs) and their 4-alkyl derivatives as the sensor material, in ISEs.The results are discussed in terms of the chemical structure peculiarities and of the possibility of complex formation between these compounds and alkali or alkaline earth metal cations. Experimental Reagents All the crown compounds investigated were obtained from appropriate cyclohexano glycols and oligoethylene glycol ditosylates by well-known cyclization procedures13 (Fig. 1). Cyclohexano glycols in turn were prepared by the acid- catalysed reaction of the corresponding epoxides with oligo- ethylene glycols.14-15 The wide variety of potential reactants makes this approach to the synthesis of trans-CCEs flexible and highly promising. .e *A f3 0 OH I OH ),) H2S04,CHC13 R AK TsO 0 OTs NaH, 1 ,4-dioxane R w R w I: n = 2, R = H VI: n = 2, R = CH3 XI: n = 2, R = (CH3I3C II: n = 3, R = H 111: n = 4, R = H V: n = 6, R = H VII: n = 3, R = CH3 VIII: n = 4, R = CH3 X: n = 6, R = CH3 XII: = 3, R = (CH3)3C XIII: n = 4, R = (CH3)3C XV: n = 6, R = (CH3)3C IV: n = 5, R = H IX: n = 5, R = CH3 XIV: n = 5, R = (CH3I3C Fig. 1 Synthesis of the crown compounds investigated * To whom correspondence should be addressed.854 ANALYST, M 4 Y 1992, VOL. 117 Table 1 Potentiometric selectivity coefficients of various membranes containing trans-cyclohexano crown ethers -logkr:.Mn+ (n = 5 ; P=O.95) trans-CCE 15C5 (I) 18C6 (11) 2 1 C7 (111) 24C8 (IV) 27C9 (V) 15C5 (VI) 18C6 (VII) 21C7 (VIII) 24C8 (IX) 27C9 (X) 15C5 (XI) 18C6 (XII) 21C7 (XIII) 24C8 (XIV) 27C9 (XV) H-CCE- CH3- CCE- C(CHs)j-CCE- Li+ Na+ 2.18 0.50 1.80 0.73 2.37 0.94 2.36 0.86 1.68 0.69 1.20 0.00 - 2.30 1.20 2.07 1.01 2.19 0.31 1.00 0.29 1.80 0.05 2.40 1.30 1.43 0.95 2.51 1.06 2.03 1.10 0.17 0.45 2.23 2.18 2.86 2.05 0.03 0.15 2.57 2.86 2.68 2.05 0.26 0.31 2.08 3.14 2.76 2.24 0.18 0.50 2.50 2.68 3.08 1.55 0.26 0.77 1.80 1.77 2.09 0.99 -0.13 0.70 1.90 2.20 1.50 1.00 0.10 0.60 2.80 2.84 2.80 2.04 0.00 0.15 2.17 2.68 2.41 1.56 0.18 0.36 2.26 2.82 2.58 1.69 0.19 0.43 0.88 1.72 1.22 0.76 0.25 0.68 1.43 1.34 1.36 0.75 0.45 1.00 2.61 3.15 2.88 2.35 0.00 0.20 2.29 1.80 2.44 1.96 0.08 0.30 2.67 3.20 3.20 2.29 0.05 0.21 2.41 2.53 2.09 1.24 0 -1 -2 + r 8: a cn -I 0 -1 -2 -3 Li + Na+ K+ Rb+ Cs+ (0.068) (0.098) (0.133) (0.149) (0.165) Mg2+ Ca2+ Sr2+ Ba2+ (0.074) (0.104) (0.120)(0.138) Fig.2 Dependence of the potentiometric selectivity coefficients (log k) on the cationic radius (Y) for (a) alkali metal cations and ( 6 ) alkaline earth metal cations. The values in parentheses on the abscissa are the cationic radii of the metal ions in nanometres. 0, n = 1; A , n = 2; 77, n = 3; 0, n = 4; and X , n = 5 Membranes The plasticized polymer membranes containing the above- mentioned macrocycles were prepared by a known proce- dure.16 the membranes included (mass ratio in percentages): macrocycle, 1.0; poly(viny1 chloride) (PVC), 33.0; and o-nitrophenyl octyl ether, 66.0. Reference PVC membranes, which did not contain macrocycles, were also prepared.The thickness of the membrane thus obtained was 0.1-0.2 mm. All the membranes were conditioned before use by soaking for 24 h in 0.02 mol dm-3 potassium chloride. Electrode System and e.m.f. Measurements The membranes were studied with use of electrochemical cells of the following type: Ag-AgCllKCI(O.1 mol dm-3)( Bridge [Sample electrolyte ~Membranel~KCl(0.1 mol dm-3)AgCl-Ag The conditioned membrane discs (10 mm in diameter) were assembled in a commercial electrode body (NPO Analit Pribor, Tbilisi). A double-junction reference electrode was also used (Model 90-02: Orion Research, Cambridge, MA, USA) and the outer filling solution was 0.1 mol dm-3 ammonium nitrate. The e.m.f. measurements were carried out at 295 t 2 K, with use of a digital Radelkis (Budapest, Hungary) OP-208 pH meter.Potentiometric selectivity coefficients for different metal cations relative to potassium ions (kgo2, Mrt+) were determined by the separate-solution method with the 0.1 mol dm-3 respective metal ion s0lutions.1~ Results and Discussion The membrane potentiometric selectivity for alkali and alkaline earth metal cations, referred to K+, was studied. Table 1 lists the selectivity coefficients obtained. It is shown that the values -log /cR"+',~"+ ( n = 1,2) for each metal cation depend significantly both on the size of the ring and on the nature of the substituent R in the cyclohexane fragment. All the trans-CCEs studied reveal selectivity towards alkali metal cations, especially K+ and Rb+.Fig. 2(a) shows that the dependence of -log /cRO+~,~+ on the cationic radius for alkali metals has a maximum at 0.133 nm, irrespective of the size of the crown ether ring (when R = H). The order of selectivity is as follows: K+, Rb+ , Cs+, Na+, Li+. The values of -log kR"tf,Li+ are 1.0-2.5 orders of magnitude lower than those for other alkali metals. It is interesting to note that the above-mentioned experimental results, espe- cially the highest potentiometric selectivity towards K+, are in agreement with those for the crown ethers not containing the cyclohexane fragment. 18 On the other hand, for alkaline earth metal cations, the analogous curves have minima at 0.098-0.113 nm [Fig. 2(b)]. The order of the selectivity in this instance is different for each macrocycle, but all log kr?,M'+ values are essentially lower than those for alkali metals, except Li+.This indicates that the known hole-size concept19-20 cannot be used to predict the potentiometric selectivity of membrane electrode systems. Fig. 3 shows the dependence of kR"+',M"+ on the size of the macrocycle. These curves are non-monotonic for all metal ions, especially for the alkyl derivatives of trans-CCEs. Hence, for tert-butyl derivatives (XI-XV), there are two distinct minima at 18-crown-6 (18C6) and 24-crown-8 (24C8) ( n = 3 and 5 ) , which alternate with three maxima at 15-crown-5 (15C5), 21-crown-7 (21C7) and 27-crown-9 (27C9) (n = 2 , 4 and 6). The highest selectivity for K+ compared with alkaline earth metal cations and Li+ is observed for mem- branes containing tert-butyl-trans-cyclohexano-18-crown-6 (XII) and tert-butyl-trans-cyclohexano-24-crown-8 (XIV).The influence of other alkali metal cations is relatively low only for compound XII. The membranes containing methyl-trans- cyclohexano-15-crown-5 (XI) are sensitive to the cations Rb+, Na+ and K+. Hence, the introduction of a fairly well-branched alkyl substituent into the cyclohexane fragment results in a signifi- cant change in the cation selectivity of the membrane.ANALYST, MAY 1992. VOL. 117 855 0 - 1 - 2 + C r 55 0 -I - 1 -2 -3 a) 1 I I 1 I b) / I I I I 1 2 3 4 5 6 I I I I I d) X 1 I I 1 I 2 3 4 5 6 R n I I 1 I I 2 3 4 5 6 Fig. 3 Dependence of the potentiometric selectivity coefficients (log kP,9' Mrt+) on the number (n) of binding-site atoms in the macrocyclic ring.