摘要:

ANPRDI 28(10) 309-356 (1991) ISSN 0144-557X President J. D. R. Thomas Publication of Analytical Proceedings is the responsibility of the Analytical Editorial Board: Editorial Secretary Claire Harris R. M. Miller B. L. Sharp Analytical Proceedings Proceedings of The Analytical Division of The Royal Society of Chemistry Analytical Division Officers: Hun. Secretary C. A. Watson Analytical Division Secretary Miss P. E. Hutchinson Ed ito ria I Manager, A n a I ytica I Jo urn a Is Judith Egan *H. M. Frey D. E. Games S. J. Hill D. L. Miles Senior Assistant Editor Paul Delaney A. G. Fogg (Chairman) K. D. Bartle D. Betteridge *J. Egan Hon. Assistant Secretary F. W. Sweeting All editorial matter should be addressed to The Editor, Analytical Proceedings, The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 4WF, UK.Telephone 0223 420066. Telex 818293 ROYAL. Analytical Proceedings (ISSN 0144-557x3 is published monthly by The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 4WF, England. All orders, accompanied by payment, should be sent t o The Royal Society of Chemistry, Turpin Transactions Ltd., Blackhorse Road, Letchworth, Herts. SG6 lHN, England. 1991 Annual Subscription price if purchased on its own: EC f l l O . O O , Rest of World €126.00, US $258.00, including air speeded delivery. Customers should make payments by cheque in sterling payable on a UK clearing bank or in US dollars payable on a US clearing bank.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. N b : Turpin Transactions Ltd., distributors, is wholly owned by the Royal Society of Chemistry. @The Royal Society of Chemistry 1991. 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 publishers."Exofficio members ANALYTICAL JOURNALS 1991 The Analyst ISSN 0003-2654 12 issues per annum plus index EC g246.00 USA $580.00 Rest of World 2283.00 Analytical Proceedings ISSN 0144-557X 12 issues per annum plus index EC 2110.00 USA $258.00 Rest of World 2126.00 ORDERING: Non-RSC Members should send their orders to: The Royal Society of Chemistry, Turpin Transactions Ltd, Blackhorse Road, Letchworth, Herts SG6 lHN, UK. RSC Members should obtain members prices and send their orders to: Membership Affairs, The Royal Society of Chemistry. Thomas Graham House. Science Park, Milton Road. Cambridge CB4 4WF. UK. Published by the Royal Society of Chemistry Journal of Analytical Atomic Spectrometry (JAAS) ISSN 0267-9477 8 issues per annum (including two special issues) plus index EC 2309.00 USA $728.00 Rest of World f355.00 Analytical Abstracts ISSN 0003-2689 12 issues per annum EC f380.00 USA $765.00 Rest of World 2420.00 SPECIAL PACKAGES (Non-RSC Members only) The Analyst, Analytical Abstracts and Analytical Proceedings Journal Ref.No. 0UO(HO124 EC 5648.00 USA $1527.0() Rest of World f745.00 The Analyst and Analytical Proceedings Journal Ref. No. OoO(r-0140 EC f313.00 USA $738.00 Rest of World f360.00 The Analyst and Analytical Abstracts Journal Ref. No. OOMN132 EC fSS1.OO USA $1299.00 Rest of World f634.00 & CHEMISTRY & SOCIETYOF ROYAL &d Services Information Turpin Transactions Ltd, distributors, is wholly owned by The Royal Society of Chemistry October 1991 Hun. Treasurer T. B. Pierce Hun. Publicity Secretary Dr. J. D. Green, BP Chemicals Ltd., Research and Development Department, Hull Laboratories, Saltend, Hull HU12 8DS Editor, Analytical Proceedings Roger A. Young

ISSN:0144-557X    

DOI:10.1039/AP99128FX037

出版商:RSC    

年代:1991

数据来源: RSC

ISSN:0144-557X    

DOI:10.1039/AP99128BX039

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

ANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 309 ~~ The Analytical Chemistry Trust An article describing the creation of the Trust and the ways in which the interest accruing from the invested capital of the Trust Fund is used to further the objects of the Trust was published in the July, 1988, issue of Analytical Proceedings. The statement of account for the year ending December 31st, 1990, is presented below. The RSC Analytical Chemistry Trust Fund Income and Expenditure Account for the Year Ended December 31st, 1990 INCOME f f Income from Investments . . . . . . . . . . . . . . . . . . . . . 176 713 Interest on Deposit Accounts (Note 1) . . . . . . . . . . . . 132 471 Sale of Video. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183 Miscellaneous Income . . .. . . . . . . . . . . . . . . . . . . . 2 008 311 375 Less: EXPENDITURE Contribution to Expenses of RSC Analytical Division. . . Student Bursaries . . . . . . . . . . . . . . . . . . . . . . . . . . Gold and Silver Medals . . . . . . . . . . . . . . . . . . . . . . Audit Fees. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Legal and Professional Fees. . . . . . . . . . . . . . . . . . . . Schools Analyst Competition . . . . . . . . . . . . . . . . . . . Donation and Grants. . . . . . . . . . . . . . . . . . . . . . . . Voluntary Service Overseas. . . . . . . . . . . . . . . . . . . . Studentships (Note 2). . . . . . . . . . . . . . . . . . . . . . . . 20 750 50 334 1 820 7 276 2 300 17 893 5 587 2 500 947 109 407 EXCESS OF INCOME OVER EXPENDITURE CARRIED TO BALANCE SHEET 201 968 Norm 1.Interest on Deposit Accounts includes accrued interest, but income on fixed interest investments is accounted for on a receipts basis. 2. The SAC Students were S. Cocks (Loughborough University of Technology), G . A. Wiltshire (University of Strathclyde), S. Summerfield (Loughborough University of Technology), 0. J. Challenger (ICI Chemicals and Polymers Ltd. and Polytechnic South310 ANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 West), A. Economou (UMIST), S. J. O’Gram (Newcastle upon Tyne Polytechnic and ICI Wilton Materials Research Centre) and K. Knight (LGC and Birkbeck College, London). Balance Sheet INVESTMENTS AT COST 2 f Quoted Investments (Market Value 23 836 405) Add: Investments Purchased During the Year. .. . . . . . Balance as at January lst, 1990 . . . . . . . . . . . . . . . . . 3 622 405 1 292 344 Less: Cost of Investments Disposed of. . . . . . . . . . . . . CURRENT ASSETS National Savings Investment Account . . . . . . . . . . . . . National Savings Deposit Bonds. . . . . . . . . . . . . . . . . National Savings Income Bonds . . . . . . . . . . . . . . . . . Deposit Account with Bank. . . . . . . . . . . . . . . . . . . . Investment Deposit Account . . . . . . . . . . . . . . . . . . . Income Tax Recoverable . . . . . . . . . . . . . . . . . . . . . Cash at Bank. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sundry Debtors. . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 914 749 1 034 565 3 880 184 147 654 148 517 50 000 670 000 46 217 9 926 54 305 13 489 1 140 108 Less: Sundry Creditors. . . . . . . . . . . . . . . . . . . . . . . . . . . 103 040 1 037 068 4 917 252 FINANCED BY: TRUST CAPITAL ACCOUNT Balance as at January lst, 1990 . . . . . . . . . . . . . . . . . Add: Profit on Sale of Investments . . . . . . . . . . . . . . . 3 311 016 156 906 3 467 922 INCOME AND EXPENDITURE ACCOUNT Balance as at January lst, 1990 . . . . . . . . . . . . . . . . . 1 247 362 201 968 Add: Excess of Income over Expenditure for the Year. . 1 449 330 4 917 252

ISSN:0144-557X    

DOI:10.1039/AP991280309b

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

310 ANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 Third Report From Sponsored VSO Teacher Since my last report, written in August, 1990, I am pleased to relate many de- velopments at the school which have improved the teaching, learning and liv- ing environments. The Mashoko mission, school, hospital and accommodation, is now receiving 24 hour mains electricity, rewiring and the running of overhead cables being com- pleted in March. This has benefited all mission residents because no electricity has been available since September when the generator broke down irreparably. The availability of lights has made work- ing during the evenings considerably easier. With electricity now available in the laboratories the range of practical work which can be attempted is much wider, particularly in physics.One of the highlights of this term was receiving the ‘A’ level results from the November, 1990, exams. In chemistry, 8 of 12 students passed with 2 Cs, 6 Ds and 2 Es; 4 Students obtained ‘0’ level passes. Considering the disadvantages ex- perienced by the class who had no quali- fied teacher for 2 terms, no laboratory for the first year, and limited resources for most of the course, they did remarkably well. Two students have obtained places at the University of Zimbabwe; others should be able to secure positions at polytechnics, on training courses and in apprenticeships. Biology obtained a 78% pass rate, the physics results were more disappointing, but in comparison with theANALYTICAL PROCEEDINGS. OCTOBER 1991. VOL 28 311 national average of 20% passing, the Department’s over-all performance was very promising. This term I have continued to develop the facilities in the laboratory with emphasis on improving safety.To this end I ordered eye protection for all students and am attempting to purchase some toughened glass to build a safety screen for use during demonstrations. I am intending to use the f50 generously donated by the International Committee in December for these purposes. Plans to fit a fume cupboard have been shelved after receiving an extortionate quotation from the main supplier. I am seeking an alternative manufacturer with more moderate prices. A more pressing need is to provide the physics laboratory and science lecture theatre with blackout curtains to enable some experiments which require darkness to be performed and slides and films to be used in lessons. The school has recently received a €200 grant from VSO and another book dona- tion from the Ranfurly Library Service and British Council.The €200 is to be spent on apparatus to assist specifically with lower school teaching, where facili- ties are much more limited. Apart from teaching and fund raising I have been actively involved in the school’s extra-curricular activitics. The ‘A’ level science club continues to thrive and recently conducted a campaign warn- ing about the dangers of mains electricity and creating some imaginative, rather gruesome, posters! I have also been coaching the school’s athletics team, sadly very unsuccessfully! I hope the football team, with which I am also assisting, will achieve better results.Since my last report I have also been involved in a number of VSO organized events. In September I co-ordinated the induction course for 25 new volunteers arriving in Zimbabwe. In December I helped to organize a very successful national conference at which we raised Z$250 for a Zimbabwean children’s charity at a concert. I presented the cheque to the Provincial Governor of Masvingo early this year. Also at the national conference I conducted a three hour sports coaching session which was enjoyed by all participants. On the basis of this I have been asked to hold a one day session later this year. My contract here ends in September, 1991, but I have been granted a three month extension in order to complete the academic year and take my classes through to their November examinations.I intend to return to the UK in December. I have requested that VSO replace me at the school as there is a low probability of finding a qualified Zimbabwean. The school physics teacher is leaving in Sep- tember and VSO have agreed to try and recruit a volunteer for that position also. I am at present busy attempting to find an MSc, or possibly PhD, in atmospheric pollution to begin in January, 1992, and have approached a number of universities and colleges in Britain, the USA and Canada. Slow communications between rural Zimbabwe and the rest of the world make this somewhat frustrating, but I hope to be able to find a place on a course or studentship in the next few months. As I approach the end of my fifth term I am pleased with the progress which has been made by the school in establishing the ‘A’ level science department. In my remaining 8 months I hope this can be consolidated and the reputation that the school is beginning to establish furthered. Mashoko continues to be a friendly and interesting place to live and work and deserving of the support given to it by VSO, the RSC, AD and other non- government and church organizations. G. T. ARCHER

