摘要:

RSC ANALYTICAL DIVISION NORTH WEST REGION A Meeting on STATISTICS IN ANALYTICAL CHEMISTRY will be held at the University of Lancaster on March 14th, 1984 This meeting is intended for practising analytical chemists who wish to familiarise themselves with the statistical techniques that should be used in the handling and treatment of data. The speakers will be Derrick Chamberlain, Richard Boddy, John Sykes, Roland Caulcutt, Trevor Lilley and Michael Gardner. For further information contact Dr. M. L. Hitchman, Department of Chemistry and Applied Chemistry, University of Salford, Salford, M5 4WT. ROYAL SOCIETY OF CHEMISTRY ANALYTICAL DlVl SlO N East Anglia Region and Automatic Methods Group INSTITUTE OF FOOD SCIENCE AND TECHNOLOGY EASTERN BRANCH NEW METHODS FOR THE QUALITY CONTROL OF FOODS AND NATURAL PRODUCTS New Hall, Cambridge, 2nd and 3rd April, 1984 A meeting to discuss near infrared and other instrumental methods for QC. Full details from A. M. C. Davies, Food Research Institute, Colney Lane, Norwich, NR4 7UA.

ISSN:0144-557X    

DOI:10.1039/AP98421FX004

出版商:RSC    

年代:1984

数据来源: RSC

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ANALYTICAL DIVISION DIARY February, 1984 Reprint of an Important Analytical Chemistry Review Standardised Thin-layer Chromatographic Systems for the Identification of Drugs and Poisons by A. H. Stead, R. Gill, T. Wright, J. P. Gibbs, A. C. Moffat The October '82 issue of The Analyst featured a review entitled Standardised Thin-Layer ChromatorrraDhic Systems for the Identification of Drugs and Poisons. The wide use of T C for the anilys'is of drugs and poisons in biological fluids and pharmaceutical preparations suggests that this article will be of great interest to many analysts working in the field. Consequently, The Royal Society of Chemistry has decided to make separate reprints available. COVERAGE This review gives criteria for good systems and applies them to the selection of the eight most effective.The selected systems are standardised by the use of standard running conditions and the use of reference compounds. Rr x 100 values are given for 594 basic, 48 neutral and 152 acidic drugs on the selected systems both in alphabetical order and ascending order of Ri for each system to aid the identification of unknown drugs. Further identification is enhanced by the inclusion of various locating procedures. Price f5.75 ($11.50)-including p Et p to all U.K. and European destinations, and Surface mail outside Europe. $1 3.00 for Airmail outside Europe. To order, send payment to the address below: The Royal Society of Chemistry The University Nottingham, NG7 2RD, England 87 Chemicals in the Oil Industry Chemicals in the I Oil lndustrr Edited bv P.H.Ogden [ Chemicals in the Oil Industry discusses some of the problems associated with oil production, particularly where the use of chemical additives is concerned. This book sets out to define those problems which can be solved through the use of chemical additives, the type of chemical currently favoured, the level of service required to supply such chemicals effectively to the oil industry, the volume of chemicals used, and the financial outlay required of the oil producer.Brief Contents: Application of Chemistry to the Drilling Operation; Chemicals for Water-based Drilling Fluids and Their Temperature Limitations; The Development and Application of Oil-base Muds; Chemical Aspects of Oilwell Cementing; The Role of Chemicals in Oil and Gas Production; Chemical Demulsification of Produced Crude Oil Emulsions; Oily Wastewater Treatment in the Production of Crude Oil; The use of Ethylene-Vinyl Acetate Copolymers as Flow Improvers and Wax Deposition Inhibitors in Waxy Crude Oil; Water Scaling Problems in the Oil Production Industry; The Chemistry of Corrosion Inhibitors Used in Oil Production; Ouarternary Ammonium Compounds: Evaluation and Application in the Control of Sulphate-resuding Bacteria; The Role of the Service Company in Offshore Operations; The Market for Chemicals in the Oil Industry; Special Publication No.45 (1983) Softcover 224pp 0 851 86 885 1 Price f12.00 (622.00) RSC Members f9.00 RSC Members should send their orders to The Royal Society of Chemistry, Membership Officer, 30 Russell Square, London WC1 B 5DT Non-RSC members should send their order to The Royal Society of Chemistry Distribution Centre, Blackhorse Road, Letchworth, Herts SG6 IHN, England Electronically typeset and printed by Heffers Printers Ltd, Cambridge, England SPECIAL PUBLICATIONS Industry and the Environment in Perspective I t-.Edited by R. E. Hester inprrrpcahn E h r d b f R t C k ~ r L J Over the past few decades environment and industrial issues have become increasingly the subject of public interest. Generally environmentalist issues (especially when linked with industrialist issues) are surrounded by controversy, and a clear viewpoint is hard to establish. Industry and the Environment in Perspective sets out to establish a clearer understanding of environmental and industrial matters by presenting chapters on a wide range of topics within this important subject. Softcover 234pp 0 851 86 895 9 Brief Contents: Policy Making in the Environmental Field; An Industrialist's View; An Environmentalist's View; The Law and its Execution; The Determination of Concentrations and Physico- chemical Speciation of Environmental Pollutants; Particulate Matter in the Atmosphere; Acid Rain in Perspective; Recent Changes in Sources of Odour Nuisance and Odour Abatement Techniques; Waste Management: Transportation and Disposal of Toxic Waste; Leachates from Mining and Landfill; The Ecological Impact of Liquid Effluents; Occupational Health Risk Assessment; Heavy Metals; Trends in the World Environment; Special Publication No. 46 (1983) Price f13.00 ($24.00) RSC Members f9.50

ISSN:0144-557X    

DOI:10.1039/AP98421BX006

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

ANPRDI 21(2) 43-88 (1984) February 1984 Hon. Secretary R. Sawyer Analvtical Proceedinas Proceedings of the Analytical Division of The Royal Society of Chemistry AD President S. Greenfield Hon. Treasurer D. C. M. Squirrel1 Hon. Assistant Secretary D. I. Coomber, O.B.E. Hon. Publicity Secretary Dr. J. F. Tyson, Department of Chemistry, Loughborough University of Technology, Loughborough, Leicestershire, LEI 1 3TU Secretary Miss P. E. Hutchinson Editor, Analyst and Analytical Proceedings P. C. Weston Senior Assistant Editors Mrs. J. Brew, R. A. Young Assistant E ditor Ms. D. Chevin Publication of Analytical Proceedings is the responsi- bility of the Analytical Editorial Board: J. M. Ottaway (Chairman) L. S. Bark G. J. Dickes L. C. Ebdon A. C. Moffat J. M. Skinner J. D. R. Thomas A.M. Ure *P. C. Weston *G. W. Kirby J. Whitehead *Ex officio members All editorial matter should be addressed to: The Editor, Analytical Proceedings, The Royal Society of Chemistry, Burlington House, Piccadilly, London, WIV OBN. Telephone 01-734 9864. Telex 268001. Advertisements: Advertising Department, The Royal Society of Chemistry, Burlington House, Piccadilly, London, WIV OBN. Telephone 01-734 9864. Analytical Proceedings (ISSN 0144-557X) is published monthly by The Royal Society of Chemistry, Burlington House, London, W1V OBN, England. All orders, accompanied by payment, should be sent to The Royal Society of Chemistry, The Distribution Centre, Black- horse Road, Letchworth, Herts., SG6 IHN, England. 1984 Annual Subscription price if purchased on its own: UK €53.00, Rest of World €56.00, US $106.00, including air speeded delivery. Air freight and mailing in the USA by Publications Expediting Inc., 200 Meacham Avenue, Elmont, N.Y.11003. USA Postmaster: Send address changes to: Analytical Proceedings, Publications Expediting Inc., 200 Meacham Avenue, Elmont, N.Y. 11003. Second class postage paid at Jamaica, N.Y. 11431. All other despatches outside the UK by Bulk Airmail within Europe, Accelerated Surface Post outside Europe. PRINTED IN THE UK. 0 The Royal Society of chemistry, 1984. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form, or by any means, electronic, mechanical, photographic, recording, or otherwise, without the prior permission of the Dublishers.Reports of Meetings Scottish Region The forty-ninth Annual General Meeting of the Region was held at 5 p.m. on Friday, November llth, 1983, at the University of Strathclyde Staff Club, Glasgow. The Chair was taken by the Chairman of the Region, Dr. G. A. Best. The following office bearers were elected for the forthcoming year: Chairman-Dr. G. A. Best. Vice-Chairman-Dr. A. F. Fell. Honorary Secretary- Dr. D. E. Wells, Freshwater Fisheries Laboratory, Pitlochry, Perthshire. Honorary Treasurer-Dr. Mary R. Masson. Honorary Assistant Secretary-Dr. Janet Warren. Members of Committee-Dr. M. Adams, Mrs. Anna Hunter, Dr. J. Marshall, Dr. A. Rowley, Mr. E. Simpson, Dr. P. Smith and Dr. J. E. Whitley (ex officio). Dr. R. A. Chalmers and Dr. B. East were re-appointed as Honorary Auditors.East Anglia Region The sixteenth Annual General Meeting of the Region was held at 1.45 p.m. on Thursday, November 3rd, 1983, at Glaxo Group Research, Ware, Hertfordshire. The Chair was taken by the Chairman of the Group, Mr. G. M. Telling, The following office bearers were elected for the forthcoming year: Chairman-Mr. G. M. Telling. Vice-Chairman-Mr . B. Woodget. Honorary Secretary and Treasurer-Mr. A. M. C. Davies, Food Research Institute, Colney Lsne, Norwich, NR4 7UA. Honorary Assistant Secretary-Mr. J. M. H. Freeman. Members of Committee-Mr. A. Anderson, Mr. B. Garwood, Mr. N. W. Joyce, Mr. R. P. Munden, Mr. J . Spencer and Dr. R. J. Whiteoak (ex officio). Mr. A. G. Croft and Mr. C. Waterhouse were re-appointed as Honorary Auditors. Biological Methods Group The thirty-ninth Annual General Meeting of the Group was held at 6 p.m. on Tuesday, November 22nd, 1983, at the Royal Society of Chemistry, Burlington House, London, W.l. The Chair was taken by the Chairman of the Group, Dr. M. E. Duncan. The following office bearers were elected for the forthcoming year: Chair- man-Dr . M. E. Duncan. Vice- Chairman-Mr . D . Sykes. Honorary Secretary-Dr. A. H. Thomas, National Institute for Biological Standards and Control, Holly Hill, Hampstead. London, NW3 6RB. Honorary Treasurer-Dr. L. Singleton. Honorary Assistant Secre- tary-Mr. G. A. Sabey. Members of Committee-Mr. D. Hossack and Dr. P. Turmer. Dr. J. H. Hamence and Dr. M. W. Parkes were re-appointed as Honorary Auditors. 43

ISSN:0144-557X    

DOI:10.1039/AP9842100043

出版商:RSC    

年代:1984

数据来源: RSC

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44 SAC SILVER MEDAL Anal. Proc., Vol. 21 Ronald Belcher Memorial Lectureship In recognition of the late Professor Ronald Belcher’s interest in education, the Council of the Analytical Division has inaugurated a Belcher Memorial Lecture to be given annually on an analytical topic by a graduate student. The award will be considered by the Honours Committee, acting on behalf of the Council of the AD. The aim of the award is to commemorate Professor Belcher as a teacher, by encouraging students to make a positive contribution to, and take an active part in, the profession of analytical chemistry. Rules I . Candidates must currently be registered postgrad- uate students of a British University or Poly- technic. 2. The merits of a particular candidate may be brought to the notice of the Honours Committee by any supervisor of postgraduate students registe- red with a British University or Polytechnic who desires to recommend the candidate, by letter addressed to the President of the Division.‘The letter shall be accompanied by a paper written by the Student. 3. The award shall be made annually in December and shall be based on an over-all assessment of the originality of the work described in the paper and the significance of its contribution to analytical chemistry. The winner of the award will be expected to present his or her work at the Research and Development Topics Meeting fol- lowing the award. 4. The award will take the form of a presentation scroll plus a sum of f150. This sum is to assist the candidate to attend a national or international conference. It will be given to the candidate, up to two years after the granting of the award, on presentation of satisfactory evidence o f the candi- date’s intention to attend such a conference. 5 . An award shall not be made if it is considered by the Honours Committee that none of the papers submitted reaches the required standard. 6. The decision of the Council of the Analytical Division shall be final. 7. Any alteration to these Rules shall be subject to the approval of the Council of the Analytical Division. Submissions should be sent to the President, Analy- tical Division. Roydl Society of Chemistry. Burlington Hou\e, London, W 1V OBN The closing date I\ Friday, September 28th, 1984.