(a) and (b): R = H; (c) and ( d ) : R = CH,; (e) and 0: R = C(CH3)3. A,'Li+; B, Na+; C, Rb+; D, Cs+; E, Mg*+; F, Ca2+; G, W+; and H, Ba2+ A B n = 0-4; R = H, CH3, C(CH3)3 Fig. 4 Two chair conformations of thc six-membered ring in trans-CCEs R = CH3, C(CH3)3 R = C(CH& Fig. 5 derivatives of the trans-CCEs Two interconverting forms of the complexes for tert-butyl Some of the teatures of trans-CCE membrane activity can be explained in terms of the conformational properties of these compounds. A six-membered ring in a trans-CCE can adopt two chair conformations, which differ in substituent orientation (Fig. 4): C-0 bonds are axial in conformation A and equatorial in conformation B. Nuclear magnetic reso- nance (NMR) studies of the equilibrium (A G= B) have shown that it is strongly shifted to the conformation B (>90%) when R = H.1"*1-23 When R = CH3, conformation A is predomi- nant to nearly the same extent, and when R = C(CH,)?, conformation A becomes virtually the sole conformation.This regularity is in accord with conformational properties of R-substituted cyclohexanes; i. e . , the destabilization of the axial position of the alkyl group (B) increases with an increase in its volume.'4 An interesting conformational feature of trans-CCEs is the alteration of the proportion of conformers with the increasing size of the macrocycle: the population of conformers (A) decreases on passing from trans-cyclohexano-15-crown-5 (I) to trans-cyclohexano-18-crown-6 (11), and increases again on passing from I1 to trans-cyclohexano-21-crown-7 (111).13-21-23 Apparently, this is a related peculiarity of macrocycles. Owing to the ring trans fusion, the chair-chair interconver- sion of the six-membered ring is accompanied by a marked change in the conformation of the macrocycle. The macro- cycle in conformer A has on oval form resulting from trans diaxial orientation of the bridge fragment 0-C-C-0. It is856 ANALYST, MAY 1992, VOL. 117 known that 18-crown-6 has an analogous oval form (with symmetry C) in the crystalline state or in a non-polar solvent.22-27 In the.complexes of 18-crown-6, with most metal cations this macrocycle adopts a ring-shaped conformation (with symmetry D), and all the fragments 0-C-C-0 have a gauche conformation (dihedral angle about 60").13.28 For trans-CCE, such a conformation of the macrocycle is attain- able only in conformer B. Indeed, it has been shown that the crown ether VII in complexes with inorganic salts adopts conformation B14,15 in spite of destabilization by the axial methyl group. The conformational energy of the tert-butyl group (23 kJ mol-1)24 is comparable to the energy difference between the chair and twist conformations of the cyclohexane ring (22 kJ mol-1).23 Therefore, one can expect two intercon- verting forms of complexes for the tert-butyl derivatives (Fig. 5 ) . The axial substituent R destabilizes the complexes of compounds VI-XV compared with complexes of unsubsti- tuted compounds I-V. As the ionophore properties of neutral carriers are determined mainly by the stability of their complexes,5J9 so it is believed that the change in complex- formation ability, with the introduction of a substituent R, is the result of conformational perturbation. The destabilization of complexes makes the substituted trans-CCE more sensitive to conformational features of the macrocycle, e.g., to the alteration of conformer population with an increase in the size of the macrocycle (see above). This results in an alteration of the selectivity coefficients, which increase with an increase in the size of the substituent R.Conclusion The potentiometric selectivity of some synthesized trans- CCEs as sensor materials in PVC-matrix membrane elec- trodes has been studied. The selectivity for potassium by some compounds is close to that of electrodes based on valinomycin.It was observed that the properties of trans-CCEs as ion carriers can be modified via conformational change caused by variation of substituents attached to the cyclohexane frag- ment. This offers the possibility of purely conformational control of crown ether complexation. References 1 Oggeneuss, P., Morf. W. E., Oesch, U., Amman, D., Prctsch, E.. and Simon, W., Anal. Chim. Acta, 1986, 180, 155. 2 Koryta, J . . Anal. Chim. Acta, 1986, 183. 1. 3 Koryta. J . , Anal. Chim. Acta. 1988, 206. 1. 4 Amman, D., Morf, W. E . , Anker, P., Meier, P. C., Prctsch, E., and Simon, W.. Ion Sel. Electrode Rev., 1983. 5 , 37. 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 Morf, W. E., in The Principles of Ion-Selective Electrodes and of Membrane Transport, ed.Pungor, E., AkadCmiai Kiado, Budapest, 1981, p. 181. Lamb, J . D., Christensen, J . J . , Ocarson, J . L., Nielsen, B. L., Asay, B. W., and Izatt, R. M., J. Am. Chem. SOC., 1980, 102, 6820. Yamauchi, M., Imato, T.. Katahira, M., Inudo, Y., and Ishibashi, N., Anal. Chim. Acta, 1985, 169, 59. Amman, D., Bissig, R., Guggi. M., Pretsch, W.. Simon, W., Borowith, J . J . , and Weis, L., Helv. Chim. Acta, 1975,58. 1535. Hara, H., and Okazaki, S . , Analyst, 1985, 110, 11. Yoshio, M., and Noguchi. H., Anal. Lett., 1982, 15, 1197. Lamb, J. D., Izatt, R. M., Raberleen, P. A., and Christensen, J. J., J. Am. Chem. SOC., 1980, 102, 2452. Shpigun, L. K.. Novikov, E. A.. and Zolotov, Yu. A., Zh. Anal. Khim.. 1986, 41, 617. Hiraoka, M., in Crown Compounds, ed.Emanuel, N. M., Mir, Moscow, 1982, p. 266. Zelenkina, 0. A., Avtoreferaz Dissertation, Moscow State University, Moscow, 1987. Samoschin. V. V., Zelenkina, 0. A., Subbotin, 0. A., Sergeev, N. M., and Zefirov, N. S . , Zh. Org. Khim.. 1988, 24, 265. Moody, G . J . , and Thomas. J. D. R., in Ion-Selective Electrodes in Analytical Chemistry, ed. Freiser, H., Plenum Press, New York, 1978, p. 287. Camman, K., in Rabota s Ionoselectivnimi Electrodami, ed. Petruchin, 0. M., Mir, Moscow, 1980, p. 71. Mascini. M.. and Pallozzi, F., Anal. Chim. Acta, 1974,73,375. Lamb, Y. Y., Izatt, R. M., Garrick, D. G., Bradshow, J. S . , and Christensen. Y. Y., J. Membr. Sci.. 1981, 9. 83. Izatt, R. M., Bradshow, J . S., Nielsen, S. A., Lamb, J . D., and Christensen, J . J., Chem. Rev.. 1985. 85. 271. Samoshin. V. V., Sybbotin, 0. A . , Zelyonkina, 0. A . , and Zefirov, N. S . , Zh. Org. Khim., 1986, 22, 2231. Samoshin, V. V.. Zelyonkina, 0. A., Yartseva. I. V., and Zefirov. N. S., Zh. Org. Khim., 1987, 23, 2244. Samoshin. V. V., Zelyonkina, 0. A.. Yartseva. I . V., Sybbotin. 0. A., and Zefirov, N. S . . Zh. Org. Khim., 1988, 24, 2458. Potupov, V. M., Stereokhimia, Khimia, Moscow, 1976, p. 339. Wipff, G., Weiner, P.. and Kollman, P., J. Am. Chem. SOC., 1982, 104. 3249. Ronghino, G., Romano, S . , Lehn, T. M., and Wipff, G.,J. Am. Chem. SOC., 1985, 107, 7873. Takenchi, H., Arai, T., and Horada. I.. J . Mol. Struct.. 1986. 146, 197. Spuillacote. M., Sheridan, R. S., Chapman, 0. L., and Anet, F. A., J . Am. Chem. SOC.. 1975.97.3244. Moody. G. J . . and Thomas, J. D. R., Chem. Ind. (London), 1975, 644. Paper 1100922B Received February 26, 1991 Accepted November 6, 1991