ISSN:0144-557X    

DOI:10.1039/AP9912800310

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

312 ANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 Report by the Analytical Methods Committee Evaluation of Analytical Instrumentation. Part VII Simultaneous Wavelength Dispersive X-ray Spectrometers Analytical Methods Committee The Royal Society of Chemistry, Burlington House, Piccadiliy, London Wl V OBN A method is provided for comparing the features of wavelength dispersive X-ray spectrometers. The Analytical Methods Committee has received and approved the following report from the Instrumental Criteria Sub-Commit tee. Introduction The following report was compiled by the above Sub- Committee of the AMC, which consisted of Professor s. Greenfield (Chairman), Dr. P. J . Potts, Mr. D. Squirrel1 and Mr. P. Warren, 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 objectives of the Instrumental Criteria Sub-committee are to tabulate a number of features of analytical instruments which should be con- sidered when making a comparison between various systems. As is explained below, it is possible then to 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 the manufacturers. The seventh* report of the Sub-Committee deals with X-ray fluorescence spectrometers that are designed as simultaneous instruments. The first six reports were as follows: ‘Atomic Absorption Spcctrophotomctcrs, Primarily for Use with Flames’. Analytical Proceedings. 1984. 21, 45: ‘Atomic Absorption Spectrophotometers. Primarily for Use With Electrothermal Atomisers’. Analytical Pro- ceedings. 1985, 22, 128; ‘Polychromators for Use in Emission Spectrometry With ICP Sources’, Analytical Proceedings, 1986, 23, 109; ‘Monochromators for Use in Emission Spectrometry With ICP Sources’, Analytical Proceedings, 1987.24, 3; ‘Inductively Coupled Plasma Sources for USC in Emission spectrometry’, Analytical Proceedings, 1987, 24, 266; ‘Wavelength Dispersive X-ray Spec- trometers’, Analytical Proceedings. 1990, 27, 324. Notes on the Use of this Document Column 1. Column 2. Column 3. Column 4. Column 5 The features of interest. What the feature is, and how it can be evaluated. The Sub-committee has indicated the relative importance of each feature and expects users to decide on a weighting factor according to their own applications. Here the Sub-committee has given reasons for its opinion as to the importance of each feature. 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/0 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 is indicated as follows: V1 (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 1 to ~ 3 , or x 1 to x 10.The factor could amount to total exclusion of the instrument. 3. (ST) Sub-total. This is obtained by multiplying PS by WF. 4. Simultaneous X-ray fluorescence spectrometers have recently developed along two pathways. As well as spec- trometers with high power X-ray tube generators, low power instruments designed with a compact excitation geometry are now available to provide a performance approaching that of the conventional high power generator instrument but in a smaller and less expensive instrument. It is therefore necessary to decide which of these two types of instrumentation is appropriate for a particular application (see section 1). Technical considerations may then be made to decide between available instruments within the particular category.The scoring of the features concerned with the number of channels, sensitivity, size, cooling, power requirements and cost should enable this choice to be made.ANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 INSTRUMENTAL CRITERIA SUB-COMMITTEE INSTRUMENT EVALUATION FORM 313 Type of Instrument: Simultaneous wavelength dispersive X-ray spectrometer Manufacturer: Model No: Feature 1. Choice of category of instrument (a) Number of channels ( b ) Sensitivity ( c ) Excitation geometry (d) Costs and services Definition and/or test procedures and guidance for assessment Score maximum for the type of instrument with sufficient channels to meet analytical requirements. Score maximum for the type of instrument which has adequate sensitivity and detection levels to meet analytical requirements.Score maximum for the provision of an excitation geometry designed to excite the lower surface of a sample if applications demand the analysis of liquid, powder or granular samples that cannot be prepared as brickettes. Score maximum for the instrument with the lowest installation and operating costs and laboratory space requirements commensurate with satisfactory analytical performance. Importana VI VI Depends 01 application I Reason Simultaneous instruments require one ‘monochromator’ channel for each element and as the provision of additional channels increases cost and complexity, the type of instrument chosen should not have more channels than is justified by the analytical application. Low power instruments (-150 W), which rely on a compact geometry, are normally restricted to 10- 14 channels, while high power instruments (3 kW) can accommodate up to 30 channels.In general, high power instruments have better sensitivity and detection limits. However, if a low power instrument can meet analytical requirements. there is little point in using a high powered instrument. Such a geometry permits the analysis of liquid and loose powdedgranular samples. This type of sample cannot be readily analysed on instruments designed to only excite the upper surface. See item l ( h ) above. Score PS WF s’r PS WF ST PS WF ST PS WF ST The following features need only be considered for the particular category of instrument selected by the above criteria. 2. Excitation ( a ) Generator (i) Conven- tional (ii) Solid state These generators are based on vacuum tube technology and use relatively large and heavy transformers which require water cooling. These generators are based on solid-state circuits using higher frequency oscillators and smaller step-up transformers and are, therefore, more compact. Both types of generator are normally satisfactory and it may be inappropriatc to score this item.This item is only of significance if thc physical size and/or weight or cooling water requirements limit application. Conventional generators are not normally used for low powered systems. Whilst some higher powered instruments are fitted with this type of generator, they are standard items for low powered systems. PS WF ST PS WF ST314 ANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 Feature (iii) Stability ( b ) X-ray tubes (i> Type (ii) Ease of replace- ment (iii Cooling and water supplies (high powered instru- ments only 1 (iv) Maximum power ( v ) Maximum voltage (c) Safety interlocks (i) Operator protection Definition and/or test procedures and guidance for assessment Score maximum for instruments with the best stability specification. End window tubes are standard for simultaneous instruments.Score maximum for convenient tube change procedure. Score maximum for minimum flow-rate and widest tolerance to temperature, supply prcssure and dissolved salts without impairing cooling efficiency. Within the category of instrument chosen, score maximum for the maximum power that can be delivered to the tube. Within the category of instrument chosen, score maximum for the highest voltage available for a tube with the anode of interest.Safety interlocks are devices to prevent injury from ionizing radiation o r hazardous voltages during stand-by, operation o r maintenance of the instrument and must satisfy current national and international regulations and be guaranteed by the manufacturer. Failure to meet national safety standards would preclude legal operation of the instrument. Importance - VI 1 I I I VI Reason The tube output in current and voltage is subject to both long term drift and short term fluctuations with mains voltage. Either source of instability will affect the frequency of recalibration and the over-all performance of the instrument. The geometry of simultaneous X-ray spectrometers precludes the use of side window tubes.Tubes have a finite operating lifc and require replacement. Therefore, it is essential that this should not be excessively time consuming o r difficult. Simultaneous instruments operate under compromise conditions and it is not necessary to change tubc anode type in the course of an analytical programme (cf. sequential XRF instruments). All tubes and some generators on high powered instruments rcquire cooling water. A tight specification for hardness, temperature control and flow will increase installation and running costs. Low power instruments have internal recirculating cooling systems which are air cooled and do not require an external cooling water supply. In general, higher power will increase sensitivity o r reducc analysis times.For higher atomic number elements. K lines are more efficicntly excited by tubes operated at higher voltages. Safety devices will include warning lights and appropriate safety switches and interlocks to prevent: (i), access to any potential source o r area of ionizing radiation whilst the excitation power is switched on; (ii), access to high power cable connections or mains voltage whilst electrically energised; (iii), operation of the tube unless it is secured in its normal position within the instrument. Score PS WF ST PS WF ST PS WF ST PS WF ST PS WF ST PS WF STANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 315 Feature (ii) Instrument protection 3 . Sample changer and presentation ( a ) Number of samples ( i ) Internal sample changer (ii) External sample changer (iii) Sample identifi- cation ( 6 ) Vacuum system bility ( i ) Reproduci- (ii) Pre-analysis pump-down (iii) Helium path option ( c ) Positioning and alignment of sample Definition and/or test procedures and guidance for assessment The flow of internal coolant and/ or external cooling water must be monitored and facilities provided for automatic safe shut- down in the event of supply failure.Score additionally if the system also monitors water temperature and, where applicable, conductivity of the deionized water supply. Score maximum for provision of an internal sample changer. Score maximum for the sample changer which can accommodate the maximum number of samples that are likely to be encountered in an analytical program. Score additionally if the sample changer can be retrofitted.Score maximum for a system of automatic identification which enables samples to be identified positively and the analytical sequence defined flexibly. Providing a level of less than 1 torr can be reached, scoring is inappropriate. In the absence of a pre-analysis evacuation compartment, score maximum for the minimum pump-down time for sample exchange. If downward facing sample geometry is employed, permitting the analysis of liquids and loose powders, score for the provision of a facility to change to an atmosphere of helium within the sample chamber/ spectrometer as an alternative to vacuum. Score additionally for economical use of helium. Score maximum for the best precision for locating samples in the excitation position for each position of the sample changer and in each sample holder.[mportance VI I I VI VI I Depends on sample type. VI Reason ~~ ~ ~ Efficient cooling is essential to prevent tube deterioration and/ or burn-out. A rise in water temperature will give early warning of partial blockage in supply lines or malfunction of heat exchanger in a recirculatory system. A rise in conductivity indicates loss of efficiency of the deionizer which, if allowed to continue, could lead to insulation break- down by high voltages present in the end-window tube. If at least two positions are available on an internal sample changer, pump down time is reduced, improving sample throughput rate. An external sample changer is often needed to give flexible and efficient operation.If a decision is taken to retrofit this device, substantial expense could be incurred if instrument design is incompatible. Essential if instrument is to be used for routine analysis, particularly if unattended operation is envisaged. Adequate vacuum within the spectrometer is essential to avoid attenuation of low energy fluorescence X-rays in the determination of light elements ( z < 20. Ca). If an instrument is not designed with a pre-analysis evacuation compartment, pump-down time must be added to sample counting time when calculating total analysis time. Pump-down time may be further extended if delays are encountered in achieving an operating vacuum owing to outgassing from the sample. This option is essential if light elements are to be analysed in either liquid samples or finely divided loose powders.This feature will affect the precision of measurements as very small errors in the position of the sample with respect to the X-ray tube will change the excitation efficiency. Score PS WF ST PS WF ST PS WF ST PS WF ST PS WF ST PS WF ST PS WF ST316 ANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 Feature (d) Sample spinning ( e ) Sample area irradiated and masking (i) Sample holder (ii) Materials and design of sample holders (iii) Masking 4. Spectrometer ( a ) Number of channels Range of mono- chromators available (c) Ease of change of channel (d) Scanning channel Definition and/or test procedures and guidance for assessment Score for the provision of a facility to rotate the sample continuously during the analysis period.Score maximum for the largest proportion of the sample area which can be excited. Score maximum for the availability of sample holders constructed in the widest range of materials and designs. Score maximum for the availability of suitable masks or apertures to match each sample size that may be used. Within the category of instrument selected (see 1 above), score maximum for the highest number of channels that can be fitted. Score maximum for the availability of the widest range of monochromators comprising the optimum combinations of diffracting crystal and X-ray counter that cover lines of all elements that are of interest. Score maximum for the easiest and most economical method of changing channel combinations. If this feature is required, score for the availability of a scanning channel which can be substituted for the minimum number of fixed channels.[mportance I I Varies according to sample types to be analysed. 1 I VI I I Reason Averaging the excitation induced at the sample surface will compensate for effects caused by minor sample inhomogeneity and surface defects. Maximizing the area on a sample from which fluorescence radiation is detected by the spectrometer will reduce sample inhomogeneity effects and enhancc detected count rates. Sample holders may also yield a fluorescence signal, particularly if liquid or thin film samples arc analysed. Selection of holder from a wide range of materials will enable the user to avoid such interferences. A range of sample holder designs is also required if non-standard small samples are to be analyscd.The use of suitable masks to restrict excitation to the sample itself will minimize scattered radiation and unwanted fluorescence from the sample holdcr. Masking is particularly important when analysing small samples to avoid excitation of thc sample support. Flexibility of application and the capacity to fit additional channels beyond the current analytical requirements. The availability of a wide range of monochromators will enable the selection of an instrument that will maximize analytical performance in the selected application. As discussed more fully under sequential XRF instruments, a range of diffracting crystals is available in XRF together with a range of X-ray detectors (including flow proportional counters, sealed proportional counters and scintillation counters).Selection from a full list of these components will allow the instrument to give maximum detection performance for the element line selected for analysis. It is convenient to be able to change quickly and economically the suite of channels to meet changing requirements. This feature allows some instrumental flexibility when non-routine analyses arc required and also provides a qualitative analysis capability. Score PS WF ST PS WF ST PS WF ST PS WF ST PS WF ST PS WF ST PS WF ST PS WF STANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 Definition and/or test procedures and guidance Feature for assessment Importance 317 Reason ~~ ~ 5 . Electronics Generul comment: Pulse processing electronics are replicated on each channel fitted to a simultaneous XRF spectrometer.Since a single set of excitation conditions must normally be used, it is important that the count rate response of channels is adequate for the very high count rates likely to be encountered in the measurement of matrix elements, as well as the low count rates that may be encountered simultaneously in other channels in the measurement of minor and trace elements. (u) Pulse processing ( i ) Window selection (ii) Pulse height depression (iii) Maximum usable count rate ( i v ) Dead-time correction and linearity (v) Total count capacity Electronic windows are selected to limit the acceptance of X-ray pulses from the detection system. Providing this facility is available on each channel. it may be inappropriate to score this feature.Score for the provision of effective electronic compensation for change of pulse height distribution as a function of count rate. Providing this facility is available on each channel, it may be inappropriate to score this feature. Score maximum for the highest count rate for which individual channels will yield a linear response. Score highly for accurate dead- time correction up to the maximum usable count rate. The accuracy of the dead-time correction may be tested by measuring the count rate response of individual channels with variations in tube current (providing the response of the tube generator is linear). Score maximum for the maximum number of counts that can be registered by individual channels. VI VI VI VI v1 A minimum voltage threshold must be set to exclude electronic noise.It is beneficial to set an upper threshold to exclude pulses that originate from higher order diffractions that will contribute to the background count rate. Pulse-height depression occurs progressively at count rates above about 10' counts per second owing to changes in electric fields within gas proportional counters. The depression. if uncorrected, may cause the analytical signal to drift out of the electronic window set by the puke height analyser when samples of varying count rate are measured. Provided detection systems retain a linear response over a wide dynamic range, a wide range of elemental concentrations can be measured under fixed excitation conditions. Analysis times are then minimized as excitation conditions do not need to be compromised by the need to constrain count rates on channeis measuring matrix elements.Dead time time arises from the ability of the counter to respond to a second X-ray event detected a short time after the first and the inability of the amplifier to distinguish between two events occurring within the resolving time of the electronic circuits fitted to individual channels. Both of these effects could lead to systematically low measurements at high count rates, unless the appropriate correction is made. The limitation on the maximum number of events that can be accumulated in any individual channel used to measure matrix elements may reduce precision in the determination of minor or trace elements being measured concurrently by other channels.This limitation arises because of the necessity to reduce count times to those appropriate to the channels with the highest count rate. Score PS WF ST PS WF ST PS WF ST318 ANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 Feature 6. Computer ( a ) Automation (i) Instrument control (ii) Operating programme (iii) Instrument (6) Data processing (i) Quantitative (ii) Qualitative analysis Definition and/or test procedures and guidance for assessment Score maximum for the greatest number of instrument functions that are under computer control. Score maximum for a user friendly operating system which permits the user to develop versatile analytical programmes tailored to the application. Score maximum for the most comprehensive display of instrument status parameters and alarm functions that monitor whether the instrument is operating within its design envelope. Score maximum for the provision.as appropriate to the application, of software features such as: (l), calibration using various data fitting modes for linear and non-linear functions; (2), matrix correction for absorption and enhancement effects by empirical and fundamental parameter procedures; ( 3 ) , facilities to output and store results for further processing and comparison. as well as editing data into a report format; (4), the facility to transfer data files in computer format to an external compatible device; ( 5 ) , statistical process control software for quality control applications. Score, if appropriate to the application, for the maximum wavelength range that can be scanned and for effective display, peak identification and hard copy facilities for the interpretation of spectral data.Importance VI I I VI May be I, depending on ipplication. Reason Computer control of instrumental parameters ensures reliable and reproducible operation of the instrument by well-trained, but non-expert operators. This prevents instrumental errors ( e . g . , conflicting settings) which could lead to instrumental malfunction, or even damage. Furthermore, computer control is essential to facilitate operation for extended periods, even when a number of parameters need resetting during such a pcriod of automatic operation. Difficult or repetitive interaction and complex access codes can lead to errors. The use of ‘soft’ keys or other devices for reducing setting up times minimizes training requirements.A comprehensive display of instrument parameters will confirm to the operator that the required analytical programme is being followed. Effective monitoring of instrument status will give early warning of malfunction. Raw data can rarely be used to calculate quantitative analyses directly. In quality control applications, statistical treatment of sets of measurements is normally essential before these data can be interpreted meaningfully. Qualitative analysis is not normally performed on simultaneous X-ray fluorescence instruments. However, when available, the capability of qualitative scans is valuable for trouble shooting ( e . g . , the assessment of unsuspected spectrum overlap interferences) and in the determination of ‘extra’ elements for which there is no fixed channel and for qualitative identification of non-routine samples.Score PS WF ST PS WF ST PS WF ST PS WF ST PS WF STANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 319 Feature 7. Installation ( a ) Instrument and services footprint ( h ) Services (i) Environ- mental con tro I (ii) Electrical (iii) Cooling water (c) Service and spares ( d ) Training facilities, effective documen- tation and technical support 8. Safety considerations 9. Extraneous leakage of radiation 10. Value for money (Points per f) Copyright: Analyti Definition and/or tcst procedures and guidance for assessment The bench and/or floor space and the floor loading required for the instrument. Score maximum for minimum requirements for environmental control (temperature, humidity, vibration, etc.) necessary to enable the instrument to operate within its design specification.Score maximum for compatibility with existing electrical supplies with regard to loading, stability, phase requirements, earth specification and tolerance to transicnts. Evaluate costs for the provision of a cooling water supply and scorc maximum for lowest costs. Enquire as to local arrangements and score accordingly. Enquire as to local arrangements and score accordingly. In the UK, construction of instrumentation and all safety interlocks must comply with the appropriate lcgislation including the Ionising Radiations Regulations 1985. Other countries have similar National Regulations with which instrument design and construction must comply. The lowest possible extraneous leakage of radiation from instrumentation should be aimed for. Score additionally for extra features that minimize extraneous radiation. Sum of the previous sub-totals divided by the purchase price of the instrument. Subject to proportional scoring and weighting factors, including ST in grand total. 11 Methods Committee, mportance Varies with ocation bul nay be VI. VI May be I. I VI I VI VI I Reason The instrument must be laboratory/plant compatible to avoid expensive alterations. Additional costs may be considerable in providing services to control environmental requirements. Service specifications may be particularly important if the instrument is to be operated in a plant environment. Provision of alternative power supplies may significantly increase installation costs. Requirements for large volumes of cooling water or the installation of a recirculating cooling water system can significantly increase running costs. Cost of consumables and spares and the availability of service may affect down-time and running costs. Good technical support can reduce commissioning time and improve the analytical efficacy of the instrument. Apart from the obvious hazard, operation of instrumentation in contravention of statutory regulations is illegal. Exposure to any X-ray radiation is undesirable and legally enforceable limits apply in most countries. ‘Simple’ instruments are often good value for money, whcreas those with unnecessary refinements are often more costly. Instrumental Criteria Sub-committee, Royal Society of Chemistry, 1991 Score PS WF ST PS WF ST PS WF ST PS WF ST PS WF ST PS WF ST PS WF ST Sumo totals sub- PS WF ST Granc Total