ISSN:0144-557X    

DOI:10.1039/AP984210044b

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

February, I984 ATOMIC-ABSORPTION SPECTROPHOTOMETERS ANALYTICAL METHODS COMMITTEE REPORT PREPARED BY THE INSTRUMENTAL CRITERIA Evaluation of Analytical Instrumentation. Part 1. SUB-COMM ITTEE 45 Atomic-absorption Spectrophotometers, Primarily for Use with Flames The Analytical Methods Committee has received and approved the following report from its Instrumental Criteria Sub-committee. Introduction The following report was compiled by the above Sub-committee of the AMC, which consisted of Professor S. Greenfield (Chairman), Professor E. Bishop, Mr. N. Barnett, Dr. L. Ebdon, Mr. D. Squirrell, Dr. P. Smith, Mr. A. Westwell and Professor G. F. Kirkbright (corresponding member) with Mr. C. A. Watson as Honorary Secretary. The purchase of analytical instrumentation is an important function of many laboratory managers, who may be called upon to choose between a wide range of competing systems which are not always easily comparable. The objective of the Instrumental Criteria Sub-committee is to tabulate a number of features of analytical instruments which should be considered when making a comparison between various systems.As is explained below, it is possible to then score these features in a rational manner, which allows a scientific comparison to be made between instruments. The over-all object is to assist purchasers in obtaining the best instrument for their analytical requirements. It is also hoped that, to a degree, it will help manufacturers to supply the instrument best suited to their customers’ needs. No attempt has been made to lay down a specification.In fact, the Committee considers that it would be invidious to do so; rather, it has tried to encourage the purchasers to make up their own minds as to the importance of the features that are on offer by manufacturers. This first report of the Sub-committee deals with atomic-absorption spectrophotometers that are primarily intended for use with flames as the atom source. A further report is being prepared for instruments that are intended primarily for use with electrothermal atomisation. Notes on the Use of This Document Column 1. The features of interest. Column 2. What the feature is, and how it can be evaluated. Column 3. The Sub-committee has indicated the relative importance of each feature and expects users to decide on a weighting factor according to their own needs.Column 4. Here the Sub-committee has given reasons for its opinion as to the importance of each feature. Column 5 onwards. It is suggested that scores are given for each feature of each instrument and that these scores are modified by a weighting factor and sub-totals obtained. The addition of the sub-totals will give the final score for each instrument. Notes on scoring 1. (PS) Proportional scoring. It will be assumed, unless otherwise stated, that the scoring of features will be by proportion, e.g., Worst/O to Best/100. 2. (WF) Weighting factor. This will depend on individual requirements. An indication of the Sub-Committee’s opinion of the relative importance of each feature will be indicated by the abbreviations VI (very important), I (important) and NVI (not very important).A scale is chosen for the weighting factor which allows the user to discriminate according to needs, e.g., X l to X3, or, X l to X10. The factor could amount to total exclusion of the instrument. 3. (ST) Sub-total. This is obtained by multiplying PS by WF.INSTRUMENTAL CRITERIA SUB-COMMITTEE INSTRUMENT EVALUATION FORM Type of instrument: Atomic-absorption Spectrophotometers for Flame Atomic Absorption Manufacturer: Model No. : Feature 1. Hollow-cathode lamp stations ( b ) Modulation (c) Method of lamp alignment 2. Burner - nebuliser assembly (a) Range of fuels and oxidants (b) Materials of construction Definition and/or test procedures and guidance for assessment Number of lamps under operating conditions should be commensurate with the analytical requirement, bearing in mind the possible use of multi-element lamps.Type and frequency-score high for electronic modulation, non-multiples of mains frequency and also for higher frequency. Two axis adjustment by accessible controls preferred. Maximum score for acetylene - hydrogen - propane and air - nitrous oxide and appropriate burner heads. Minimum score for air - acetylene only. Maximum score for titanium burner with inert plastic spray chamber and tantalum nebuliser. Reduce score for other materials such as stainless steel or glass unless demanded by a special application. Importance VI VI Reason Economic (speed of analysis versus financial commitment). Suppression of unwanted d.c. signals, rejection of mains noise and low frequency noise from nebuliser and gas flows.Alignment of source on optical axis. Correct selection of fuel - oxidant enables maximum analytical coverage and minimises matrix effects. Safety, durability and resistance to corrosion and memory effects. Score PS WF ST PS WF ST PS WF ST PS WF ST PS WF ST(c) Spray chamber characteristics (d) Methodof positional adjustment of the burner ( e ) Nebuliser uptake adjustment cf) Dissolved solids capability 3. Monochromator - optics ( a ) Temperature stability ( b ) Focal length andfnumber Maximum score for spray chamber with impact-bead - baffle assembly, which gives maximum production of small particles with narrow distribution. Maximum score for calibrated scales for lateral, vertical and rotational adjustment and accessible controls for each.Maximum score for widest range. Reduce score for fixed uptake rate with minimum score for highest fixed uptake. Plot signal and standard deviations of a series of solutions containing 5 p.p.m. of lead and 0-20% of magnesium chloride. Aspirate each solution for 5 min. AhPC, change in wavelength per degree. If instrument required for trace analysis, score maximum for long focal length and highf number. For instruments required for analysis of major constituents, score maximum for short focal length and low fnumber. VI I I I VI I interferences are minimised and noise is also reduced with mall droplets. Alignment for maximum absorption and/or linear :ahbration. Uptake rate affects sensitivity, freedom from interferences and usage of sample.(NOTE: Drop size and distribution are also affected by uptake rate.) A low tolerance to solids will result in burner - nebuliser blockage with degradation of sensitivity and precision. Elimination of instrumental drift, particularly important when high-temperature flames are used. Long focal lengths and high fnumbers lead to a narrow beam in the flame. This results in only the central reducing region being examined and gives improved sensitivity and greater freedom from interferences. Short focal lengths and lowfnumbers fve high light throughput and ence good signal to noise ratio, but this may be invali- dated by increased shot noise due to extra flame emissions reaching the detector. PS WF ST PS WF ST PS WF ST PS WF ST PS WF ST PS WF STFeature (c) Slits ( d ) Grating ( e ) Wavelength (i) Read-out precision (ii) Repeatability (f) Numberof reflective and refractive elements (8) Background correction ( h ) Dispersion, resolution and resolving power Definition and/or test procedures and guidance for assessment Score minimum for fixed slits, intermediate for stepwise adjustment and maximum for continuously variable.Additional score for height adjustment facility. Mount and blaze angle. Modified Czerny-Turner mount generally preferred to Ebert or Littrow as stray light characteristics better. Maximum score for blaze angle nearest the wavelengths of most interest. Four figure digital read-out preferred. Maximum score for smallest range of transmission values following re-setting to a previously located line.Score maximum for minimum number of o tical elements. Score extra &r quartz-coated optics. Score maximum for widest wavelength range for which background can be corrected. Additional score for ease of replacement of source if present. (See NOTE 111.) Score maximum for: small angular deviation; high angular dispersion; small reciprocal linear dispersion; small resolution; high resolving power. Importance I I I I I I NVI Reason Spectral discrimination and control of luminous flux. Suitable blaze angle required to ensure adequate radiation throughput throughout the range of interest. The useful working range is approximately from two-thirds to three times the blaze wavelength, the fall of efficiency being particularly sharp at the short wavelength limit.Ease of re-setting instrument. Ability consistently to locate analytical wavelength. Maximum energy throughput with minimum scatter. Coated optics will increase the useful life of the instrument. Particularly important if ETA is contemplated or when samples with very high solids content are analysed by flame. Normally adequate for AAS, but important if emission measurements are contemplated. Score PS WF ST PS WF ST PS WF ST PS WF ST PS WF ST PS WF ST PS WF ST(i) Slewing speed (j) Scanning speed ( k ) Singleor double beam 4. Gas control unit (a) Pressure for stable operation (b) Safety features (c) Manual/ electronic Score maximum for maximum speed. For automatic instru- ments score maximum for speed and accuracy and ability to identify a line unambiguously. Score maximum for lowest speed and greatest range of speeds.Double beam preferred for long continuous sample runs. Single beam for lower cost and better sensitivity. Score maximum for wide range consistent with safe working practice. Score zero if minimum acetylene pressure is above 9 Ib in-2. Score maximum for “Auto Shut Down,” “Pressure Dro Sensors,” “Gas Cylinder VaEe Heaters,” and correct burner type and location sensor. Electronic preferred in general. NVI NVI NVI VI VI I Economic, speed of analysis for automatic instruments. Normally adequate for “peaking” in AAS, but important if emission measurements are contemplated. Double beam eliminates any residual drift resulting from the source. This is only of importance when extended sample runs are contemplated, e.g., auto-sampling.Single beam systems may be preferred for lower costs and minimum detection limits. Wide range enables best fuel - oxidant usage under conditions that minimise fluctuations in flow-rates. The operation of acetylene flames at over 9 Ib in-2 is generally not permitted in the UK. Safe operation of instrument. Electronic control provides an extra element of safety and convenience for most situations, but may not be compatible with hydrogen as fuel, as gases are shut off in the wrong order and many flame sensors fail to detect hydrogen flames. PS WF ST PS WF ST PS WF ST PS WF ST PS WF ST PS WF STFeature ( d ) Number of fuel and support gas inlets (e) Flow-rate indication (f, Auto-ignitors 5 , Detectors 6. EHTsupply (a) Voltage range ( b ) Meansof adjustment 7 .Amplifier ( a ) Type ( b ) T‘ ime constants Definition and/or test procedures and guidance for assessment Score maximum for maximum number. Score maximum for digital indication and wide range of flow for each gas line. Score according to preference. Score maximum for the availability of a photo- multiplier tube which meets most requirements, and for the ease of interchange. Score maximum for wide range and digital read-out of applied voltage. Adjustment by calibrated control preferred. Automatic continuous adjustment of EHT is not desirable. Synchronously demodulated “lock-in” normal; score maximum for above type with largest number of attenuation ranges. Score maximum for widest available range and number Importance I T NVI I VI I Reason Enables instrument to be conveniently operated with desired fuel - oxidant combinations.Ease of reproducing conditions. Convenience of operation. A suitable photomultiplier is required to cover the wave- length range of the elements of interest. Where one photomultiplier cannot give sufficient spectral range, ease of interchange is important, as is the ability of the replacement to attain working stability rapidly. Reproducible instrument operation and minimum detector noise. Consistent signal to noise ratio can only be achieved by operation at constant EHT. Operational versatility and removal of unwanted d.c. signals. Minimising noise, consistent with signal type and efficient sample utilization. Score PS WF ST PS WF ST PS WF ST PS WF ST PS WF ST PS WF ST PS WF ST PS WF ST( c ) Integration 8.output ( a ) Read out type (b) Interface 9. Amenities (a) Modular construction ( b ) Bench space required (c) Services (d) Automation ( e ) Servicing and spares Importance of ability to change integration times is dependent on the particular instrument. Score maximum for avail- ability of analogue, digital, printer and graphics output. Score maximum for suitable interface, e.g., RS232, IEEE, BCD or ASCII. Self explanatory. Self explanatory. Electrical, plumbing, drainage. Various items such as sample presentation, lamp selection, wavelength setting, slit setting and burner operation may be automated. Enquire in detail as to local arrangements. I These items will have varying importance to different users and should be scored and rated accordingly.If the signal contains a high proportion of (white) noise, integration will improve the precision. Signals that are dominated by proportional (pink) noise will not show much improvement in precision upon integration. Digital read-out and printer are particularly suitable for quality control applications and measurement of small signals, while analogue and graphics outputs are beneficial when measuring transient peaks. Compatibility with available printers or microprocessors/ computers. Allows expansion of system to meet changing needs. The instrument must fit the laboratory or expensive modifications may be needed. Installation of additional services (e.g., 3-phase power supply) will increase cost of installation. Items such as auto-samplers are essential for some users (e.g., ETA), while other automation may be desirable if large numbers of samples are to be handled.Automation also reduces operator errors and invariably improves precision. Cost of service and downtime may severely alter over-all running costs. PS WF ST PS WF ST PS WF ST PS WF ST PS WF ST PS WF ST PS WF ST PS WF STFeature U, Applications support (g). Availability of major accessories and updates 10. Over-all performance ( a ) Baseline stability (b) Figures of merit (i) Precision (ii) Sensitivity (slope of calibration curve Definition and/or test procedures and guidance for assessment En uire as to availability of appqications support in field(s) of interest. Enquire about manufacturer’s policy on updating software and compatibility of present and future accessories. Allow 30 min to warm up then take readings at 1-min intervals for 30 min with only the lamp running (electronic stability).Repeat the above measurements with the flame running and water aspirating to give over-all base-line stability. Calculate the standard deviations and score accordingly. Use concentrations of test elements to give nominal absorbances of 0.0,0.005, 0.01,0.02.0.05,0.1,0.2,0.5, 1 .O and 2.0 based on the manufacturer’s sensitivity data, assuming that a linear relationship exists. Measure each solution at least 6 times, using scale expansion for readings below 0.1. Solutions of 0.01 and 0.2 should be measured at 1-min intervals for 30 min to give precision data. Calculate standard deviation and score maximum for lowest.Calculate slope of line and score maximum for highest slope. Importance I VI VI VI Reason Time and facilities for method development may add significant costs. Future analytical requirements. Affects accuracy and precision. A shorter warm-up period may be suitable for double-beam instruments but, although base lines stabilise more rapidly, sensitivity will still vary until the system has stabilised. Self explanatory. Self explanatory. Score PS WF ST PS WF ST PS WF ST PS WF ST PS WF ST(iii) Linear range (iv) Detection limit (v) Curve correction 11. Value for money Points per % Calculate from calibration curve. Score high for widest linear range. Calculate from twice standard deviation of reagent blank. Score maximum for lowest. Use the curve correction facility to linearise the calibration function in ( b ) above and spray a solution with a known concentration and a nominal absorbance of 1.2-1.5.Score maximum for the most accurate result. ~~ ~ Sum of previous sub-totals divided by the purchase price of the instrument. Subject to proportional scoring and weighting factor, as for previous features. Include ST in grand total. VI VI NVI I Self explanatory. Self explanatory. Self explanatory. Simple instruments areoften good value for money, whereas those with many refinements are often costly. PS WF ST PS WF ST PS WF ST Sum of totals sub- PS WF ST Grand total NOTES (I) Measurement can either be made using a recorder, which should be run for 10 s with a time constant of 1-2 s, or usin integration and a digital read-out, in which case the integration time should be no more than 5 s and should be as similar as possible for ah instruments being evaluated. (11) The user can employ any element(s) that he feels may be of importance; however, the following elements are thought to be particularly useful: Arsenic at 193.7 nm-Evaluate performance at the far ultravioiet end of the instrument range. Lead at 217.0 nm-Short wavelengths, widely determined, often difficult at levels of interest. Nickel at 232.0 nm-Requires good resolution to achieve good sensitivity and linearity. Aluminium at 309.3 nm-Uses N 2 0 - C2Hz flame which has high emission in this region. Will test flame stability and effectiveness of Copper at 324.7 nm-Typical “easy” mid-range element for which fi ures of merit are quoted by some manufacturers. Calcium at 422.7 nm-Similar to copper, above, but uses N 2 0 - CJf2 flame. Potassium at 766.7 nm-Most common long wavelength element, requiring red sensitive photomultiplier tube. modulation in excluding flame noise. (111) The efficiency of background correction depends on the availability of equal time constants in both channels of the amplifier and the ability to balance the source intensity for both channels. A test for the efficiency can be made by evaluating the positive interference caused by 10 000 p.p.m. of sodium chloride on 5 p.p.m. of lead at 217.0 nm and 10 p.p.m. of arsenic at 193.7 nm.