ISSN:0003-2654    

DOI:10.1039/AN9921700853

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

ANALYST, MAY 1992, VOL. 117 857 Simple Voltammetric Method for the Determination of p-Carotene in Brine and Soya Oil Samples at Mercury and Glassy Carbon Electrodes B. Valentin Pfund and Alan M. Bond Department of Chemistry, La Trobe University, Bundoora, 3083, Victoria, Australia Terence C. Hughes* Denehurst Ltd., 961 Glenhuntly Road, Caulfield South, 3162, Victoria, Australia (3-Carotene is a naturally occurring yellow-orange pigment, which can be derived from saline micro-algae marine phytoplankton and some plant-derived natural oils. In this work, a simple method for the determination of (3-carotene, involving only solvent extraction from brine (or soya oil) samples into dichloromethane, followed by addition of electrolyte and direct measurement of the differential-pulse polarogram at mercury electrodes or differential-pulse voltammograms at a glassy carbon electrode, is described, based on the extremely well-defined two-electron oxidation process that occurs in non-aqueous solvents.The method has been applied t o soya oil and brine reference concentrates and t o feed and effluent samples associated with the production of 6-carotene via marine micro-algae. Excellent agreement with a well-established spectrophotometric method has been obtained, confirming that the simple voltammetric method should be a useful addition t o the analytical methodology available for monitoring the production of (3-carotene concentrates. Keywords: Voltammetry; analytical determination; (3-carotene (3-Carotene is a precursor of vitamin A and contributes to the colour of many plants.It is particularly well known as a yellow-orange pigment found in carrots. (3-Carotene contains 11 carbon-carbon double bonds in conjugation (Fig. 1) and owes its colour to absorption at the violet end of the visible spectrum (A = 451 nm).] In view of its colour, it is not surprising that spectrophotometric methods for the deter- mination of p-carotene have been widely used.2 In addition to being present in many plants, carotenoids, including p-carotene, are present at relatively high concentra- tions in saline micro-algae marine phytoplankton and other marine matter. For the determination of (3-carotene in marine samples, high-performance liquid chromatographic separa- tion procedures, coupled with spectrophotometric detec- tion3.4 and a very sensitive and specific resonance Raman method, have been described.5-6 In view of the presence of @-carotene in many natural products, it is not surprising that commercial products are usually derived from sources such as micro-algae. In the production of (3-carotene from natural sources, it is necessary to have quality control at every stage of the plant production process.Simple methods for the rapid determination of b-carotene are, therefore, required as an alternative or in addition to the commonly used spectrophotometrie methods,*-h which generally require relatively time-consum- ing separation procedures to achieve adequate selectivity. Despite the fact that the extensive series of conjugated double bonds that are present in the structure of (3-carotene should indicate that carotene is likely to be electroactive, the analytical use of voltammetric oxidation and/or reduction processes, known to occur on a range of electrode surfaces,7-13 has been rather limited.However, the amperometric detec- tion of (3-carotene in irradiated fruits after chromatographic separation has been described in detail.14 In this method, carotene esters and other carotene compounds are hydrolysed by addition of KOH, mixed with ethanol containing pyrogal- late and extracted into light petroleum for injection on to the chromatographic column. This application refers to the determination at naturally occurring trace levels, where interference from many other related electroactive com- pounds is expected to occur without inclusion of a chromato- graphic or other form of separation into the analytical methodology.In this work, we have investigated the possibil- ity of developing a more direct voltammetric method in which the @-carotene present in relatively high concentrations in the commercial concentrate is simply extracted into dichloro- methane containing an electrolyte, and a direct voltammetric determination is then undertaken in the non-aqueous solvent. Results from this very simple procedure are then compared with a spectrophotometric method to confirm the validity of the voltammetric method. Experimental A 30% (3-carotene sample (Hoffman-La Roche, Basle, Switzerland) was used to prepare a 10-2 mol dm-3 standard solution in dichloromethane (electrolyte). The reference material samples (technical-grade quality), provided by Betatene (Melbourne, Australia), were as follows: (i) 30% (3-carotene in soya oil, and (ii) 1.5% fi-carotene in brine.Samples obtained from various stages of the production of (3-carotene, and examined in this work, were also supplied by Fig. 1 Structure of &carotene * Present address: Unichema Australia Pty Ltd., 164 Ingles Street, Port Melbourne, 3207, Victoria, Australia.858 ANALYST, MAY 1992, VOL. 117 Betatene. The reference method for the determination of (3-carotene was a spectrophotometric procedure, which is approved by the AOAC.15 It consists of a sample homogeniza- tion step, separation and purification of the (3-carotene by quantitative solvent extraction into hexane, and a spectropho- tometric determination under standardized conditions at 436 nm.Before use, analytical-reagent grade dichloromethane was distilled with use of a 30 cm Vigreux column. The electrolytes were electrometric grade (G. Frederick Smith Chemicals, Columbus, OH) tetrabutylammonium perchlorate (Bu4N- C104) or tetrabutylammonium tetrafluoroborate (Bu4NBF4), used at a concentration of 0.1 mol dm-3 in distilled di- chloromethane. Initial vol tamme tric (polarographic) investigations to con- firm the mechanism of the electrode processes and to establish optimal conditions for the analytical procedures were under- taken with a PAR Model 174A polarographic analyser, equipped with a dropping-mercury or platinum-disc working electrode, a platinum-wire auxiliary electrode and an Ag- AgCl (dichloromethane; saturated LiCI) reference electrode. Analytical determinations of (3-carotene were carried out with a Metrohm Model 646 VA processor and Model 647 VA stand, with use of a multi-mode mercury working electrode operated in the dropping-mercury mode, or a glassy carbon working electrode, with a glassy carbon auxiliary electrode and the same Ag-AgC1 reference electrode as above.All experiments were undertaken at 20 k 1 "C and where necessary (for reduction studies) solutions were de-gassed with high-purity nitrogen to remove oxygen before comment- ing a voltammetric experiment. Results and Discussion Details of the Electrode Processes in Dichloromethane Fig. 2 shows a differential-pulse polarogram for a 1 mmol dm-3 solution of 0-carotene in dichloromethane (0.1 rnol dm-3 Bu4NC104).At a (peak) potential (Ep) of -1.70 V versus the Ag-AgC1, a well-defined reduction wave is observed, and at +0.59 V versus the Ag-AgC1, a narrower and larger oxidation process is observed. As also shown in Fig. 2, the reference compound ferrocene (Fc) exhibits a reversible one-electron oxidation process (Fc # Fc+ + e-) with a peak potential of 0.50 V versus the Ag-AgC1 under the same conditions. At a mercury electrode, no other waves were observed prior to the solvent limit (negative potential limit) or mercury electrode oxidation (positive potential limit). Mairanovsky et a1 .I3 report a reversible one-electron reduction with a half-wave potential (E; value) of -1.68 V versus the SCE in non-aqueous solvents to produce a L 1 I 1 I I 0.50 0 -0.50 -1.00 -1.50 -2.00 €N versus Ag-AgCI-LiCI Fig. 2 Differential-pulse polarogram (drop time = 1 s, pulse amplitude = 50 mV) for reduction and oxidation of p-carotene in dichloromethane (0.1 mol dm-3 Bu4NC104). 