2 ANALYTICAL PROCEEDINGS. JANUARY 1989, VOL 26 Research and Development Topics in Analytical Chemistry The following are summaries of seventeen of the papers presented at a Meeting of the Analytical Division held on July 18th-I 9th, 1988, in the Polytechnic, Plymouth. Summaries of a further sixteen papers will appear in the February issue. Lithium Ion-selective Electrodes: Optimisation Studies for Blood Serum Analysis C. W. Beswick, G. J. Moody and J. D. R. Thomas School of Chemistry and Applied Chemistry, University of Wales College of Cardiff, P.O. Box 972, Cardiff CF7 3TB Lithium therapy is the most widely used treatment for the control of recurrent depression or manic depression. The safe conduct of such therapy involves the measurement of serum lithium levels which should be maintained between 0.5 and 1 .O mM.At about 2.0-2.5 mM. adverse side-effects appear and higher levels are fatal.' Hence its facile assay in blood samples from patients prescribed lithium salts is clinically desirable. A clinical lithium analyser based on ion-selective electrodes (ISEs) is available for potentiometric analysis,* but there is much room available for improvement. ISE measurements under appropriate conditions have the advantages of compact- ness, low sample minimal operating volume consumption, rapG analysis' and costs. UNQ A Sodium is the main ionic interferent in blood fluid assays, its concentration being about 1400 times higher than the lowest level of 0.1 mM lithium. In the first instance the ISE should be capable of measuring 0.1 mM lithium in the presence of about 150 mM sodium.Other parameters, particularly interference from organic serum components, constitute further problems in the realisation of a suitable lithium ISE. Lithium ISEs have been designed with sensor membranes of diverse materials, but the most encouraging results have been reported for electrodes with sensor membranes consisting of neutral carrier molecules,' admixed with compatible solvent mediators in poly(viny1 chloride) (PVC) matrices. Two broad groups of neutral carrier ionophores are recognised. namely. O D C H 0 0 0 0 OH HO O C H Y X 0 0 U C x: D Fig. 1. University of Sheffield; D = a gift (ETH 2137) from Professor W. Simon, Zurich. Switperland Formulae of lithium sensors studied. A = a gift from Beckman RTIC. High Wycombe.Bucks.: B and C = gifts from Dr. J . F. Stoddart,ANALYTICAL PROCEEDINGS. JANUARY 1989. VOL 26 3 cyclic (crown ethers) and acyclic (lipophilic diamides). However, a third class of ionophores. based on poly- propoxylate adducts,J should also be mentioned. In this study, five different sensor cocktails [A-D in Fig. 1 and a sensor E based on the tetraphenylboratej of a barium propoxylate (PPG 1025)] are described, each having previously been optimised with respect to solvent mediator (Table 1). Table 1. Membrane compositions for lithium ISEs Composition. 'Yo Solvent Sensor mediator* Sensor Mediator PVC A . . . . 2-NPOE 1 .o 66.0 33.0 B . . . . DOPP 1 .o 66.0 33.0 C . . . . DOPP 1 .o 66.0 33.0 D . . . . BBPA 2.0 65.6 32.4 E t . . . . DOPP 1 .o 66.0 33.0 * NPOE = 2-nitrophenyl octyl ether; DOPP = dioctyl phenylphosphonate: BBPA = his( 1-buty1pentyl)adipate; and TPB = tetraphenylborate.+ E = Ba (PPG 1025),,,,, TPB,. Elect rode Opt imisat ion Lithium calibrations performed in aqueous solutions of lithium chloride (Fig. 2) show that each electrode has a nearly Nernstian slope. Sensor D has the best detection limit and, therefore, seems to be the best lithium sensor. However. the 0 > E > 2 -100 0, Q, 0 - - - 200 -6 - 4 -2 Log(lLi+l/M) Fig. 2. standards Calibration of lithium ISEs using aqueous lithium chloride selectivity coefficients, kE.b, show sensor A to be the most selective towards lithium over sodium, although this still does not meet the ideal value required, namely, log(kK.h,) = -3 (Fig. 3). r 1 0 .