ISSN:0144-557X    

DOI:10.1039/AP9912800312

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

320 ANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 Analytical Viewpoint The following is a member of a continuing series of articles providing either a personal view of part of one discipline in analytical chemistry (its present state, where it may be leading, etc.), or a philosophical look at a topic of relevance to chemists in general or analytical chemists in particular. These contributions need not have been the subject of papers at Analytical Division Meetings. Persons wishing to provide an article for publication in this series are invited to contact the editor of Analytica! Proceedings, who will be pleased to receive manuscripts or to discuss outline ideas with prospective authors. ~ Evaluation of the Foss-Heraeus Macro N for the Determination of Nitrogen in a Wide Range of Foodstuffs, Ingredients and Biological Materials and Comparison with the Ian D.Smith Analytical Department, Unilever Research Laboratory, The Kjelfoss (KF) ,' manufactured by Foss-Electric, Denmark, was introduced in 1973 as an alternative to the traditional manual Kjeldahl method for the determination of total nitrogen ( x factor = protein). While not deviating in principle from the standard Kjeldahl method developed in the mid-19th century, it offered fast sample throughput: 12 min from sample insertion to result and a sample every 3 min. Although the instrument was highly automated, it exposed the operator to hazardous and toxic chemicals and still required considerable operator attention and preventative maintenance. Running repairs were frequent and costly. More recently, a serious implication of the KF is that some local authorities are reluctant to dispose of the toxic waste produced by the instrument, and. with the increased awareness by analysts working with hazardous chemicals. the use of the Kjeldahl method on a large scale has become less desirable.These factors prompted Foss-Electric to cease production of the KF and seek an alternative technique. In collaboration with a German company, Heraeus, they have recently marketed a new instrument called the Foss-Heraeus Macro N (MN). The aim of this study was to evaluate the MN and make a direct comparison against the KF using a wide range of foodstuffs, ingredients and biological materials. Principle and Description of the MN The sample i s completely burned at temperatures in excess of 1000°C in the presence of oxygen.The resulting combustion gases are swept through a series of absorption tubes by a stream of carbon dioxide; this procedure removes unwanted gases. Finally, the nitrogen is passed through a thermal conductivity detector where it is quantified. The basic unit is contained in a glass-fronted cabinet 560 mrn (width) x 610 mm (depth) x 1620mm (height) weighing 196 kg. The upper compartment houses a sample arm/lance which collects samples from a 90-position chain feed situated under a Perspex cover to the side of the sample arm housing. The lower compartment houses all the electrical and mechan- ical components in addition to the absorption tubes. The unit operates from a 240 V power supply. It requires carbon dioxide of 99.9% purity and oxygen of 99.995% purity.An IBM compatible PC and printer are required to control the instrument and collect data. A balance with 0.1 mg resolution i s required; most modern balances with RS232 output can be linked directly into the system. Measurement is possible between I and SO0 mg of nitrogen with a detection limit of 0.05 mg of nitrogen. Detailed Operation of the MN The sample. up to 3 g of solid material or 5 ml of solution, is weighed into a re-usable stainless-steel crucible. The mass is Kj elf oss Colworth House, Sharnbrouk, Bedford MK44 I L Q transferred to the PC automatically from the balance, or it can be entered manually. The crucible is loaded into a 90-position sample chain, its identity and run parameter code (see under Parameters) are entered into the PC and the system is instructed to run.Firstly, a system pressure reduction is performed which prevents 'blow back' when the sample arm retracts from the combustion tube. The sample arm collects the crucible and inserts it into the combustion zone; during this operation carbon dioxide is blown from the arm to purge atmospheric gases from the crucible and prevent air from entering the system. The sample is heated to ll00"C and oxygen is injected to aid cornbustion; the resulting combustion gases are transported by the carrier gas, carbon dioxide, through a series of heated absorption tubes containing the following chemicals: copper oxide and platinum catalyst breaks down hydrocarbons; silver wool removes halogens; lead chromate absorbs suIphur dioxide: and copper wire removes excess of oxygen and reduces nitric oxides to nitrogen. Before passing over the copper wire the oxygen content of the gas mixture is measured; this enables the instrument to adjust and optimize the oxygen dosing for the following sample.Water vapour is removed by a condenser unit and self indicating diphosphorus pentaoxide. The resulting mixture of carbon dioxide and nitrogen is then passed through a thermal conductivity detector where the nitrogen content is quantified and the concentration as a percentage of the starting sample mass is calculated by the PCand recorded on the printer. The sample arm then discards the used crucible before collecting the next sample. The whole operation takes between 7 and 11 min.Parameters Different combinations of parameters such as oxygen injection timehate, and heating and cooling periods can be used to accommodate samples with various moisture, oil and carbo- hydrate levels. During evaluation, six different sets of par- ameters were used to cover all the materials analysed; hence oxygen optimization was not necessarily as effective as it could have been. Batching of similar samples within a run would help to overcome this effect. The furnace temperatures remained the same for all the standards and samples throughout the investigation. Calibration Full linear calibration is carried out on installation of the instrument using a range of different masses of analytical- reagent grade aspartic acid; then, with each set of samples a small number of standards of aspartic acid are run to produce a daily correction factor.Full re-calibration is only required if the daily factor falls outside set limits or if a major component such as the detector is replaced.ANALYTICAL PROCEEDINGS, OCTOBER 1991. VOL 28 Maintenance Various absorption tubes require changing at approximately 100 and 500 assay intervals; these are signalled by the PC. Tubes can be pre-packed then changed in a matter of minutes. Long-term service maintenance is claimed to be minimal. Experimental Reproducibility of the MN was checked with finely ground samples of wheat flour and skim milk powder, sample size approximately 1 g. The effectiveness of the MN for analysis over a range of nitrogen levels was evaluated using a series of freshly opened organic compounds (99+ % purity). The effectiveness of the MN for determining nitrogen in a range of foodstuffs, ingredients and biological materials was measured on 220 different samples using both the MN and the KF (KF using mercury catalyst).Samples were homogenized or ground thoroughly using appropriate mixers and grinders. All the determinations were performed in duplicate on both instruments using the same sample source. Sample masses were varied depending on matrix and nitrogen contents-dry samples: >8% N, ~ 0 . 5 g; <8% N, =1 g; wet samples, -2.5 g; liquid samples, 4 g. Results All the results for nitrogen content are expressed on an 'as received' basis. Reproducibility of the MN Results for reproducibility for ten analyses of both wheat flour and skim milk powder are reported in Table 1.Table 1 Reproducibility of the MN Nitrogen (%)* Wheat flour 1.72 1.70 1.71 1.72 1.71 1.71 1.72 1.71 1.72 1.70 Mean: 1.712 SDt: 0.008 * Ten consecutive assays. t SD = Standard deviation. Skim milk powder 5.78 5.77 5.78 5.76 5.74 5.74 5.78 5.79 5.77 5.75 5.766 0.018 Evaluation of the MN for a Range of Nitrogen Standards Table 2 compares results for theoretical nitrogen versus nitrogen determined with the MN for a range of compounds. Table 2 Results for theoretical nitrogen versus nitrogen determined with the MN Nitrogen (%) Acetanilide Ammonium sulphate Benzamide Creatinine Glycine Lysine Nicotinic acid Tryptophan Tyrosine Theoretical 10.36 21.10 11.56 37.13 18.65 15.33 11.37 13.71 7.73 MN 10.38 21.35 11.56 36.56 18.56 15.38 11.32 13.75 7.82 Recovery 100.2 100.8 100.0 98.5 99.5 100.3 99.6 100.3 101.2 32 1 ~ ~~ Table 3 Determination of nitrogen with the KF and the MN Nitrogen" (YO) Raw meats- Chicken.white meat Chicken, red meat Turkey steaks Lamb, neck Lamb, lean Pork. head meat Pork. 90 : 10 (lean : fat) Pork. 50 : S O (lean : fat) Pork, fat Beef, 95 : S (lean :fat) Beef. subcutaneous fat Beef, intermuscular fat Beef. lean Beef, lean Beef, lean Beef, wet Beef, milled hide Kidney, pig Liver. pig Cured meat- Bacon, smoked back Bacon, smoked middle Bacon. plain streaky Cold shelf meats- Garlic sausage Chicken and mushroom pie Chicken roll Chicken spreading p2te Cornish pasty Ham sausage Liver and bacon spreading p2te Liver and garlic spreading p2te Liver sausage Polony slicing sausage Pork luncheon meat Pork pie Sausage.economy Sausage, beef Sausage, pork Sausage, pork and beef Sausage roll Smoked ham Steak pie Steak and kidney pie Chicken breast Chopped ham and pork Corned beef Pork luncheon meat Dried meats- Chicken Beef Beefburgers, with onion Beefburgers, 100% Protein hydrolysates - Bovril Ox0 Stock cube, beef Stock cube, chicken Stock cube. pork Stock cube. fish Cheese, Cheddar Cheese, Cheshire Cheese, Leicester Cheese, Stilton Cheese, spread Annatto Canned meats- Frozen meat- Dairy and fats- KF 3.69 3.32 4.05 2.97 3.21 2.55 3.65 1.67 0.71 3.60 0.73 0.94 3.38 3.55 3.56 0.13 5.30 2.65 3.63 2.86 2.25 2.56 2.55 1.17 2.74 2.35 1.11 3.04 2.29 2.23 2.41 1.53 2.33 1 .55 1.49 1.50 1.84, 1.78 1.34 3.56 1.64 1.41 2.53 2.38 4.26 2.13 8.73 8.97 2.81 2.72 7.11 3.40 1.73 2.30 1 .so 3.10 3.97 3.54 3.97 3.59 1.88 2.26 MN 3.69 3.32 4.Oh 3.01 3.22 2.57 3.63 1.71 0.7 1 3.61 0.84 1.02 3.49 3.51 3.55 0.18 5.25 2.66 3.63 2.91 2.27 2.62 2.56 1.32 2.80 2.34 1.21 3.07 2.35 2.25 2.42 1.58 2.38 1.63 1.51 1.46 1.93 1.84 1.45 3.50 1.60 1.51 2.5 1 2.32 4.25 2.13 8.76. 8.91 2.83 2.73 7.11 3.42 1.72 2.25 1.47 3.07 4.09 3.63 4.12 3.69 1.94 2.27322 ANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 Table 3 -continued Nitrogen* (YO) - Table 3 -continued Nitrogen* (YO) Cows’ milk Goats’ milk Buttermilk powder Creamer powder Skim milk powder Spray dried fat Whey powder Yoghurt Ice cream, coconut fat Ice cream, dairy Ice cream, choc ice Mousse, choc Eclair, fresh cream Non-dairy cream Non-dairy cream Superwhip Low fat spread Low fat spread Low fat spread Couverture Whole egg Canned fish (contents drained) - Clams Cod roe Crab, dressed Crab, white meat Mussels, smoked Mackerel Prawns Salmon, pink Sardines Tuna Frozen fish - Cod Coley Haddock Plaice Fish products- Fish burgers, in batter Fish burgers, cheese filling Fishfingers Fishfingers Dried vegetables, cereals and seeds- Asparagus Beans, green Cabbage Carrot Garlic Leek Lentils, red Mushroom Onion Peas Peppers, green Peppers, red Swede Sweet corn Tomato Potato Waffles Instant potato Croquette potato Potato chips Mustard Sesame seeds Sunflower seeds Fenugreek Rice, long grain Rice, brown Rice, Pilau, cooked KF 0.52 0.46 4.04 0.48 5.68 0.68 1.99 0.69 0.56 0.57 0.64 0.65 1.11 0.21 0.38 0.54 1.48 0.43 1.37 1.11 2.03 2.66 2.02 2.49 2.45 3.02 2.72 3.12 3.45 3.93 4.47 2.73 3.03 3.13 2.77 2.04 1.85 1.85 2.11 2.85 2.62 2.13 1.53 1.49 2.79 4.41 5.06 1.73 3.79 2.07 1.67 1.22 1.76 2.26 0.36 1.43 0.60 0.32 4.55 3.56 3.94 4.88 1.28 1.46 0.65 MN 0.53 0.47 4.14 0.52 5.78 0.74 2.04 0.70 0.58 0.58 0.64 0.65 1.11 0.27 0.48 0.60 1.54 0.46 1.39 1.14 2.08 2.62 2.04 2.41 2.49 2.99 2.73 3.18 3.47 3.86 4.55 2.72 3.04 3.15 2.72 2.02 1.90 1.91 2.10 2.89 2.72 2.23 1.60 1.58 2.99 4.55 5.12 1.79 3.89 2.17 1.75 1.31 1.84 2.36 0.43 1.42 0.63 0.36 4.59 3.63 3.98 4.94 1.33 1.50 0.66 Wheat gluten Bread, white Bread, wholemeal Almonds, sweet Brazil nuts Coconut, desiccated Hazelnuts All Bran Corn Flakes Porridge oats Weetabix Canned soups - Asparagus Chicken Mushroom Tomato Packet soups- Minestrone Oxtail Spring vegetable Vegetable and beef Canned ready meals- Baked beans in tomato sauce Chicken in white wine Spaghetti in tomato sauce Stewed steak with gravy Frozen ready meals- Beef roganjosh Bolognese sauce Chicken chasseur Chicken and ham lasagne Chicken pie Chicken tikka Fish pasta au gratin Fish pie Lasagne a1 forno Pizza Seafood lasagne Seafood tagliatelle, sauce Spaghetti, cooked Tagliatelle, cooked Biological materials- Cow faeces Cow urine Fish faeces Goat faeces Goat urine Pig faeces Pig urine Poultry droppings, dried Cow blood Finished diets (animal feeds) - Cattle diet Cattle diet Baby pig diet Broiler diet Fish diet Fish feed Fish meal Layers diet Pig diet Pig reworks Turkey diet Raw materials- Barley Clover Cocoa residue Cocoa shell KF 12.85 1.69 1.87 4.31 2.97 1.14 2.58 2.24 1.40 2.03 2.14 0.10 0.16 0.17 0.16 1.85 1.86 1.49 1.53 0.83 1.55 0.31 3.11 2.29 1.10 1.82 1.24 1.19 1.65 1.57 1.17 1 .oo 1.71 0.87 1.31 0.72 0.67 0.49 0.99 1.45 0.72 0.66 0.91 0.54 12.12 2.68 2.81 2.16 3.62 3.61 10.28 7.95 10.55 2.80 2.98 2.86 3.32 1.68 2.95 2.78 2.64 MN 13.17 1.71 1.92 4.25 2.91 1.23 2.63 2.22 1.42 2.05 2.10 0.10 0.18 0.19 0.17 1.90 1.89 1.51 1 .58 0.84 1.58 0.34 3.09 2.31 1.11 1.79 1.23 1.21 1.66 1.56 1.17 1.00 1.71 0.92 1.33 0.72 0.65 0.48 1.01 1.52 0.73 0.68 0.91 0.55 12.12 2.73 2.89 2.21 3.68 3.72 10.44 8.04 10.77 2.86 3.01 2.9.5 3.37 1.73 2.97 2.81 2.65ANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 323 Table 3-continued Nitrogen* (%) Concentrate Corn gluten Distillers by-product Extracted palm kernel Extracted rape Extracted sheanut Extracted soya Extracted sunflower Feather meal Field beans Grain screenings Grass silage, dried Groundnut Hay Kale Lettuce roots Linseed Lucerne Lupin Maize Meat and bone meal Meat and bone meal Nutritionally improved straw Paddy meal Pea Rapeseed, whole Salseed Sugar beet Sugar beet, tops Wheat Wheat feed Wheat flour * Mean of duplicate determinations.KF 7.66 3.11 4.32 2.15 5.57 2.22 6.49 5.14 12.89 3.84 1.90 2.71 7.61 1.33 1.59 1.68 3.62 2.29 5.19 1.41 8.25 11.33 0.67 2.59 3.34 3.40 1.51 1.60 2.03 1.67 2.64 2.26 MN 7.71 3.16 4.36 2.22 5.70 2.26 6.62 5.23 12.91 3.95 1.96 2.69 7.62 1.37 1.63 1.80 3.72 2.32 5.44 1.47 8.29 11.41 0.72 2.73 3.41 3.45 1.52 1.67 2.09 1.71 2.71 2.32 Evaluation of the MN for a Range of Materials Results for the determination of nitrogen with the KF and the MN are listed in Table 3.The correlation between per cent. nitrogen determined with the KF and the MN is illustrated graphically in Fig. 1. Statistical analysis of the results for the complete range of materials is summarized in Tables 4 and 5. All the results were recorded to two decimal places, and many duplicates were identical. An allowance for rounding error was made in the analysis of precision by assuming that notionally exact measures were uniformly distributed over the range: recorded value f0.005, i.e., a variance component for rounding error of 0.012/12 was added. 14 13 - 11 g l 2 5 10 E ; 5 7 g 4 F 6 z 2 a 5 .= 3 1 0 0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 Nitrogen, KF mean (%) Fig. 1 Correlation between per cent.nitrogen determined with the KF and the MN (duplicate means). The broken line is the line of equality. Correlation, 0.99975; n = 220 Data were allocated to groups on the basis of matrix similarity, within product categories. Each category was analysed separately to assess consistency of bias, if any, across groups. On the basis of this analysis soups and meat products were split into the sub-sets used above. Discussion The MN is a straightforward instrument to use; maintenance operations are easy and can be carried out rapidly, thereby eliminating excessive down time. Although the MN has a slower sample throughput than the KF, it can be fully loaded to operate outside normal working hours. It is less labour intensive than the KF and does not subject the operator to hazardous chemicals; the instrument produces no toxic waste.The MN is likely to be less problematic than the KF and mechanically more reliable. The controlling software for the MN, although adequate, would benefit from more sophistica- tion and screen information. Parameters Different combinations of parameters such as oxygen injection timehate, and heating and cooling periods can be used to accommodate samples with various moisture, oil and carbo- hydrate levels. During evaluation, six different sets of par- ameters were used to cover all the materials analysed; hence oxygen optimization was not necessarily as effective as it could have been. Batching of similar samples within a run would help to overcome this effect. The furnace temperatures remained Table 4 Comparison of nitrogen contents with the MN and the KF on 220 materials.Comparison of means Nitrogen (%) Sub-set Animal feeds and raw materials Biological materials Meat products (cold) Meat products (canned and dried) Soups (canned) Soups (packet) Dairy and fats Dried vegetables Fish Meats Ready meals * Values in parentheses are the standard errors. T P significance level GO.001. 3: P significance level 60.05. § P significance level ~ 0 . 0 1 . fl P significance level >0.05. n 47 9 22 12 4 4 27 37 18 22 18 Range 0.7-12.9 0.5-12.1 1.1-3.5 1.5-9.0 0.1-0.2 1.5-1.9 0.2-5.8 0.3-1 3.2 1.9-4.6 0.1-5.5 0.3-3.1 MN 4.05 2.30 2.10 3.99 0.155 1.72 1.71 2.64 2.77 2.79 1.34 KF 3.98 2.28 2.07 4.01 0.148 1.68 1.66 2.58 2.77 2.77 1.34 MN - KF 0.069 (0.008)* 0.021 (0.009) 0.028 (0.009) 0.008 (0.005) 0.036 (0.008) 0.050 (0.008) 0.058 (0.010) 0.003 (0.011) 0.016 (0.007) 0.006 (0.005) -0.019 (0.008) t-value 9.1t 2.4$ 3.25 -2.3$ 0.27 4.45 6.2i 5.6f 0.37 2.25 1.37324 ANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 Table 5 Precision of the MN and KF on 220 materials.Comparison of replicate standard deviations Sub-set n Animal feeds and raw Biological materials Meat products (cold) Meat products (canned Soups (canned) Soups (packet) Dairy and fats Dried vegetables Fish Meats Ready meals materials dried) 47 9 22 12 4 4 27 37 18 22 18 and Over-all - * P significance level >0.05. ? P significance level S0.05. MN 0.019 0.017 0.021 0.019 0.008 0.018 0.013 0.017 0.014 0.026 0.018 0.019 KF 0.019 0.013 0.024 0.015 0.003 0.011 0.011 0.015 0.020 0.016 0.014 0.017 F-value (MN/KF) 1.1* 1.8" 0.8" 1.7" 7.0* 2.8" 1.4* 1.4* 0.5* 2.61 1.8" 1.3" The MN uses a variety of consumables including chemicals, crucibles, tubes and sample lances; these are purchased in a 2000 assay kit. The current kit might not be in the correct proportions for all the applications and consequently it might be more cost effective to purchase items separately. Over-all costs per assay would appear to be similar for both the MN and the KF. Comparison of the KF and the MN produced a good correlation between duplicate means of 0.999 75. However, as can be seen from Table 4, the MN gives a slightly higher figure for most samples; this is probably due to combustion tech- niques giving recoveries of nitrogen close to 100% when compared with the KF. This effect is particularly noticeable for samples which contain nitrate because the Kjeldahl method does not measure nitrate-nitrogen quantitatively. The author thanks L. Aspinall (Unilever) for statistical interpretation of the data and P. Wilson (Foss-Electric) for technical backup during evaluation. the same for all the standards and samples throughout the investigation. Reference 1 Montag, A., Gordian, 1974, 74, 203.