ISSN:0144-557X    

DOI:10.1039/AP9842100045

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

ATOMIC SPECTROSCOPY SYMPOSIUM 54 Anal. Proc., Vol. 21 Atomic Spectroscopy Symposium The following are summaries of four papers presented at a Meeting of the Analytical Division held on May 24th, 1983, at the Scientific Societies’ Lecture Theatre, Savile Row, London, W.l. Coherent Forward Scattering and its Application to Elemental Analysis Alan Walsh 11 Dendy Street, Brighton, Victoria, 3 186, Australia An interesting spectrochemical development of the past decade has been the study, mainly by Japanese scientists, of methods of elemental analysis based on the measurement of forward scattering. Such methods involve passing a beam of radiation through an atomic vapour, as in atomic-absorption and atomic-fluorescence methods, and measuring the forward scattering, i. e . , the radiation scattered in the direction of the transmitted beam.From an analytical point of view, one of the most interesting characteristics of forward scattering is that it is coherent, ie., the scattering from different atoms is in phase. As a consequence the intensity of the forward scattering is, for low values of atomic absorption, proportional to W , where N is the number of atoms. By contrast, the lateral scattering by an assembly of atoms, as measured in conventional atomic-fluorescence methods, is non-coherent and its intensity is proportional to N . It is perhaps surprising that spontaneous radiation processes can lead to coherent radiations, but R. H. Dicke has pointed out that when atoms are interacting with a common radiation field they cannot be treated as being independent.1 Various methods233 have been used to isolate the forward scattered radiation from the incident beam. The arrangement used in all analytical applications of coherent forward scattering (CFS) measurement is that used by Corney et al.3 in their detailed investigation of the forward scattering of resonance radiation. In this arrangement the incident radiation is linearly polarized before passing through the atomic vapour, which is subjected to an external magnetic field. The transmitted radiation passes through a second polarizer having its plane of polarization perpendicular to that of the first polariser. Thus, in the absence of a magnetic field, no radiation from the light source is transmitted by the crossed polarisers. When the field is switched on, however, the polarization state of the incident light is modified, resulting in an output signal which is due to forward scattered radiation.Corney et al. have given a theoretical derivation of the intensity of this scattering and shown that it can be expressed in terms of the absorption and dispersion of the incident beam by the atomic vapour in the magnetic field. In this respect it may be recalled that in the classical wave theory of the propagation of light through a polarisable medium the absorption and refraction of light are explained in terms of the scattered waves. Similarly, the theory can be extended to propagation through an atomic vapour in a magnetic field, which results in each atomic absorption line having a Zeeman fine structure.The absorption by each of these Zeeman components depends on the polarisation of the incident radiation. Thus, the polarisation of the incident radiation is modified by passage through the atomic vapour. Similarly, the refractive index, and therefore the phase velocity of the radiation, at the wavelength of each Zeeman component also depends on the polarisation state of the incident radiation. Thus, dispersive effects also modify the polarisation state of the radiation transmitted by the atomic vapour. Accounts of such classical explanation of magneto-optical effects are to be found in standard texts on physical optics.4.5 When the field is parallel to the direction of propagation the plane of polarisation is rotated. This is the Faraday effect. With a transverse field, the transmitted radiation is elliptically polarised.This is the Voigt effect. The profile of the forward scattered radiation depends on the Zeeman pattern of the resonance line and on the orientation of the magnetic field. For parallel fields this profile shows a minimum at the wavelength of the resonance line. Thus, sharp-line atomic spectral lamps, as used in atomic-absorption measurements, are not well suited to the measurements of the Faraday effect. It therefore becomes necessary to operate the lamps at much higher powers than usual if satisfactory signals are to be obtained. In this instance, however, the resulting self-reversal effects are not harmful. With transverse fields, the line profile may exhibit a pronounced maximum at the resonance line wavelength.In this instance, any self-reversal effects are obviously deleterious. As in conventional atomic fluorescence methods, a continuum light source can be used, and has the advantage of being applicable to the determination of several elements. The intensity at the various resonance line wavelengths, however, is usually much less than can be obtained with line sources.February, 1984 ATOMIC SPECTROSCOPY SYMPOSIUM 55 CFS methods of elemental analysis have several attractive features. First, as already mentioned, there is the possibility of improved detection limits and sensitivity due to the W dependency. Secondly, any molecular species or scattering particles in the atomic vapour will not usually be optically active and will not therefore give any output signal.They may, however, attenuate the magneto-optical signal and this attenuation must be measured in order to obtain the tone signal due to atomic scattering. Thirdly, a continuum light source can be used and will thus permit simultaneous multi-element analysis. However, this requires that one set of atomisation conditions be used for the various elements, a requirement that is difficult to meet whether using furnace or flame methods of atomisation. Most of the investigations of CFS methods of analysis have been carried out in Japan and references 6-11 list some of the key papers. Many further papers on the subject have been published in the past 5 years in The Analyst and Spectrochirnica Acta B. Most have been concerned with furnace methods of atomization, the apparatus used being only slightly different from that using atomic-absorption methods incorporating Zeeman methods of background correction.To date, CFS methods have not yielded detection limits greatly superior to those obtained by atomic absorption methods and any improvement is limited to a small number of elements. A multi-element CFS system has recently been reported. 12 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. References Dicke, R. H., Phys. Rev., 1954, 93, 99. Griffiths, R. B . , and Dicke, R. H., Rev. Sci. Instrum., 1957, 28, 646. Corney, A., Kibble, B. P., and Series, G. W., Proc. R. SOC. London, Ser. A , 1966, 293, 70. Wood, R. W . , “Physical Optics,” Macmillan, New York, 1934. Jenkins, F. A . , and White, H. E . , “Fundamentals of Physical Optics,” McGraw Hill, New York, 1937 Church, D.A . , and Hadeishi, T., Phys. Rev. A , 1973, 1866. Church, D. A., and Hadeishi, T., Appl. Phys. Lett., 1974, 24, 185. Ito, M., Anal. Chem., 1980, 52, 1592. Kitagawa, K . , Shigoyasu, T., and Takeuchi, T., Analyst, 1978, 103, 1021. Yamamoto, M., and Murayama, S . , J . Opt. SOC. Am., 1979, 69, 781. Yamamoto, M., Murayama, S., Ito, M., and Yasuda, M., Spectrochim. Acta, 1980, 35B, 43. Win, P., Debus, H., Hanle, W., and Scharmann, A , , Spectrochirn. Acta B, 1982, 1013. Recent Inn ovations in Electro t he rm a I Atomisa t io n John M. Ottaway Department of Pure and Applied Chemistry, University of Strathclyde, Cathedral Street, Glasgow, G1 1 XL Commercial electrothermal atomisation systems have now been available for about 12 years and provide significant analytical advantages in terms of sensitivity, reduced sample size, low gas consumption and the possibility of direct atomisation of solid samples, which have been well documented elsewhere.1-3 The major problems that have arisen concern chemical interferences caused by the sample matrix, particularly where the matrix contains large amounts of chloride ions. Many publications have described such interference effects in detail and the literature has recently been the subject of an excellent review by Slavin and Manning.4 Although background absorption interference is serious for some sample types, modern graphite furnace and spectrometer design has generally alleviated this problem. Over the past 12 years, numerous procedures have been devised in attempts to overcome chemical interferences in a wide range of analytical applications.Amongst the most popular have been (a) pre-separation of the analyte from the matrix, (b) the use of the standard additions procedure, (c) the coating of graphite tubes with materials such as tantalum or zirconium carbides, which have been shown to reduce interferences from chloride matrices, (d) matrix modification and (e) the use of matrix matched standards. Even where the effects of chemical interferences can be minimised by matrix modification, it has become common practice to use either matrix matched standards or the standard additions method in routine analytical procedures, in order to overcome residual variations in sensitivity caused by variations in the sample matrix.Such methods are generally inconvenient and more time consuming. Whilst electrothermal atomisation is now used widely for routine analysis, particularly in the clinical and environmental fields, the limitations of the method have become well known and are often stressed by authors involved in the development of competitive techniques. It should be emphasised, however, that the high sensitivity of electrothermal AAS methods means that,56 ATOMIC SPECTROSCOPY SYMPOSIUM Anal. Proc., Vol. 21 in the absence of the pre-separation of the analyte from the matrix, the analyte to matrix ratio is often much lower than that used or even contemplated with other techniques, and under these conditions matrix interferences could be expected to be more serious. A single example can be used to illustrate the current practice of electrothermal atomisation methods.The most popular method for blood lead determinations is probably that described by Stoeppler et al.5 This involves pre-treatment of the sample with nitric acid, which subsequently acts as a matrix modifier. Analysis is then normally carried out, for example, by our col!eagues in the Glasgow Royal Infirmary, using specially prepared blood standards. In other laboratories with a smaller workload, standard additions to each sample are often preferred. Thus, two of the procedures identified above, either (d) and (e), or (d) and (b), are combined to produce results of the highest accuracy, but with some inconvenience owing to the need for sample pre-treatment and special standardisation routines.The major cause of matrix interferences mentioned above is the release or formation of the analyte as gas phase molecular species rather than atoms. In the Massmann furnace design6 incorporated in almost all commercial atomisers, the sample is deposited on the internal wall of the graphite tube when cold, and is consecutively dried, ashed and atomised by heating the tube. The analyte is therefore released into the vapour phase when the tube temperature reaches the appropriate evaporation or atomisation temperature of analyte molecules or atoms. For volatile elements such as cadmium, lead and thallium evaporation temperatures are below 1 200 “C and the vapour temperatures available for the dissociation of molecular species during their residence time in the furnace are consequently low.The formation of molecular species may occur from oxyanion media, for example for aluminium and boron, but for most elements molecule formation is increased from solutions containing high concentrations of chloride salts, such as are prevalent in many clinical and environmental samples? When introduced, the Massmann furnace design provided both a convenient method of sample introduction for the user and simplicity of operation, and has undoubtedly been responsible for the widespread application of the technique in routine laboratories. However, the Massmann design represented a departure from the early L’VOV~ and WoodrifP-10 designs of electrothermal atomisers, which allowed atomisation under the constant high-temperature conditions more favourable for molecular dissociation.Higher temperatures will obviously increase the dissociation of gas-phase molecules, but constant temperature conditions are also required to reduce the effects of the variation of matrix element concentrations on the rate of evaporation and hence the time and temperature conditions experienced by the analyte vapour. The problem with the early designs of constant- temperature furnace was that they appeared to be severely impractical and too complex for use in routine laboratories. In a key publication that appeared in 1978, L’vovll refocused attention on the need to modify electrothermal atomiser design to allow constant-temperature atomisation conditions to be approached more closely in order to reduce interference effects.Since then a number of new and innovative designs have been investigated, many of them being derived from suggestions made in L’vov’s paper. Two general approaches can be identified. In the first the heating rate of the graphite furnace is increased so that high and constant temperature conditions are achieved during the lifetime of the analyte species. The use of capacitive discharge heating with a bank of condensers has been investigated by Chakrabarti and co-workers,12,13 who have achieved heating rates up to lo5 K s-1. Significant reductions in interference effects from chloride salts have been demonstrated. Recent commercial atomiser systems can also achieve significantly increased heating rates of up to 2 000 K s-1.However, the use of high heating rates alone is not satisfactory, as it is then less likely that the vapour inside the tube will remain in local thermal equilibrium with the tube wall temperature. L’vovll suggested the use of a graphite platform installed inside the furnace to delay the production of analyte vapour to a time when the furnace and vapour exist at higher and more constant temperature conditions. Since then much effort has been devoted to the investigation of the optimum use of platforms with commercial atomisers, most notably by Slavin and co-workers. 1 4 ~ 5 As Slavin covered this topic in his paper, only a few comments are made here on the use of platforms. The platform method is undoubtedly the simplest and most convenient method of improving the performance of the current range of commercial instruments.In my opinion it should be adopted in most routine analytical procedures, the only exceptions being methods for the most involatile elements where the lower temperatures and slightly lower heating rate of the platform seriously impair sensitivity. In our own laboratory, a platform method has been investigated for blood lead determinations. l6 Pre-treatment/ matrix modification with nitric acid is still required, but standardisation can be achieved with aqueous (nitric acid) lead solutions without the need for either matrix matched standards or the standard additions technique. This is typical of most platform methods. Although greater freedom fromFebruary, 1984 ATOMIC SPECTROSCOPY SYMPOSIUM 57 interference results from the use of the platform, a matrix modifier is still required in most procedures15 to remove residual interference effects.The second general approach to the achievement of constant-temperature atomisation conditions is conceptually more ideal than the above, in that the sample is introduced directly into the pre-heated furnace operating at constant temperature as in the L'vov8 and Woodriff designs.") In the L'vov furnace8 the sample was evaporated/atomised from an electrode heated by an auxiliary power supply, and was then passed into the centre of the tube. In recent work, Frech and Jonsson17 have reported an analogous system using two CRA 90 power supplies'to heat the tube and a sample cup separately. In the Woodriff furnace samples are either introduced by nebulisation into the pre-heated furnace or via an electrode which is heated by contact with the tube wall.") Another approach, which is analogous to the platform, uses a ballast18 or plug19 of tantalum or graphite held in a slot in the wall of the graphite tube.When samples are evaporated from the tube surface they condense on the surface of the colder ballast, which is heated more slowly owing to its relatively large mass. As with the platform, the ballast is then heated by radiation from the tube wall and the analyte is re-vaporised but into an atmosphere that is at a much higher temperature than the ballast. If the plug or ballast was effectively water or air cooled, the second release of the analyte vapour could be controlled and made to occur when the furnace and the furnace vapour had reached the required constant-temperature conditions.Such control would be superior to that of the platform and would bring the plug method into the second category defined above. What is required is a simple and convenient method of introducing samples into a constant- temperature furnace, and this is most closely achieved at present by the so-called probe methods. L'vov and Pelieva2"Jl described the introduction of samples on a tungsten wire mounted on the arm of a Perkin-Elmer AS-I autosampler. A solution of the sample was injected on to the wire and dried, and subsequently the wire was introduced into the furnace through the normal injection hole after the furnace had reached the maximum or optimum atomisation temperature.Slavin and c o - w o r k e r ~ ~ ~ , ~ ~ also used a small tungsten wire coil mounted on the AS-I autosampler, and noted significant reductions in interference effects compared to atomisation from the tube wall. A similar system has also been described by Matousek.24 It is also interesting to note that the filament-in-furnace atomiser or FIFA system,25 which uses a series of tungsten coils introduced longitudinally into a CRA 90 atomiser, could be operated under constant-temperature conditions. Results from such use have not been reported, however. Although useful results have been achieved with tungsten probes, the disadvantages of all the above systems are that only small sample volumes can be used conveniently (1-2 pl), which reduces the relative sensitivity, and that lifetimes of tungsten wires when operated at the high temperatures required to reduce interferences are short.Two graphite probes have recently been described that offer greater potential for the wider adoption of the probe technique. Slavin and Manning26 introduced 5-pl samples on a flat probe of pyrolytic graphite through a side-arm added at the centre of the graphite tube. Drying and charring were performed outside the tube and significant advantages were demonstrated for refractory metals such as titanium, vanadium and molybdenum. At the University of Strathclyde, we have also experimented with probes made of 100% pyrolytic graphite and have introduced these manually through the open end27 and through a hole cut in the side of an HGA-72 graphite furnace.28.29 We have been fortunate to have available a new form of pyrolytic graphite manufactured by Philips30 that has high mechanical stability and gives long lifetimes of both tubes and probes.With this probe, volumes as high as 50 p1 have been conveniently used. In operation, the injected sample is dried by holding the probe head at a suitable position near the heated graphite tube. Ashing can be carried out in the same position or by complete insertion into the tube. The probe is then removed and re-introduced into the HGA-72 graphite tube after the selected atomisation temperature has been reached. Apparent heating rates in excess of 4 000 K s-1 have been observed and interferences have been shown to be much reduced.27-29 The determination of lead is free of interference from up to 2% magnesium chloride and 2% calcium chloride provided a sufficiently high vapour temperature is selected.As a result, it has been possible to develop an ETA AAS procedure for the determination of lead in blood using only dilution of the sample with water and simple aqueous Although the original work was carried out with the HGA-72 atomiser, which has a larger tube than current commercial systems, probe systems have also been successfully developed for the smaller Pye Unicam SP9 and Perkin-Elmer HGA-500 furnaces and similar sensitivities have been achieved. Before such a device could be used for routine analysis, an automated version will be required. A simple automated probe system has now been developed in our laboratory that operates with the normal commercial types of autosampler provided for injection of samples into the graphite tube.31 InATOMIC SPECTROSCOPY SYMPOSIUM Anal.Proc., Vol. 21 58 this case injection of the sample on to the probe is carried out whilst it is positioned inside the graphite tube. Drying and ashing are carried out in this position and the probe is then withdrawn using a solenoid assembly triggered by the recorder output of the furnace programmer. The tube is then heated to the pre-selected constant-temperature conditions, when the power to the solenoid is switched off, which allows the probe rapidly to re-enter the tube for sample atomisation. Up to 2O-pl sample aliquots could be used conveniently in the HGA-500 atomiser and reproducibilities in the range 0.8-2.3% were achieved under fully automatic operation.Electrothermal atomisation provides a method of high sensitivity with good precision under automatic sample dispensing, and is suitable for a wide range of elements. The problem has always been in achieving adequate accuracy without considerable inconvenience in sample treatment and standardisation. The platform and probe methods offer the possibility of achieving high accuracy with much less difficulty than in the past, using simple standardisation procedures. The potential for rapid multi-element analysis at trace levels is therefore greatly increased. As the electrothermal atomiser provides a transient signal response, rapid multi-element analysis could only be achieved by simultaneous measurement, as sequential measurement would involve inconveniently long analysis times.Simultaneous measurement of more than a few elements is not possible with current instrumentation based on line-source (hollow-cathode lamp) AAS, but has been reported using continuum-source atomic-absorption spectrometry32333 and furnace atomic emission spectrometry.34.35 In both instances automatic background correction is achieved by wavelength modulation at positions very close to the analyte line. High accuracy background correction is therefore obtained as a consequence of avoiding the requirement accurately to match two dissimilar light sources. The important features of the continuum source AAS instrument are the high intensity xenon arc light source and a high resolution echelle spectrometer.32 These allow sensitivities similar to those obtained with line-source instrumentation to be achieved for all elements except those with resonance wavelengths in the low UV region where the intensity of the xenon arc is too weak.The continuum-source wavelength-modulated system also allows the measurement of the whole of the atomic-absorption line