-1 ' -m 0 2 W -2 ' m -I -3 - 4 I I 1 I Li Na K Mg Ca Pictorial representation of selectivity coefficients. keyg B Fig. 3. The same sensors examined using lithium calibrations in a fixed background of 140 mM sodium chloride (Fig. 4) confirm that sensor A has the best selectivity towards lithium over sodium. This superior selectivity is further emphasised in a + 40 1 0 > E E oi 5 -80 u -40 - -120 -7 -6 -5 - 4 - 3 -2 - 1 Log([ Li + ]/M) Lithium ISE calibration using lithium chloride standards in a Fig. 4. background of 140 mM sodium chloride direct comparison between A and D over the required lithium concentration range of 0.7-1.5 mM as described by Metzger er al.5 Fig. 5 indicates that over this range electrode A shows a difference of 12.5 mM compared with only 6.5 mV for D.Sensor A was, therefore, chosen as the optimum system. - 4 -3 - 2 Log([Li-];~) Fig. 5. Lithium ISE calibration using lithium chloride standards over the clinical range in 140 mM sodium chloride 2 rnM Fig. 6. FIA recording of lithium ISE (electrode A ) response for lithium standards in artificial serum A injected into a carrier stream of artificial herum A4 ANALYTICAL PROCEEDINGS. JANUARY 1989. VOL 26 Studies on Serum by Flow Injection Analysis The sensor cocktail A was incorporated into a flow injection analysis system (a gift from Professor A. A. S. C. Machado, Oporto, Portugal) as described by Alegret et al.6 The carrier solution used was an artificial electrolyte, serum A, as described by Gadzekpo et al.7 The system gave reproducible data (Fig.6) and pH interference studies on a 1 mM lithium standard indicated no interference between pH 5.5 and 9 (Fig. 7). 111111 - 2 4 6 8 10 12 I1 2 60 L Y 3 20 a . . I l l I I II I , I 10min I I I I I I I I I Fig. 7. by FIA pH interference profile of lithium ISE (electrode A) obtained The analysis of serum samples from patients on lithium therapy (Fig. 8) gave reproducible sample peaks, and there was no fouling of the electrode. However, certain samples showed negative peaks, probably because of the low sodium concentra- tion as previously observed by Gadzekpo er d . 7 The data for 1.55 mM A I l l 0.9 mM 0.2 mM I 0.7 mM Fig. 8. Serum analysis by FIA using electrode A with 200 mm7 of samples injected into a carrier stream of artificial serum A interspersed by standards containing 0.5 mM lithium chloride positive FIA lithium peaks were correlated for ISE and the flame photometric data (Fig. 9).Neglecting the outlier in the square box. the relationship was LilsE = 0.81Lin,,,, - 0.09. with a correlation of 0.984. This compares favourably with previously reported results. Conclusions From this study, diamide sensor A appears to be the optimum on the basis of its selectivity towards lithium. However, the study has demonstrated that there are still problems that need 1.2 1 .o 0.8 E . 0.6 - 0.4 0.2 - t .- -1 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 [Li * 1flarne’m~ Fig. 9. Correlation plot of data obtained with lithium ISE (electrode A) by FIA for serum samples neglecting samples showing negative peaks attributable to the level of low sodium content.Equation for full line taking in all points: [LiIISE = 0.81[Lijndmc - 0.11 (correlation 0.952) Equation for dotted line, neglecting outlying square box point: [Li],,, = 0.81[Li]fl,,c - 0.09 (correlation 0.983) to be solved in lithium determinations using ISEs. e.g., insufficient selectivity over sodium, the effect of biochemical serum components on the electrode, the effect of varying electrolyte levels in serum samples and the effect of sample storage .H The Science and Engineering Research Council is thanked for financial support, including a studentship (to C. W. B.). Professor A. A. S. C. Machado. University of Oporto, Portugal, is thanked for helpful discussions, made possible by NATO travel grant (0069184). Finally. Dr. K. Davies of the University Hospital of Wales.Cardiff, is thanked for providing serum samples. References 1. Johnson. F. N.. Editor. ”Depression and Mania: Modern 2. 