ISSN:0144-557X    

DOI:10.1039/AP9912800320

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

ANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 Gas and Headspace Vapour Analysers 325 The following are summaries of thirteen of the papers and posters presented at a Meeting of the Analytical Division held on March Ist, 1991, in the University of Wales College of Cardiff. Competitive Adsorption on to Coated Surface Acoustic Wave Sensors John F. Alder Department of Instrumentation and Analytical Science, UMIST, P.O. Box 88, Manchester M60 ?OD Surfacc acoustic wave sensors (SAWS) rely for their action on the adsorption of vapour from the ambient atmosphere on to an active surface layer in intimate contact with the piezoelectric substrate, which is usually quartz. Specificity is achieved largely through the choice of active surface and one endeavours to find a surface which selectively holds the target vapour as an adlayer while releasing, or not adsorbing, other concomitant gases.Adsorption of trace amounts of atmosphere constituents on to surfaces exposed to normal ambient atmospheres is a complex process. Any surface exposed to humid air will have water vapour and probably permanent gases adsorbed on it. Adsorption of trace amounts of vapour (in the work carried out in this laboratory they are usually polar organic compounds) will always be in competition with adsorption of water. The adsorption process is therefore influenced by the history and current hydration statc of the surface. When a hydrophobic surface is exposed to a humid atmosphere, the adsorption isotherm often resembles a type 111 in the BDDT classification1 which is approximately quadratic in form.This type of isotherm arises when the adsorbent- adsorbate forces are relatively weak. 1 Hence water molecules adsorbing o n the surface experience greater attraction to the water filled sites than the sites on the hydrophobic surface. As the partial pressure of water in the atmosphere increases the surface becomes covered with water and hence becomes more hydrophilic. The amount of water adsorbed from the increasingly humid atmosphere is consequently greater. This phenomenon arises because of the greatcr enthalpy of adsorp- tion of water on to the water filled sitc than on the sites o n the hydrophobic surface. Although type 111 isotherms are not confined to water vapour, water exhibits this behaviour markedly. As an example, Gregg and Sing1 reporting Barrer' cite the adsorption of water vapour on to €3-chabasite at 298 K.The dispersion interaction energy of water with H-chabasite is 0.633 kJ mol- I compared with the molar enthalpy of condensa- tion of water of 2.53 kJ mol-1, giving rise to such a type I11 isotherm. On SAWS one tends to use polymeric materials of high relative molecular mass as adsorbents, in order to minimize coating cvaporation and to provide a well defined stable surface. These are often hydrophobic materials, having weak interaction with water vapour. Gregg and Sing (reference 1, p. 250), citing several workers, have shown type I11 isotherms for hexane and octane on polytetrafluoroethylene (PTFE) and water on poly(methy1 methacrylate) and bis (A-polycarbonate).The heats of adsorption of water vapour on the last two polymers were measured as a function of increasing coverage. In both instances these increase from initially lower values to the molar enthalpy of condensation of water at high surface coverage (apart from initial high values for water on methyl methacrylate, attributed to surface heterogeneity) . Clearly, adsorption o f water vapour on the surface alters the characteristics of that surface significantly. This is as true for water adsorption on to surfaces exposed to water as it is for other vapours. Surface modification, achieved by equilibrating the surface with a particular compound, is well known; one regularly coats glass surfaces with silicones in order to minimize adsorption. Surface characteristics can be significantly altered by many other classes of compound to produce different isotherm types and degree of adsorption.' It is true to say, therefore, for the SAWS and piezoelectric crystal techniques amongst others, that the history of the sensor is going to influence its sensitivity and adsorption/response characteristics significantly. In work carried out by Fox and Alder,3 measuring the adsorption of water vapour on to SAWS coated with PVP [poly(vinylpyrrolidone)] and FPOL (a fluorinated pol yether/ polyol), it was demonstrated that one could obtain BDDT type I11 isotherms at ambient temperature and 30 "C, but that at 40 and 50 "C, the isotherms up to 80% RH (relative humidity) take on a rather different shape, indicating much reduced water adsorption at the higher temperatures.Indeed, for FPOL, there was indication that the isotherm had a negative slope. This seems unlikely and it is possibly a result of the combined effect of alteration of the viscoelastic properties of the polymers, combined with rather low water uptake which led to this result. Surfaces that are already covered with water will behave in a rather more complex way towards challenge from a gas which is also adsorbed by the coating. There will be an equilibrium set up between the challenge gas, the concomitant water vapour and the water vapour adsorbed on the surface. Although we have not yet achieved the desired experiments with coated SAWS, one can draw analogies from work on charcoal surfaces. While accepting that there are significant differences between activated carbons and polymers, there is bound to be a similarity in the adsorption and desorption processes. Anstice et aZ.4 demonstrated that chloropicrin in 80% RH air displaced water vapour from charcoal equilibrated with water vapour at the same RH.The ratio of the number of moles of water desorbed to the number of moles of chloropicrin adsorbed changed as the mass of chloropicrin adsorbed increased from about 2.5 : 1 at low chloropicrin uptake to a limiting value of about 5.6 : 1 at high uptake. This is a characteristic property of microporous materials; the change in molar ratio can be associated with a change between displacement of water molecules adsorbed in a monolayer, and water vapour dis- placed from bulk liquid trapped in the microporous structure.It is likely that such behaviour will also occur on coated SAWS, particularly if the coating were microporous, but also if there were multilayer formation as would be the situation at higher surface coverages on a hydrophobic surface of the types described above. Clearly, therefore, the net mass changes would be different for these varying conditions, and if the SAWS was being used326 ANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 for gas analysis, the sensitivity of the SAWS would be a function of the state of the coating surface. 1 In working with mass-sensitive devices such as SAWS and piezoelectric crystals, care must be taken to define the state of the surface. One will need to work at low RH in order to minimize the effects of competitive binding.3 4 minimize the amount of water adsorbed on the surface, and to References Gregg, S. J . , and Sing, K. S . W., Adsorption, Surface Area and Porosity, Academic Press, London, 2nd edn., 1982. Barrer, K. M., J . Colloid Interface Sci., 1966, 21. 415. Fox, C . G., and Alder. J. F . . Anal. Chirn. Acta, submitted for publication. Anstice, P. J. C., Halliday. N., and Alder, J . F., Adsorpt. Sci. Technol.. submitted for publication. Evaluation of Sensors Coated on Piezoelectric Quartz Crystals for the Determination of Aromatic Compounds M. A. F. Elmosalamy, N. K. Harris, G. J. Moody and J. D. R. Thomas* School of Chemistry and Applied Chemistry, University of Wales College of Cardiff, P.O. Box 912, Cardiff CFI 3TB Piezoelectric crystals coated with a variety of sensor materials have been developed as a sensitive and selective technique for measuring a range of air and gaseous pollutants.1-3 The quantitative basis of the method relates to the mass of coating material deposited on the quartz surface and the change in frequency according to Sauerbrey's equation9 where AF is the frequency change (Hz), F is the initial frequency (MHz) of the quartz crystal, Am is the mass of the deposited material and A is the area (cm2) coated.The Sauerbrey equation can be simplified to AF= -2.3 X 106PAmIA ( 1 ) A F = Kc (2) where cis the concentration (e.g., mg m-3) of sample gas and K a constant relating to the basic frequency of the quartz, the area coated and a factor to convert the mass of analyte sorbed into its gas-phase concentration.Hence, piezoelectric crystals suitably coated with appro- priate sensors have been used to determine a variety of aromatic hydrocarbons, e.g., toluene," 2-methyltoluene,' ethylbenzene,s hexane,s xylenes6 and benzene ,7 and also nitrobenzene8.9 and 2-nitrotoluene. 10 This paper describes some preliminary results from the evaluation of seven cyclic sensors (Fig. 1) coated on quartz crystals for the detection of benzene and nitrobenzene and the influence of selected possible interferents. Experimental The apparatus and detector cell used have been described elsewhere," measurements being taken at 25 -t 1°C. The AT-cut quartz crystal with gold-plated electrodes on each side (Quartz Crystal, Wellington Crescent, New Malden, Surrey; now available from Webster Electronics, Rosemills, Hart- bridge, Ilminster, Somerset) has a resonant frequency of 9 MHz.The sensor materials (1)-(VII) were gifts from Professor J. F. Stoddard, Chemistry Department, Sheffield University (now of the University of Birmingham). Coating Technique The compounds dissolved in acetone or chloroform (0.64% d v ) were brush-coated on each side of the quartz crystal surface. After air drying, the coated crystal was positioned into the detector compartment of the apparatus." The coatings applied caused a decrease of 12-20 kHz in the frequency of the crystal. All the sensors could be readily stripped from the crystal with chloroform or acetone and air-dried for reloading. ~ * To whom correspondence should be addressed. Test Samples Nitrobenzene vapour samples were obtained with a previously flushed out 10 cm3 syringe from the headspace of its liquid under dry air in a thermostat at 25°C.Its concentration was calculated to be 1.84 x 103 mg m-3 from the quoted12 equilibrium pressure of 0.277 mmHg. The samples of the other six vapours were obtained similarly. 12-14 Serial dilutions of the headspace samples were effected by the syringe dilution method,6 modified as described pre- viously. 11 Successive dilutions of samples using air dried over silica gel were delayed by 30-60 s in order to allow the vapour to diffuse throughout the air in the syringe. Operation of Piezoelectric Quartz Crystal Detection Apparatus The operation of the system is essentially a flow injection analysis involving the gas phase. Gas samples were injected into an air stream, dried by passing through silica gel and carried through the quartz crystal cell at 20cm3 min-1 by a Pitman Instruments Model 7069 air sample pump.Derivatives (1)-(VII), when individually coated on the piezoelectric crystals, interacted to various extents with the concentrations of the chosen vapour standards. This was manifested by a decrease in the frequency of the oscillation of the crystal; the extent of these decreases was then simply related to the concentration of a particular vapour. Results and Discussion Sensors (1)-(111) Calibration data were collected for many nitrobenzene runs over 58 d with crystals coated with the hexaepoxyoctacosahy- dro[ 12lcyclacene derivative (I). The frequency change ranged, respectively, from 11 1 Hz for nitrobenzene headspace vapour to 10 Hz for headspace vapour diluted with dry air 5 X 2.42 times (the factor 2.42 allows for the volume of vapour left in the needle and connector in addition to 4 cm3 left in the syringe after evacuating 6 cm3 of vapour from the 10 cm3 syringe).A linear calibration was observed for all the 5 x 2.42 times dilutions. Various interferences were also tested (Table 1). Except for 2- and 3-nitrotoluene, there were no significant interferences from benzene, bromobenzene, chlorobenzene or toluene. Sensor (11), on the other hand, failed to show selectivity towards any of the seven aromatic vapours (Table 1 ) and it represents a more universal type of sensor. In fact, the response of 2-nitrotoluene exceeds that of nitrobenzene.The same trends arc also apparent with the triallyl macrocyclic derivative (111) (Table 1). It is interesting that three molecules of toluene can be trapped" in the cavity of the sensor (111) molecule (Fig. 1).ANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 327 I II 111 ,OR , OSiMe2CMe3 1V R = F G C O VI R = tea-butyldimethylsilyl V OH 0 BzO Bz = benzyl VII Fig. 1 Formulae of sensors for coating quartz crystals Sensors (1V)-(VII) Sensor (VI) has previously been shown to be very suitable for the measurement of benzene when coated on quartz crystals. Recovery times were short (<2 min), lifetimes long (about 20 d) and little interference arises from nitrobenzene or toluene (Table 2). The response profiles of cyclodextrins (IV) and (V) to benzene are poorer than that for (VI), whereas interference from the other six vapours is more serious.Response to toluene, however, is improved but the recovery times for benzene, and toluene, benzene, chlorobenzene and bro- mobenzene, are excessively long (Table 2). A delayed blip in the recovery pattern for bromobenzene in each instance is unusual. The other feature concerns the irreversible response of sensor (IV) to ammonia whereas recovery times using sensor (V) were about 40 min. The proton nuclear magnetic resonance (NMR) spectrum of 2,3-dodecabenzyl-~-cyclodextrin (VII) in d6-acetone showed the expected six-fold symmetry but in CDC& this symmetry was halved and the number of signals expected in the IH and 13C NMR spectra were doubled. Lehn et proposed a new conformation in CDCI, wherein each alternate glucose unit had flipped out of its normal position.Such a change would adversely affect the formerly larger cavity of (VII) in d6- acetone and in turn its ability to form inclusion compounds. In order to test this proposition the cyclodextrin (VII) was evaporated on to the quartz crystal from acetone (cavity intact) and chloroform (cavity distorted), respectively, thus possibly realizing sensor species with two very different conformations. The difference in their ability to sense vapours was tested with a range of nitrobenzene and 3-nitrotoluene standards. Except for the responses at the highest level of nitrobenzene (1840 mg m-3) and 3-nitrotoluene (1500 mg m-3), respectively, the responses at all other levels do not support this concept (Table 3).Table 1 Piezoelectric responses of two cyclacene derivatives and a triallyl macrocyclic compound to seven aromatic vapours Trial1 y 1 macrocyclic Cyclacene (I) Cyclacene (11) (111) Headspace vapour Toluene 2-Nitrotoluene 3-Nitrotoluene Nitrobenzene Chlorobenzene Bromobenzene Benzene Concentration*/ 103 mg m-3 30.0 1.4 1.5 1.84 15.4 4.18 19.2 Response ( AF/Hz) for coating 1st 19 48 38 111 16 30 10 2nd 10 46 37 109 18 31 12 Response (A F/Hz) for coating 1st 86 136 85 121 81 75 125 - Response (A F/Hz) for coating 1st 128 111 72 98 100 86 130 * Headspace concentrations calculated from vapour pressure data at 25 “C.328 ANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 Table 2 Piezoelectric responses of three chemically modified cyclodcxtrins (IV-VI) to seven aromatic vapours Cyclodextrin (IV) Cyclodextrin (V) Hcadspacc Concentration*/ Response, Recovery/ Response.Recovery/ vapour 103 mgrnp3 A FIHirI- min A FIHzt min Toluene 30.0 155 >60 191 -50 2-Nitrotoluenc 1.4 38 ==S 37 = I 0 3-Nitrotoluene I .5 36 -7 49 - 10 Nitrobenzene 1 .84 50 -6 59 = 10 Chlorohenzenc 15.4 77 > 60 83 > 60 B romobc nze ne 4. I8 44 >SO$ 56 >60$ Ben 7 en c 19.2 283 >60 335 >60 * Headspace concentrations calculatcd trom vapour prcqsurc data at 25 "c'. -t Mean result from runs with three separate coatings. $ Two-phase rccovcry process. initially fast, then much slowcr. Cycliodextrin (VI) Responsc A FIHz 64 - - 14 - - 743 Table 3 Piezoclectric responses ( AFIHz) of cyclodcxtrin (VII) to nitrobenzenc and 3-nitrotoluene Nitrobcnzcne/mg m -3 3-Nitrotoluene/mg m-3 Sensor system 1840 760 314 130 54 1500 620 256 106 44 (VII)-CH,COCH, 92 40 23 I I 5 73 35 12 8 6 (VI1)-C HCI 3 99 4.4 21 10 5 87 38 20 10 7 The authors are grateful to the Science and Engineering Research Council (under the auspices of their initiative on chemical sensors) for a grant and to Zagazig University, Egypt, for leave of absence and financial assistance to M.A. F. E. References 1 2 3 4 5 6 7 Hlavay. J . . and Guilbault, G. G.. Anal. Chem.. 1977.49, 1890. Guilbault. G. G.. Ion-Sel. Electrode Rev., 1980, 2, 3. Alder, .I. F., and McCallum. .I. J . . Analyst. 1983. 108, 1169. Ho. M. H.. Guilbault. G. G.. and Reitz. B.. Anal. C!zem., 1980. 52. 1489. Edmonds, 7'. E., and West. T. S . . Anal. Cliim. Acra, 1980.117, 147.Karmarkar, K. H.. and Guilbault, G. G., Environ. Lett.. 1985, 10, 1489. Lai, C. S.-I., Moody, G. J . , Thomas, J . D. R.. Mulligan, D. C., Stoddart. J . F., and Zarzycki. R.. J. Chcm. Soc., Perkin Trans. 2, 1980. 319. 8 9 10 1 1 12 13 14 15 16 Sanchez-Pcdrano. J . A . 0.. Drew, P. K. P., and Alder, J . F.. Anal. Clzim. Acra, 1986, 182, 285. Elmosalamy. M. A. F., Moody, G. J . , Thomas, J . D. R., Kohnkc, F. A., and Stoddart, J. F., Anal. Proc., 1989,26, 12. Tomita, Y . , Ho. M. H., and Guilbault, G. G., Anal. Chem., 1979, 51, 147.5. Lai. C. S.-I, Moody, G. J . , and Thomas. J . D. R., Analyst, 1986. 111, 511. Kirk, R. E., and Othmer, D. F.. Encyclopedia of Chemical Technology. Wiley, New York and Chichester, 3rd edn., 1983, Handbook o,f Chemistry and Physics, eds.Weast, R. C., and Astle. M. J . , CRC Press, Boca Raton, FL, 65th edn., 1985, pp. D204 and D206. Lange's Handbook of Chemistry, ed. Dean, J . A.. McGraw- Hill, New York, 12th edn., 1979, pp. 10-52. Stoddart, J . F., and Williams, D. J . , unpublished results. Boger. J . , Corcoran, J . R.. and Lehn, J.-M., Helv. Chirn. Acta, 1978, 61, 2190. vol. 21, p. 387. Use of Ion Mobility Spectroscopy for the Detection and Analysis of Vapours Peter Watts Chemical Defence Establishment, Porton Down, Salisbury, Wiltshire SP4 OJQ Although the first paper o n ion mobility spectroscopy (IMS) was published as long ago as 19701 and there have been many papers2 and one book3 published since then, it is a relatively little known technique. It is therefore necessary to outline the general principles of IMS.Ion mobility spectroscopy is concerned with the formation of ion-molecule clusters in air or other gases and their movement in an electric field. When clean air is ionized, generally using a radioactive source, a number of positive and negative ions are formed. If, in addition, an organic vapour is present in the air it can react with the parent positive or negative ions (often these are known as the reactant ions) to form new ions, product ions. If these ions are introduced into an electric field they will migrate. The smaller, lighter, more mobile ions will migrateANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 329 Ion-molecule reaction region Radioactive source Ion collector \ Ionization / Screen grid Gas flow outlet Fig.1 Component parts of a typical IMS cell faster than the heavier ions and hence separation occurs. The equipment in which these processes are studied is known as an ion mobility spectrometer and its output a mobility spectrum. The single peak normally observed in 'clean' air is called the reactant ion peak or RIP. A schematic diagram of a typical spectrometer is shown in Fig. 1. There are six important parts to an ion mobility spectrometer: the ionization region; the ion-molecule reaction region; the shutter; the drift region; the screen grid; and the ion collector. The cell is usually constructed from a series of metal rings, guard rings, separated from each other by insulators. These guard rings are connected to a resistance network and a high voltage is attached to the ends of the resistor chain to produce a unitorm electric field along the cell.At atmospheric pressure the field is usually about 250 V cm-1 but it can be altered to vary the reaction time. The polarity of the field is selected according to whether positive or negative ions are being studied. The ionization region contains the ion source. Nowadays this is usually a h3"i radioactive foil, the same type as is used in electron-capture detectors. Other ion sources that have been used include tritium foils, corona discharge, photo-emission and photo-ionization. The ions drift under the influence of the electric field through the reaction region where, it is hoped, the reactions will attain equilibrium. The ions then arrive at the Bradbury-Nielsen shutter. The purpose of this shutter, which is normally closed, is to allow a 100.0 87.5 75.0 62.5 s .= 50.0 - >.C a, C 4- - 37.5 25.0 12.5 0 06 - 1 - ! L 100.0 87.5 75.0 8 62.5 I > 2 50.0 - 37.5 25.0 c, .- a, I; 4- 12.5 pulse of ions into the drift region. The Bradbury-Nielscn shutter is constructed from a pair of interdigitated electrodes or wire grids. When the potential on the two grids i s the same the ions travel through the spaces between the wires under the influence of the drift field. When a potential difference, typically about 40 V, is applied between the two sets of wires the field is greater than the drift field and the ions migrate to one or the other sets of wires and are annihilated, i.e., the shutter is closed. When injecting a pulse of ions into the drift region it is typically opened for 200 ps.Having entered the drift region the ions then travel down towards the collector electrode or Faraday plate and separate out into different peaks according to their mobilities. Immediately prior to the collector electrode there is a screen grid; this is to screen the collector electrode from the approaching ion cloud which would otherwise induce a charge on the collector. Without it the current peak measured is distorted and not a direct representation of the actual cloud of ions. Finally, the ions hit the collector electrode and a current flows producing a chromatogram. Typical chromatograms are shown in Figs. 2(a) and 3(u). Fig. 2(a) is of clean air and Fig. 3(u) is of air containing trace amounts of DPM (dipropyl methyl ether, CH30C3H6QC3H60H, a common solvent).The number over each peak is the reduced mobility for the ionic species giving rise to that peak. The reduced mobility, KO, is calculated from the equation K , = K (76Olp) (77273) Ib) '3 101 55 73 91 101 117 119 129 157 129 10.00 13.8 16.5 19.3 22.31 25.38 28.46 0 22.2 44.3 66.5 88.6 110.8 132.9 ' 166.2 188.3 210.5 232.6 254.8 276.9 Time/ms 144.0 Mass Number Fig. 2 tion. 4 ppm; drift water concentration, 4 ppm; and temperature, 36 "C ( a ) Positive rcactant ion peak (water chemistry); and ( h ) mass analysis of positive RIP (water chemistry). Carrier water concentra-330 100.0 87.5 75.0 62.5 - s 'iij 50.0 - > C a, .I- .I- - 37.5 25.0 12.5 0 I I I I 1.62 2.06 1 I I I I .23 100.0 87.5 75.0 62.5 h > v) * .