1, Solvent (baseline); 2, solvent and 1 X lo-' rnol dm-3 6-carotene; and 3, solvent with 1 x 10-3 mol dm-3 /3-carotene and 1 X 10-3 rnol dm-3 ferrocene. A.6-Carotene and B, ferrocene moderately stable anion radical (and other reduction processes at a more negative potential, which are outside the dichloromethane solvent range) and a two-electron oxidation process with an Eh value of +0.61 V versus SCE. The oxidation process is a reversible two-electron charge transfer with an irreversible chemical step, following charge transfer being observed with long-term domain experiments.The separation between the E,-values in this work and the El values reported in ref. 13 agree completely, confirming that the solvent is not particularly important in determining the potential for reduction or oxidation nor probably the mechan- ism for either process. In agreement with Mairanovsky et al., we find that the reduction process is a one-electron step and the oxidation a two-electron step, which disagrees with Takakachi and Tachi,8.9 who described the reduction as a four-electron process. The reduction process is chemically and electrochemically reversible in dichloromethane under conditions of cyclic voltammetry in the sense that it has a AE, value at both platinum and mercury electrodes for separation of reduction and oxidation components identical to that obtained for the known reversible one-electron oxidation of Fc.The product of the reduction in dichloromethane can, therefore, be postu- lated to be the anion radical as is the case in other non-aqueous solvents.13 As required for the proposed mechanism, the direct-current polarographic limiting current for the reduction step is one-half that of the oxidation process (ignoring sign differences), and, in agreement with Mairanovsky et al. ,I3 the oxidation process, while having electrochemically reversible two-electron characteristics with respect to charge transfer, exhibits some degree of chemical irreversibility at both platinum and mercury electrodes at a scan rate of 500 mV s- 1 (Fig. 3). Oxidation processes at more positive potentials are not discussed in this paper.The mechanism for oxidation proposed by Mairanovsky et a1.13 involves the initial formation of a dication by a two-electron charge transfer process followed, in longer time domain experiments, by proton loss to form a monocation and the one-electron reduction of the monocation to a neutral, but unstable, radical is then detected on the reverse scan of cyclic voltammograms (Fig. 3). The oxidation mechanism in di- chloromethane, while not studied in detail in this work, therefore appears to be the same as in other solvents examined by Mairanovsky et al. 13 In dichloromethane, the above data demonstrate that, in principle, either of two processes could be used for analytical purposes. However, the oxidation process is a more sensitive two-electron step, has a narrower half-width, is better resolved from the solvent limit process, and because it is an I I 0 0.2 0.4 0.6 0.8 1.0 E N versus Ag-AgCI Fig.3 Cyclic voltammograms (scan rate = 500 mV s-I) obtained at 1, a slowly growing mercury electrode and 2, at a platinum disc electrode for oxidation of 1 x 10-3 mol dm-3 (%carotene in dichlo- romethane (0.1 rnol dm-3 Bu4NCI04)ANALYST, MAY 1992, VOL. 117 859 oxidation occurring at relatively positive potentials, rather than a reduction step, its use does not require the removal of oxygen. This process was, therefore, employed in subsequent analytical studies. Differential-pulse Polarography at the Dropping-mercury Electrode Calibration With a drop time of 1.0 s and a pulse amplitude of 50 mV, a plot of differential-pulse peak height for the oxidation of (3-carotene in dichloromethane (0.1 rnol dm-3 Bu4NC104) was linear over the concentration range of 2 x to 10-3 rnol dm-3 with a correlation coefficient of 0.9997 (slope 27.5 pA/pmoi dm-3, intercept 0.006 PA).At concentrations above 2 x 10-3 rnol dm-3, non-linearity was observed in the calibration curve, which may have been a result of Ohmic iR drop. Consequently, determinations were confined to concen- trations up to 10-3 rnol dm-3 and were undertaken by the method of standard additions to avoid matrix effects. Determination of [%carotene in a reference soya oil sample A 100 mg sample of (3-carotene in soya oil (30%) (Betatene) was weighed into a 100 ml calibrated flask containing distilled dichloromethane (0.1 rnol dm-3 Bu4NCI04).After dilution to 100 ml with dichloromethane, the differential-pulse polaro- gram (drop time = 1.0 s, AE = 50 mV) was recorded for the solution over the range +0.20 to +0.85 V versus the Ag- AgCl. A well-defined differential-pulse peak corresponding to oxidation of 0-carotene was observed at +0.55 V versus the Ag-AgCI [ Fig.4(a)]. The standard-additions method was used to determine the @carotene in the sample with a value of 34.7 -t 0.8% of 0-carotene being obtained from four determina- tions, which is in satisfactory agreement with the manufac- turer’s nominal value of 30%. Recoveries of 100 k 3% were obtained for soya oil samples spiked with known amounts of fi-carotene, which also suggests that a valid procedure has been developed for the voltammetric determination of p- carotene in soya oil.10 pA I I 0.80 0.50 0.20 0.6 0.4 0.2 EN versus Ag-AgCI Fig. 4 Differential-pulse polarograms (drop time = 1 s, pulse amplitude = SO mV) obtained for the dctcrmination of (3-carotenc in ( u ) soya oil rcfcrcncc sample: 1. sample; 2, after addition of 5 x IO-’ rnol dm - j 13-carotene; and ( h ) brinc reference sample: 1. sample; 2. after addition of S x 10Y rnol dm-3 @-carotene; and 3 , after addition of 1 x 10-3 rnol dm-3 p-carotcnc. For details see text Determination of (3-carotene in brine A 2.50 mg sample of (3-carotene concentrate in brine (1.5%) (Betatene) was extracted with distilled dichloromethane (4 x 10 ml) and, after addition of 3.4 g of Bu4NC104, the extract was diluted to 100 ml with distilled dichloromethane. Dif- ferential-pulse polarograms [Fig.4(b)] obtained for the solu- tion and analysed by the standard-additions method, as for the soya oil sample, gave a value of 1.9 k 0.1% of (3-carotene, based on four determinations, which again is in satisfactory agreement with the manufacturer’s nominal value of 1.5%. As was the case with the soya oil sample, recoveries of 100 2 3% were obtained for samples of brine spiked with known amounts of @carotene. Table 1 Data for the differential-pulse polarographic determination of B-carotene in brine samples obtained at various stages of production from marine micro-algae (details of the method used are given in the text) Results (@-carotene concentration) Voltammetry Sample origin Mass used/g ( n = 4) Product 2 0.625 3.06 f 0.07 Product 3 50.0 37 f 3 Product 1 510 1.47 k 0.02 Effluent 1 46 1 0.06 k 0.02 Effluent 2 509 <0.05 (g kg- (mg kg-’) (mg kg- 1 (mg kg- 1 (mg kg- l ) t Y E 2 u 0.8 0.6 0.4 0.2 0.8 0.6 0.4 0.2 EN versus Ag-AgCI Fig.5 Differential-pulse polarograms (drop timc = 1 s, pulse amplitude = 50 mV) for the determination of (S-carotene in brinc plant feed samples. ( a ) product 2 sample: 1, sample; 2, sample plus 200 yl of 1 x 10-2 rnol dm-3 8-carotene standard; 3 , sample plus 400yl of 1 x 10-2mol dm-3 /3-carotene standard; ( h ) product 1 sample: 1, sample; 2, sample plus 100 yI of 1 x 10-2 rnol dm-3 B-carotene standard; 3 , sample plus 200 yl of 1 x lo-’ rnol dm-3 @-carotene standard. For details see Table 1 and textANALYST, MAY 1992, VOL.117 0.8 0.6 0.4 0.2 0.8 0.6 0.4 0.2 E N versus Ag-AgCI Fig. 6 Differential-pulse polarogram for the determination of (3-carotene in brine plant effluent samples. (a) Effluent 1 sample; (b) Effluent 2 sample. Curve 1 is the sample and curves 2 and 3 correspond to addition of 100 and 2001.