3. 3. 5 . 6 . 7. 8. Lithium Therapy,” IRL Press. 1987. McCurdy. W . . Chi. Chem.. 1988. 34. 367. Gadzekpo. V. P. Y . , Moody. G. J . , Thomas. J . D. R.. and Christian. G. D.. ton-Selectii~e Elecrrode Re\$.. 1986. 8. 173. Gadzekpo. V. P. Y . , Moody. G. J . . and Thomas. J . D. R.. Analyst, 1985, 110, 1381. Metzger. E . . Dohner. R.. and Simon. W.. A d . C’hrnz.. 1987. 59, 160& 1603. Alegret. S . , Alonso. J . , Bartroli. J.. Lima. J . L. F. C.. Machado. A. A. S. C.. and Paulis. J . M.. Anal. Leu., 1985, 18. 2291. Gadzekpo, V. P. Y . . Moody. G. J.. and Thomas. J . D. R.. Analysl. 1986. 111. 567. Mulryan. G . . Brazil.N . . Day-Cody. D.. and McKeon. P.. Clin. Chern.. 1987. 33. 1943. Development of a Flow Injection Manifold for the Extraction of the Perchlorate Ion with Brilliant Green D. Thorburn Burns, N. Chimpalee and M. Harriott Department of Analytical Chemistry, The Queen’s University of Belfast, Belfast BT9 5AG A flow injection manifold (Fig. 1) has been developed for the the design of Townshend and Alwehaid.3 The optimum spectrophotometric determination of perchlorate as Brilliant reagent and flow conditions are given in Table 1. The Green perchlorate at 640 nm based on an earlier manual interferences are similar to those for the manual procedures’; solvent extraction procedure’.’ and a membrane separator of the principal effects due to chlorate. nitrate. bromide, iodideANALYTICAL PROCEEDINGS, JANUARY 1989.VOL 26 1 2 3 5 - and thiocyanate can be removed by evaporation of samples with concentrated hydrochloric acid. The calibration graph was linear over the range 0-2.5 pg ml-1 of perchlorate and the relative standard deviation for the determination of 1 .O pg ml-1 perchlorate was 1.2% (ten replicates). The procedure has been applied to the determination of perchlorate in reagent and laboratory grades of potassium chlorate. The procedure gave similar accuracy and precision to earlier methods but is more rapid (sampling rate for prepared solutions. 20 h-I); the Table 1. Conditions for the determination of perchlorate ion Mixing coil . . . . . . . . . . Extraction coil . . . . . . . . . . Sampleinjectionvolume . . . . .. Flow-rates: Carrier stream (buffer pH 6) . . . . Reagent stream (Brilliant Green) . . Extraction solvent stream (benzene) . . Organicwastestream . . . . . . Flow-through cell (30 111) . . . . . . Wavelengthofdetection . . . . . . . . . . . . 2 5 0 ~ 1 20cm x 0.5 rnm i.d. 200crn x 0.5 mm i.d. . . 0.95ml rnin I . . 0.85rnlmin 1 . . 0.65rnl rnin I . . 0.68ml min-1 . . . . 640nrn 10 rnm path length detection limit, 0.036 pg ml-l (three times base-line noise), is superior to that for the only other reported FIA method4 for the perchlorate ion based on the AAS determination of copper after extraction of copper(1) - 6-methylpicolinaldehyde azine into isobutyl methyl ketone. We acknowledge with thanks the Du Pont Science Grant 1987-88 used in support of this work. References 1. 2.3. 4. Fogg. A. G.. Burgess, C.. and Thorburn Burns, D.. Analw. 1971. 96, 854. Thorburn Burns. D.. and Tungkananuruk. N . . Anal. Chm. A m . 1987. 199. 237. Townshend. A.. and Alwehaid. A.. personal communication. Gallego, M.. and Valcarcel. M.. Anal. Chim. A m . 1985. 169. 