- 50.0 37.5 .I- C - 25.0 12.5 n " 10.0 14.62 19.23 23.85 28.46 33.08 37.69 Time/ms ANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 2.06 1.62 v- RIP Monomer 101 100.0 87.5 75.0 62.5 s 6 50.0 - >- v) .I- .- 4d C - 37.5 25.0 12.5 0 73 10.8 132.9 144.0' 188.3 210.5 232.6 254.8 166.2 Mass Number I 276.9 I 288.0 Fig.3 (a) Mobility spectrum of DPM in a water-based system; and ( b ) mobility spectrum of DPM in a water-based system i . e . , it is the mobility normalized for temperature and pressure. K , the mobility, is the velocity of the ion in a unitary field and has the units of cm* V-1 s-1. This mobility is characteristic of a particular ion species and depends on the mass, the shape, the ion structure and the drift gas. It is not just an empirical characteristic of the ion but is related to the diffusion coefficient ( D ) of the ion by the Nernst-Townsend-Einstein relationship K = qDIkT where q is the ionic charge and k the Boltzman constant.Because the mobility is dependent on shape and size, and some other workers have shown that ions from closely related structural isomers can produce ions of differing mobilities, it is Acetone monomer 77 = Ac.HZO.H+ 95 = Ac.(H20)2.H+ 105 = Ac.HzO.Nz.H+ 1 13 = Ac.( H20)3H + 123 = Ac.(H~O)~.N~-H+ 133 = Ac.H~O(N~)~.H+ 151 = Ac.(H~O)~(N~)~.H+ 2.01 1.85 Acetone dimer 117 = (Ac)~H+ 145 = (Ac)~.(N)~.H+ 173 = (Ac)~.(N)~.H+ 20.00 20.92 21.85 22.77 23.69 24.42 25.54 Time/ms Fig. 4 Acetone in air: ion mobility spectrum with mass analysis 100.0 87.5 75.0 37.5 25.0 12.5 0 ' a ) 1.87 15.00 17.31 19.62 21.92 24.23 26.54 28.85 Time/ms 100.0 87.5 75.0 s .g 62.5 E 50.0 h - C a, - 37.5 25.0 12.5 0 16) 59 ' 117 114 .173 1 r 145 1.23 n Dimer i 299.1 Fig. 5 analysis of acetone RIP (a) Reactant ion peak (acetone chemistry); and ( b ) massANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 331 misleading to call IMS atmospheric pressure mass spec- trometry as has been done in the past. In order to determine which ions are in a peak a combined IMS-MS system is used. In the IMS-MS the collector electrode is pierced and ions are sampled through a pinhole into a quadrupole mass spectrometer. There is a commercial instru- ment on the market manufactured by PCP (FL, USA) and our laboratory is equipped with this instrument. This is useful, but RIP i OH b e t c I 1.82 1.67 etc.- [&+ 2 etc. 12.00 15.08 18.15 21.23 24.31 27.38 30.46 Time/rns Fig. 6 Mobility spectrum of cyclohexanol for more fundamental studies we have built a more versatile machine in collaboration with Birmingham University. By using these instruments we find that only rarely does a peak contain a single species of ions. More usually the ions are associated or clustered with neutral species, a sort of gas-phase solvation. Fig. 2(b) shows the mass spectrum of the ion species occurring in the water RIP. Not only are polar water molecules clustered around the protonated species but also nitrogen molecules. The proportions of the various species depend on the concentration of water vapour and temperature. The DPM spectrum in Fig. 3(6) shows that the high mobility peak consists of the water clusters, the second peak consists of protonated DPM molecules associated with various numbers of water and nitrogen molecules and the third, low mobility peak, essen- tially of ions containing two molecules of DPM and one proton.Acetone gives some interesting results. At very low concen- trations of acetone a diminution of the water RIP occurs and two new peaks are seen (see Fig. 4). At higher acetone concentrations only a single peak consisting of the acetone dimer is seen as in Fig. 5. Not all the reactions occurring in the positive mode are simple and consist of protonated clusters of the sample molecules. More complex reactions can occur and an example is shown in Fig. 6 for cyclohexanol. The species that mass analyse for cyclohexanone do not originate from any cyclo- hexanone in the sample.In the negative mode a similar range of behaviour is observed . In conclusion, the introduction of neutral molecules into an IMS system produces new ions characteristic of the sample. These ions can be separated to produce a spectrum that can be used to identify the vapour and give an estimate of its concentration. Variables that, although important, have not been discussed here, include the effects of temperature, the addition of a dopant or reactant gas to vary the selectivity of the system or any mention of the physics of the system and how the over-all performance is determined, e . g . , the sensitivity and resolution. References 1 2 Cohen, M. J., and Karasek, F. W., 1. Chromatogr. Sci., 1970,8, 330. Hill, H.H., Jr., Siems, W. F., St. Louis, R. H.. and McMinn, D. G., Anal. Chem., 1990. 62, 1201A, and rcfcrences cited therein. Plasmu Chromatography, ed. Carr, T. W., Plenum, New York, 1984. 3 Fuel Cell Sensor Design for Carbon Monoxide and Methanol Using Platinum Single Crystals Garv A. Attard Schbol of Chemistry and Applied Chemistry/ University of Wales College of Cardiff, P.O. Box 912, Cardiff CFI 3TB Recent investigations of electro-oxidation reactions at plati- num electrodes have shown that many of these are extremely sensitive to both the crystallography and composition of the electrode surface. 1 For example, the initial electro-oxidation current obtained from methanol in aqueous acid is observed to be in the order (110) = (100) > > {lll}, where (loo}, etc., refers to the Miller indices of the particular platinum crystal plane exposed to the electrolyte.2 Further, a number of adsorbed intermediates have been identified both electro- chemically3 and spectroscopically4 during the reaction. In particular, these include a poisoning intermediate1 which, if not removed at higher overpotentials, will eventually quench all electro-oxidations occurring at the platinum anode.1 This poisoning intermediate has been identified unambiguously as adsorbed carbon monoxide using electrochemically modulated infrared spectroscopy (EMIRS) .4 Unfortunately, the surfaces that give rise to the largest electro-oxidation currents are precisely those which poison most readily, hence precluding the use of, for example, platinum { l l O } surfaces as superior electrocatalysts for methanol electro-oxidation.332 In spite of this, strategies have been developed to enhance the catalytic activity of platinum-based fuel cells while limiting the degree of self-poisoning observed during electro-oxidation.One of these strategies is to adsorb foreign metal adatoms on to the surface of the platinum electrode irreversibly. This technique, developed originally by Janssen and Moolhuysen ,5 involves dipping the platinum electrode into a salt solution of the metal to be adsorbed. The modified electrode is then rinsed in pure water and returned to the electrochemical cell containing the organic fuel. It is found that an irreversibly adsorbed layer of adatoms is formed on the surface, the coverage of which is determined by the time elapsed during immersion of the electrode in the salt solution.Well defined coverages of adsorbate can be prepared by this method. Irreversible adsorption also has advantages over more classical techniques of metal plating, such as underpotential deposition, because the coverage of adsorbate is not a function of potential and the electrochemical response is not influenced by the presence of adsorbate anionskations in the background electrolyte. Certain metals (lead, tin and antimony) when adsorbed on platinum have been shown to increase the activity of the electrode surface by an order of magnitude.6 The mechanism by which this is achieved, however, is still a subject of some controversy.' One model involves blocking of adsorption sites by the adatom (geometric effect) .7 As the surface reaction which leads to the formation of adsorbed carbon monoxide (poison) is believed to require at least two adjacent platinum surface atoms, whereas the reaction which produces useful electro- oxidation currents requires only one, physical blocking of the second platinum surface site by the adsorbed metal atom precludes the formation of any adsorbed carbon monoxide as a viable reaction pathway.Hence, the rate of electro-oxidation is increased significantly. The second models proposes that metals such as lead and tin can adsorb hydroxide species at lower overpotentials than clean platinum. Therefore, the Langmuir-Hinshelwood surface recombination reaction between adsorbed carbon monoxide and surface hydroxide can proceed readily at lower overpotentials to produce carbon dioxide and water.This mechanism facilitates an efficient way of removing adsorbed carbon monoxide from the electrode surface. These models at present are purely speculative although evidence exists in support af both. A number of recent examples of irreversible metal adsorption on platinum single crystal electrodes will be discussed, which try to elucidate the various reaction mechanisms operating and in particular the need to stabilize the adatom while electro- oxidation proceeds if commercial exploitation of these surface- modified electrodes is to be achieved. This is necessary as it is found that prolonged electro-oxidation can lead to a decrease in the amount of irreversibly adsorbed material and a subsequent loss of catalytic activity.Increasing between anions and the surface, however, appears to stabilize the metal adatom on platinum to some extent. Although an economically viable power source (fuel cell) based on single crystal platinum electrodes is still some time off, the possibility of utilizing the properties of single crystal platinum electrodes in analytical devices might hold some promise. For example, a significant proportion of gas and ANALYTICAL PROCEEDINGS, OCTOBER 1991. VOL 28 headspace analysers are reported to be of a fuel cell design9 employing platinum as the anode. Presumably, similar surface electro-oxidation processes are occurring in these devices as those alluded to earlier. Hence, gas and headspace analysers are amenable to improvement by chemical/structural modifica- tions of the electrode surface.However, a number of technical disadvantages are encountered, if one wishes to use single crystal platinum electrodes in sensor design. The first of these is their cost, as crystal manufacture is both time consuming and complicated. As it is only the surface of the electrode that is active for the electro-oxidation, macroscopic single crystal platinum samples also represent a wasteful use of a valuable and scarce resource. Ideally, one requires a highly dispersed electrode (good signal), such as platinum black, which pos- sesses a large surface to volume ratio. The two requirements of high dispersion and monocrystallinity appear at first sight to be mutually incompatible. Nonetheless, electrochemical methods for transforming polycrystalline platinum electrodes into preferentially orientated ({ 11 l} , { loo}, { 110)) samples have been developed with some degree of success.The technique of electrochemical facetting was developed originally by Arvia et af.1° The term refers to the crystallographic orientation of grains in polycrystalline metals when they are subjected to a periodic potential perturbation. The potential perturbation itself consists of a rapid triangular sweep (>lo00 Hz) between potential limits in the hydrogen and oxide electrosorption regions. The particular potential limits and sweep rates used determine the eventual crystallographic orientation of the platinum electrode. The procedure can be successfully applied to other metals including gold, rhodium and palladium.Hence, knowledge gained via fundamental studies of electro-oxidation at platinum electrodes leading to improved electrocatalytic activity can readily be extrapolated to high surface area platinum black samples (as used in fuel cell sensors) orientated by using electrochemical facetting. I t is also suggested that the singular electrochemical response of different crystallographic planes might present a method of selectively oxidizing gas mixtures by utilizing the slightly different overpotentials for electro-oxidation exhibited by each plane for a given gas. 1 2 3 4 5 6 7 8 9 10 References Clavilicr. J . , J . Electroanal. Chern., 1987, 236, 87. Lamy, C., Lcgcr. J . M., Clavilier, J . . and Parsons. R . . J .Electroanal. Cliem.. 1983. 150, 71. Capon, A., and Parsons, R., J. Electroanal. Chern.. 1973, 45, 205. Beden. B . , Bewick, A., and Lamy, C . , J . Electroanal. Chern., 1983, 148, 147. Janssen, M. M. P . . and Moolhuysen, J . , Electrochim. Acta, 1976, 21, 869. Clavilier, J . , Fernandcz-Vega, A . , Fcliu, J . M., and Aldez, A.. J . Electroanal. Chem.. 1989, 258, 89. Shibata. M.. Furuya, N., Watanabc. M., and Motoo, S . , J . Electroanal. Chem., 1989, 263, 87. Motoo, S . , and Watanabc. M.. J . Elrctroanal. Chem., 1976,69, 429. Criddle, W. J . , Separation, 1988, 2, 1 1 . Arvia, A . J . , Canullo, J . C., Custiadiano, W., Perdriel, C. L.. and Triaca, W. E., Electrochim. Acta, 1986, 31, 1359.ANALYTICAL PROCEEDINGS, OCTOBER 1991. VOL 28 333 Gas Monitoring Based on Phthalocyanines and Related Materials Colin L.Honeybourne and Julie O'Donnell Molecular Electronics and Surface Science Group, Bristol Polytechnic, Coldharbour lane, Frencha y, BristolBS76 IQY The semiconductive properties of the phthalocyanines (Pcs) were first observed independently by Eleyl and Vartanyan' in 1948. Since that time, the study of the semiconductive and photoconductive properties of Pcs has become an intensely researched area3-4 which has been extended to include the allied macrocyclic ligands porphyrins5 and [dibenzotetraaza- cyclotetradecines (dibenzotetraazaannulenes, TAAS)],~ and a where the in-plane dark d.c. conductivity of the LB films of copper(I1) mesoporphyrin IX diol was found to increase by up to 4 orders of magnitude in the presence of 10 ppm of NO,.In addition to measuring conductivity changes, other tech- niques for studying the change in physical properties of organic thin films during gas adsorption have been employed; for example, surface acoustic wave (SAW) device^,*^,*^ quartz PPPCCU PPPCVO OPPCCU NPPcCu APPcCu CPPCVO COOC2H5 CPTAACo kOOCZHS TM COZTAA Fig. 1 Sensors for oxides of nitrogen: ( a ) , substituted Pcs; ( b ) , TAAs range of dyes, conjugated hydrocarbons and conjugated oscillators,l" surface plasmon resonance (SPR)," infrared polymers.7-lo Recently, Eley has reviewed the past 40 years of spectroscopy18 and the Seebeck effect.19 Independent of the development in the theory of electrical conductivity in organic sensing technique used, the structure and morphology of the solids.I 1 thin film itself greatly influences its gas sensing characteristics.3 The dark d.c. conductivity and the photoconductivity of conjugated solids are both greatly influenced by the presence of dopant gases and vapours, properties which have led to extensive research into the use of thin films of such solids as gas and vapour sensors. For the phthalocyanines, in particular, the topic of gas sensing has recently been reviewed in considerable detail,-? and groups worldwide are continuing to expand the knowledge of the use of vacuum sublimed films of Pcs as sensors for oxidising gases.12 The effects of oxidising gases on the conductivity of vacuum sublimed films5ah and Langmuir- Rlodgett (LB) films13 of porphyrins has also been reported s! 5 mm Fig. 2 Interdigital gold electrodes334 ANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 Table 1 Comparison of the performance of Pc and TAA films exposed to NO, and monitored using three sensing techniques Performance with each sensing method Conductivity SAW SPR Sample Response Reversal Response Reversal Response PPPcVO "Good Good90-99Yo "Good 30% No response CPPCVO <PPPcVO Good 95-100% Good 18-36% No response CUPCS V.good Poor 46-92% Good - PPPcCu good TMC02TAA Small 90-100% Low 744% "V. good CPTAACo Good 84- 100% Good - Small response "Denotes the compound that is judged to be the best compound for that method, taking reversibility and reproducibility into consideration. initially 4 % Fig. 3 Gas flow system components and electrical components Much attention has been paid to optimising the structure of vacuum sublimed films by means of carefully controlled deposition and successive treatments of heat and gas exposure in order to increase sensitivity and reproducibility.2o.21 It is in the control of film structure where the use of thin LB films may be advantageous, their order and thinness providing a better defined and simpler structure for gas adsorption .22 Recently, an extensive study of the use of LB films of new substituted Pcs [Fig.l(a)] and TAAs [Fig. 1(6)] as sensors for NO,, in which dark d.c. conductivity, SAW devices and SPR were used to monitor the response to gas sorption, has been conducted at Bristol Polytechnic.23 Significant responses to the presence of NO, were observed from both groups of macro- cycles using all three sensing systems.Some comparative conclusions have been tabulated (Table 1). Dark d.c. conductivity measurements were made on LB films of a range of thickness from 2 to 30 layers, prepared from a mixture of the macrocycle with cadmium stearate, and deposited on interdigitated gold electrodes (Fig. 2). RF amplifier - - - - T T - r T 1 Low-pass filter I Acoustic - absorber - - - -- RF amplifier Fig. 4 SAW device14 A selection of the results obtained on exposure of the films to 1-10 ppm of NO, in a computer controlled gas flow rig (Fig. 3) at atmospheric pressure and 30 "C is shown in Table 2. For most of these compounds a plot of log conductivity versus log NO, concentration was linear and a high degree of reversibility was attained, particularly for the oxovanadium Pcs and the TAAs.The SAW device (Fig. 4) sensor operates by recording a change in its resonant frequency in response to the change in mass induced by adsorption of the gas on to the thin film coating. Typically, frequency changes of a few hundred Hz (Table 3) were observed for 10-20 layer LB films on exposure to 10-30 ppm ofbNO,. Reversibility was generally lower than that exhibited by conductivity measurements, but this is thought to be a result of the retention of NO, in the cadmium stearate matrix. Reproducible results and linear log frequency versus log concentration plots were obtained for the thinnest films of 2 and 5 LB layers. I Fig. 5 SPR experimentANALYTICAL PROCEEDINGS, OCTOBER 1991. VOL 28 335 Table 2 Conductivity changes observed for Pc and TAA LB films exposed to NO, NO, Film concentra- Exposure Max.current Sample layers tion (ppm) time/min change/A PPPCVO 10 1 30 2.7 x 10-10 50 mol% * 2 30 4.6 x 10-10 5 20 1.4 x lo-' 20 1 30 3.8 x lo-'() 2 30 2.3 x 10-10 PPPcCu 2 2 20 5.6 x 10-11 50 mol % 5 20 1.7 x 10-l0 5 20 1.3 x 10-9 10 2 20 1.8 x 10-9 5 20 9.7 x 10-9 20 2 20 2.7 x 10-9 5 20 6.0 x 10-8 TMC02T A A 20 2 20 5.3 x 10-12 50 mol % 5 20 2.1 x 10-11 10 2 20 3.1 x 10-11 5 20 9.9 x 10-10 10 20 2.4 x CPTAACo 20 2 20 2.0 x 10-9 5 20 2.3 x 10-9 10 20 1.9 x 10-9 11 2 20 7.1 x lo-" 5 20 2.3 x 4 2 20 7.1 x 10-12 5 20 2.0 x 10-10 10 20 1.4 x 10-lo * Figures for mol% are given as mol% of Pc in a mixed film with cadmium stearate. Reversal (Yo ) 95 95 97 95 95 94 86 92 85 77 46 94 89 100 100 93 91 84 100 100 100 100 100 - - Table 3 Frequency changes observed for SAW devices coated with LB films of TAAs and Pcs and exposed to NO, NO, Frequency concentra- change over Reversal Sample tion (ppm) 20 mins/Hz (%) TMC02TAA 20 layers 20 64 7 50 mol% 30 152 18 40 350 12 so 349 30 60 187 64 CPTAACo 2 layers 10 12 32 20 52 13 30 112 32 20 113 30 112 CPTAACo 10 layers 5 72 10 102 20 485 CPTAACo 20 layers 10 405 50 mol% 30 195 40 40 707 46 50 870 48 10 60 19 20 252 36 CPPcVO 10 layers 10 495 36 50 mol% 5 132 18 20 moly0 - CPTAACo 5 layers 10 120 - - - - - - PPPcVo 20 layers 20 167 > 100 PPPcVO 5 layers 5 22 45 NPPcCu 10 layers 10 175 - By using the arrangement shown in Fig.5 , at a particular incident angle, 8, a surface plasmon can be excited optically on a thin metal film and is characterized by a sharp fall in the intensity of light reflected from the metal surface (Fig.6). The position at which this resonance occurs is dependent upon a number of factors, including the refractive index and thickness of the organic overlay. Thus, any change in these parameters, Table 4 Changes in reflectance observed for LB films of Pcs and TAAs exposed to NO,, monitored at constant angle, 8 Reflec- tion of change Concentra- tivity Sample NO, (FPm) (Yo 1 Comment TMCOZTAA 0-50 9.3 0-100 4.9 Total change 14.2% 100-120 21.2 120-200 2.3 Total change 23.5% PPPcCu 1 layer &loo 0.5 100- 1 50 13.6 150-200 2.9 Total change 17% PPPcCu 20 mol% 0-200 17.8 No change at 100 ppm (2 layers) PPPcCu 20 mol% 0-150 1.4 (4 layers) 150-200 7.6 Total change 9% A ng I el@ Fig.6 a before exposure to gas and b after exposure SPR resonance dip and associated reflectance at fixed angle, 8,336 ANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 caused by the adsorption of a gas on the organic film, will result in a shift in the position of the resonance and an increase in the reflectance observed at angle 8 [Fig. 6(b)]. The changes in reflectance exhibited by Pc and TAA films exposed to 50-200 ppm of NO, are shown in Table 4. The authors gratefully acknowledge the help of Professor G. G. Roberts FRS and his research group at the Department of Engineering Science, Oxford University, in providing the SAW devices and laboratory facilities necessary for the SAW device research to be carried out. References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 I5 16 17 18 19 20 21 Eley, D.D., Nature, 1948, 612, 819. Vartanyan, A. R., Zhur. Fiz. Khim. 1948,22, 769. Wright, J . D., frog. Surf. Sci., 1989, 31, 1. Willis, M. R., Mol. Cryst. Liq. Cryst.. 1989, 171, 217. Honeybourne, C. L., Houghton, J . D., and Jones, 0. T. G., Chemtronics, 1986. 1, 83. Honeybourne. C. L., Hill, C. A. S . , Ewen. R. J.. Collings. M. S . , and Clarke, W. C., J. Phys. Chem. Solid, 1988,49,1003. Honeybourne, C . L., J . Phys. Chem. Solid, 1987.48, 109. Ratcliffe, N. M., Anal. Chim. Acta, 1990, 239, 257. Meier, H., Organic Semiconductors. Wiley. New York. 1967. Simon, J., and Andre, J . J . , Molecular Semiconductors. Springer-Verlag, Heidelberg, 1985. Eley. D. D., Mol. Cryst. Liq. Cryst., 1989, 171, 1. Sadoaka, Y., Sakai, Y., Jones, T.A., and Gopel, W., J. Muter. Sci., 1990, 25, 3024. Li, J. P., Tredgold, R. H., Jones, R., and Hodge, P.. Thin Solid Films. 1990, 186. 167. Holcroft. B., and Roberts, G. G., Thin Solid Films, 1988, 160, 445. NeuwenHuizen, M. S., Nederlof, A. J., and Barendez, A. W., Anal. Chem., 1988, 60, 230. Ross, J. F., and Roberts, G. G.. Proceedings of the Second International Conf. Chem. Sensors, Bordeaux. 1986. 704. Lloyd, J. P., Pearson, C., and Petty, M. C., Thin Solid Films, 1988, 160,431. Schoch, K. F., and Ternofonte, T. A., Thin Solid Films, 1988, 165, 83. Willis, M. R., Markland, K. J . , and Fahy, M. H., Synth. Met., 1989. 