profile (or atomic spectrum) and Harnly and O’Haver33 have used this facility to extend the linear calibration range of atomic-absorption measurements to 4-6 orders or magnitude of concentration. At concentrations 2-3 orders of magnitude above the detection limit, where line-source instruments show strong curvature of atomic-absorption calibration graphs, measurements are made in the wings of the line profile with the continuum-source instrumentation without any modification or change in the operation of the system.In our laboratory we have been particularly concerned with the development of carbon furnace atomic-emission spectrometry over the past few years. With purpose-built instrumentation34 we have demonstrated high sensitivity for this technique for a wide range of elements despite the relatively low temperature of the atom source. The low temperature can be seen as an advantage as it limits ionisation and spectral interferences and allows accurate automatic background correction to be achieved more easily than with higher temperature sources. The platform34.36 and probe27-28 are particularly suited to furnace emission as the higher vapour temperatures available to the atoms increase the excited state populations and hence emission sensitivities of all elements, but particularly of volatile elements that only experience low temperatures under normal atomisation conditions.In a joint project with Harnly and O’Haver, we have been able to realise the potential of carbon furnace atomic-emission spectrometry for simultaneous multi-element analysis.35 The multi-channel kchelle spectrometer system designed for use in continuum-source AAS was used with only minor modification to the computer software. In addition, it was found possible to extend the calibration range of furnace atomic emission in an analogous manner to that used for continuum-source AAS. The low temperature of electrothermal atomisers limits the sensitivity for elements of high excitation potential such as cadmium, zinc and selenium.This difficulty has been overcome by the incorporation of an auxiliary low-pressure discharge inside an evacuated electrothermal atomiser.37J8 This device, known as FANES (furnace atomisation with non-thermal excitation), allows much lower detection limits to be achieved for these elements but at the cost of some increase in the complexity of the background spectrum owing to the higher excitation temperature. The fact that electrothermal atomisation is now a widely used method for trace element analysis might suggest that instrumental developments should be few and far between. Having introduced more adequate and flexible control of heating rates and temperatures a few years ago, most instrument manufacturers have since then concentrated on software rather than hardware developments.Not all the hardware innovations described in this paper should be expected to appear in the form of commercial instrumentation. However, I believe it is clear that the present designs of commercialFebruary, I984 ATOMIC SPECTROSCOPY SYMPOSIUM 59 atomiser do not allow the performance of electrothermal atomisation under ideal conditions, and that relatively simple modifications could bring this a lot closer. With reference to research described from the University of Strathclyde, the author acknowledges the contribution made by research students and colleagues, notably D. Littlejohn and J. Marshall. Financial support from many sources, including particularly that from Pye Unicam and Perkin-Elmer, is also gratefully acknowledged. 1.2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22 * 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. References Ottaway, J. M., At. Spectrosc., 1982, 3, 89. Fuller, C. W., “Electrothermal Atomisation for Atomic Absorption Spectrometry,” Royal Society of Price, W. J., “Spectrochemical Analysis by Atomic Absorption,” Heyden, London, 1979. Slavin, W., and Manning, D. C., Prog. Anal. At. Spectrosc., 1982,5, 243. Stoeppler, M., Brandt, K., and Rains, T. C., Analyst, 1978, 103,714. Massmann, H., Spectrochim. Acta, Part B, 1968, 23,215. Ottaway, J. M., Proc. Anal. Div. Chem. Sac., 1976, 13, 185. L’vov, B. V., Spectrochim. Acta, 1961, 17, 761. Woodriff, R., and Ramelow, J., Spectrochim.Acta, Part B, 1968, 23, 665. Lawson, S. R., Dewalt, F. G., and Woodriff, R. Prog. Anal. At. Spectrosc., 1983,6, 1. L’vov, B. V., Spectrochim. Acta, Part B, 1978, 33, 153. Chakrabarti, C. L., Hamed, H. A., Wan, C. C., Li, W. C., Bertels, P. C., Gregoire, D. C., and Lee, S.,Anal. Chakrabarti, C. L., Wan, C. C., Hamed, H. A., and Bertels, P. C., Anal. Chem., 1981, 53, 444. Slavin, W., Manning, D. C., and Carnrick, G. R., At. Spectrosc., 1981, 2, 137. Slavin, W., Carnrick, G. R., Manning, D. C., and Pruszkowska, E., At. Spectrosc., 1983, 4, 69. Hendry, J. B. M., Ottaway, J. M., and Fell, G. S., to be published. Frech, W., and Jonsson, S., Spectrochim. Acta, Part B, 1983,37, 1021. Katskov, D. A., and Grinshtein, I., Zh. Prikl. Spektrosk., 1978, 28, 968.Holcombe, J . A., and Sheehan, M. T., Appl. Spectrosc., 1982,36,631. L’vov, B. V., and Pelieva, L. A., Zh. Anal. Khim., 1978, 33, 1572. L’vov, B. V., and Pelieva, L. A., Zh. Anal. Khim., 1980,35, 1744. Manning, D. C., Slavin, W., and Myers, S., Anal. Chem., 1979, 51,2375. Slavin, W., and Manning, D. C., Anal. Chim. Acta, 1980, 118,301. Matousek, J. P., Prog. Anal. At. Spectrosc., 1981, 4, 247. Garnys, V. P., and Smythe, L. E., Anal. Chem., 1979, 51, 62. Slavin, W., and Manning, D. C., Spectrochim. Acta, Part B, 1982, 37, 955. Marshall, J., Giri, S. K., Littlejohn, D., and Ottaway, J . M., Anal. Chim. Acta, 1983, 147, 173. Giri, S. K., Littlejohn, D., and Ottaway, J. M., Analyst, 1982, 107, 1095. Giri, S. K., Shields, C. K., Littlejohn, D., and Ottaway, J.M., Analyst, 1983, 108, 244. Lersmacher, B., Ger. Offen., 2949275 (Cl.GOINZI/03), 25 June, 1981. Littlejohn, D., Marshall, J., Carroll, J . , Cormack, W., and Ottaway, J. M., Analyst, 1983, 108, 893. Harnly, J. M., O’Haver, T. C., Golden, B., and Wolf, W. R., Anal. Chem., 1979, 51, 2007. Harnly, J. M., and O’Haver, T. C., Anal. Chem., 1981, 53, 1291. Ottaway, J. M., Bezur, L., and Marshall, J., Analyst, 1980, 105, 1130. Marshall, J . , Littlejohn, D., Ottaway, J. M., Harnly, J . M., Miller-Ihli, N. J . , and O’Haver, T. C., Analyst, Bezur, L., Marshall, J . , Ottaway, J. M., and Fakhrul-Aldeen, R., Analyst, 1983, 108, 553. Falk, H., Hoffman, E., and Ludke, Ch., Spectrochim. Acta, Part B, 1981,36, 767. Falk, H., Hoffman, E., Ludke, Ch., Ottaway, J.M., and Giri, S. K., Analyst, 1983, 108, 1459. Chemistry, London, 1977. Chem., 1980, 52, 167. 1983, 108, 178. Modern Graphite Furnace AAS Technology and Standardless Analyses Walter Slavin Perkin-Elmer Corporation, Main Avenue, Norwalk, CT 06856, USA The modern graphite furnace has been improved in the last few years at a remarkable rate. Papers using the new platform technology are now beginning to appear in the literature in increasing numbers. While these papers use many aspects of the new furnace technology, the technology as a whole is still rarely used. The opportunity to study the system as a whole for the past year or two, including Zeema background correction, has convinced us that we can now think of “standardless” or absolute analyses60 ATOMIC SPECTROSCOPY SYMPOSIUM Anal.Proc., Vol. 21 The Stabilised Temperature Platform Furnace, STPF, technique is made up of a number of individual instrumental characteristics’ which are mutually dependent upon each other. While the technique is not specific to a particular piece of hardware, the hardware that is used must be compatible with the complete STPF system. Many of the published papers that have used only portions of the STPF system have not achieved the level of control of interferences that we experience and report.2-5 Furnace signals occur very rapidly and fast digital electronics are required to follow the absorbance profile. The earlier spectrophotometers used for AAS were designed for steady-state signals from the flame and therefore had time constants which were rarely shorter than 1 s.In the instrument which we use, on 60-cycle power lines, each fully corrected absorbance signal is collected in a time interval of about 8 ms. The integrated signal is the sum of these individual values over the time interval selected by the analyst. For the integrated signal to be accurate, the base line of the absorbance profile before and after the analyte is vaporised must be established very accurately. Earlier AA instruments did not have this facility and these instruments produced very imprecise results in absorbance seconds signals. The integration of the absorbance signal is one of the properties of the STPF that is often omitted by workers who use the other aspects of this technique. If peak absorbance signals are used to prepare a standard curve for quantitation, the result will usually be in error,6 even if the rest of the STPF technology is used.The equality of peak absorbance values in the standards and the samples requires that the rate of analyte vaporisation be independent of the matrix, a characteristic rarely experienced. Zeeman-effect background correction7 has been of particular importance in achieving this performance. Use of the Zeeman/5000 with the STPF technique has made the analytical situation more independent of the size and character of the matrix. Detection limits do not appear to be different when aqueous solutions are compared with solutions having matrices that provide a large background, except for the calculable reduction of the light intensity. For this system to approach the theoretical conditions, there are several additional characteristics necessary for the STPF.The analyte must not react strongly with the graphite tubes, thus high quality pyrolytically coated tubes are necessary. Matrix modifiers are an important part of this technology. Most analytes require a matrix modifier to stabilise the analyte until the thermal conditions have become more nearly constant. This practice will permit the use of higher char temperatures without loss of analyte. For cadmium, lead, tin and zinc, phosphate will permit higher char temperatures and an alkali or alkaline earth salt will permit still higher char temperatures. Magnesium nitrate, by itseif, provides the same advantage for aluminium manganese, beryllium, chromium, etc.Obviously it would be convenient if a single matrix modifier were satisfactory for all elements. The combination of phosphate and magnesium nitrate is very useful for most of the analytes typically determined with the graphite furnace. Except for the semi-metals, which are all preferably modified with nickel, an appropriate combination will probably prove to be useful. We have used the concept of “characteristic amount” as a measure of analytical sensitivity for each analyte. It is the amount of analyte, in picograms, that produces a signal of 0.0044 A s. Used in conjunction with the STPF technique, it has proved to be a remarkably stable value for each analyte. Experimental data for cadmium that support the reproducibility of the characteristic amount to better than 20% are shown in Table I.Some of the data pre-date our understanding of characteristic amount and the values shown are calculated from data in the papers. The data were from instruments that were different in design, some were Zeeman corrected, some continuum corrected. Different conditions and different matrix modifiers were used and certainly the sample matrices were very different. TABLE I CADMIUM DATA C* References 0.35 Slavin and Manning,8 1980 0.32 Suitch, P-E, 1980 0.31 Manning, P-E, 1982 0.45 Manning, P-E, 1982 0.31 Manning, P-E, 1982 0.4 Manning and Slavin,’ 1983 0.35 Pruszkowska, etal.,4 1983 0.35 Pruszkowska, etal.,3 1983 0.36 Viillkopf, P-E, 1983 0.44 Viillkopf, P-E, 1983 * Characteristic amount/pg (0.0044 As). Conditions Platform paper Test of Model 5000 Test of CSIRO lamps Test of PO4 and Mg(N03), In seawater In stream water In seawater, Zeeman In urine, Zeeman Waste water In fishFebruary, 1984 ATOMIC SPECTROSCOPY SYMPOSIUM 61 Similar results have been found for most of the elements frequently determined in the graphite furnace and these data will be reported elsewhere.Theory suggests that the characteristic amount should be reproducible to within a few per cent. and more recent experimental data are beginning to support this result. Obviously, standardless analyses can be run with the same accuracy. There are potential problems that must be considered before relying on an absolute method. If the amount of matrix exceeds the capacity of the background corrector, or if vapour phase bonding of the analyte alters the efficiency of atomisation, errors will arise. The presence of halides increases the volatility of most analytes, and errors are caused by premature volatilisation of the analyte during the char step.It is usually necessary to use recoveries to confirm a determination in a new matrix for which there was no prior experience. There will be some effects due to variations in atomisation temperature, but these should be small. In conclusion, then, we are very encouraged by the results of this preliminary work with standardless furnace AA. However, this is very much a report of work in progress. We have to become more sure of the stability of the results between instruments and over a period of time. However, there seems no doubt that for most metals these standardless analyses are feasible to at least an accuracy of 10%. It also seems possible that this work should lend encouragement to equivalent analyses on solid samples.We hope to start such experiments soon. References 1. 2. 3. 4. 5 . 6. 7. 8. Slavin, W., Manning, D. C . , and Carnrick, G. R., At. Spectrosc., 1981, 2, 137. Carnrick, G. R . , Slavin, W., and Manning, D. C . , Anal. Chem., 1981, 53, 1866. Pruszkowska, E . , Carnrick, G. R., and Slavin, W., Clin. Chem., 1983, 29, 477. Pruszkowska, E., Carnrick, G. R., and Slavin, W., Anal. Chem., 1983, 55, 182. Manning, D. C . , and Slavin, W., Appl. Spectrosc., 1983, 37, 1 . Pruszkowska, E., Carnrick, G. R . , and Slavin, W., At. Spectrosc., 1983, 4, 59. Fernandez, F. J . , Bohler, W., Beaty, M.M., and Barnett, W. B . , At. Spectrosc., 1981, 2, 7 3 . Slavin, W., and Manning, D. C . , Spectrochim. Actu, 1980, 35B, 701. Inductively Coupled Plasmas in Atomic Fluorescence Spectrometry S. Greenfield Department of Chemistry, Loughborough University of Technology, Loughborough, Leicestershire, LE11 3TU As early as 1975 Montaser and Fassel,] using an inductively coupled plasma (ICP) as an atomiser in atomic fluorescence spectrometry (AFS) , obtained detection limits superior to those obtained using the same ICP for emission spectrometry. Furthermore , long dynamic ranges were found, together with relative freedom from scattering and matrix interferences. In early 1981 Baird introduced a commercial instrument2 based on similar principles. In the meantime a number of workers3 had demonstrated the use of an ICP as a line source in AFS.Their work was foreshadowed by other workers435 who had used ICPs as sources in atomic absorption (AA) and in AFS. Although Doppler broadening of the emission profile of an ICP might be expected, if the over-all profile is narrower than the absorption profile in a flame, this would not preclude its use as a line source. Human and Scott6 found, by interferometry, that the spectral lines emitted by the plasma are Doppler broadened, but show considerably less collisional broadening than in flames. Winefordner el af.3 showed that experimental curves of growth produced by an ICP as a source and a flame as an atomiser in AFS had the expected shape for a line source. The commercial AFS instrument that has been mentioned has a circular arrangement of twelve assemblies, each of which consists of a hollow cathode lamp, an interference filter and a photomultiplier tube surrounding a centrally placed low power argon plasma (Fig.1). Each hollow cathode lamp is pulsed at a frequency of 500 Hz, and each is on in sequence; that is to say, only one hollow cathode lamp is on at any one time, so that only one element at a time is excited. The experimental results obtained by Demers and Allemand,7 who designed the instrument, show that the detection limits obtainable are similar to those obtainable by ICP-OES, except when the refractory elements are the elements of interest; here ICP-OES is superior. There is little to choose between the detection limits when compared with flame atomic absorption. The dynamic range of the Baird instrument is some four to five orders of magnitude and the technique is relatively free from chemical matrix effects and ionisation interference.62 ATOMIC SPECTROSCOPY SYMPOSIUM Anal.Proc., Vol. 21 Fig. 1. Atomic-fluorescence spectrometer with inductively coupled plasma and hollow cathode lamps. There is no doubt that the technique of emission spectroscopy is subject to spectral interferences, such as instrument broadening of the spectral lines leading to spectral overlap of close-packed spectral lines. As an example of the spectral overlap problem, the major zinc resonance line at 213.856 nm is subject to direct spectral interference by the copper 213.853 nm non-resonance transition. It has been pointed out8 that it would be impossible to determine 5 p.p.m., for instance, in pure copper at this wavelength by OES (1% mlV copper solution, zinc concentration 50 p.p.b.).The other zinc line at 206.2 nm is four times less sensitive. Demers has shown9 that this level of zinc in copper can be determined easily by AFS, either directly or by standard additions. Lawson and Fassel'o have shown that approximately 2 500 p.p.m. of aluminium will produce a background shift of one order of magnitude in the 190-220 nm region in ICP-OES due to radiative recombination continua. On the other hand, Demers has shown that since the AFS instrument employs a.c.-coupled electronics, which discriminate against changes in d.c. signals, it is not susceptible to this type of background interference and 10 000 p.p.m.of aluminium have no effect. Optical emission spectrometry is subject to background interference from collisional broadening; a good example is the interference of strong ionic emission from the calcium lines at 393.4 and 396.8 nm on the aluminium lines at 394.4 and 396.2 nm, respectively. In the presence of 1 000 p.p.m. of calcium the background shift extends as far as 10 nm from the ionic lines. This phenomenon is also potentially present in AFS and would not be avoided by a.c. coupling of the electronics. In fact, Demers found no interference on the background when 10 000 p.p.m. of calcium were aspirated with an aluminium hollow cathode lamp running. In 1968, the author4 was working with ICPs as both line sources and atomisers in atomic absorption and postulated the use of two ICPs in one instrument.In 1981 he returned to this idea of using one plasma as a source and another as an atomiser, this time in AFS. It was felt that this technique, viz., atomiser , source, inductively coupled plasmas in atomic fluorescence spectrometry (ASIA), apart from having relative freedom from spectral interference, had the advantage of versatility. The ICP that is used as a source is powered by a Radyne generator of nominal 15 kW output at 7 MHz; the atomiser plasma is powered by a smaller Radyne generator, of 2.4 kW output at 36 MHz. The source is a nitrogen - argon - argon plasma induced in a Greenfield torch. 11 A reduced image of the tailflame of this plasma, of sufficient size to fill the aperture of the light chopper, is brought to focus at the aperture with a concave quartz lens of 6.5 cm focal length.The light chopper is a Brookdeal, with 30 blades and a chopping frequency of 5 Hz to 1 kHz; it also supplies a reference signal for the lock-in amplifier. A second concave quartz lens of similar focal length is used to focus the image at the chopper on to the tailflame of the atomiser plasma. This second image has a breadth less than that of the tailflame of the second plasma. The atomiser is an argon - argon - argon plasma induced in a Scott torch11 with an extended outer tube. The fluorescent radiation is focused by means of two lenses on to the entrance slit of a modified Optica spectrometer. A photomultiplier power supply, an integrator and a recorder complete the system (see Fig.2 ) . Winefordnerlz has also assembled an atomic fluorescence system, using two low power plasmas. TheFebruary, I984 ATOMIC SPECTROSCOPY SYMPOSIUM 63 results obtained by this team are somewhat disappointing. The detection limits reported for elements which normally give good results by AFS, and do not include any refractory elements, are not as good as can be obtained by ICP-OES or by the Baird system; in many instances they are orders of magnitude worse. Matrix effects were observed. Aluminium produced an enhancement of the calcium ion fluorescence, sodium also produced an enhancement, whereas phosphorus produced a slight suppressive effect. However, relative freedom from spectral interferences was clearly demonstrated. Given this relative freedom from spectral interference, and given that the matrix effects found by Winefordner, but not by Demers, can be avoided, there remains only the problem of the inferior detection limits for the refractory elements before the use of ASIA will be acceptable. Demers did, at least, a partial optimisation for minimum interference when he was looking at matrix effects. It is conceivable that a true optimisation, in a mathematical sense, for maximum interference may well be successful in overcoming the type of matrix'effects that have been discussed. Lock-in amplifier Monochromator PM supply Recorder Fig. 2. ASIA spectrometer. With regard to detection limits, the use of a large torch and higher powers may well do something to reduce these to acceptable levels for refractory elements, as more concentrated solutions can be nebulised into the source plasma than is the case with low powered plasmas and Scott torches. Higher power in the source will certainly increase the signal and the background may not be as important as it is in ICP-OES. However, there is the possibility of the continuum exciting other matrix elements to fluoresce. The present set-up is only regarded as a test bed to ascertain the operating conditions and to test the principles. Given a successful conclusion to this part of the project the author believes a practical system is entirely possible. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. References Montaser, A., and Fassel, V. A., Anal. Chem., 1976, 48, 1490. Demers, D. R., and Allernand, C. D., paper presented to the Pittsburgh Conference, Atlantic City, N.J., Omenetto, N., Nikdel, S., Bradshaw, J. D., Epstein, M. S., Reeves, R. D., and Winefordner, J. D., Anal. Greenfield, S., Smith, P. B., Breeze, A. E., and Chilton, N. M. D., Anal. Chim. Acfa, 1968,41, 385. Hussein, Ch. A. M., and Nickless, G., paper presented to the 2nd ICAAS, Sheffield, England, 1969. Human, H. G. C., and Scott, R. H., Spectrochim. Acta, 1976, 31B, 459. Demers, D. R., and Allemand, C. D., Anal. Chem., 1981, 53, 1915. Epstein, M. S., Nikdel, S., Ornenetto, N., Reeves, R., Bradshaw, J., and Winefordner, J. D., Anal. Chem., Demers, D. R., personal communication. Lawson, G. F., and Fassel, V. A., Appl. Spectrosc., 1979, 33, 592. ZCP Znf. Newsl., 1975, 1, 49. Kosinski, M. A., Uchida, H., and Winefordner, J . D., Anal. Chem., 1983, 55, 688. USA, 1981. Chem., 1979, 51, 1521. 1979,51, 2071.