~1 of 1 X 10-2moldm-3 (3-carotene standard, respectively. For details see Table 1 and text Determination of (3-carotene in brine during various stages of production In the commercial production of (3-carotene from marine micro-algae, a substantial number of determinations are required at each of various stages of the manufacturing process. Results for the determination of (3-carotene in the production samples and effluent samples are in excellent agreement with values determined spectrophotometrically, as indicated in Table 1.For the brine samples cited in Table 1, an aliquot of each sample was extracted with distilled dichloromethane (4 X 5 ml) with use of a centrifuge to speed up the separation of the aqueous and dichloromethane phases. After addition of 0.85 g of Bu4NC104, the extract was diluted to 25 ml with dichloro- methane, and a differential-pulse polarogram was recorded. In the ‘product 2’ sample, where (3-carotene concentrations are very high, the polarograms are well defined and equivalent to that in Fig. 3(b). In the ‘product 3’ and ‘product I’ feed samples, resolution from neighbouring peaks can be achieved and the expected peak from oxidation of (3-carotene is observed at +0.5-+0.6 V versus the Ag-AgCI (Fig.5 ) . In the example of the ‘effluent 1’ sample [Fig. 6(a], the (3-carotene levels are low and near the detection limit, whereas for the ‘effluent 2’ sample, the @-carotene concentration is below the detection limit [Fig. 6(b)]. However, in all instances, excellent agreement is obtained with the spectrophotometric method. Consequently, it is confirmed that the simple and direct method can be used for the determination of (3-carotene in these commercially important samples without the need for separation procedures other than those introduced via the solvent-extraction step. Of course, if samples at naturally occurring (3-carotene levels were being examined, where many compounds at much higher concentrations than (3-carotene would be present, chromatographic methods with amperometric detection, as described in ref.14, would almost certainly be required to 12 8 Q, .k 4 0 t a 7 6 5 0.3 0.6 0.9 0.3 0.6 0.9 E N versus Ag-AgCI 0.3 0.6 0.9 0 20 40 60 80 100 [p-Carotene]/mg I-’ Fig. 7 (a)-+) Differential-pulse voltammograms and ( d ) calibration graph for oxidation of (3-carotene at a glassy carbon electrode in 25 ml of dichloromethane (0.1 mol dm-3 Bu4NC104) after addition of an aliquot of a 1.00 g 1-1 @-carotene standard solution. Curve 1 is for a sample containing no (3-carotene, curves 2-5 are for samples containing 4 x 100 p1 additions of the @-carotene standard, and curves 6-10 are for samples containing 5 x 400 p1 additions of the (3-carotene standard. Duration between pulses = 0.8s. Pulse amplitude = 50 mV. For details see text achieve adequate resolution from overlapping peaks that could arise from the presence of compounds having similar oxidation (reduction) potentials to (3-carotene.In contrast to naturally occurring matrices, the sample we have been interested in, (3-carotene, is present at very elevated levels and is a major constituent. In this sense, it may, therefore, be surprising that no significant interference has been encountered in the analytical procedure. Differential-pulse Voltammetry at a Glassy Carbon Electrode and with Bu4NBF4 Electrolyte In a production plant environment, analytical procedures requiring mercury, as used with the technique of polar- ography, may not be considered to be suitable from the aspect of occupational health. Consequently, differential-pulse vol- tammetry at a glassy carbon electrode was examined as an alternative to polarography at a dropping-mercury electrode.As shown in Fig. 7, well-defined curves and linear calibration curves can be obtained at the glassy carbon electrode, and data essentially indistinguishable from those obtained at the mercury electrode are observed for all samples. Use of a glassy carbon electrode for asymmetric detection after chromato- graphic separation obviously also would be preferred in a liquid chromatography-electrochemical detection method. Similarly, replacement of a 0.1 mol dm-3 Bu4NC104 by 0.1 mol dm-3 Bu4NBF4 was examined and also found to be equally suitable for the determination of P-carotene. Perchlor- ate electrolytes are often regarded as potentially explosive materials, and the use of tetrafluoroborate electrolyte as anANALYST, MAY 1992, VOL.117 861 alternative may, therefore, be preferred. Fortunately, the peak potentials and characteristics of the voltammetric curves obtained at carbon, platinum or mercury electrodes with either perchlorate or tetrafluoroborate electrolytes are essen- tially the same as described in detail for those at the dropping-mercury electrode, with 0.1 mol dm-3 Bu4NC104 as the electrolyte, so that the occupationally safer alternatives are also viable. Conclusions A simple voltammetric procedure for determining 6-carotene, involving extraction from brine into dichloromethane, fol- lowed by direct determination in dichloromethane with 0.1 mol dm-3 Bu4NCI04 or 0.1 mol dm-3 Bu4NBF4 as the electrolyte, has been shown to be applicable to solutions relevant to the production of 6-carotene from marine micro- algae and phytoplankton.Equivalent results to those of methods based on the well-established spectrophotometric procedure are obtained, and the method should be useful for monitoring important stages of plant production of p-caro- tene. Equivalent and equally useful voltammetric methods could also probably be developed for other components in the increasingly important field of the determination of caro- tenoids and porphyrins in foods and natural products. We thank Betatene Ltd. for the supply of the S-carotene samples. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 References Morrison, R. T., and Boyd, R. N., Organic Chemistry, Allyn and Bacon, Boston, MA, 4th edn., 1983, p. 1153. Walton, H. F., and Reyes, J., Modern Chemical Analysis and Instrumentation. Marcel Dekker. New York, 1973. Crompton, T. R., Determination of Organic Substances in Water, Wiley Interscience, Chichester, UK, 1985, vol. 2, p. 498. Abaychi, J. K.. and Riley. J. P., Anal. Chim. Acta, 1979,107, 1. Beyermann, K., Organic Trace Analysis, Ellis Honvood. Chichester, UK, 1985. p. 235. Hoskins, L. C., and Alexander, V., Anal. Chem., 1977,49,695. Takahashi, R.. Rev. Polarogr., 1961, 9. 247. Takahashi, R., and Tachi, 1.. Agric. Biol. Chern., 1962,26,771, 777. Takahashi, R., and Tachi, I . , Abh. Dtsch. Akad. Wiss. Berlin, Kl. Med., 1966, 589. Kuta, E. J., Science, 1964, 144, 1130. Kuta, E. J., and Yu, M., Lipids, 1967, 2, 411. Mairanovsky, V. G., Engovatov, A. A., and Samokhvalov, G. I., Zh. Org. Khirn., 1970,6, 632. Mairanovsky, V. G., Engovatov, A. A., Ioffe, N. T., and Samokhvalov. G. I.. J. Electroanal. Chem., 1975, 66, 123. Argneessens, R., Nangniot, P., Lacroix, J. P., and Muri, D., Bull. Rech. Agron. Gembloux, 1989, 24, 85; Chem. Abstr., 1989, 111, 1327012. Official Methods of Analysis of the Association of Official Analytical Chemists, ed. Williams, S . , AOAC, Washington, DC, 14th edn., 1984, Nos. 4301443023. p. 834. Paper I fO3684J Received July 19, I991 Accepted November 11, I991