28, C781. Archer, P. B. M., Chadwick. A. V., Miasik. J. J . , Tarnize, M., and Wright, J. D., Sensors Actuators, 1989, 16, 379. Jones, T.A.. Bott, B., and Thorpe, S. C., Sensors Actuators, 1989, 17, 467. 22 23 Baker, S., Petty, M. C., and Roberts, G. G., IEEE Proc., 1983. 131( l), 260. O'Donnell, J., PhD Thesis, Bristol Polytechnic, 1990. Bibliography J. Phys. D, 1990, 23. 79. * Sens. Act., 1989, 19, 159." Mukromol. Chem., 1989, 190(7), 1573." J. Muter. Sci. Lett.. 1989. 8. 1095.* WisJ. Z. Tech. Univ. K.-M.-Stadt, 1989, 31, 393." Z . Chem.. 1989, 29( 12). 454." Chem. Expr.. 1990. 5(3), 153. Sens. Act.. 1989, 17. 475." Anal. Chem., 1990. 62, 353." J. Phys. D, 1989, 22, 1162. Le Contellec. M.. Eur. Pat.. EP-316230-AL 89.05.17." Chem. Muter., 1990, 2, 110. Synth. Met., 1989, 29, F37-F44. Synth. Met., 1989. 29. F31-F35. J. Am. Chem. SOC., 1989, 111,5271. J . Chem. Soc., Perkin Trans. I , 1989, 1071.BCS Japan, 1989,62,3359. J. Am. Chem. SOC.. 1990, 112, 1757. Recl Trav. Chim. Pays-Bas, 1990, 109. 208. Thin Solid Films, 1990, 186, 167.* Dyes Pigm., 1990, 13, 81. J. Am. Chem. Soc., 1990, 112,5388. J . Chem. Soc., Perkin Trans. I , 1990. 1169. Polym. Commun., 1990, 31(6). 232. Phil. Mag. B. 1990. 61(5). 843. Mol. Cryst. Liq. Cryst.. 1990, 183, 387. J. Mol. Catal.. 1990, 59(3), 291. J. Chem. Soc. Dalton Trans., 1990, 1115. J . Muter. Sci., 1990, 25, 3024. Synth. Met.. 1990. 38. 121. Intern. J. Electron.. 1990, 69, 3. J. Muter. Sci. Lett., 1990, 9. 1324. Chem. Phys. Lett.. 1990. 172, 299. Anal. Chem., 1990, 62. 2357.* J. Am. Chem. Soc.. 1990, 112,8064. J. Phys. Chem., 1990.94, 8019. Synth. Met., 1990, 37, 327. J. Electroanal. Chem.. 1990. 290, 203 and 15. Sens.Act., 1990, B2, 33." Chem. Abstr.. 1990, 114, 16630. Solid State Zonics, 1990, 40(1), 421." Sens. Act., 1990. B1.408." * Papers on sensors. Developments in Membrane-covered Gas Sensors for Oxygen and Other Gases in Relation to the Clinical and Biomedical Field Clive E. W. Hahn Nuffield Department of Anaesthetics, University of Oxford, Radcliffe Infirmary, Oxford OX2 6HE The measurement of the partial pressures of gases in inspired and expired air, and in the blood, is an extremely important procedure in the clinical and biomedical fields. The quest for stable, accurate and reproducible electrochemical techniques to perform these tasks has occupied the energy and talents of many physical and medical scientists over the past four decades. The polio epidemics in the USA and Europe in the early 1950s focused attention sharply upon the need firstly to measure the partial pressures of the physiological gases, oxygen and carbon dioxide, in severely ill patients' blood, and secondly to develop apparatus for the artificial ventilation of patients.These needs tied up the facilities and resources of very many medical departments until the epidemic was finally controlled, and physical scientists played a great part in developing the gas and blood-gas electrochemical sensors necessary for the continuing care of patients with respiratoryANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 337 failure requiring artificial ventilation; they anticipated, in some measure, the following explosion in the development of modern anaesthetic, surgical and intensive care techniques.Thus, the management of critically ill patients by the careful control of inspired and expired gases, and by the concomitant measurement o f these gases in the blood, has heralded the modern era of neonatal and adult intensive care and therapy. The story of the development of these sensors is, however, very chequered and, remarkably, their design and working principles have changed very little over the past decades since Stow introduced the first carbon dioxide electrode (developed in 1953) to an incredulous conference audience in August, 1954.1.2 Within weeks, this potentiometric membrane-covered sensor was improved upon dramatically by Severinghaus,3 was then commercially produced and has remained essentially unchanged ever since.Almost simultaneously, Clark devel- oped the oxygen electrode,‘,s completely independent of Stow’s sensor, but was then deprived of any patent rights because his sensor was judged to have been a derivative of Stow’s idea. Thereafter, chemists and clinical scientists have continued to refine the working principles of these two devices without, however, seriously challenging their dominance of the gas and blood-gas measurement field by designing newer or better sensors, working on different principles. Indeed, merely ‘refining’ these devices has often been a slow and painful exercise because their successful use in medicine demands strict performance criteria, such as reliability? stability, accu- racy and biocompatibility. Different design criteria are required for in vitro (gas and blood) and in vivo (blood) sensors.Moreover, in vivo sensors can be designed to be either invasive ( i . e . , intravascular) or non-invasive (transcutaneous). Although the same chemical principles determine the mode of action for in vitro and in vivo membrane covered sensors, in vivo sensors suffer from the well known problems of miniaturisation, fragility, bioincompatibil- ity and cost. These problems are minimised in in vitro sensors. Sensor response time can also differ greatly for different applications.68 Some oxygen sensors are required for gas analysis, in flowing gas streams, where the oxygen concentra- tion is constant (i.e., pipeline analysis, anaesthetic machine output and patient face mask inspired gas analysis); others are required for dynamic analysis, such as in expired air or in flowing blood streams (arteries), and so present membrane technology problems. This problem is not so pronounced for dynamic gas phase measurements, but, for blood-gas analysis, it is compounded by the physical fact that oxygen sensors work on amperometric principles and therefore consume oxygen from the blood sample.A thin, highly permeable membrane is idcal for fast time response, but is disastrous for blood sample oxygen consumption and depletion. Developments, in recent years, for tackling this problem have included miniaturizing the sensor cathode and/or pulsing the cathode polarising voltage, with varying degrees of success and failure.’.lO The ideal solution of a compromise between fast sensor response time and low sample oxygen consumption has yet to emerge. Conversely, the C02 sensor, because it works on poten- tiometric principles involving a ‘dynamic equilibrium’ between [C02] and [H+] in a hydrogen carbonate layer adjacent to a standard glass [H-+I electrode, does not consume C 0 2 from a clinical blood sample.9 The glass electrode sensor is, however, fragile and cannot be miniaturized for intravascular use,10 thus limiting its true usefulness to in vitro blood gas analysis.Neither can it be used for dynamic C 0 2 analysis in expired air because of its very slow (many seconds) response time. Other clinical gases, or vapours, which can be analysed by electrochemical (amperometric) techniques include the anaes- thetic agents dinitrogen oxide and halothane.Work in our laboratory, and elsewhere, demonstrated that N 2 0 could be reduced electrochemically on silver surfaces,’ 1 in an aqueous solvent, if a high negative polarizing voltage was applied (circa -1.3 V). If O2 and N 2 0 are reduced simultaneously on the same silver electrode, there is clear separation of the 0 2 and N 2 0 polarographic reduction waves, and the second (N20) wave is additive to the O2 wave.” However, the product of the N 2 0 reduction is nitrogen, and N2 bubbles will form on the cathode surface if the high negative voltage is applied constantly, thus causing the sensor to stop working. The solution to this problem is to pulse the electrode with a series of ‘low’ (02 reduction) and ‘high’ (02 + N 2 0 reduction) negative voltages, and so relate the sensor response through the Cottrell equation to O2 and N 2 0 partial pressures.12 Halothane (1-bromo- l-chloro-2,2,2-trifluoroethane; CF3CHCIBr) can be reduced on silver and gold surfaces, but not on platinum.Work on silver surfaces has indicated that the reduction process involves the breaking of the C-Br bond, with bromide as the reduction product.13 Thus, a halothane sensor can be constructed by use of silver cathodes. Unfortunately, halothane is reduced at the same polarising voltage as oxygen and it is impossible, as yet, to separate the two gas components on silver. This is possible on gold, and a membrane-covered gold cathode sensor has been developed, but its performance is not yet sufficient for accurate and reliable routine use.13 The clinical field is therefore ripe for innovation and exploitation, if new chemical sensors can be developed to meet the various undoubted, but different, clinical needs. The amperometric (and simultaneous) measurement of both 0 2 and CO2 on a single working electrode, as developed in recent years in the laboratories of the author and of Professor W.J . Albery, may yet pave the way for a new generation of electrochemical sensors, but the technique is not without its own problems.15 This technique depends upon the reduction processes occurring when O2 is reduced in non-aqueous solvents, such as dimethyl sulphoxide (DMSO). When O2 is reduced under such circumstances, the reduction product is the superoxide ion, 02--. If the cathode surface is pulsed negatively, the amount of 02.- produced is proportional to the O2 partial pressure.If a ‘reversed’ oxidising pulse is applied, the 02.- is oxidized back to oxygen. If C 0 2 is present, it reacts with the 02.- present to form peroxycarbonate, C2062-. Thus, in the presence of C02, the oxidising pulse will produce a smaller sensor current output (for the same prevailing O2 concentration) because some of the 02*- has reacted with C 0 2 , leaving less 02*- to be oxidized. The practical sensor therefore measures O2 on the reduction pulse, and infers the measurement of C 0 2 on the oxidising pulse, i.e., a pulse titration technique. Practical problems include the difficulties of working with non-aqueous solvents, the complex mathemat- ical relations between current response and gas concentration, and the fact that the C 0 2 and 02*- reactions involve not only the production of C2062-, but also the production of O2 itself.Thus, the sensor oxygen response is elevated by the reaction between C 0 2 and 02‘-, complicating the current response to the true prevailing oxygen partial pressure in the gas (or blood) sample. Similarly, the new generation of optical sensors,16-17 utilizing firstly the fluorescence quenching properties of O2 with specific dyes, and secondly, the spectral changes induced in other dyes by changes in pH, have yet to realise their full potential. The chemistry of these sensors (as opposed to simple amperometric devices) is determined by the laws governing fluorescence intensity and mass action, and so their signal versus analyte concentration relationships are complex.Unlike amperometric measurements, the optical measurements are not ‘direct’ and so suffer from the same drawback as the conventional (potentiometric) CO2 sensor, i.e. , [CO,] is inferred from induced pH changes in a hydrogen carbonate layer. Optical sensor designers must also overcome the problems of ambient light interference, slow response time and possible interference from ( i ) , anaesthetic agents, and (ii), dyes injected into patients for investigative purposes, before they can become truly clinically useful.338 ANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 Thus, new developments of either direct electrochemical, or 8 9 10 indirect chemical optical, sensors are still needed for accurate and fast analysis in both the gaseous and blood-gas phases. 11 12 678.Stow, R. W.. Baer, R. F., and Randall, B.. Arch. Phys. Med. 13 Rehabil., 1957, 38, 646. Severinghaus, J. W., and Bradley, A. F., J . Appl. Physiol., 14 19.58, 13, 515. 15 Physiol.. 1953, 6, 189. Clark, L. C.. Trans. A m . SOC. Artif. Intern. Organs, 1956,2.41. 16 Fatt. I . . Polarographic Oxygen Sensors, CRC Press, Boca Raton, FL, USA, 1976. 17 Hitchman, M. L., The Measurement of Dissolved Oxygen, John Wiley, Chichester, 1978. References Stow, R. W.. and Randall, B . F., Am. J. Physiol.. 1954, 179, Clark, L. C., Wolf, R., Granger, D., and Taylor. Z., J. Appl. Koryta. J., Medical and Biological Applications of Electrochem- ical Devices, John Wiley, Chichester, 1980. Hahn, C. E . W., J. Phys. E: Sci. Instrum.. 1980. 13, 470.Hahn, C. E . W., J. Phys. E: Sci. Instrum., 1981, 14, 783. Albery, W. J., Brooks, W. M., Gibson. S. P.. and Hahn, C. E . W., J . Appl. Physiol.. 1978, 45, 637. Brooks, W. N., Hahn, C. E. W., Foex, P.. Maynard. P.. and Albery. W. J . , Br. J. Anaesth.. 1980, 52. 715. Albery. W. J., Hahn, C. E. W., and Brooks, W. N.. Br. J . Anaestli.. 1981. 53, 447. Tebbutt, P., and Hahn, C. E . W., J . Electroanal. Chem., 1989, 261, 205. Clark. D., Electrochemical Sensors for Medical Gases, DPhil. Thesis, University of Oxford, 1987. Opitz. N., and Lubbers, D. W., PJlugers Arch., 1975. 355, R120. Gehrich, J. L., Lubbers, D. W., Opitz, N., Hansmann, D. R., Miller, W. W., Tusa, J. K.. and Yafuso, M., IEEE Trans. Biomed. Eng.. 1986, 33, 117. Use of Platinum-based Fuel Cells for Ethanol Analysis W.J. Criddle’ School of Chemistry and Applied Chemistry, University of Wales College of Cardiff, Cardiff CFI 3TB T. P. Jones Lion Laboratories Ltd., Barry, South Glamorgan CF6 3BE It is only rarely in the field of analytical chemistry that a particular field of study can legitimately claim to have had a serious impact on social attitudes and behaviour. Probably the most outstanding area has been the study of the compounds of tobacco smoke and its linking with chest disorders, in particular the development of lung cancer. Other studies, particularly in the area of environmental pollution, are making the general public aware of other areas of danger and much public money is being spent on bringing the results of these studies to the public notice.In much the same way, the abuse of ethanol, particularly as a result of excessive consumption of alcoholic beverages prior to driving on the public highway, has become a serious social issue in recent years for many governments worldwide, and not least in the UK. The analytical require- ment in the event of a suspected drinking and driving situation is two-fold. Firstly, to establish a prima facie case, with a screening test, that the suspect is at or over the legal limit (which varies from country to country, the UK value being 80 mg of ethanol per 100 cm3 of blood) and then to provide evidence that will be accepted by a court of law, i.e., an evidential test. How this screening test is achieved is discussed and evaluated below. Ethanol Distribution in the Body When ethanol is taken by mouth, the vast bulk of the ethanol is, of course, transferred to the stomach and small intestine, where the ethanol is absorbed into the bloodstream and distributed rapidly through the body. The adverse physiolog- ical effects occur when the blood containing the ethanol reaches the brain, although the time it takes to reach the brain will vary depending on several factors, which will be discussed later in this paper.However, the ethanol, when in the bloodstream, will rapidly achieve an equilibrium with the air in the lungs and thus a measure of the lung ethanol will be proportional to the blood ethanol , I over-all equilibrium being achieved between 20 and 40 min after the final ethanol consumption. It is important to * To whom correspondence should be addressed.note that breath sampling should not be attempted imme- diately after consumption of the alcoholic beverage, as results will be artificially high owing to residual mouth ethanol, and it is equally important that a deep lung sample be taken to ensure that a vapour sample representative of the lung air-blood ethanol equilibrium is obtained. The rate of transfer of the ethanol to the brain is dependent firstly on the concentration of ethanol in the stomach and small intestine and secondly on the food content of these organs, which will slow the ethanol assimilation process. The Screening Test It is perhaps important to point out that both screening and evidential tests are carried out by police personnel who, by definition, are not trained analysts, and therefore the equip- ment design has to allow for simple operation. The ultimate method of blood ethanol analysis is gas chromatography, but this is now only occasionally resorted to when there is a dispute over data and, of course, such analyses are carried out on blood samples taken by medically qualified personnel and carried out by trained analysts.The first approach to the problem of breath analysis was the yellow to green colour change when ethanol is oxidised using acid potassium dichromate crystals contained in a tube. The deep lung sample was achieved by the subject being required to blow up a bag of fixed volume and this method achieved a considerable degree of success worldwide. However, there were deficiencies in the system, in particular the lack of accuracy of the method, resulting in the main from the difficulty of assessing the colour change which was difficult to determine, particularly in poor lighting conditions.The search for a new approach to this problem required that a sensor be developed that was generally specific for ethanol. Studies carried out in this School”.” led to the development of platinum-based fuel cells which have been used successfully, particularly in small, hand-held roadside screeners which have become the main unit used by police forces worldwide. The Platinum-based Fuel Cell The basic principles of the fuel cell are well documented in theANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 339 literature,J although it is only in recent times that they have been used as analytical sensors.536 However, they have several advantages in the context of breath ethanol analysis.Firstly, in the form used in the screening units, they are extremely robust, and the instruments themselves can withstand considerable shock treatment, as is sometimes incurred when violent suspects are screened. Secondly, the fuel cell has a high degree of specificity with respect to ethanol, particularly when this point is taken in the context of organic substances likely to be found in breath. The, platinum fuel cell catalyses the oxidation of ethanol to acetaldehyde according to the equation CH3CH20H + CH3CHOH + H+ + e- (1) (2) J. CH3CH0 + H+ + e- and thus a potential is created which is directly proportional to the ethanol concentration. The potential developed is propor- tional to concentration over a substantial concentration range, the cell used in the screener showing a linear response up to at least 100 pg ethanol per 100 cm3 of breath.Note that the maximum permitted level in the UK is 35 pg per 100 cm3 of breath, i . e . , equivalent to 80 mg ethanol in 100 cm3 of blood. The precise make up of these fuel cells is, of course, a commercially sensitive area, but in essence they consist of a porous poly(viny1 chloride) disc approximately 3 cm in diameter, which is coated on both sides with a catalytic grade of platinum expecially prepared for this purpose. The disc is mounted in a plastic (ABS) case, which makes it virtually indestructable in field use. The cell is fitted with a sampling tube, through which the subject blows, and a sampling valve designed to draw a precise volume of breath into the cell chamber from the sampling tube, whence the output can be displayed digitally in a variety of units depending on legislative requirements.In most countries, the display is normally in mg of ethanol per 100 cm3 of blood, but in the UK a series of coloured lights indicate the ethanol level, red alone indicating a value at or over the legal limit. A deep lung breath sample is achieved by the use of a pressure switch, which illuminates indicator lights indicating that sufficient volume of sample has been exhaled. Turning to the specificity of the fuel cell, its main advantage over many other sensors is its semi-specificity. In the context of breath analysis, breath in the main consists only of exhaled air and carbon dioxide, the latter being fuel cell inactive. Acetone is a component of breath taken from diabetics but, here again, the substance is fuel cell inactive. It must be pointed out, however, that most primary and secondary alcohols, aldehydes and aliphatic unsaturated compounds are fuel cell active, and when such substances are suspected, e . g . , in cases where solvent abuse is suspected, results should be treated with caution. The actual potential achieved by a fuel cell is dependent not only on the actual size of the sample, but also on the load resistor associated with the cell circuit. As the load through which the cell is discharged increases, so the cell develops the characteristic of a capacitor, and the discharge profiles change. As the load resistor value is increased, so does the potential developed. However, the discharge time increases also, thus increasing the analysis time. The chosen value for load resistors is, therefore, a compromise between analysis time and signal value, and also the sophistication of the signal amplification circuitry. The output profile for fuel cells has been studied by Huck,7 who derived an expression for the potential developed based on the assumption that equations 1 and 2 represented the only relevant first order processes. While this expression superficially seems to be in reasonable agreement with experimental data, it is our opinion that this may well be an oversimplified approach, and we hope to publish a more detailed study of this area of fuel cell science in due course. In conclusion then, the fuel cells initially developed in this School and further developed commercially by Lion Labora- tories have now been proved over two decades to be reliable, robust and inexpensive sensors, are long lived (up to 2 years) and have a high calibration stability, particularly for the application described here. It is worth pointing out, however, that similar cells are now the basis of a range of more sophisticated ethanol analysers, including evidential breath analysers, and instrumentation Por the measurement of ethanol in body fluids (blood, saliva, urine) and in alcoholic beverages8 and it is clear that cells of this type have great potential as sensors for analytical applications outside the ethanol field. References Jones, A. W., PhD Thesis, University of Wales, 1974. Williams, P. M.. PhD Thesis, University of Wales. 1978. Neame. M. J . H., PhD Thesis. University of Wales, 1981. Criddle, W. J . , Jones, T. P.. and Neame. M. J. H., J . Inst. Meus. Control. 1984, 17, 10. Criddle, W. J.. Jones, T. P., and Parry, K. W., Analyst, 1986, 111,507. Criddle, W. J., Jones, T. P., and Parry, K. W., Analyst, 1987, 112. 615. Huck, H., Electroanal. Chem. Inter. Electrochem.. 1974.53,121. Lion Laboratories Ltd., Barry. South Glamorgan.