ISSN:0144-557X    

DOI:10.1039/AP9842100054

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

64 FOOD ANALYSIS Anal. Proc., Vol. 21 Food Analysis The following are summaries of two of the papers presented at a Meeting of the East Anglia Region, the Chromatography and Electrophoresis Group and the Essex Section of the RSC held on April 14th, 1983, at the College of Technology, Southend. Isotachophoresis-a Method of High Potential A. M. C. Davies ARC Food Research Institute, Colney Lane, Norwich, NR4 7UA Developments in food analysis only rarely open up new areas of research, the identification of food flavours made possible by gas - liquid chromatography is an example, but very often the scope and scale of a research programme is markedly influenced by the analytical chore involved. Thus, at FRI, we are always on the lookout for new developments, especially of techniques that minimise sample clean-up before analysis.Although isotachophoresis is not new, recent advances in the detector system make it applicable to all ionic species. The time seemed ripe for investigation of its potential uses for food components being analysed in FRI research projects. Principles of Isotachophoresis Isotachophoresis (ITP) is the electrophoretic separation of ionic species according to mobility, which is achieved in a capillary column under conditions of constant current. Although it is a modern technique the principles were first proposed by Kohlrausch in 1897.1 The ITP experiment consists in placing a sample containing a mixture of ions of varying mobilities in a capillary at the interface between an electrolyte containing an ion of high mobility (the leading electrolyte) and an electrolyte containing an ion of low mobility (the terminating electrolyte), all three having a common counter ion [see Fig.1 ( a ) ] . When the appropriate electric field is applied to the system the sample ions of highest mobility (A) will move faster than ions of lower mobility (B). However, as this change is taking place the resistance along the capillary that contains sample will increase because of the relationship between mobility and resistance. This change in resistance should result in a reduced current, but as there is a constant current device in the circuit the applied voltage increases and this will automatically result in an increase in the applied field along the separating sample. These increases will be such that the effective mobilities of all ions will be the same.The net effect will be that the ions will be separated into zones according to their mobilities and then continue to move along the capillary, as in Fig. 1 ( b ) . There L- T- o L- T- 0 Fig. 1. Isotachophoresis theory: (a), initial state, sample between leading electrolyte, L-, and terminating electrolyte, T-; (b), separation of sample ions A- and B-; (c), spacer technique to give complete separation of A- and B-.February, 1984 FOOD ANALYSIS 65 cannot, of course, be any physical gap between one ion and its next neighbour because there would then be nothing to carry the current. However, the conditions do favour very good separation between neighbouring ions because of two effects which can be predicted from the Kohlrausch equation, namely, the concentrating effect and the zone sharpening effect.The Kohlrausch Equation The Kohlrausch equation describes the conditions at a boundary between two ions, A and B, with mobilities MA amd MB. The ratio between the concentrations CA and CB is given by: where MR is the mobility of the common ion R (of opposite sign). mobility, the effective ionic mobilities are all constant and the equation can be simplified to Under the conditions employed for ITP, where the applied field is increased to compensate for lower where k is a constant which will be dependent on the concentration of the leading electrolyte. The Concentrating Effect It follows from the modified Kohlrausch equation that if a sample contains an ion at very low concentration it will be concentrated until it fulfils the equation.(Conversely, if the sample contains an ion at high concentration it will be diluted.) The Zone Sharpening Effect If an ion of lower mobility should move into the next zone containing ions of higher mobility it will experience a sudden drop in the field strength, which will cause a reduction in its velocity and it will be recaptured by its own ionic zone. Conversely, if an ion of higher mobility should move into the next zone containing ions of lower mobility it will experience a sudden increase in field strength which will increase its velocity until it catches up with its own zone. It is important to note that in ITP there is nothing that will cause concentration of an ion within its zone and thus the concentration of the ion is constant.It follows that the zone length is proportional to the amount of the ion in it. If it should be important to obtain a physical separation between one zone and another then this can be achieved by the addition of “spacer” ions to the sample. A spacer ion is one with a mobility intermediate between the two ions. The effect is shown in Fig. 1 (c). Instrumentation LKB developed the 2127 Tachophor from the work of Everaerts,* and Martin and Everaerts,3 and it is shown diagramatically in Fig. 2. The capillary is a 0.5 mm PTFE tube which must be carefully thermostatted. This is achieved by Peltier elements and separations can be carried out over the range 3-29 “C. The initial sharp boundary between the leading and terminating electrolytes is obtained by alternately forcing the leading and terminating electrolyte through a common exit. The sample is placed at the boundary by careful injection through a septum. The constant current device allows a choice of current from 0 to 500 PA at a voltage of up to 30000 V.Detection It is possible to demonstrate ITP with the use of food colours because they are anions and it is possible to see the zones being established and sharpened. For practical applications detectors are required. Ultraviolet detectors have the advantage of being very sensitive but they are also specific and a non-specific detector is always required. Initially, thermometric detectors were employed but these have been replaced by conductimetric devices which, when used in the differential mode, match ultraviolet detectors for sensitivity .66 FOOD ANALYSIS Anal.Proc., Vol. 21 Quantification and Identification of Zones As has already been demonstrated quantification is simply a matter of measuring zone length and comparing this to a calibration graph obtained under the same conditions. With trace ions which are ultraviolet absorbing the separated zone may be so short as to appear as a single peak. Under these conditions it is possible to use the peak height for quantification. Although the detector outputs are recorded on a normal two-pen recorder, the identity of any component cannot be determined on a time basis. The analysis time is dependent on the amount of ions. Instead, identification is made by reference to the electric field or to the relative conductivity as indicated by the conductivity detector.ITP traces are thus different from normal chromatography recordings and at first they may appear rather confusing (see Fig. 3). Constant current power supply Terminating Leading electrolyte \ electrolyte / Conductivity UV Fig. 2. Schematic diagram of apparatus for isotacho- phoresis. The boundaryisformedat the point I. Potato ?xtract plus ascorbic acid I Potato extract ascorbic acid Fig. 3. Isotachophoresis trace of a potato extract. Upper trace, relative conductivity; lower trace, ultraviolet absorption. Application to Food Analysis ITP has received little attention from food analysts in the UK, but several papers have appeared in the European literature, for example, ascorbic acid in orange juice,4 saccharin and cyclamate in diet beverages, organic acids in wine and preservatives in mayonnaise ,S amino acids ,6 B vitamins7 and hydroperoxides in linoleic acid.8 At FRI, major nutritional studies involve the potato, in which the ascorbic acid is difficult to determine. Fig.3 is a recording obtained from a potato extract before and after the addition of ascorbic acid. We have only had the apparatus for a short time but this first example looks promising and I am confident that it will live up to its high potential. For further reading the review by Hjalmarsson and Baldesteng is recommended. 1. 2. 3. 4. 5. 6. 7. 8. 9. References Kohlrausch, F., Ann. Physik., 1897, 62, 209. Everaerts, F. M., Thesis, Technische Hogeschool Eindhoven. Eindhoven, The Netherlands, 1968.Martin, A. J. P., and Everaerts, F. M., Anal. Chim. Acta, 1967, 38, 233. Baldesten, A., Hjalmarsson, S.-G., and Neumann, G., Fresenius’ 2. Anal. Chem., 1978, 290, 148. Kaiser, K.-P., and Hurpf, H., Dtsch. Lebensm. Rundsch., 1979, 75, 346. Everaerts, F. M., Beckers, J. L., and Verheggen, Th. P.E.M., “Journal of Chromatography Library,” Everaerts, F. M., Mikkens, F. E. P., and Verheggen, Th. P.E.M., Sep. Purif. Methods, 1977, 6 , 287. Kopwillem, A., and Eriksson, K., “LKB Application Note No. 111,” LKB-Produkter AB, Bromma, Hjalmarsson, S.-G., and Baldesten, A., CRC Crit. Rev. Anal. Chem.. 1981, 261. Volume 6, Elsevier, Amsterdam, 1976. Sweden, 1974.February, 1984 FOOD ANALYSIS Chromatographic Techniques for the Analysis of Food Additives and Contaminants N.P. Boley 67 Department of Industry, Laboratory of the Government Chemist, Cornwall House, Stamford Street, London, S E l 9 N Q High-performance liquid chromatography and gas - liquid chromatography are now well established techniques for the analysis of food additives and contaminants. These are a diverse group of compounds and this is reflected in the variety of chromatographic methods used in their determination. High-performance Liquid Chromatography High-performance liquid chromatography (HPLC) has been an accepted tool of the food analyst for some years now. The applications of this technique in the analysis of food additives and contaminants include synthetic food colours, mycotoxins, fatty acid anilides, polynuclear aromatic hydrocarbons and post-harvest fungicides.At the Laboratory of the Government Chemist we developed a comprehensive method for the determination of all 15 permitted synthetic colours in food.1 The samples are pre-treated with an enzyme to release any colour bound to the food matrix, if necessary,2 then the colours are extracted into a liquid anion-exchange resin dissolved in butan-1-01. Following back-extraction into an alkaline salt solution, the extract is concentrated and cleaned up by polyamide column chromatography, prior to HPLC analysis. The dyes are separated on a reversed-phase C8 column, such as SAS-Hypersil, using ion-paired chromatography. The mobile phase consists of methanol and water containing cetyltrimethylammonium bromide (cetrimide) as the ion-pair reagent. A separate method for the unstable food colour Indigo Carmine was also developed, which involved the use of a Sep-Pak sample preparation cartridge (Waters Associates Ltd.) to extract and concentrate the colour.3 The recent problems encountered with toxic Spanish olive oil led to interest being shown in fatty acid anilides.It was thought that these occurred in contaminated samples of this oil, following the use of aniline as a denaturant in the processing of the oil, and that these could be the toxic agents that led to such tragic consequences. An HPLC method was developed for these contaminants. Following extraction of the anilides from the sample with methanol, clean-up and concentration, separation and quantification is achieved using reversed-phase HPLC on an ODS C18-bonded column with 100% acetonitrile as the mobile phase and ultraviolet detection.Mycotoxins are a very important group of contaminants that may be determined by HPLC and are secondary metabolites produced during the growth of certain moulds. They are amongst the most carcinogenic substances known to man. One of the major types of mycotoxins are the aflatoxins. In the determination of these, the sample is macerated with a mixture of methanol, water and hexane, centrifuged and the aqueous layer extracted with chloroform. Silica-gel column chromatography is used to clean up the extract, the aflatoxins being finally eluted from the column with 3% methanol in chloroform. Following concentration, this is analysed on a silica-gel HPLC column with 10% acetic acid in water-saturated chloroform as the mobile phase.The eluted aflatoxins are determined by fluorescence detection and a detection limit of 1 ng is possible. This may be lowered if the detector flow-cell is first dry-packed with silica gel.4 Alternatively, reversed-phase HPLC can be used to determine aflatoxins, using an ODS C18-bonded column and a mobile phase of water, methanol and acetonitrile. Confirmation of the presence of aflatoxins can be shown using this system if they are converted to their hemiacetal form with trifluoroacetic acid, prior to HPLC analysis. Another important mycotoxin is ochratoxin A. This is usually found in the kidneys of animals that have eaten contaminated feed. The sample is extracted as previously described, and determined on an ODS column using a methanol - water mobile phase buffered to pH 4.5 with acetic acid using fluorescence detection.Unfortunately, ochratoxin A fluoresces weakly in acidic solution. To increase the sensitivity the pH of the column effluent must be increased before detection. This is achieved by introducing a stream of dilute ammonia between the column and the detector, and a ten-fold increase of sensitivity can be obtained.5 Other mycotoxins that may be determined by HPLC include patulin, sterigmatocystin and penicillic acid. Post-harvest fungicides are usually found in citrus products. They are sprayed on to the fruit after it has been picked to protect it during packaging and transportation, but migrate into the rest of the fruit. Gas - liquid chromatography has been used in the past to determine the two main compounds in this68 FOOD ANALYSIS And.Proc., Vol. 21 group, biphenyl (BP) and 2-hydroxybiphenyl (OPP or orthophenylphenol), but HPLC is now the preferred technique. A silica-gel column is used; a hydrocarbon mobile phase (hexane or heptane) is used to chromatograph biphenyl, but acetic acid needs to be added to the mobile phase for 2-hydroxybiphenyl as this compound tends to ionise, which leads to a very strong affinity to the silica column. Addition of acid suppresses any ionisation, enabling 2-hydroxybiphenyl to be chromato- graphed successfully. Gas - Liquid Chromatography There are two main applications of gas - liquid chromatography (GLC) in food additive and contaminant analysis: residual monomers and flavour compounds.As many compounds that fall into these categories are volatile, the use of headspace gas chromatography (HSGC) is often desirable. In this technique the headspace above a liquid or solid sample is injected on to the GLC column. It has the advantages of minimising sample preparation and being easily automated. Residual monomers are frequently toxic, and in the case of vinyl chloride, highly carcinogenic. They occur either in plastic packaging material, or in food and drink that has been stored in such packaging. Other examples of these residual monomers are styrene, acrylonitrile and vinylidene chloride. Food and drink samples can be determined directly; the sample is placed in a vial, which is then septum sealed and capped, equilibrated and sampled.An apolar column phase such as OV-1 is frequently used. Packaging samples, however, need to be dissolved in a suitable solvent. Dimethylformamide or dimethylacetamide are often used. They have high boiling points, low vapour pressures and dissolve plastics easily. To increase the concentration of monomer in the headspace, water is added to the vial. This reduces the solubility of the monomer in the solvent, and therefore increases the concentration in the headspace. Residual monomers can be determined by manual injection GLC, but problems usually arise. Dissolved polymeric material precipitates out in the injection liner, which results in poor reproducibility, and late eluting components lengthen analysis time, or can interfere with later injections.Food flavouring compounds can arise from either natural or synthetic sources. Compounds from both sources are the subject of proposed EEC legislation, but methods for their determination are in their infancy. The use of capillary GLC is particularly useful in flavour analysis because of its greater resolution and efficiency. At the Laboratory of the Government Chemist we have been assessing proposed methods for some flavouring components known as active principles. These usually arise from natural sources, and are therefore contaminants, and need to be restricted due to their toxicity. Examples include safrole, thujone, coumarin and beta-asarone. Packed-column GLC methods are used to determine these compounds, but in some instances co-eluating peaks can lead to false results. In such instances, for example the analysis of thujone in bitter aperitifs, capillary GLC methods need to be studied. Conclusions Food additives and contaminants are a very diverse group of compounds. Chromatographic techniques are widely used in their determination, but the diversity of the compounds of interest is reflected in the diversity of analytical techniques, using both HPLC and GLC. The field of additive and contaminant analysis is very wide and only a fraction of the applications have been looked at here. New developments in apparatus and technology continue to grow and the spread of capillary GLC should lead to improvements in methodology, especially in the area of flavour analysis. References 1. 2. 3. 4. 5 . Boley, N. P., Bunton, N. G., Crosby, N. T., Johnson, A. E., Roper, P., and Somers, L., Analyst, 1980, 105, Boley, N. P., Crosby, N. T., and Roper, P., Analyst, 1979, 104, 472. Boley, N. P., Crosby, N. T., Roper, P., and Somers, L., Analyst, 1981, 106, 710. Palanaks, T., and Scott, P. M . , J . Assoc. Off. Anal. Chem., 1977, 60, 583. Hunt, D. C., Philp, L. A., and Crosby, N. T., Analyst, 1979, 104, 1171. 589.