ISSN:0003-2654    

DOI:10.1039/AN9921700857

出版商:RSC    

年代:1992

数据来源: RSC

摘要:

ANALYST, MAY 1992. VOL. 117 863 Monitoring and Assay of Water Treatment Additives by Alternating Current Tensammetry and Voltammetry: Scope and Limitations Pierre M. Bersier" Central Analytical laboratory, Ciba-Geigy Ltd., Basle, Switzerland William Neagle and David Clark Ciba- Geig y In d us tria I Ch em ica Is, Tra ffo rd Park, Man ch ester, U K The alternating current (a.c.) tensammetric behaviour of different commercially available water treatment additives is described. Possibilities and limitations of their routine determination by a.c. tensammetry at low levels (0.510 ppm) in different aqueous media are discussed. Indirect differential-pulse voltammetry via the 12-molybdophosphate derivative allows a classification between phosphino-containing and phosphorus-free water treatment compounds.Practical examples are given. Keywords: Water additives; routine determination; alternating current tensammetry; voltammetry The whole spectrum of industry, manufacturing and engineer- ing, textiles and chemicals, food and drinks, even leisure and service industries, depend on pure water. Pure water is, however, not widely or readily available. It often has to be chemically treated or obtained from sea-water in limited amounts. World-wide efforts to develop chemicals for water treat- ment are being made. These chemicals are essential for modern industry and desalination technology for controlling problems such as: (i) scaling: scaling is a build-up of solid material formed on the inner surface of boilers, for instance, when the concentration of the impurities in the water used exceeds their solubility limit and precipitation occurs;' (ii) microbiological fouling: fouling is the deposition of materials, normally in suspension, onto heat-transfer or other surfaces such as boilers; and (iii) corrosion: corrosion is the destruction of a metal by electrochemical reaction with its environment.' These problems must be dealt with in a safe and environmen- tally sound manner.* The need to control the concentration of water treatment chemicals and to control operating costs make it desirable to use cost-effective water treatment products that can be applied with minimum operator involvement.Chemicals used in non-precipitation programmes are either: (i) chelants, forming complexes with calcium and magnesium;' or (ii) sequestrants (solubilizing agents), which, in the same way as chelants, keep calcium and iron in solution, but are less corrosive.' These formulations typically contain phosphonates, poly- (acrylates), poly(methacrylates), poly(ma1eates) and poly- meric dispersants.Operating at threshold levels as opposed to the stoichiometric reaction of chelant reaction programmes, the polymers and phosphonates function primarily by altering o r distorting the crystalline structure of hardness precipitates. The new technology eliminates corrosion problems associated with chelants and the excessive precipitation common with phosphate treatments. Typical sequestering compounds include aminotri(methy1- enephosphonic acid) or NTP (Wayplex NTP) and hydroxy- ethylenediphosphonic acid (HEDPA) (see ref.3). Poly- (acrylates) and poly(methacrylates) play leading roles in todays most advanced treatment programmes.' Belasol, for instance, was developed to meet the needs of the oil industry. It ensures that sea-water pumped through permeable rock to push the oil up will not leave impurities behind which would clog the rock and block oil recovery.* Belgarde EV, on the * Present address: Gstaltenrainweg 61, 4125-Riehen. Switzerland. other hand, is a liquid polymer scale control additive based on an entirely new branch of poly(ma1eic acid) chemistry.* A factor that is common to all the compounds used is that their monitoring and the control of their concentration is difficult owing to the low levels encountered. In some instances this has led to their being blended with low levels of heavy metal ions as markers.' Analytical Methods Commonly used methods for determining phosphonate-con- taining additives are reported to be difficult, time consuming and plagued by interferences.3 A rapid method based on the ultraviolet (UV)-catalysed oxidation of the phosphonate moiety to orthophosphate has been reported.The phosphate compound is determined as (3-12-molybdophosphate by UV absorption at 700 nm.3-4 This method is, however, not specific for the intact phosphonate additive and additives as such. For the assay of some additives without a phosphonate moiety, fluorescence has been pro- posed.4 This method, although very sensitive, is not specific for the intact additive. Poly(acry1ates) and poly(methacry1ates) are highly surface- active and hence have a strong influence on the differential capacity of the electrical double layer at the mercury-water interface.5--8 They exhibit strong alternating current (a.c.) tensammetric signals.The a.c. tensammogram of a poly- (acrylic acid) such as PAA-1 is shown in Fig. 1 (cf. Table 1). Pospisil and Kuta studied the behaviour of maleic acid at a mercury electrode by a.c. polarographyg and its influence on the electrocapillary curve.") The present study shows that the direct a.c. tensammetric assay of poly( maleic compounds) is feasible. This paper discusses the possibilities and limitations of a.c. tensammetry for the direct assay and monitoring of selected modified phosphinocarboxylic acids (PCA- 1 , PCA-2, PCA-3 and HPA, cf.Table 1). A.c. tensammetry is also applicable to poly(acry1ic acids) (PAA-1-PAA-5, cf. Table 1) and poly- (maleic acids) (PMA-1, PMA-2, cf. Table 1), additives which contain no phosphinate groups, cf. Table 1. Experimental Apparatus A.c. tensammetric measurements were carried out with a Metrohm Polarecord 506 in conjunction with a Metrohm VA 633 multielectrode stand, using a hanging mercury drop as the working electrode.864 ANALYST, MAY 1992, VOL. 117 For the indirect differential-pulse voltammetric measure- ments of the p-12-molybdophosphate, a Polarecord 506 or 626 or an Amel Model 471 Multipolarograph and a dropping mercury electrode as the working electrode were used. A platinum wire was used as the counter electrode and a saturated calomel electrode as the reference electrode.The latter was connected to the cell by means of a double salt-agar bridge. All tensammetric and voltammetric measurements were made at room temperature (23 k OS'C), in de-aerated solutions. Reagents and Equipment The additives studied, which are summarized in Table 1, were all of the purest grade available. All other chemicals were of analytical-reagent grade and were used as received. Generic structures of the three groups of water treatment additive (I, I1 and 111) examined in this work are given in Table 2. The chemical composition of the model waters, viz., Ca-50, Ca-300, an artificial sea-water formulated according to DIN I I I I 0 - 500 - 1000 - 1500 NmV versus SCE Fig. 1 A.c. tensammograms of poly(acry1ic acid), PAA-1, recorded in this laboratory.Supporting electrolyte: model water Ca-300- 0.12 mol dm-3 sodium perchlorate, pH 4.4. Working electrode, hanging mercury drop. Applied alternating voltage, 15 mV (root mean square) at 75 Hz. Curve 1, supporting electrolyte; 2 , l ; 3,2; 4,4; 5.10; 6,20; 7,50; 8,100; 9,200; and 10,500 mg I-' of PAA-1. A, Rest current depression; and B, desorption peaks 5090011 and an artificial oil-loaded formation water, is given in Tables 3-5. Sep-Pak CI8 cartridges (Waters), when used for the assay of water treatment additives in polluted sea-water, were acti- vated with 5 ml of methanol and washed with 10 ml of doubly distilled water. Direct a.c. tensammetric assays were performed by adding the stock solution or sample solution to the appropriate supporting electrolyte.Procedure For the a.c. tensammetric assay, the following experimental procedure was applied. Stock solutions of each additive were prepared by dissolving approximately 150 mg of the additive of known concentration in 10 ml of doubly distilled water. For the indirect voltammetric determination of phosphonate-con- taining