ISSN:0144-557X    

DOI:10.1039/AP9912800325

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

ANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 339 Odour Detection Using Sensor Arrays Krishna C. Persaud DIAS, UMIST, P.O. Box 88, Manchester M60 ?OD The proliferation of patent applications for gas sensing devices in the last decade would indicate that present day sensor technology does not fulfil current needs. Among the require- ments of many users of gas detectors is an electronic device which has high sensitivity, is capable of discrimination of large numbers of volatile chemicals, with the capacity for evaluation of complex mixtures, and is flexible in adapting to continuously changing environments. The chemoreception system in ani- mals has long ago evolved to achieve these characteristics and is far superior to man-made gas sensing systems. Hence, there is much intcrest in emulating biological chemosensory systems, such as the human nose, electronically.This presentation focuses on recent developments towards this goal. In the human nose, the sensor array consists of about 100 million odour transducers. The signals generated when odor- ant molecules interact with these sensors are fed to the olfactory bulb, the part of the brain responsible for olfactory information processing (Fig. l).' There are at least thirty types of odour transducers, which are characterized by broadly overlapping specificity of interaction towards different classes of volatile chemicals. Hence, what the brain interprets as an odour is actually the complex pattern of responses resulting from interaction of odorant molecules with the sensor array.Discrimination of odours results from comparison of the incoming odour pattern with patterns previously learnt. Any electronic model of the human nose can only be a simplification of the concepts encoded in the biological system. The core components of the system are the sensor array, a feature extractor. and a pattern classification system, as shown in Fig. 1. From our understanding of the biological system we340 ANALYTICAL PROCEEDINGS, OCTOBER 1991. VOL 28 Organic Semiconducting Gas Sensors Organic materials, such as phthalocyanine, have long been identified as potential gas sensors. Bott and Jones’ have developed sensitive and reliable sensors for oxides of nitrogen for use in the mining industry. Much interest has been aroused in conducting polymers for gas sensing purposes.Nylander et al.3 developed an ammonia sensor using polypyrrole. Persaud and Pelosi.t-6 have synthe- sized several polymers and investigated their gas sensing properties, while Bartlett et ai.73 have characterized poly- pyrrole for methanol detection. Pattern classification Sensor array Information processing feature extraction Fig. 1 The olfactory system. In the human, this consists of an array of 100 million odour sensors. These sensors have rather broad speci- ficities of response. In the array, there are about thirty different types of sensor which display differing odour specificities to particular classes of odours. The outputs of these sensors are taken to the olfactory bulb, which carries out substantial amounts of information processing, extracting pattern features which can be classified by higher centres of the brain can determine the specifications of suitable transducers for gases and odours in an electronic model nose.These would include the following items. Firstly, high specificity towards individual volatile molecules is not necessary, but there should be some specificity toward particular classes of chemicals. Secondly, specificity towards particular chemicals can be achieved by arrays of such transducers, each with different but overlapping response characteristics. Thirdly, ideally, the response to different concentrations of a volatile chemical should be monotonic. Fourthly, they should respond rapidly and reversibly at ambient temperatures. Fifthly, signals pro- duced by the transducers should be simply and easily processed.The types of sensors to be used in an artificial system must necessarily be different from those incorporated in the biological system. Numerous types of gas sensors have been developed for applications such as combustible or hazardous gas detection. These include metal oxide gas sensors, catalytic gas sensors, solid electrolyte gas sensors, mass-sensitive devices, fibre-optic devices and devices based on Langmuir- Blodgett films coupled to various metal oxide semiconductor devices. We have concentrated on alternative materials based on electrically conducting organic polymers, which are dis- cussed later. Many of these sensors may be suitable for incorporation into an electronic nose, albeit mimicking very poorly the charac- teristics of the biological sensors.Assuming that we start with a collection of at least 30 different types of odorant transducers, which are characterized by an overlapping specificity toward different classes of volatile chemicals, it does not matter too much which types of sensors are used. There are constraints on sensor performance, such as stability with time, temperature and humidity changes, susceptibility to poisoning, and reversi- bility of response, which dictate which types of sensors are suitable for particular applications. Py rro I e Thiophene Aniline Fig. 2 Structures of commonly used heterocyclic substances that can be polymerized using chemical or electrochemical means to produce sensors that change reversibly in resistance on exposure to polar volatile chemicals.In general, chemical derivatives of these substances may confer greater specificities on particular groups of chemicals Realization of an Electronic Nose Metal oxide semiconductors can be used in the design of sensor arrays and several researchers have used these materials. However, they suffer from two major disadvantages. They need to be heated to high temperatures in order to attain rapid adsorption-desorption kinetics, and they are easily poisoned by gas molecules adhering irreversibly to the surface of the sensors. One useful set of materials that can be utilized as sensors in an electronic nose is that of electrically conducting organic polymers based on heterocyclic chemicals, such as pyrrole, aniline and thiophene (Fig. 2).Persaud and Pelosi have investigated the gas sensing properties of over twenty conduct- ing polymers. These display reversible changes in conductivity when exposed to polar volatile chemicals. Unlike many commercially available gas sensors, rapid adsorption and desorption kinetics are observed at ambient temperatures, as illustrated in Fig. 3. The materials do not display high specificity to individual gases. However, they can be chemic- ally tailored to enhance differences in response to particular classes of polar molecules. The concentration-response pro- files are almost linear over a wide concentration range. This is advantageous as simple computational methods may be used for information processing. 100 fm a, -400 -500 - 600 Q CT - 700 0 5 10 15 20 25 30 35 40 45 Timels Fig.3 Responses of conducting polymer sensors to pulses of carbon monoxide gas (75 ppm v/v). The polymers display rapid adsorption and desorption kinetics Different polymers made from modified monomer units show broad overlapping response profiles to different volatile compounds. Hence, arrays of these sensors should behave very similarly to olfactory sensor arrays in the biological system. Miniature arrays consisting of up to twenty different conduct- ing polymer materials have now been realized. A microproces- sor driven circuit measuring changes in resistances of individual sensor elements interrogates the sensor array at user defined intervals and data are stored in memory. Each sensor element changes in resistance when exposed to a volatile compound.However, the degree of response to a given substance depends on the type of polymer element used so that a pattern of resistance changes can be recorded which can be processed to produce a set of descriptors for that particular substance. The sensor responses are normalized to account for differences inANALYTICAL PROCEEDINGS. OCTOBER 1991. VOL 28 34 1 basal resistance and record the ratio of the change in resistance to the absolute resistance (dRIR). A concentration-indepen- dent pattern can be produced. Such patterns are illustrated in Fig. 4, which shows the relative responses of individual sensor elements to carbon monoxide, acetone and methanol vapours presented to the array at room temperature. It can be seen that the patterns of all three compounds are different, carbon monoxide being opposite in polarity to the other two.Methanol and acetone cannot be distinguished by some sensor elements, but other elements show substantial differences. Taken over the whole array there are enough statistical differences for acetone and methanol to be differentiated from each other. The problem is to produce a computer program that will carry out this task automatically in the same way in which the human nose recognizes odours in all kinds of adverse conditions. Background odours may be present, temperature and humidity may be cycling up and down and sensor ageing effects may also be interfering. 0 c 0 n v) n 8 6 4 2 0 -2 -4 -6 irbon I ianol e future gas monitoring applications in a number of situations. The low cost and flexibility of the system leads to easy learning of volatile chemicals which the user wishes to monitor.Industrial monitoring A variety of needs exist in the chemical and process industries for sensor systems for the detection and identification of a number of volatile chemicals. These include maintenance of clean environments to achieve protection of personnel from toxic or hazardous substances. The simplest application is that of leak-seeking equipment to detect sources of minor vapour emissions. The effectiveness of this can be enhanced if many multi-component array sensors may be located remotely across an entire industrial site and coupled to a central monitoring facility. This can only be implemented if the cost of such installations is not prohibitive. An electronic nose may be used as a personal monitor worn by personnel working in potentially toxic environments.The sensor arrays can detect gases which are normally odourless to humans. Such gases include carbon monoxide and oxides of nitrogen. Odour quality monitoring All industries involved in food processing, fermentation and brewing, cosmetics and perfumes for aromatised products rely on panels of expert human noses in order to maintain product quality. The use of an electronic nose tuned to the normal patterns describing the characteristic odour quality of a particular product may serve as a means of automatic monitoring of a particular product. monoxide This work was supported by Cogent Ltd., Bucklersbury House, 3 Queen Victoria Street, London. 0 2 4 6 8 10 12 14 16 18 Sensor number Fig.4 Concentration-independent patterns produced by an array of conducting-polymer sensors in response to carbon monoxide, acetone and methanol. The patterns can be used to discriminate between these compounds Pattern Recognition Techniques Many computational methods have been applied to processing data from multi-sensor arrays in order to achieve discrimina- tion between substances measured and also to determine concentration. We have applied three techniques, which, with variations, can be applied to many sensory situations. These are correlation methods, partial least squares methods and neural networks. Applications of an Electronic Nose The realization of an electronic nose has profound impact in References Lancet, D., Ann.Rev. Neurosci., 1986,9, 329. Bott. B., and Jones, T. A., Sens. Actuators. 1986, 9, 19. Nylander. C.. Armgarth, M., and Lundstom, I., Anal. Chem. Symp. Series, 1983, 17,203. Persaud, K. C., and Pelosi, P., Truns. A m . Soc. ArfiJ Organs, 1985,31,297. Pelosi, P., and Persaud, K. C., Sensors and Sensory Systemsfor Advanced Robots, NATO A S I Series: Series F: Computer and Systems Science, ed. Dario, P., Springer-Verlag, Berlin, 1988, Persaud, K. C., Bartlett, J., and Pelosi, P., Robots and Biological Systems, NATO A S I Series, eds. Dario, P., Sandhi, G., and Aebischer, P., Springer-Verlag, Berlin, 1990. Bartlett, P. N.. Archer, P. B. M., and Sim, K. L.-C., Sens. Actuutors, 1989, 19, 12.5. Bartlett. P. N., and Sim, K. L-C.. Sens. ActuatorLr. 1989, 19, 141. pp. 361-382.Vapour Sensing by the Technique of SurFace Plasmon Resonance Henry C. Jaggers, J. Spencer Shaw and Stanley J. Peacock British Gas London Research Station, Michael Road, Fulham, London SW6 2AD Since the pioneering work of Kretschmann and Raetherl there has been a rapid growth in interest in the phenomenon of surface plasmon resonance (SPR). It was Nylander and his group2 who first showed that the technique could be applied to gas and vapour sensing when they used a thin film of silicon-glycol copolymer to detect halothane anaesthetic gas in concentrations down to about 10 ppm v/v. With SPR high sensitivity is possible and detection of @ RSC and British Gas. changes in the refractive index of the sensing layer of 10-5 have been reported.3 The SPR Technique In many respects a metal or semiconductor can be treated as a plasma where the positive charges are fixed in the crystal lattice and are surrounded by a sea of conduction electrons.A plasmon is a quantized charge oscillation in such a system. Surface plasmons exist in the boundary of the metal, the electric field of the surface wave being produced by an342 oscillation of the surface charge. It is possible to excite surface plasmon oscillation by the electric fields of light. This occurs when the wave vector and frequency of the light equals the wave vector and frequency of the plasma wave; energy is then coupled from the light to the plasma wave. The effect is called surface plasmon resonance. There are a number of ways in which the light wave can be matched to the surface plasmon for resonance to be observed.The most common way, and that used in this paper, is called the Kretschmann configuration and uses a prism for coupling the light into the metal film. Other methods involve perturba- tion of the light and plasma wave by surface roughness or by a regular surface pattern such as a grating.4 Sambles and co-workers have also shown that by using two polarisers at 45" to each other a maximum instead of a minimum at the position of resonance can be observed.4 ANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 Conducting polymer vapour absorbing films were deposited by electrochemical techniques, using low deposition potentials of 0.7 V to prevent oxidation of the film. The gold film formed the electrode on which the polymer was to be deposited.Application of SPR to Gas and Vapour Sensing SPR can be used for gas and vapour sensing because the coupling conditions and the depth of the resonance are very sensitive to the dielectric properties of an additional layer placed on the metal surface. SPR can thus provide a very sensitive means of probing any change brought about in this layer by the species to be detected. Apart from the gas specific sensing material used for this layer, the sensitivity of an SPR based sensor can be tailored by the use of the correct metal film and choice of wavelength of light; for example, a very much sharper resonance, and hence higher sensitivity, is observed in the infrared compared with the visible region. Also, work in this region enables many more metals to be investigated as substrates for SPR.536 This is because in the infrared, specifically at 3391 nm, the dielectric constant of most metals is hardly influenced by inter-band transitions and free electron contributions dominate.Thus, Sambles and co-workers have shown that metals such as palladium and vanadium5 and nickel and platinum6 will exhibit SPR as well as the more commonly used gold and silver substrates. This, of course, extends the range of sensing applications for SPR. Experimental The bulk of our work has been carried out by using gold films in the visible part of the spectrum using the Kretschmann configuration, although some work was carried out in the infrared. The gold (approximately 40 nm thick) was deposited on a microscope slide by vapour deposition and coupled to a glass prism with glycerol as an index matching fluid.The whole was attached to a gas and vapour cell and mounted on a rotation stage controlled by a computer operated stepper motor. The signal was collected by a photodiode, processed by a lock-in amplifier and fed into the computer. The light sources in both the visible and infrared experiments were 6 5 5 2 G 3 .- m 2 $ 4 S - I0 S m 1 0 Fig. 1 he6um neon lasers operating at 633 and 1520 nm -14 -12 -10 -8 -6 -4 -2 0 2 4 Relative angle of incidence/" SPR graphs for gold (40 nm thick) and gold coated with a conducting polymer. Both exposed to air and monoethylene glycol. A, Gold coated with conducting polymer; B, gold coated with conducting polymer while exposed to MEG vapour; C, gold film alone exposed to air and MEG Results The gas industry is interested in detecting monoethylene glycol because it is used as a gas conditioning agent to protect joints in the gas distribution pipeline system.The experiments reported here used this as a target vapour. Some typical SPR curves are shown in Fig. 1. It can be seen that a gold layer on its own gives no shift when exposed to monoethylene glycol, whereas when the gold is coated with the conducting polymer a large shift in the position of SPR is observed. This is coupled with a shallowing of the resonance: the greater the shift the greater the shallowing. In this instance, when the slide is exposed to monoethylene glycol a further shift is observed. For the technique to be of use as a sensor reversibility is desirable, and in each experiment described here the position and shape of the resonance returned to the original position when clean air was passed over the slide.Working in the infrared region offers distinct advantages over working in the visible. In our experiments with plain gold the width of a resonance in the visible region at half its depth is about 2 degrees, whereas in the infrared this reduces to less than half a degree. Even for slides coated with the conducting polymer, where a considerable widening of the resonance would be expected, the width is still only about half a degree. The increased sharpness and hence greater separation of the resonances leads to greater sensitivity for a sensor towards the target gas or vapour. For the technique to be used for sensor development ideally a system with no moving parts is needed. This can be achieved by an alternative approach which uses a convergent beam technique (Fig.2) first proposed by Kretschmann.7 In this the light source is first expanded and then focused on the gold film. Provided that the range of angles covered includes the SPR angle the SPR position is seen as a sharp absorption line on the fan of light reflected from the gold film. Any gas or vapour interaction with a detecting layer on the gold film is seen as a shift in the position of the absorption in a completely analogous situation to the rotating stage technique. Similarly, any \ Gold film with Prism I Diode laser Cylindrical lenses Position of the S P R Y - Detector Fig. 2 Diagrammatic representation of the converging beam tech- nique shallowing of the absorption is seen as the absorption line becoming less distinct.Thus it can be seen that SPR is a very sensitive technique for gas and vapour sensing, with sensitivity being increased by using infrared radiation. Specificity towards a specific gas or vapour can be achieved by using a gas absorbing layer and in the work described here we have reported the use of a conducting polymer for the sensing of monoethylene glycol vapour. The authors would like to thank British Gas for permission to publish this work. References 1 Kretschmann, E.. and Raether, H.. 2. Nuturfursch., 1968, 23a, 2135.ANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 343 5 Yang. F.. Bradberry. G. W., Jarvis, D. J.. and Sambles, J .R., J. Mod. Opt., 1990,37, 977. 6 Yang, F., Bradberry, G. W., and Sambles. J . R., J . Mod. Opt., 1989, 36. 1405. 7 Kretschmann, E., Opt. Commun.. 1978, 26, 41. 2 Nylander, C., Liedberg, B., and Lind, T., Sens. Actuators, 1982, 3, 79. 3 Liedberg, B., Yylander. C., and Lundstrom, I., Sens. Actiiators, 1983, 4, 299. 4 Vukusic, P. S . , Bryan-Brown, G. P., and Sambles, J. R., Sens. Actuators, submitted for publication. Gas Analysis Using Mass Spectrometry Paul Nicholas VG Gas Analysis Systems Ltd., Aston Way, Holmes Chapel Road, Middlewich, Cheshire CWlO OHT Mass spectrometry has the advantages of rapid analysis speed, high accuracy and full automation. These features, together with the ability to analyse several (up to 64) gas streams for up to 16 components, have established mass spectrometry as one of the most powerful single techniques for industrial process and environmental gas analysis.In essence, the function of any mass spectrometer is to identify a sample by producing characteristic 'fingerprints' of its constituents. The fingerprint, or cracking pattern, of a particular compound or mixture of compounds can be repro- duced for a given set of conditions. This allows the user to store and compare knowns with unknowns, thereby simplifying identification. In addition, mass spectrometry is used to quantify compounds by measuring their cracking pattern intensities. Applications of mass spectrometry in industry are diverse, ranging from furnace and mixing station gas monitoring in the steel industry to ultra-high-purity gas analysis in the fabrication of semiconductor devices.Other applications include head- space analysis for fermentation, acrylonitrile and ethylene oxide production, hydrocarbon analysis in the petrochemicals industry and the low level detection of volatile organic compounds in the environment. Good hardware design and powerful software have enabled individuals with no experience of mass spectrometry to exploit the benefits of the technique which, until relatively recently, had been restricted to the confines of the laboratory. Warwick Electronic Nose Philip N. Bartlett," George H. Dodd,* Julian W. Gardnert and Harold V. Shurmert Departments of Chemistry* and Engineeringt, University of Warwick, Coventry CV4 7AL The ultimate goal of our research is to design an instrument that will mimic the human sense of smell.' This ambition distinguishes our approach from that in the related area of gas sensing.In the latter area, the object is usually to detect a single gas or vapour which has exceeded a pre-defined threshold and thus monitor or control a chemical process. In contrast we use our nose in a much more sophisticated manner for classifying and grading smells. The human sense of smell is still the primary 'instrument' used for process evaluation in a broad range of industrial products including: beverages ( e . g . , wine, beer, whisky, fruit juice, tea, coffee); foodstuffs (e.g. , cheese, butter, fish); and perfumes ( e . g . , extrait perfumes, soaps, cosmetics). In the industries associated with these products, the physico- chemical variables of the finished goods are usually measured by using conventional analytical instrumentation, but the crucial flavour or odour is measured subjectively by sensory analysis.This defines a need for an electronic nose. Our electronic nose is comprised of an array of discrete chemical solid-state sensors with an output that is analysed by a microprocessor. The system architecture is based upon the same principles that have been identified in the mammalian olfactory system. The mammalian olfactory system, which is able to detect as little as one part of odorous substance in 10'2 parts, has about 50 million olfactory receptor cells. These synaptically couple into several thousand glomeruli nodes which feed the mitral cells in the olfactory bulb of the human brain.This vast number of olfactory sensors and complex neuronal architecture helps to explain why the nose can be so sensitive and discriminating. Our electronic nose is relatively modest as the number of chemical sensors is limited by present microelectronic technol- ogy to perhaps a few hundred elements in an array. The sensor array is, however, in an early stage of development with reports made on a practical nose that contains only twelve discrete sensing elements. Each element has a partially overlapping sensitivity to a range of odorous substances, as observed in the biological system. This greatly enhances the dynamic range of the electronic nose from only twelve odours for an odour specific element to the large number classifiable in 12-dimensional space.The performance of the Warwick electronic nose to recog- nize odours was discussed and results were presented on the discrimination of tobacco odours, alcohols,* beers and spirits.3 References 1 Gardner, J. W., Bartlett. P. N., Dodd, G. H . , and Shurmcr, H. V., in Chemosensory Information Processing. ed. Schild, D., Springer Verlag, Berlin, 1990, pp. 131-173. 2 Shurmer. H. V.. Gardner. J . W., and Chan, H. T.. Sens. Actuators. 1989, 18, 361. 3 Shurmer. H. V., Gardncr, J . W., and Corcoran, P.. Sens. Actuators, 1990, B1, 256.344 ANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 Exam inat and Cond on of Ammonia-Polypyrrole Interactions by Piezoelectric uctivity Measurements Jonathan M. Slater and Esther J. Watt Analytical Science Group, Birkbeck College, University of London, 29 Gordon Square, London WCW OPP Conducting polymers, e.g., polypyrrole, have been proposed as the active sensing element for a range of conducting gas sensors. 1 Prototype devices reported in the literature have shown some usefulness, especially as possible array com- ponents; however, their response is neither characterized nor understood.2 An alternative approach to sensing with poly- pyrrole is to make the polymer layer a coating on a quartz crystal microbalance. In this way it is possible to measure mass changes, such as gas sorption,3 and compare them with conductivity responses. The utility of this dual approach is that mass change profiles may be a useful tool for elucidating conductivity change mechanisms.4 I I I 0 10 20 30 Time/mi n Frequency response of the sensor to 5 min pulses of I% Fig. 1 ammonia gas The sensor device, a piezoelectric crystal coated with polypyrrole, was found to be sensitive to gas such as NO,, CH4, COZ and CO, showing particular sensitivity to ammonia gas (Figs. 1 and 2). Not only was the device sensitive to gas concentrations between 0.05 and 1% but the response corresponds to simultaneous parallel conductivity measurements. 1% NH3 0.5% NH3 1 1 Conductivity response I5ookQ 1 Fig. 2 Correlation between piczoelcctric and conductivity responses to 10 ml injections of ammonia gas Our results show that by controlling pyrrole polymerisation regimes useful sensor selectivities and sensitivities can be achieved. Results for a selective ammonia sensor will be presented. References 1 Nylander, C., Armgarth, M . , and Lyndstrom, I . , in Proceedings of the International Meeting on Chemical Sensors, Fukuoka, 19H3, ed. Seiyama, T.. Fueki, K . , Shiokawa. J . , and Suzuki, S., Elsevier, Amsterdam, 1983, pp. 203-207. 2 Miasik, J . J . , Hooper. A., and Tofield, B. C., J . Chem. Soc., Faraday Trans. I , 1986.82, 1117. 3 McCallum, J . J . , Analyst. 1989, 114. 1173. 4 Slater, J . M., and Watt, E. J . , Analyst, 1991, in the press.