ISSN:0144-557X    

DOI:10.1039/AP9842100064

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

February, 1984 ENVIRONMENTAL MONITORING 69 Environmental Monitoring The following is a summary of one of the papers presented at a Meeting of the East Anglia Region held on May l l t h , 1983, at Warren Spring Laboratory, Stevenage. Remote Monitoring by Lasers in the Troposphere R. H. Partridge Division of Quantum Metrology, National Physical Laboratory, Teddington, Middlesex, 7Wl 1 OL W The emergence of tunable lasers over the last decade has led to the development of a versatile new atmospheric gas measurement device, the laser remote monitor. Atmospheric gas detection has hitherto been carried out largely by physical or chemical point sensors and much useful information has been gained concerning problems of a spatially localised nature. However, where large area coverage is required involving distances of from hundreds of metres to many kilometres, point sensors are far less practical as they must then be employed in large arrays or, for vertical coverage, be flown in balloons or aircraft.For such applications a remote monitor using a laser beam that can be scanned from a single convenient location is clearly more attractive. The tunable laser has four outstanding characteristics for remote monitoring applications: it can provide considerable radiation intensity in a very narrow spectral band width, so allowing the selective excitation of a particular type of “target” molecule; it can produce a well collimated beam of radiation that will propagate through the atmosphere without excessive divergence and so can define a compact measurement region even at distances of several kilometres; many lasers can emit very short pulses of very high power which, as will be seen, are essential in making range-resolved radar-like measurements of gas concentration; and laser sensing of gases in the atmosphere is a non-intrusive technique as the sample volume is not disturbed.This last point means that there is no risk of gas adsorption or degradation (except for very high laser power densities) such as can occur with some point sensing methods that rely on drawing a sample into a container or instrument for subsequent measurement. Ultraviolet, visible and infrared lasers have all been used for remote sensing but some wavelength limitations are set by atmospheric absorption. Thus, ultraviolet wavelengths shorter than about 230 nm are considerably attenuated by oxygen absorption, while in the infrared region absorption by carbon dioxide and by water prevents operation around 4.2 pm and between about 5 and 8 pm.This is not a serious limitation, however, and the ability to operate over much of the infrared molecular “fingerprint” region is a great advantage. Laser radiation can interact with atmospheric species in a variety of ways, as summarisedl in Table I. The typical interaction cross-sections cited are defined through the equation Z = Z, exp (-aNL) where Zo is the initial intensity of a radiation beam that traverses a path of length L through a concentration of N species per unit volume of absorption cross-section a, while Z is the intensity of the beam that emerges without having interacted.The values shown in Table I are actually given as cross-sections per unit solid angle Q as the scattering processes are not isotropic. The various interaction processes in Table I will now be considered in turn, in relation to their use in remote monitoring applications. Elastic Scattering and Lidar The Mie and Rayleigh scattering processes are shown in Table I as “non-spectroscopic” because they occur at all wavelengths and, being elastic, give no frequency shift to the scattered radiation by which the scatterers might be identified. Mie scattering (due to particles of a size comparable to the wavelength of the radiation) is much stronger in the troposphere than Rayleigh (molecular or very small particle) scattering. Although elastic scattering gives no spectroscopic information it can be used to great effect by a laser remote sensing system to measure spatial aerosol distribution over and around suitable sites such as power stations, industrial plants and individual effluent stacks of all types.The technique used is known as LIDAR (the optical analogue of radar) in which a brief optical pulse is launched into the atmosphere, some of which is scattered by aerosol particles into a detector usually situated, for convenience, near the laser. The scattered light intensity at the detector is measured as a function of the70 ENVIRONMENTAL MONITORING Anal. Proc., Vol. 21 time elapsed since the launching of the laser pulse. From this time delay and a knowledge of the speed of light, the distance of the scattering species from the laser can be determined and it is thus possible to measure the relative atmospheric scattering efficiency at intervals all along the length of the laser beam until at extreme distances the signal becomes lost in system noise.Further, by scanning the laser beam horizontally and/or vertically it is then possible to build up a qualitative two-dimensional or even three-dimensional map of the aerosol distribution in a region up to several kilometres square. Such capabilities go far beyond anything possible with simple point sensors and represent a new dimension in aerosol measurement technique.2 Raman Scattering Lidar Raman scattering is, in principle, more attractive than elastic scattering for laser remote sensing because the radiation scattered from each molecule is shifted in frequency by an amount characteristic of that molecule. Thus, the combination of a pulsed laser of any frequency with a series of detectors tuned to selected optical frequencies should allow the simultaneous detection of a wide range of atmospheric molecules together with the range-resolution capability of the lidar system described earlier.Unfortunately, the Raman interaction cross-section is very small (see Table I), which necessitates the use of very high power lasers and very large aperture detectors. Development of Raman lidar systems was pursued up until about the rnid-l970s, particularly by Hirschfeld et af.3 However, since this time they have been largely superseded by the absorption techniques described later because absorption generally exhibits a far larger interaction cross-section than Raman scattering.TABLE I TYPICAL OPTICAL CROSS-SECTIONS Process Rayleighscattering . . . . . Miescattering . . . . . . Ramanscattering . . . . . Atomic fluorescence . . . Molecular fluorescence UV: Vis) Absorption UV,Vis) . . . . AbsorptiontIR) . . . . . Non-spectroscopic- Spectroscopic- Molecular fluorescence )IR) . . 10-27 10-8-10-27 10-25 1e13 quenched 10-16 1017)quenched 10-23 l @ l y (quenched 10-22 I 10- 13-10- 17 1 0 1 7- 10-21 Spectral region UV, Vis UV-midIR UV, Vis UV, Vis UV, Vis IR UV, Vis IR Fluorescence Lidar Fluorescence from excited electronic or vibrational states produced by a laser pump beam may also be employed in the general lidar technique.Fluorescence usually has a much higher interaction cross-section than Raman scattering (see Table I) even after allowance has been made for quenching of the excited states by other atmospheric molecules. However, a tunable laser is required in order to produce the excited states efficiently by setting the laser beam wavelength to one of the strong absorption lines of the target molecule. Calibration of the fluorescence lidar signals in terms of molecular concentration is difficult because the intensity of the laser pump beam at each sampling region is not known and neither is the degree of fluorescence quenching. Depletion of the pump beam by other absorption and scattering processes is also a problem and for these reasons very little remote tropospheric sensing has yet been performed in this way.However, detection of alkali metals, such as sodium, in the stratosphere has been very successful4 as here fluorescence quenching is negligible while the atomic concentrations are not so high as to unduly reduce the intensity of the laser pump beam. Absorption Methods In recent years the radiation - interaction process predominantly used by laser monitoring systems has been absorption. This is due to its generally large interaction cross-section (Table I), giving good detection sensitivity for many common pollutants, coupled with the relative ease of calibrating the signal from the system in terms of gas concentration. It is also due to the advent of tunable lasers, particularly dye lasers in the ultraviolet - visible and diode lasers in the mid-infrared regions.With such lasers the wavelength of the output beam can be matched to that of known strong absorption lines of the target molecule.February, 1984 ENVIRONMENTAL MONITORING 71 Absorption remote monitoring systems can be divided into single-ended or double-ended configurations. In the latter instance the emitted laser beam is returned to a suitable detector by a mirror or, where high power lasers are being used, by a convenient topographic target such as a wall, tree or hillside. Measurement of the reduction in beam intensity due to absorption by the target molecules together with knowledge of their absorption coefficient then enables the average molecular concentration along the beam to be determined, providing that compensation can be made for beam losses due to scattering, divergence and local changes in atmospheric refractive index.By contrast, the single-ended remote monitoring configuration requires no artificial element to return the laser beam to the detector but relies purely on local aerosol scattering which, in the troposphere, is always present to some extent. This configuration, which is used by all true lidar systems, is thus more versatile than the double-ended system in that the laser beam may be pointed freely in any direction or steadily scanned. However, it does of course require much more laser power because the amount of laser radiation fortuitously scattered into the detector is far smaller, by a factor of typically 106-108, than that returned by a specular reflection aimed directly at the detector.Absorption remote monitoring systems have been produced by many laboratories around the world but typical of these are the two systems presently under development at the National Physical Laboratory, a double-ended diode laser absorption system and a differential absorption lidar (DIAL) single-ended system. Diode Laser System Solid-state diode lasers based on lead salt compounds have revolutionised high resolution mid-infrared spectroscopy by providing a tunable, narrow band (approximately le3 cm-l) source of laser radiation that, by use of a series of lasers of different chemical composition, can span the entire mid-infrared region from about 3 to 30 pm. Individual lasers can typically be tuned over 100 cm-1 or more and all can be easily frequency modulated with amplitudes of up to several tenths of a wavenumber. Operation in this molecular “fingerprint” region of the infrared allows the possibility of detecting many molecules of environmental interest, although the low power of these lasers (typically tens of microwatts for single frequency operation) requires the use of the double-ended configuration employing a retroreflecting mirror to return the beam to a detector situated close to the laser.In remote monitoring operations the diode laser is first tuned to a particular absorption line of the target molecule that is known to be free from interference by absorption lines of other gases present in the atmospheric region that is being sampled. The laser is then frequency modulated across the chosen absorption line, which causes an amplitude modulation of the beam intensity that can be detected by a lock-in amplifier as long as the target molecule is present in sufficient concentration in the path sampled by the laser beam. In order to compensate for beam intensity fluctuations owing to atmospheric turbulence the total returned beam power at the detector is also measured, using amplitude chopping of the emitted laser beam and a second lock-in amplifier. The ratio of these two different signals is then taken, as this provides a signal that is linear with target molecule concentration and beam path length, but in which beam intensity fluctuations due to atmospheric turbulence or laser power changes have been cancelled out.Calibration of the ratio signal in terms of gas concentration is easily achieved by placing in the beam a gas cell of known length containing a known concentration of the target gas.With this system average target gas concentrations of just a few parts in l o 9 are typically measurable over ranges of several hundred metres, although this naturally depends upon the absorption strength of the particular gas line being employed. Such measurements also have the advantage of being made continuously in real time. The NPL diode laser system is mounted in a modified Ford Transit van equipped with an optical table for the laser optics and a periscope by which the laser beam emerging from the van can be steered in any desired direction. The system has been designed with the aim of performing long path gas measurements at places such as oil refineries, industrial plants and motorway sites.Field testing in these environments is already underway. Later extensions of the technique could involve gas absorption and emission by agricultural field and forest systems and also mineral prospecting by detection of sub-soil gas evolution. DIAL System The DIAL system is similar to the basic lidar system described earlier in that it measures and displays the intensity and time delay of laser radiation that has been emitted in a brief pulse and returned to a detector via aerosol scattering in the atmosphere. It differs, however, in using laser beam pulses of two72 ION-PAIR CHROMATOGRAPHY And. Proc., Vol. 21 different wavelengths, where one is arranged to be on a known absorption line of the target molecule while the other is in a region of negligible absorption.If the two laser pulses are sufficiently close together both in wavelength and in time then it can be assumed that each is scattered to the same extent by the aerosol. Ratioing of detector signals of equal time delay from the two pulses then cancels out intensity variations due to scattering and leaves just the intensity reduction due to true target molecule absorption in the “on-line” beam. As the effective optical path-length can be calculated from the time delay it is then possible, knowing the absorption strength of the molecular line being used, to compute the concentration of the target molecule at distance intervals all the way along the laser beam.This ability to make “range-resolved” concentration measurements in a single-ended configuration gives the DIAL system a unique measurement capability. In particular, it becomes possible, by sweeping the laser beam horizontally or vertically, to build up a two- or even three-dimensional map of target gas concentration over a chosen region of up to several kilometres. Alternatively the beam can be held in a fixed position, usually vertical, whilst the mobile laboratory housing the DIAL system is driven along a selected path; this can be most useful in following individual plumes from industrial stacks over long distances to observe their rates of dispersion. Of course the system can also be employed just as a conventional lidar for aerosol detection. The DIAL system developed at the NPL uses twin dye lasers whose outputs are frequency doubled to bring them into the ultraviolet region to allow detection of sulphur dioxide and ozone.The output pulse energy of around 10 mJ for a pulse width of just 10 ns is sufficient to allow range-resolved measurements of up to about 3 km from the laser in range increments only 10 m wide. Typical detection sensitivity is about 0.1 p.p.m. per 10 m range element for sulphur dioxide and ozone. As with the diode laser system the DIAL apparatus is built into a mobile laboratory, which is currently being used in a variety of field measurements both in the UK and abroad. The DIAL system provides an interesting contrast to the diode laser system in that it is clearly more versatile in its applications by virtue of its range resolution capability and single-ended configuration, but has to pay for this in somewhat reduced sensitivity and in much greater complexity and cost.These latter factors are due to the high laser power required and to the sophisticated computerised detection system. The detection system needs to digitise and store some 400 range element intensity values returned from each laser pulse, followed by averaging of values from a series of pulses and then taking logarithmic ratios of on- and off-resonant pairs of range elements to provide the range-resolved molecular concentrations. Future development of the NPL DIAL system will include operation in the infrared region of the spectrum so that it will be able to detect the wide range of molecules currently available to the diode laser system. A number of DIAL systems are now in operation around the world. These include one operated by NASA5 in the USA, which is being flown in an aircraft so that it can look vertically downwards upon the troposphere. This system is in a turn a prototype for one to be fitted to the Space Shuttle in several years time to obtain a continuous view of the entire atmosphere. Truly, when it comes to applications of the DIAL technique the sky is no longer the limit! References 1. 2. 3. 4. 5. Byer, R. L., Opt. Quant. Electron., 1975, 7 , 147. Carswell, A. I . , Can. J . Phys., 1983, 61, 378. Hirschfeld, T., Schildkraut, E. R . , Tannenbaum, H . , and Tanenbaum, D., Appl. Phys. Lett., 1973,22,38. Chanin, M., and Hauchecorne, A . , J. Geophys. Res., 1981,86, 9715, and references therein. Browell, E. V., Carter, A. F., Shipley, S. T., Allen, R. J . , Butler, C. F., Mayo, M. N., Siviter, J . H., and Hall, W. M., Appl. Opt., 1983, 22, 522.