161. Some Recent Developments in Electrochemical lmmunoassays Eileen Buckley and Malcolm R. Smyth School of Chemical Sciences, NlHE Dublin, Glasnevin, Dublin 9, Ireland William R. Heineman and H. Brian Halsall Department of Chemistry, University of Cincinnati, Cincinnati, OH 45227-01 72, USA Immunoassays exploit the molecular recognition properties of biological systems such as immunoglobulins and enzymes. I An antibody can therefore be used as a selective analytical reagent. allowing detection and quantification of an antigen species.The popularity and feasibility of any immunoassay technique depends on the label used. Until recently. radioimmunoassays (RIA) using radioisotopes as labels have been the most popular and have resulted in the most sensitive assays. However, such labels have been declining in popularity owing to problems associated with waste disposal, licensing of laboratories. training of personnel and the high cost of equipment. Labels currently being investigated and used clinically include enzyme labels. chemiluminescent groups. fluorescent groups. metal atoms, electrochemical labels, etc. In electro- chemical immunoassays. both direct and indirect approaches to labelling are currently being investigated. An indirect method would be based on the labelling of the antigen with a non-electrochemical label.e.g.. an enzyme. and would involve monitoring the conversion of a substrate to product following the reaction of the antigen with its corresponding antibody.' A more direct method would involve labelling the antigen with an electrochemical label. e.g.. a heavy metal ion such as indium- (111). and monitoring the response of that label following interaction with the corresponding antibody.3 Alternative methods have been reported recently based on monitoring the changes in the electrochemical response of an antigen (or antibody) adsorbed or immobilised at a charged surface when the corresponding antibody (or antigen) is introduced into the solution.4 The direct approach to monitoring an antigen - antibody interaction is by no means a new concept.Breyer and Radcliffs studied the interaction of an azoprotein with its antiserum using d.c. polarography. No decrease in peak current was observed for the antigen unless the specific antiserum for the azoprotein was added to the cell. Hertlh has studied the dose-response curves for IgG and anti-IgG. anti-ferritin and S. aureus cells and suggested that the formation of surface antibody - antigen complexes disturbs the electrical double layer resulting in a change in the current. Smyth er 01.7 have used adsorptive stripping voltammetry to study the interaction of mouse IgG with anti-mouse IgG. The protein was adsorbed at a hanging mercury drop electrode in a stirred solution at a slightly positive potential and the potential then scanned in a negative direction.Parameters such as accumulation time (la,,), accumulation potential (Ezlc,) and scan rate were optimised and the influence of conditions such as drop size, stirring rate and buffer concentration were also studied. Each protein yielded two faradaic peaks at similar potentials, i.e., at -0.25 and -0.56 V (versus Ag - AgCl). The optimum conditions for the determination of these proteins were: EL,,, +0.05V,r;,,,J00sandscanrate 10mVs-1. Alonger accumulation time gave rise to greater sensitivity but also resulted in a longer analysis time. As increasing amounts of anti-IgG were added to a solution of mouse IgG. the peak currents for mouse IgG decreased in size. If a non-specific antiserum was added, however.no such decrease was ob- served. This is analogous to the behaviour reported by Breyer and Radcliffs for their azoprotein system.6 ANALYTICAL PROCEEDINGS, JANUARY 1989, VOL 26 We have also become interested in learning more about the nature of the electrochemical response of adsorbed proteins, as there is some debate in the literature as to whether this is a faradaic or a non-faradaic process. We have therefore used proteolytic enzymes to produce Fab and F(ab')* fragments of the anti-mouse IgG molecule in an attempt to attribute the observed peaks to a faradaic response (presumably due to reduction of the disulphide link) or a non-faradaic response (such as perturbation of the electrical double layer) or even to a combination of these