ISSN:0144-557X    

DOI:10.1039/AP9912800339

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

ANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 345 ~ Equipment News Mass Spectrometers The new Senatorr range of quadrupole mass spectrometers features two models: 3BQ and VGA. Senatorr-3BQ has just three buttons to control all functions. Data are displayed on a 5 in monochrome VDU, and standard software includes bar graph scan, peak select, leak detection and automatic analysis. Senatorr-VGA has a 10 in VDU and colour graphics. Additional software includes 16-channel trend analysis, split screedlibrary and data-logging. Both models use the TIMS analyser to give accurate total pressure measurement. RS232 is standard. Spectramass Ltd., Radnor Park Indus- trial Estate, Congleton, Cheshire CW12 4XR. Mass Analyser The Lasermat mass analyser uses the matrix-assisted laser desorption time-of- flight mass spectrometry pioneered by Hillenkamp and Karas at the University of Munster.This enables rapid and accur- ate relative molecular mass measurement of biopolymers up to 200000 u and beyond at the low picomoie level. A brochure is available. Finnigan MAT, 355 River Oaks Park- way, San Jose, CA 95134-1991, USA. Software for Measurement of Intra- celIuIar Ionic Concentrations The measurement of intracellular con- centrations of Ca2+ and other ions using the makers' Model LS-50 luminescence spectrometer can be performed automati- cally with the addition of Perkin-Elmer Intracellular Biochemistry software. Ana- lysts can select either excitation wave- Iength ratios for probes such as FURA-2 or emission wavelength ratios for probes such as INDO-1. Method calibration and autofluorescence correction are auto- mated.Intensity ratios are plotted during data collection and calibration for process monitoring. Post-run viewing capabilities of the software include all calibration and raw analysis data in addition to concen- tration versus time. Perkin-Elmer Ltd., MaxweIl Road, Beaconsfield, Buckinghamshire HP9 1QA. Columns for Non-aqueous GPC Phenogel GPC columns provide excellent resolving power for a wide range of samples in a reproducible manner. Phe- nogel materials are highly cross-linked, offering excellent mechanical and chemi- cal stability. This results in wide solvent compatibility and resistance to gel shrink- ing and swelling during solvent switching. In a competitive comparison performed by an independent laboratory only Phe- nogel survived the following sequential solvent changes while providing good efficiency at each step: THF + DMF + toluene + CHC13 + DMF (ambient temperature) + DMF (50 "C) -+ THF + NMP + DMSO -+ THF -+ DMSO -+ THF + methanol.HPLC Technology Ltd., Wellington House, Waterloo Street West, Maccles- field, Cheshire SKll 6PJ. LC System for GPC Applications A new low-cost GPC system integrates highly sensitive liquid chromatography instrumentation for routine gel-permea- tion chromatography applications in quality control of plastics, resins and polymer formulations or blends. Linear calibration for a relative molecular mass range from 500 to 5 millionu ensures rapid, high precision analysis of relative molecular mass averages and distribu- tions of polymers and copolymers in organic solvents.The system consists of the makers' Model 250 isocratic pump, the LC-30 refractive index detector, Rheodyne 7125 manual injector, Model 101 column oven, 2 1 polythene bottle kit and the PLgel mixed-bed column and gel guard column. Dedicated and flexible GPC 2900 software on chromatography data systems is available. Perkin-Elmer Ltd., Maxwell Road, Beaconsfield, B uckinghamshire HP9 1QA. System for HPLC and Capillary Electrophoresis Featuring Beckman Gold software, the latest in the P/ACE Series, the System 2100, encourages the use of a single PC to control both the P/ACE System 2100 for capillary electrophoresis and a Beckman System Gold for high-performance liquid chromatography. Data from both systems can be displayed simultaneously and can be overlaid for efficient comparison of results.Although electropherograms resemble chromatograms in form, accu- rate and reproducible quantification of results requires special data analysis only available with the Version 6.0 Gold software. Beckman, Progress Road, Sand Indus- trial Estate, High Wycombe, Buckingharnshire. HPLC Consumables A comprehensive range of HPLC con- sumable~ is announced. A full selection of branded packing materials can be sup- plied in any size of column from 1 cm guard cartridges to full prepscale col- umns. Hardware items include tubing, connectors, nuts and ferrules, injectors and spares, autosampler vials and solvent filters. Applied Chromatography Systems Ltd., The Arsenal, Heapy Street, Macclesfield.Cheshire SK11 7JB. Sparging Vials for Purge and Trap Soil Sample Handling A new line of sparging vials minimizes the time required to prepare soil samples for purge and trap analysis. The vials can be retrofitted to all Tekmar (LSC and ALS models) and 01 (4460 and MPM16 models) purge and trap samplers and autosamplers. The vials are offered in ten different sizes. J and W Scientific, 91 Blue Ravine Road, Folsom, CA 95630-4714, USA. Direct Injection Columns Small hydrophobic molecules in complex mixtures such as plasma, urine, milk and cerebrospinal fluid can be determined quantitatively with minimal sample prep- aration using Ultrabiosep direct-injection columns, which operate on a similar principle to internal surface reversed- phase columns. Applications of the col- umns include the study of the in situ behaviour of pesticides and their metab- olites in non-deproteinated fluids from various animal species and the quantita- tive determination of drugs in plasma.Shandon Scientific Ltd., Chadwick Road, Astmoor, Runcorn, Cheshire WA7 1PR. Carbamate Analysis System A new HPLC system is available specifi- cally for the analysis of carbamate pesti- cides. It is designed for use with the LC- 240 fluorescence detector and the Picker- ing post-column reaction module. The system includes a Series 250 binary LC pump, 7125 manual injector, helium manifold, two 11 glass-lined polythene bottles and installation manual. It can be linked to the ISS-200 autosampler and a Model 1020 integrator for optimum sample and data handling.Perkin-Elmer Ltd., Maxwell Road, Be aconsfield, Buckinghamshire HP9 1QA. System for Biomolecular Purification and Analysis A basic bioseparations system provides a complete package of high-quality inert instrumentation for biomolecular purifi- cation and characterization. It consists of the Model 250 binary biocompatible pump, biocompatible LC-290 variable- wavelength ultraviolet-visible detector and the biocompatible 7125 Rheodyne injection valve and provides ail the instru-346 ANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 mentation required for separation of pro- teins by size-exclusion or hydrophobic interaction. Perkin-Elmer Ltd., Maxwell Road, Beaconsfield, Buckinghamshire HP9 1QA. Service Contracts for HPLC Instruments For as little as f100, HPLC users can obtain a 12 month service contract for their instruments. It is available for most manufacturers’ products.Instruments that can be serviced include ovens, pumps, fraction collectors, integrators, autosamplers, all detectors and chart recorders. Emergency cover and other related services are also available. HPLC Technology Ltd., Wellington House, Waterloo Street West, Maccles- field, Cheshire S K l l 6PJ. Gas Chromatograph The PU4600 GC system, designed and developed to BS 5750 and I S 0 9000 standards, occupies only 582 mm of bench space and couples guaranteed perform- ance with outstanding analytical flexibil- ity. Standard features include method stores, GLP files, timed events, auxiliary oven control and an RS232 link, all contained in an instrument weighing as little as 30 kg.Five types of detector are offered as standard, all optimized for capillary chromatography and with fast response times and small dead volumes. Unicam Ltd., York Street, Cambridge CB12PX. Upgrade for Gas Chromatography- Mass Spectrometry Systems Owners of the makers’ chromatography- mass spectrometry systems using the HP 5970A or HP 5971A mass-selective detec- tor and MS ChemStation (Pascal series) can now upgrade to the new, multi- tasking, PC ChemStation-based GC-MS system offering PC-based ChemStation control under the Windows 3.0 multi- tasking environment. A data sheet de- scribes the upgrade. Hewlett-Packard SA, 150 route du Nant d’Avril, CH-1217 Meyrin 2, Switzerland. Protease and Blood Protein Purification The lack of stability at high alkalinity of current affinity adsorbents used for pro- tease and blood protein purifications has been a matter for concern.The develop- ment of stable bonding strategies has now resulted in a range of lysine, arginine and benzamidine ligands bonded to highly stable 6% cross-linked agaroses, which virtually eliminates ligand leakage up to pH 13 and beyond. These products ensure more repeatable separations and extended column lifetimes. Affinity Chromatography Ltd., Free- port, Ballasalla, Isle of Man. Electrodes Micro Electrodes Inc. have developed a new range of micro flow-through elec- trodes for the measurement of 02, C 0 2 , pH, reference and NH3. They are ideal for monitoring these parameters in a continuous fluid flow such as water, blood and urine. A micro flow-through elec- trode for conductivity continuous measurement has also been developed.V. A. Howe and Co. Ltd., Beaumont Close, Banbury, Oxfords hire OX16 7RG. pH Meters The FE253 and FE257 are the first two meters in the Series 2 range of portable meters. The former is manually controlled, the latter fully automatic. Both have optional temperature probes that not only measure the initial buffer temperature but also avoid further manual adjustment if the temperature of the sample changes. There is a choice of electrodes. The accurate 0.1 mV range of the FE257 opens the door to on-site ISE measurement. A brochure is available. EDT Instruments Ltd., Lorne Road, Dover, Kent CT16 2AA. Water Monitoring System Watersure is a pollution monitoring system which detects the presence of undesirable substances in the water sup- ply.It can be used in the laboratory or mounted on a specially adapted vehicle for monitoring ’on the beat’. Based on the makers’ Uvikon range of spectropho- tometers, the system is controlled by an advanced Scorpion microprocessor, which can be programmed to carry out an almost infinite range of user-defined monitoring tasks. Kontron Instruments Ltd., Blackmoor Lane, Croxley Centre, Watford, Hert- fordshire WD1 SXQ. Software and Peripheral Units for Titration With the new DL70 titrator, titration methods can be developed and stored. Data can be interchanged between the DL70 and a PC via the new RS232C PC/ LIMS interface, and the SW70 software takes over control of the data communica- tion on the PC side. A variety of peri- pheral units are available.Via the RS232C interfaces of the DL70 a dot- matrix graphics printer, PC/LIMS or the ST20 sample changer are attached. Via the PC/LIMS interface and with the SW70 software package the DL70 operates together with robots or external sample preparation stations. Mettler-Toledo AG, CH-8606 Greifen- see, Switzerland. Microbalance The ergonomically well devised construc- tion of the MT5 microbalance provides the guarantee of achieving weighing results rapidly and simply. After taring of the vessel the sample is loaded and the result read off. All other manipulations are performed by using the ergonomically designed control unit separated from the weighing cell. The DeltaTrac analogue graphic indicator provides continuous information on the weighing range in use and that still available, even after taring, and with the vibration adaptor disturbing vibrations or drafts are filtered out. With the weighing process adaptor the display behaviour of the balance is matched to the weighing work.Mettler-Toledo AG, CH-8606 Greifen- see, Switzerland. Scales The DigiTOL scales are battery operated as standard and can therefore be used at any location. ‘DigiTOL’ means digital signal processing; a microprocessor digi- talizes the sensitive analogue signals in the load cell, allowing disturbing influences such as material drift, non-linearity and temperature changes to be continuously detected and eliminated. The scales cover a weight range from 6 to 600 kg with a readability of 2 g up to 200 g.They are also available in a certified version follow- ing OIML, Class 111. Mettler-Toledo AG, CH-8606 Griefen- see, Switzerland. Temperature Probes Three new probes measure and hence control the effects of temperature change on viscosity readings from both Visc- ometers UK and Brookfield viscometers. These effects are one of the principal areas of error identified by the Merck technical support department. Two of the probes are designed to replace the base plugs in the Viscometers UK small sample adaptor and low centipoise adaptor; the third is a pencil-type probe for use with samples in open containers. BDH Laboratory Supplies, Poole, Dorset. Microcentrifuges The 5415C is the latest in a long line of Eppendorf ambient bench microfuges with some impressive features.Re uired speeds of up to 14000 rev min3 are rapidly reached and easily maintained; a rotor lid reduces noise levels to a mini- mum and a fan ensures that samples do not overheat even during long runs. The refrigerated 5402 incorporates a digital display of running parameters to provide total control over the centrifugation process. BDH Laboratory Supplies, Poole, Dorset. Filter for Gas Cleaning Carbon Cap is a capsule filter designed to provide an extremely large surface area of activated carbon within a high-purity polypropylene housing. Odours, oil mistsANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 347 and contaminants are simply and effi- ciently removed. Two sizes offer different capacities: Carbon Cap 75 with 2200m’ and Carbon Cap 150 with 7000m2 fil- tration areas.Both types are intended for in-line use and feature hose barb connec- tions to fit 12 mm i.d. hose. Whatman Scientific Ltd., Whatman House, St. Leonard’s Road, 20/20 Maid- stone, Kent ME16 OLS. Capsule Filters A range of ready-to-use capsule pleated cartridge systems for the safe and eco- nomic filtration of small and medium volumes are designed for critical and non- critical liquid gas applications. Available in three sizes, 0.05, 0.1 and 0.2 m2, they come with a wide choice of end fittings. Sartorius Filtration, Longmead Busi- ness Centre, Blenheim Road, Epsom, Surrey KT19 9QN. Laboratory Information Management Systems Labsys and IBM (UK) announce the conclusion of an agreement under which IBM will support the marketing of the Labsys LIMS/400 in the UK.In develop- ing their system, Labsys chose IBM’s AS/ 400 range of computers and used IBM strategic applications software such as IBM Office/400. LIMS/400 is particularly suited to the pharmaceutical, chemical, petrochemical, food and fast-moving con- sumer goods industries. IBM United Kingdom Ltd., P.O. Box 118, Normandy House, AlenCon Link, Basingstoke, Hampshire RG12 1EJ. Multiloop and Logic Control System An alternative operator interface has been added to the Gardsman ML2 multi- loop and logic control system. This system offers the system integrator the ability to design a control scheme where true inter- action between analogue and digital control is required using up to 32 PID control blocks and 48 digital I/O. The new interface allows the operator to view and adjust all of the logic and control par- ameters using an extremely simple ‘touch and see’ approach, achieved by using the pressure-sensitive overlay and some clearer ergonomic design. Standard features of the advanced operator display include user-defined tag names.Mark IV Instruments Ltd., The Hyde, Brighton, East Sussex BN2 4JU. Laboratory Information Management Systems EasyLIMS Revision 2.0 allows users to customize laboratory reports by using a graphical report writer. On-line connec- tion of laboratory instruments is made possible by the availability of an add-on module called Acquire, which uses the same windows-based interface as Easy- LIMS. The latest addition to the Digital VAX-based Lab Manager LIMS includes Beckman Synergy co-ordinated connectivity plan new reporting and screen configuration features.An event-triggered reporting (ETR) module allows users to configure automatic reporting activities based on LIMS sample and test events. Synergy, a co-ordinated connectivity plan, allows the user to connect individual components such as System Gold HPLC and P/ACE with other instrumentation to EasyLIMS and/or Lab Manager LIMS. An SQL pipeline is now provided with the new Lab Manager LIMS software, allowing users to develop new and/or integrated appli- cations using the commercial 4GL tools that MIS professionals are most com- fortable with. A new ‘hot key’ reporting facility enables users to configure LIMS screen function keys to allow immediate print-out of customized reports, graphics and statistical quality control charts during data entryheview sessions without having to interrupt or leave the current screen.Also announced is CHROM Link, which couples chromatographs to computers running the new PeakPro chromatography system software. Beckman, Progress Road, Sands In- dustrial Estate, High Wycombe, Buckinghamshire . Laboratory Information Management System The current installations of the LIMS package Unilab are running on HP 9000 computers under the Unix operating system HP/UX. Unilab version 2.0, sche- duled for October 1991, will be available on both HP and DEC platforms. Compex nv, Brusselsesteenweg 356, B-9402 Ninove, Belgium. Laboratory Information Management System A new data-drive result exporting pack- age has been added to the latest release of the VAX-based Accomplis LIMS.The package, which requires no programming to set up, makes it easy to transfer analytical results directly to external computers of almost any type. Appli- cations range from communicating with mainframe production information systems to supplying data for analysis in PC spreadsheets. A brochure is available. ICI Chemicals and Polymers Ltd., P. 0. Box No. 1, Billingham, Cleveland TS23 1LB. Laboratory Information Management System Fisons Instruments and Digital Equip- ment Corporation have announced a co- operative development programme to provide the VG LIMS product on Digi- tal’s new ULTRIWRISC workstation family of computers. Fisons have also announced the availability on VG LIMS of EasyTalk, an Oracle-based natural language query system developed by Intelligent Business Systems.Also announced is the new chromatography data system, Multichrom 2, which has been designed to operate on the complete range of DEC VAX computers. A bro- chure describes VG LIMS. VG Laboratory Systems, Number 1 St. George‘s Court, Hanover Business Park, Altrincham, Cheshire WA14 5TP. Chart Recorder Serial data I/O options, ramp and soak facilities and enhanced flow recording form a package of new features to increase further the capability of the PX105 circular chart recorders and recorder-controllers. The Serial I/O options, available for most PXlOS vari- ants, allow the recorder to be interrogated remotely via a personal computer or a pre-programmed host computer. ABB Kent-Taylor Ltd., Howard Road, Eaton Socon, St.Neots, Huntingdon, Cambridgeshire PE 19 3EU. Molecular Modelling Software Oxford Molecular announces that it has abolished licence fees on its software products for all academics in North Amer-348 ANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 ica and Europe, including the United Kingdom. Oxford Molecular Ltd., Terrapin House, South Parks Road, Oxford OX1 3UB. Chemical Reaction Hazards Chilworth Technology has expanded its testing and consultancy services in techni- cal safety with the addition of new labora- tories for evaluating chemical reaction hazards. The new facilities enable Chil- worth’s safety consultants to provide a comprehensive hazard evaluation service that includes thermochemical testing, data interpretation, specification of safety measures and reactor vent sizing.Chilworth Technology Ltd., Beta House, Chilworth Research Centre, Southampton SO1 7NS. Foam to Suppress Hazardous Vapour Vapor Shield is a new foam which sup- presses hazardous vapour from dangerous chemical spills from road tankers, storage tanks, processing areas or in-plant sites. It is intended as a first-aid measure, protect- ing emergency crew and buying critical time for the initiation of clear-up techniques. Chubb Fire Engineering, Lancaster Road, Cressex Industrial Estate, High Wycombe, Buckinghamshire HP12 3QF. Literature Two brochures describe the latest scan- ning ultraviolet-visible spectropho- tometers, the first illustrating the PUS730 Professional-UV Series, the second devoted to the PUS710 Personal-UV instruments.Unicam Ltd., York Street, Cambridge CBl2PX. A brochure gives details of the DU-7000 Series ul traviole t-visible spec tro- photometer. Beckman, Progress Road, Sands Indus- trial Estate, High Wycombe, Buck- inghamshire HP12 3BR. A brochure describes SQL*LIMS, a fourth generation laboratory information management system designed to provide seamless integration with a variety of other widely used scientific software applications. Perkin-Elmer Ltd., Maxwell Road, Beaconsfield , Buckinghamshire HP9 1QA. A brochure introduces two members of the System Gold ultraviolet-visible detec- tor range, both operating as fully pro- grammable, stand-alone units. Beckman Instruments (United King- dom) Ltd., Progress Road, Sands Indus- trial Estate, High Wycombe, Buck- inghamshire HP12 4JL. A sample preparation products catalogue and up-to-date applications bibliography has been issued for users of solid-phase extraction technology. European Technical Centre, Varian Sample Preparation Products, P.O. Box 234, Cambridge CB2 1PE. Literature is available on Qualiprobe ion- selective electrodes. EDT Instruments Ltd., Lorne Road, Dover, Kent CT16 1BR. A catalogue describes a full range of space-saving , high-speed analytical evap- orators and extractors together with related glassware and accessories. Organomation Associates Inc., 266 River Road West, Berlin, MA 01503, USA. A brochure describes Sam, a sample and analysis management software system. Radian Corporation. 8501 Mo-Pac, P.O. Box 201088, Austin, TX 78720- 1088, USA. A brochure gives information on the Labvantage laboratory information man- agement system, version 3.2 of which has recently been released. Laboratory MicroSystems, Coronation Road, High Wycombe, Buckinghamshire HP12 3SY. A catalogue supplement covers the many innovative products added to the range covered in the current catalogue. It includes double-chambered ovens, incu- bators, oven-incubator combinations, safety bins for storing and disposing of radioactive waste, hot-plates with digital displays, furnaces, a platform shaker, a melting-point apparatus and a biotechno- logical reactor. Stuart Scientific Co. Ltd., Holme- thorpe Avenue, Holme thorpe Industrial Estate, Redhill, Surrey RH1 2NB.

ISSN:0144-557X    

DOI:10.1039/AP9912800345

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

348 ANALYTICAL PROCEEDINGS, OCTOBER 1991, VOL 28 ELINCS New Substances List The Commission of the European Com- munities has published the European List of Notified (New) Chemical Substances (ELINCS). It contains all substances noti- fied between September 18th, 1981, and June 30th, 1990, under the notification scheme established by the sixth amend- ment to the EC Dangerous Substances Directive. In the UK that scheme is implemented by the Notification of New Substances Regulations 1982, which are adminstered by the Health and Safety Executive (HSE) in co-operation with the Depart- ment of the Environment. Generally, substances are subject to notification if they do not appear in the European Inventory of Existing Commer- cial Chemical Substances (EINECS) , which contains those substances which were on the EC market between January lst, 1971, and September 18th, 1981.A notification should be made to the HSE before a manufacturer or importer places a new substance on the market for the first time. Included in a notification is a technical dossier of information on the hazardous properties of the substance. That information is scrutinized by the HSE and the Department of the Environ- ment so that the effects of new substances on human health and the environment may be assessed. For each substance included on ELINCS, the following information is given: EEC number; notification dossier numbers; trade names; classification (only if the substance has been officially classified at EC level and the classification published in Annex I to the Dangerous Substances Directive); and chemical name (unless confidentiality has been granted to it).For many substances, more than one notification has been submitted by differ- ent manufacturers or importers and there are different trade names. An indication is given if a substance has not yet been officially classified at EC level but has been provisionally classified by the notifier. Inclusion of a substance in ELINCS does not mean that it is exempt from the notification scheme. Any manufacturer or importer planning to supply a new sub- stance for the first time is obliged to notify it whether or not it has been notified previously by someone else. ELINCS will be updated yearly to take account of substances notified after June 30th, 1990. ‘European List of Notified (New) Chemical Substances’, pubiished in the Official Journal of the European Communities, C139, Volume 34 of May 29, 1991 (ISBN 0 11 968831 X), is available from HMSO, 51 Nine Elms Lane, London SW8 SDR, price 29.

ISSN:0144-557X    

DOI:10.1039/AP9912800348

出版商:RSC    

年代:1991

数据来源: RSC