ISSN:0144-557X    

DOI:10.1039/AP9842100069

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

72 ION-PAIR CHROMATOGRAPHY And. Proc., Vol. 21 Ion-pair Chromatography The following is a summary of one of the papers presented at a Joint Meeting of the Midlands Region and the Chromatography and Electrophoresis Group held on May 12th, 1983, at Fisons Pharmaceutic- als, Loughborough.February, I984 ION-PAIR CHROMATOGRAPHY "Ion-pair" Chromatography of Metal Ions 73 Roger M. Smith Department of Chemistry, Loughborough University of Technology, Loughborough, Leicestershire, L El 1 3TU The traditional method for the chromatography of metal ions has been resin-based ion-exchange chromatography. With a few exceptions little use has been made of gas - liquid chromatography because of the poor volatility of most metallic compounds and most instrumental analysis has concentrated on the use of atomic-absorption spectroscopy.However, with the advent of HPLC, these restrictions have largely been removed and there is an increasing interest in liquid chromatographic determinations in which metal ions are involved. The techniques can be divided into two broad areas, firstly the use of metal ions to enhance the chromatography of organic compounds, and secondly the analysis of metal ions by complexation with organic compounds. At first sight many of the methods are similar to ion-pair chromatography as they use a mobile phase incorporating an ionic species, which interacts with the ionic sample molecules and changes their separation. Metal Ions in the Separation of Organic Compounds There are two main areas in which metal ions have been used, enhancement of separation and the specific area of the separation of chiral compounds. Silver ions have long been used to enhance TLC separations of organic compounds containing olefinic groups, and in recent years the concept has been applied to HPLC.If silica columns coated with silver nitrate were used only limited success was achieved,l but on the incorporation of silver nitrate into the mobile phase of a reversed-phase column cis and trans isomers of fatty acid methyl esters could be separated.* The mechanism is not, however, ion pairing and the JC complex formed is ionic and elutes more rapidly than the uncomplexed compound. Other methods have made use of the ability of some metal ions to undergo chelation. In a study of the degradation of dazomet, it was desirable to quantify N-methyldithiocarbamate; however, because of its instability at low pH ion suppression could not be employed.Conventional ion-pair reagents for acids caused loss of the sample. If nickel or cobalt ions were included in the mobile phase, sharp peaks could be obtained for the corresponding dithiocarbamate chelate, rapid formation of the complexes taking place on injection of the sample.3.4 The complexes had the same retention times as complexes prepared off the column and then injected, suggesting that complex formation was rapid and complete. The retention times were insensitive to metal ion concentration. Consequently, despite the similarity to an ion-pair separation in which an ionic sample reacts on the column to give a neutral, organic soluble complex, the dithiocarbamate complex formation is essentially complete and the method is probably best described as on-column derivatisation.Other authors have used zinc ions to improve the separation of aminobenzoic acids5 and nickel ions to alter selectively the retention of aminophenols.677 In each instance the ortho isomers interacted more strongly than the meta or para isomers, suggesting that chelation was occurring. In these instances the effect was dependent on the metal ion concentration and pH, although the complexes were apparently ionised as the retention times were reduced compared with the other isomers. A more subtle use of chelation has occurred in the use of partially masked metal ions such as nickel - DPM [bis(2,2,6,6-tetramethylheptane-3,5-dionate)] chelates to form outer complexes with aminesg or of dodecyldien - zinc complexes to alter the separation of amino acids and sulphonamides.9 An alternative approach to produce a solublised metal complex has been to use a metal ion-modified stationary phase.By using an amino-bonded column and an eluent containing cadmium sulphate, it was possible to obtain separations of amino sugars and peptides.10 A similar method with a diamino-bonded column and cobalt ions was used for the separation of dihydroxybenzenes. 1 1 In these examples there is a ternary complex between the column amino group, the metal ion and the sample. A special example of this technique has been used by a number of workers to achieve chiral separations of amino acids by adding a copper - amino acid complex to the mobile phase.12 An alternative approach has used the formation of a mixed dithiocarbamate complex formed by the in situ reaction of carbon disulphide with the chiral analyte amine ephedrine and copper ions.'3,14 Separation of Metal Ions Many workers have examined the separation of metal chelates.15-16 In almost all instances these have been formed and isolated in separate chemical reactions before injection into the HPLC system. The74 ION-PAIR CHROMATOGRAPHY And. Proc., Vol. 21 complexes have included phenanthrolines, dithiocarbamates, dithizones and acetylacetonates and either normal- or reversed-phase columns have been used. These pre-column derivatisations of the metal ions are time consuming and require multiple steps before analysis.Sometimes they have the advantage that a concentration step can be included. It seemed that it should be possible to carry out an on-column reaction to give the same products by injecting an aqueous solution of the metal ions into an eluent containing the appropriate reagent. This method has similarities to ion-pair chromatography but, as earlier, the equilibrium strongly favours the complex. Initial studies examined the use of PAN (pyridylazonaphthol) in the eluent but they were not very successful. Subsequently other workers have separated the pre-formed PAR (pyridylazoresorcinol) and PAN complexes.17 Following the earlier successful study of the separation of dithiocarbamates by using their rapid complexation with metal ions, their application as on-column reagents was studied.Ammonium tetramethylenedithiocarbamate was first examined but gave poor results. However, if sodium diethyldithiocarbamate was added to the mobile phase, copper, cobalt, cadmium and lead ions could be readily separated and detected (Fig. 1).18The responses varied with the wavelength of detection and because of the similarity of the ultraviolet spectra of the chelates to that of the reagent cadmium and lead were only poorly detected. A more detailed study was carried out using copper ions and good sensitivity and reproducibility could be obtained. 18 Lead ions have subsequently been re-examined and it was found that in order to obtain reproducible results the mobile phase had to contain chloroform to prevent precipitation of the complex.19 A similar method had been used for the pre-formed complexes.20 0 5 10 Ti me/m i n Fig.1. Chromatogram of a sample containing copper (100 p.p.m.), cobalt (5 p.p.m.), lead (10 p.p.m.) and cadmium (100 p.p.m.) ions injected into 70 + 35 methanol - water containing 0.05% m/V sodium diethyldithiocarbamate with detection at 350 nm, 0.05 a.u.f.s.18 The same reagent has been employed by Bond and Wallace21-23 for the determination of copper, cobalt, chromium and nickel in water. They used electrochemical detection and achieved a similar sensitivity to the spectroscopic detector. In both of these studies the concentration of reagent had no effect on the retention times, indicating that the process is effectively derivatisation rather than an equilibrium ion-pair formation.Other chelating groups can also be used and Berthod et d . 2 4 employed quinolin-8-01 in the mobile phase to separate a range of metal ions using atomic-absorption, spectroscopic and electrochemical detection. A particular advantage of HPLC over other instrumental methods for the analysis of metal ions is the ability to separate metal ions in different oxidation states. A number of workers have examined the separation and determination of Cr(II1) and Cr(VI).25,26 Of particular interest was recent work in which Hoffmann and Schwedt27 compared the effectiveness of pre- and on-column derivatisation for the analysis of Mn(I1) and Mn(II1) using quinolin-8-01 as a reagent. References 1. 2. 3. Smith, E. C., Jones, A. D., and Hammond, E. W., J . Chromatogr., 1980, 188, 205.Chan, H.-S., and Levett, G., Chem. Ind. (London), 1978, 578. Smith, R. M., Morarji, R. L., Salt, W. G., and Stretton, R. G., Analyst, 1980, 105, 184.4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. February, 1984 EQUIPMENT NEWS 75 Smith, R. M., Morarji, R. L., and Salt, W. G., Analyst, 1981, 106, 129. Walters, V., and Raghavan, N. V., J. Chromatogr., 1979, 176, 470. Sternson, L. A., and DeWitte, W. J., J. Chromatogr., 1977, 137, 305. Sternson, L. A., Dixit, A. S., Riley, C. M., Seigler, R. W., and Schoech, D., J. Pharm. Biomed. Anal., 1983, Lochmuller, C. H., and Hangac, H. H., J. Chromatogr. Sci., 1982, 20, 171. Cooke, N. H. C., Viavattene, R. L., Eksteen, R., Wong, W., Davies, G., and Karger, B.L., J . Chrornatogr., Dua, V. K., and Bush, C. A., J. Chromatogr., 1982,244, 128. Chang, C. A., and Tu, C.-F., Anal. Chem., 1982, 54, 1179. Grushka, E., Levin, S., and Gilon, C., J. Chromatogr., 1982, 235, 401. Low, G. K. C., Haddad, P. R., and Duffield, A. M., Chromatographia, 1983, 17, 16. Low, G. K. C., Haddad, P. R., and Duffield, A. M., J. Chromatogr., 1983, 254, 123. Willeford, B. R., and Veening, H., J. Chromatogr., Chromatogr. Rev., 1982, 251, 61. Schwedt, G., Chromatographia, 1979, 12, 613. Schwedt, G., and Rudde, R., Chromatographia, 1982, 15,527. Smith, R. M., and Yankey, L. E., Analyst, 1982, 107, 744. Smith, R. M., Yankey, L. E., Thakur, A., and Butt, A., in preparation. Drasch, G., von Meyer, L., and Kauert, G., 2. Anal. Chem., 1982, 311, 695. Bond, A. M., and Wallace, G. G., Anal. Chem., 1981, 53, 1209. Bond, A. M., and Wallace, G. G., Anal. Chem., 1982, 54, 1706. Bond, A. M., and Wallace, G. G., Anal. Chem., 1983, 55, 718. Berthod, A., Kolosky, M., Rocca, J.-L., and Vittori, O., Analusis, 1979, 7, 395. Tande, T., Petterson, J. E., and Torgrimsen, T., Chromatographia, 1980, 13, 607. Schwedt, G., 2. Anal. Chem., 1979, 295, 382. Hoffmann, B. W., and Schwedt, G., J. High Resolut. Chromatogr. Chromatogr. Commun., 1982, 5, 439. 1, 105. 1978, 149, 391.