additives via fi-12-molybdophosphate, the UV-cata- lysed oxidation of the phosphino group to orthophosphate was performed in a glass cell with a thermostatically controlled heating mantle connected to a Lauda water-bath, using a high-pressure 125 W Hg tube. To 2 ml aliquots of the sample were added 1 ml of 1 mol dm-3 NaOH, 1 ml of concentrated H2S04 and 1 g of ammonium peroxydisulfate, and the volume was made up to 20 ml with doubly distilled water.The pH of this solution is about 7. The solution was irradiated for 20 min at 70 "C. The irradiated sample was then transferred quanti- Table 2 Generic structures of the three groups of water treatment additive (I, I1 and 111) examined (cf. Table 1) Group I : Phosphinocarboxylic acids- Group I1 : Poly(acry1ic acids)- Group 111: Poly(ma1eic acids)- +:&-::& Table 1 General and a.c. tensammetric data for the water treatment additives examined Compound PCA-1 (1) PCA-2 PCA-3 PAA-1 (11) PAA-2 PAA-3 PAA-4 PAA-5 PMA-1 (111) PMA-2 PMA-2 PMA-2 TCA ( W HPA P(MA/ME) P( MNSSA) Group Phosphinocarboxylic acid Phosphinocarboxylic acid Phosphinocarboxylic acid Poly(acry1ic acid) Poly(acry1ic acid) Poly(acry1ic acid) Poly(acry1ic acid) Poly(acry1ic acid) Poly(ma1eic acid) Poly(maleic acid) Poly(ma1eic acid) Poly(maleic acid) Triazinecarboxylic acid Hydroxyphosphonocarboxylic acid Maleic acid-ethylacrylate copolymer Maleic acid-st yrene-sulfonic acid copolymer Relative molecular mass -3000 -2700 -700 -2000 -5000 -4500 -2000 -3000 =700 500-550 -500 - 1250 -470 156 800-850 -1500 Lower detection limit (ppm) 1 1 1 1 1 1 1 1 1 1 1 1 0.1 10 1 1 Linear range 1-10 1-10 1-600 ( P P 4 - - - - - 1-(50) 1-600 1-60 0.1-20 10-50 1-10 - -ANALYST, MAY 1992, VOL.117 865 Table 3 Composition of Ca-50 and Ca-300 model waters Concentratiodmmol dm-3 Water S042- HC03- Ca*+ Mg2+ c1- Ca-50 0.4 0.3 0.5 0.5 1.1 Ca-300 0.4 6 3.0 3.0 6.1 Table 4 Composition of artificial sea-water11-12 Constituent Amount present*/g NaCl 28 MgSO4.7H20 7 MgC12.6H20 5 NaHC03 0.2 CaCI2.6H20 2.4 * In 985 ml of distilled water.Table 5 Composition of artificial oil-loaded formation water (density, 1.029 g cm-3; pH, 5.5; and ionic strength, 0.77 mol dm-3). The water is prepared by shaking artificial Gullfaks formation water with Gullfaks crude oil (SO + 50, v/v) for 24 h at ambient temperature Constituent Concentratiodmg I-L Na+ K+ Ca’+ Mg*+ Sr*+ Ba2+ c1- HC03- co32-- sop 14 570 330 1 040 305 260 50 25 600 400 0 0 Total dissolved solids: 42 555 tatively into a 50 ml calibrated flask and made up to the mark with doubly distilled water. To a 4 ml aliquot of this solution were added 4 ml of acetone and 2 ml of a solution containing 30 g of Na2MoO4-2H20, 24 g of tartaric acid and 90 ml of HCI (32%) per litre, and the solution was transferred into the polarographic cell.After de-aeration with pure nitrogen (99.998%) for 10 min, the voltammograms were recorded in the differential-pulse mode. The exact experimental conditions are given in the figure legends. Results and Discussion Quantitative Determinations Typical a.c. tensammograms of the different classes of additive (cf. Table 1) are illustrated in Fig. 2. The four additives, as shown in Fig. 2, were measured under identical experimental conditions. For a practical assay, however, the optimum conditions for each class and com- pound must be established. Variation in the concentration of the surface-active com- pound affects the depth of the depression of the current of the pure supporting electrolyte (curve 1 in Fig. 1) in addition to the change in the height and the position of the desorption peaks (cf.Fig. 1). Both effects can be exploited for the quantification of water treatment additives. Tensammetric waves frequently behave differently to fara- daic processes (cf. refs. 5-8, and references cited therein). A characteristic of polarographic and voltammetric techniques is the broad linear dependence over six or more decades. It is therefore much broader than that of most other instrumental methods. In tensammetry the dependence of the value of the measured capacity current on the concentration of the surfactant generally has a non-linear character. Hence a linear t c ‘ I I I I I I I I I I I 0 -500 -1000 -1500 -2000 0 -500 -1000 -1500 -2000 NmV versus SCE Fig. 2 I , PCA-I; (b) Group 11, PAA-1; (c) Group 111, PMA-1; and ( d ! Group IV, TCA.Curve 1: supporting electrolyte, 0.9 N lithium sulfate, pH 4; curve 2: additive concentration, 1.5 x mol dm-3 (cf. Table 4) A.c. tensammograms of water treatment additives: (a) Grou E I5O E a r 0, a z .- Y 100 a 2 a c 3 50 c .- 0 cz 9 t 0 1 2 3 4 5 [PCA-I]/pg ml-’ Fig. 3 Calibration graph for the determination of PCA-1 (‘as is’), measured in 0.1 mol dm-3 sodium fluoride, pH 11. Different symbols indicate replicate measurements dependence between the measured signal height (depression of the current of the supporting electrolyte) or the height of the desorption peaks, respectively, is observed only at comparably low concentrations and over a narrow concentra- tion range (Fig. 3). The calibration graphs obtained under the experimental conditions exhibit linear ranges that depend on the compound under study and are therefore a characteristic of each additive.The linear ranges observed are summarized in Table 1. At low concentrations the standard additions method can be applied for the quantitative determination of the water treatment additives, provided that the sum of the analyte present after addition of the reference substance still falls within the linear part of the graph. For quantitative determination in low concentration ranges, the depression of the rest current (cf. Fig. 1) was exploited in most instances. The lowest detection limits are in the range 0.1-1 ppm (cf. Table 1). At higher additive concentrations, a dependence of the peak potential (Ep) on the logarithm of the additive concen- tration (log c) is observed in most instances (Fig.4).866 w 0 a 0 - s $ -20 u* -60 -40 . ANALYST, MAY 1992, VOL. 117 - - - +60 I 1 + +40 20 I -80 1 ,/ , , I -100 10 100 1000 10000 [PCA-Il/pg rnl-1 Fig. 4 Peak potential ( E J versus log of the PCA-1 concentration. The first desorption peak is exploited for quantitative assay. Different symbols indicate replicate measurements +215 rnV 1 NmV versus SCE --t Fig. 5 Differential-pulse voltammograms of 6-12-molybdophos- phate (procedure according to Fogg and co-workers.13.*4 Curve 1, sample after UV-catalysed oxidation; and curves 2 4 , sample after standard additions of phosphate (equivalent phosphate concentra- tions = 3.07, 6.08 and 9.03 pg ml-1 of polarographic solution) Classification of the Different Additives A serious shortcoming of tensammetry and of all electrochem- ical methods in general is their limited specificity and selectivity .The difficulties in determining surfactants in mixtures and interference problems are certainly among the drawbacks hindering the use of tensammetry as a viable analytical technique. The potential of the tensammetric peaks must differ by at least 0.2-0.3 V. The concentration of the most strongly adsorbed component must be such that the coverage of the electrode surface is less than 100%. Otherwise, only the peak of the most strongly adsorbed compound will be detected on the tensammograms.5-8 A direct differentiation between the four classes of water treatment additive (I-IV, cf. Fig. 2 and Table l), viz., (I): additives with phosphino groups; (11): without phosphino groups; (111): poly(maleic acids) (cf.Table 2); and (IV): miscellaneous, in the concentration range of interest [O. 1( 1)- 10 ppm] is not possible with the a.c. tensammetric procedure described here. The UV and voltammetric determination of the phosphino group, however, allows a preliminary classifica- tion between group I and groups I1 and 111. The voltammetric wave of (3-12-molybdophosphate ob- served at a glassy carbon electrode (cf. Fig. 5 ) has been used as Table 6 Comparison of differential-pulse voltammetric and spectro- photometric results for the determination of the phosphorus content of selected commercial water treatment additives after UV-catalysed oxidation of the phosphinate or phosphonate moiety to orthophos- phate Phosphorus (%) Polarographic Sample assay PCA- 1 0.92 0.97 0.98 PCA-2 0.98 0.76 PCA-3 2.54 2.67 HPA 9.4 9.3 9.1 Spectrophotometric assay 1.02 0.98 0.93 0.77 2.41 2.21 11.16 11.08 - - 80 I I E E .4 d J= P) r Y m a m 60 .