ISSN:0144-557X    

DOI:10.1039/AP9842100072

出版商:RSC    

年代:1984

数据来源: RSC

摘要:

February, 1984 EQUIPMENT NEWS 75 Equipment News Infrared Spectrophotometer The Model 9836 is based on the Model 983 but has increased data handling capacity. It has two full spectral memories and the capability to display spectra on the visual display unit. Spectra can be recorded directly on to the screen, processed, formatted, then plotted on the thermal recorder. There is a choice of cursor modes for peak picking and an interactive difference routine which simplifies spectral subtraction. Perkin-Elmer Limited, Post Office Lane, Beacons- field, Buckinghamshire, HP9 1QA. Clinical Micro-flow Spectrophotometer The Shimadzu CL720 visible-only instrument incorpor- ates a microcomputer, which performs enzyme kinetic assays and colorimetric end-point assays automatically. Up to 18 individual measurement programmes can be stored.A two-channel 5-point working curve lineariser permits handling of non-linear data direct in RATE or CONC modes. The UV-730 is an ultraviolet - visible micro-flow instrument, which can be used for rate assays with an optional thermoelectric cuvette. V. A. Howe & Company Limited, 12-24 St. Ann’s Crescent, London, SW18 2LS. Spectrophotometer Software A software package, available on disc and tape, for the HP85 computer provides a wavelength scanning facility for the PUS600 series of ultraviolet - visible spectro- photometers. Pye Unicam Ltd., York Street, Cambridge, CB12PX. Tablet Dissolution Monitoring Systems An automatic six-cell programmer accessory for the PUS800 range of ultraviolet - visible spectrophoto- meters enables automatic measurement of up to six samples against a reference. With the aid of a dissolu- tion software package for the HPS5 computer, full data handling facilities are provided.Pye Unicam Ltd., York Street, Cambridge, CB1 2PX. Infrared Spectrophotometers Four new models of the SP3 series of instruments, the S76 EQUIPMENT NEWS Anal. Proc., Vol. 21 series, feature the Spectraset system, which automatic- ally optimises instrument operating parameters accord- ing to the type of sample. Pye Unicam Ltd., York Street, Cambridge, CB12PX. Infrared Spectroscopy Software The CDS-3 data processing package for the Model 7500 Professional Computer contains established routines, such as spectral storage, digital smoothing and base line flattening.Perkin-Elmer Limited, Post Office Lane, Beacons- field, Buckinghamshire, HP9 1QA. X-ray Fluorescence Spectrometer Accessory An automatic changeover accessory for the PW1400 sequential X-ray spectrometer system automatically switches the spectrometer from vacuum path to helium path for the analysis of liquid samples, and the reverse. Pye Unicam Ltd., York Street, Cambridge, CB12PX. Atomic-absorption Spectrophotometry Accessory The Slotted Tube Atom Trap (STAT) gives enhanced sensitivities for elements such as arsenic, selenium, cadmium, lead, zinc and copper. It can be fitted to the maker’s SP9 50 mm or PU9000 burners. Typical applica- tions include analysis of effluents and potable waters for Gas Chromatograph The Model 8300 is suitable for routine or process laboratories and features automated bleed compensa- tion, optional real-time display of a chromatogram and optimal integral data handling facilities.Interchange between the injector and the detector is easy. Perkin-Elmer Limited, Post Office Lane, Beacons- field, Buckinghamshire, HP9 1QA. Gas Chromatograph Accessory The cool on-column capillary injector is an accessory to the SP7100 Gas Chromatograph. It is for use with fused silica and glass capillary columns. Spectra-Physics Ltd., 17 Brick Knoll Park, St. Albans, Hertfordshire, AL1 5UF. Syringes for Gas Chromatography Hamilton microlitre syringes are available with remov- able, de-activated fused silica needles. The interchange- able needles, with precision hubs and ferrules, are available for 700 series microlitre syringes in 5 and 10 y1 capacities.V. A. Howe & Company Limited, 12-14 St. Ann’s Crescent, London, SW18 2LS. HPLC Autosampler The Model 504 autosampler can hold up to 102 samples. It is designed so that the column is mounted close to the point of sample injection. It can inject 1 PI of sample from a total sample of 1 0 ~ 1 . The sample volume is adjustable from 1 to 100 yl. Beckman-RIIC Ltd., Progress Road, Sands Indus- trial Estate, High Wycombe, Buckinghamshire. Sample Injector for Liquid Chromatography The LDC/Milton Roy Model 713 automatic injector can inject 60 samples with up to 3 injections per vial. Only 100~1 of sample is required for a 10-yl injection. It is available with a choice of injection valves, the Rheodyne 7010A, 7126 and 7140A. Laboratory Data Control (UK) Limited, Milton Roy House, High Street, Stone, Staffordshire, ST15 8AR.Radio-HPLC Flow Cells A range of cells is available for use with the LB503 (Analytical) and LB504 (Preparative) Radio-HPLC polluting elements, such as arsenic, lead and cadmium, and the determination of copper and zinc in serum and lead in whole blood. Pye Unicam Ltd., York Street, Cambridge, CB1 2PX. X-ray Fluorescence Analyser The Lab-X 2200 microprocessor-controlled analyser has customised software which makes possible the program- ming of correction facilities into the microprocessor memory for complex applications such as the deter- mination of ash in coal and tin in ores. Oxford Analytical Instruments Limited, 20 Nuffield Way, Abingdon, Oxfordshire, OX14 1TX.February, 1984 EQUIPMENT NEWS 77 detectors. The cells offer flow-rates of up to 60 ml min-’ at 30 bar.Laboratory Impex Limited, Lion Road, Twicken- ham, Middlesex. Sample Preparation System Hamilton’s Chrom-Prep system consists of a small disposable cartridge which fits into a holder fitted to one of the maker’s syringes. It can be used to concentrate either by absorption of specific compounds while eluting undesired materials or by absorbing the interfer- ing materials while eluting the analyte. V. A. Howe & Company Limited, 12-14 St. Ann’s Crescent, London, SW18 2LS. Glass Cartridges for HPLC CGC glass cartridges can withstand internal pressures of 8 700 Ib in-2 when replacing steel columns. Sorbents offered are LiChrosorb Si60, RP18, RP8, DIOL and NH2. BDH Chemicals Ltd., Broom Road, Poole, Dorset, BH12 4”.HPLC Pump The microMetric pump provides pulse-free flow from 1 to 500 p1 min-1. It has a 5-ml capacity and can achieve 700 bar. Laboratory Data Control (UK) Limited, Milton Roy House, High Street, Stone, Staffordshire, ST15 8AR. Automated Peak Height Analysis The PHA-82 can be used with practically any analyser that produces a regular series of peaks. Based on the Commodore CBM/PET computer, it can handle up to 16 channels of analogue data. DMA, 906 Woodborough Road, Nottingham, NG3 5QR. Flow Injection Analysis The FIAstar is a fully automated system intended for the analysis of water, soil, fertilisers and beverages. It can carry out 300 analyses per hour with response times as low as 5 s. Tecator Ltd., Tecator House, Cooper Road, Thorn- bury, Bristol, BS12 2UP. pH Analyser The F-80 can produce a resolution of one thousandth of a pH unit. Calibration is achieved by immersing the electrode in any of the standard pH 4, 7 or 9 buffer solutions.The instrument identifies which standard is being used and automatically calibrates, making adjust- ments for temperature changes. Horiba Instruments Ltd., 5 Harrowden Road, Brack- mills, Northampton, NN4 OEB. Ammonia Electrode The 95-12 has a 2-year warranty and its simplified assembly eliminates the need for O-rings and spacers. MSE Scientific Instruments, Manor Royal, Crawley, Sussex, RHlO 2QQ. Analysers for Gas and Vapour Monitoring The Binos range uses infrared, visible, ultraviolet, thermal conductivity and mass spectrometry analytical techniques.Specialised versions can be developed for individual needs. For example, the Binos infrared analyser has been developed to monitor carbon dioxide andor water in photosynthesis research. Leybold-Heraeus Ltd., 16 Endeavour Way, Durns- ford Road, London, SW19 8UH. Glucose Analyser The new Stat glucose analyser uses the oxygen accelera- tion method and requires only a 10-pl sample. One hundred and twenty samples per hour can be analysed and results are displayed digitally 6 s after injection. Camlab Limited, Nuffield Road, Cambridge, CB4 1TH. Balance The AEl00 electronic analysis balance has a capacity of 109 g and a readability of 0.1 mg. Mettler Instrumente AG, CH-8606 Greifensee, Swit- zerland. Balance The BlOO is a top loading analytical balance with 100-g capacity.Its precision is +O.O007g and its linearity is Ohaus Scale Europe Ltd., Broad Lane, Cottenham, kO.001 g. Cambridgeshire. Balances A new range of electronic precision balances is announ- ced. There are two basic categories: those accurate up to one part in 5000 and those accurate up to one part in 20 000. Salter Industrial Measurement Ltd., George Street, West Bromwich, West Midlands, B70 6AD. Balances The 4504 MP8 ultra-micro balance offers an electronic weighing range of 120 mg to a readability of 0.1 pg. The 4503/MP6 microbalance gives an electronic weighing range from 300mg to 1 pg. Both feature dial-in tare weights. Sartorius Instruments Limited, 18 Avenue Road, Belmont, Surrey, SM2 6JD. Thermal Printer The GA44 alphanumeric thermal printer can be connec- ted to all the maker’s electronic balances that are78 EQUIPMENT NEWS A n d Proc., Vol.26 equipped with a serial data output. It can also be connected to such instruments as the DL40RC Memo- Titrator or the FP8O controller in the FP800 thermosy- stem. Its capacity is 20 characters per line and the transfer rate is adjustable in four steps from 110 to 2 400 baud. Mettler Instrumente AG, CH-8606 Greifensee, Swit- zerland. Scanning Electron Microscopes The lowest continuous operating voltage range on the SEM 505 has been extended from the previous 1-3 kV to 0.2-3 kV. The minimum operational voltage of 200 V makes possible micrography in the sub-1 kV region. The extension of the lower end of the EHT range can be fitted to existing instruments, Pye Unicam Ltd., York Street, Cambridge, CB12PX.Microscope The Labovert is an upright inverted microscope. It has a working distance of 95 mm and a field of view index of up to 22.5. A range of objectives and condensers is available. E. Leitz (Instruments) Ltd., 48 Park Street, Luton, LU13HP. Thermometer The Hl00 thermocouple digital instrument is pocket- sized and has a range from -50 to +500”C. Hale Instruments Ltd., Manor House, Manor Road, A1 trincham, Cheshire , WA 15 9QX. Porosimeter The Pore Sizer 9305 features an RS232 port that allows automatic acquisition, reduction and reporting of poros- ity data. Data can be presented in tabular and graphic formats and can be stored and retrieved for comparison purposes. Micromeritics Instrument Corporation, 5680 Goshen Springs Road, Norcross, Georgia 30093, USA.Graphic Recorder The RMS GR33 offers three standard modes of operation: 32 channel chart recording, 132 column alphanumeric line printing and 1240 element Raster Graphics printing. It weighs 11 kg and is contained within a 5 x 19in rack mounting. GSE Limited, Unit A, Cradock Road, Luton, Bed- fordshire, LU4 ONF. Oscilloscope The 5110 is a 100-MHz instrument with a built-in IEEE-488 interface. It has comprehensive trigger facili- ties, including delay by events up to 999 999 999 and delay by time with a 10 ns resolution up to a maximum of 344 s . Gould Instruments Limited, Roebuck Road, Hain- ault, Ilford, Essex. Literature A catalogue gives details of the PFP-1 flame photo- meter, which gives readings between 0 and 199.9 in relation to full-scale readings of 3-100 p.p.m.for potassium and sodium, 5-100 p.p.m. for lithium, 1W200 p.p.m. for barium and 5-100 p.p.m. for calcium. Reproducibility is to a coefficient of variation of 1% for 20 consecutive samples, with 10 p.p.m. set to read 100 on the scale, using samples of 2-6 ml. Jenway Ltd., Gransmore Green, Felsted, Dunmow, Essex, CM6 3LB. A brochure describes the SFM25 spectrofluorimeter, which is available in four wavelength operating modes: fixed wavelength, excitation scan, emission scan and synchroscan. Information is also available on automatic blood grouping techniques, an HPLC technical consul- tancy and the MDA312 multi-well gamma counter. Kontron Instruments Limited, P.O. Box 88, St. Albans, Hertfordshire. A brochure deals with automatic sample injection systems for liquids, solid materials and gases in gas chromatography.Packard Instruments Limited, 13-17 Church Road, Caversham, Berkshire, RG4 7AA. A brochure describes a range of gas chromatography columns fabricated in a glass lined stainless-steel tubing. Scientific Glass Engineering (UK) Ltd., 1 Potters Lane, Kiln Farm, Milton Keynes, MKll 3LA. A leaflet gives details of the Model MVlOl electric valve actuator, designed to give fast operation for various valves, such as the Rheodyne 7000 series, without the use of pneumatics. Avok Ltd., 1 Willow Road, Poyle Industrial Estate, Colnbrook, Berkshire, SL3 OBS. A brochure presents the P105M microprocessor-based circular chart recorder. Kent Industrial Measurements Limited, Industrial Instruments, Howard Road, Eaton Socon, St.Neots, Huntingdon, Cambridgeshire, PE19 3EU. A brochure describes the Fryka range of low tempera- ture cooling units. Four model ranges are available: cold plates, immersion coolers, throughput coolers and cooling baths. Camlab Limited, Nuffield Road, Cambridge, CB4 1TH. A brochure describes the Cygnair 1000 laboratory fume cupboards. These incorporate many design safety fea- tures stemming from the BSI provisional paper “Draft for Development of Laboratory Fume Cupboards, DD80 (Parts 1, 2 and 3).” Cygnet Joinery Limited, Higher Swan Lane, Bolton, Lancashire, BL3 3AH. A brochure covers the Variomag range of electronic magnetic stirrers. Two types are offered: Type 1 for general laboratory use and in incubators up to 50°C; and Type 2 with remote controls enabling them to be used in water-baths, incubators, fume cupboards, etc. There is also a submersible micro-stirrer. Camlab Limited, Nuffield Road, Cambridge, CB4 1TH.February, 1984 COMPUTER SOFTWARE IN ANALYTICAL CHEMISTRY 79 A brochure describes the Anika Series L021/L022 transient signal recorders for high frequency recording. Ramtech Electronics Ltd., Southbank House, Black Prince Road, London, SE1 7SJ. A brochure presents TriM, a sandwich consisting of a high strength zirconia grain stabilised (ZGS) platinum on the outer layers with a palladium core. TriM can be successfully used at up to 1 300 "C in oxidising environ- ments. Johnson Matthey Metals Limited, South Way, Exhi- bition Grounds, Wembley, HA9 OHW.

ISSN:0144-557X    

DOI:10.1039/AP9842100075

出版商:RSC    

年代:1984

数据来源: RSC