- Y 40 2 3 >- m a "u 20 0" 0 2 4 6 8 10 [AdditiveVpg rnl-' Fig. 6 Influence of the relative molecular mass of phosphinocar- boxylic acid additives on the slope of the peak height. A, HPA; B, PAC-3; C, PCA-2; and D, PCA-1 the basis of a method for determining orthophosphate.13.14 Hence, voltammetry can serve as an alternative method to spectrophotometry for the determination of phosphate. Good agreement was found between phosphate concentrations determined by voltammetry and data obtained by a spectro- photometric method,*4 as shown in Table 6.The advantage of the electroanalytical methods used here is that both the voltammetric and the a.c. tensammetric assays can be carried out with the same instrument (a polarograph). Qualitative Determination Based on Tensammetric Measurements The shape and peak potentials of the tensammetric waves depend on the nature of the compound studied and, therefore, can, in some instances, give qualitative information. Jehrings showed that with increasing relative molecular mass (M,) (1000,3000, 5000,20 000) the peak potential of poly(ethy1ene glycols) moves progressively towards more negative values. The shift is a linear function of the reciprocal average M , . Resolved peaks are obtained. The peak width decreases with increasing M,.Bagdasarov et ~ 1 . 1 5 observed that the slope of the plot of capacity current versus concentration increased with increas- ing length of the hydrocarbon chain, for instance from C8 to CI2 (cf. Fig. 2 in ref. 15). Distinct differences between the slope of the measured capacity current (ic) (depression of theANALYST, MAY 1992, VOL. 117 867 1000 1 1 v v C 10 20 Additive/mg dm-3 Fig. 7 Adsorption of a water treatment additive onto the surface of iron powder as a function of the amount of additive. Medium: model water Ca-SO containin 4 g l - l of Fe (20h at 40°C). 0, pH 5.5 de-aerated solution; 8, pH 7.5 de-aerated solution; 0, pH 7.5 aerated solution; V, pH 9.5 de-aerated solution; and A, pure water, pH 7.5. A, 0.79; B, 1.2U1.26; and C, 1.53 mg g-l of Fe rest current or the peak height, respectively) and the M , of different phosphonate-containing additives of group I are observed (Fig.6). Hence, for the phosphinocarboxylic acids HPA (average M , =: 160), PAC-3 (average M , = 700) PCA-2 (average M , = 2700) and PAC-1 (average M , = 3000), provided that the amount of phosphonate present is known (the determination is carried out with the indirect procedure via (3-12-molybdo- phosphate described above), the slope of a graph of i, versus c furnishes qualitative information on the additive present, as is revealed by an inspection of Fig. 6. A distinction between the phosphinocarboxylic acids containing PCA-1 and PCA-2 with an average M , of 3000 and 2700, respectively, is, however, not possible, as shown in Fig.6. In practical applications, such as monitoring, not all of these compounds are present in a mixture. Hence, tensammetric measurements should permit both their determination down to fairly low levels and, for phosphinocarboxylic acids, with greatly different M , values, their qualitative determination/ identification. Applications In addition to the examples mentioned above, the following examples serve to illustrate the possible applications of the a.c. tensammetric techniques to the routine practical analysis of water treatment additives . Tensammetric determination of additives in the presence of iron (powder) performed on model waters Experiments run in the presence of iron powder (4 g per litre of model water Ca-50, for instance) showed no direct influence of iron or iron ions on the tensammetric assay of the additive examined.Changes in the concentration of the additive at the ppm level, owing to adsorption onto the surface of the iron powder, could be followed in de-aerated and aerated media. Hence the determination of the amount of an additive adsorbed on a surface and the thickness of the adsorbed layer can be monitored as a function of the given experimental conditions, such as temperature, composition, pH of the bath and time. The adsorption of an additive onto the surface of iron powder as a function of the amount of the additive is shown in Fig. 7. Determination of scale inhibitors in oil-loaded artificial sea- water and oil-loaded artificial formation water Concentrations of 0.5-400 ppm (range tested) of the scale inhibitor PCA-1 could be determined in oil-loaded artificial I I 1 1 10 100 1000 PCA-l/pg ml-1 Fig.8 Recovery of PCA-1 added to artificial sea-water (0, 0) and artificial formation water (A). Measured by the proposed a.c. tensammetric procedure sea-water and formation water (for details of the composition of these waters see Tables 4 and 5 ) in the presence of a de-emulsifier (100 and 200 ppm) by a.c. tensammetry. The assay was performed in 0.1 mol dm-3 NaF, pH 11, supporting electrolyte, after separation and accumulation on a Sep-Pak CI8 cartridge with or without prior extraction of the oil with CCI4, using the procedure described by Gruenfeld.16 A hanging mercury drop electrode was used as the working electrode. In pure 0.1 mol dm-3 NaF, pH 11, a linear relationship between the height of the desorption peak and the additivehhibitor concentration was found in the range 0.5-5 ppm.In the range 100-9000 ppm, a linear (EP versus log c ) dependence was observed. Recoveries of PCA-1 added to oil-loaded artificial forma- tion water and artificial sea-water are illustrated in Fig. 8. Conclusions A.c. tensammetry, combining high sensitivity with good precision, appears to constitute a very convenient electro- chemical procedure for the assay of water treatment additives, which represent a particularly important group of compounds in modern technology. It is therefore a valuable alternative to the methods commonly used. However, the practical analytical chemist, who is interested in the potential applications of tensammetric techniques, should be familiar with the variability of these techniques towards medium effects.The type and concentration of electrolyte may influence the wave shape and position far more than faradaic processes (cf. refs. 5-8). As stressed by Bond,” extreme care and commonsense must, therefore, be employed in using tensammetric techniques in routine analysis. The skillful technical assistance of H. G. Wenzel and J.-P. Worch is gratefully acknowledged. References Sendelbach, M. G., Chem. Eng., 1988, August, 127. Sykes, S . , Ciba-Geigy J.. 1988, 3, 10. Hach’s Water Analysis Handbook. Hach, Loveland, CO, Phosphonates. Range: 0-20 mg 1- 1 . Persulfate/UV Oxidation Method for Water, pp. 2-234-2-236. Clark, D., unpublished work. Bersier, P. M., and Bersier, J . , Analyst, 1988, 113, 3. Kalvoda, R., Pure Appl. Chem., 1987. 59. 715. Kalvoda, R.. and Parsons, R., Electrochem. Res. Dev., (Proc. UNESCO Forum. 1984, Publ. 1985; Chem. Abstr.. 105, 690 15n).868 ANALYST, MAY 1992, VOL. 117 8 Jehring, H . , Elektrosorptionsanalyse mit der Wechselstrompol- arographie, Akademie Verlag, Berlin, 1974. 9 Pospisil, L., and Kuta, J., Collect. Czech. Chem. Commun., 1968,33, 3040. 10 Pospisil, L., and Kuta, J., Collect. Czech. Chem. Commun., 1969,s. 3047. 11 Deutsche Industrie Norm (DIN) 50900, November, 1960. 12 Rompp Chemie Lexikon, eds. Falbe, J., and Regitz, M., Georg Thieme, Stuttgart, 1991, vol. M-PK, p. 2669. 13 Fogg, A. G., and Bsebsu, N. K., Analysr, 1981, 106, 369. 14 Fogg, A. G., Bsebsu, N. K., and Birch, B. J., Talanta, 1981,28, 473, and references cited therein. 15 16 17 Bagdasarov, K. N., Lokshina, G. A., Sadimenko, L. P., and Sokolov, V. P., Zh. Anal. Khim., 1986, 41, 171. Gruenfeld, M., Environ. Sci. Technol.. 1973, 7. 636. Bond, A. M., Modern Polarographic Methods in Analytical Chemistry, Marcel Dekker, New York, 1980. Paper 1 I01 516H Received April 2, 1991 Accepted December 13, 1991

ISSN:0003-2654    

DOI:10.1039/AN9921700863

出版商:RSC    

年代:1992

数据来源: RSC