factors.8 However, this study was inconclusive and there remains some doubt as to the nature of the responses exhibited by proteins at mercury electrodes.9 An example of an indirect electrochemical immunoassay is that developed by Wehmeyer et ,/.,lo which employs an enzyme label.The enzyme is used to catalyse a substrate to product reaction and the latter is detected electrochemically following high-performance liquid chromatography (HPLC) or flow injection analysis (FIA). This technique has been applied to a competitive heterogeneous immunoassay. A new substrate has recently been developed for the above assay. 1 * The conversion of p-aminophenylphosphate (PAPP) to p-aminophenol (PAP) is catalysed by the metalloenzyme alkaline phosphatase (obtained from calf intestine). This conversion of substrate to product has been investigated in a series of different buffer systems and the enzyme activity ( i .e . , substrate to product turnover) was determined in instance. 1.0 - 0.8 - 2 0.6 - F c. C 3 0.4 0 - 0.2 - 0 - each 100 200 300 400 500 Applied potentialimV Fig. 1. buffer (pH 5.0) mobile phase Hydrodynamic voltammogram of PAP in ammonium acetate As both the substrate and product are electroactive, a hydrodynamic voltammogram (Fig. 1) was generated to ascertain the optimum oxidative potential at which to monitor PAP without interference from PAPP (which is also electro- active) and other possible interferents, such as ascorbic acid, that may be present in a biological matrix. The buffer systems studied were tris(hydroxymethy1)aminomethane (THAM), glycine, ammonium hydrogen carbonate, diethanolamine (DEA) and ethylaminoethanol (EAE).The optimum condi- tions for the highest enzyme activity with respect to pH and molarity within each buffer system were determined first and then each buffer system was optimised further to obtain the highest activity between the systems. All enzymic reactions were carried out at a constant temperature of 37°C. After separation by HPLC on a Brownlee C8 reversed-phase column using 0.1 M ammonium acetate buffer (pH 5.0) as eluent. detection of PAP was achieved with a thin-layer electrochem- ical detector. The highest enzyme activity, determined from values of K , and VmaX,, and estimated from direct graphical plots as described by Cornish-Bowden," was exhibited in the EAE ( 0 .1 ~ , pH 9.8) buffer solution. It is anticipated that this enzymic system will give rise to a more sensitive electrochem- ical immunoassay in the future. In conclusion, an examination of the recent literature has revealed that there is increasing interest in the area of electrochemical immunoassays and that there are a wide range of different strategies, both direct and indirect, which are currently (sic) being investigated in this regard. It is anticipated that this area of research in electroanalytical chemistry will continue to blossom in the years to come. 1. 2. 3. 4. 5 . 6. 7. 8. 9. 10. 11. 12. References Kricka. L. J . , in Edmonds. T. E.. Editor. "Chemical Sensors." Blackie, Glasgow and London, 1988, pp. 3-14. Heineman, W. R.. Deutsch, E . , and Halsall. H.B., in Smyth. M. R.. and Vos. J . G.. Editors. "Electrochemistry, Sensors and Analysis," Elsevier, Amsterdam, 1986. pp. 345-353. Doyle, M. J . , Halsall, H. B., and Heineman. W . R.. Anal. Chem., 1982, 54. 2318. Smyth, M. R . , Buckley, E . , Rodriguez Flores. J.. John. R.. and Wallace, G. G., paper presented at "ElectroFinnAnaly- sis." Turku. Finland, June, 1988. Breyer, B., and Radcliff. F. J.. Nature (London). 1951. 167. 79. Hertl, W., Bioelectrochem. Bioenerg.. 1987. 17. 89. Smyth, M. R., Buckley. E.. Rodriguez Flores, J . . and O'Kennedy. R.. Analyst. 1988, 113, 31. Buckley. E., Reilly, P.. Rodriguez Flores, J . , O'Kennedy, R.. and Smyth, M. R., J. Phurm. Biomed. Anal.. submitted for publication. Emons, H.. Werner, G.. and Heineman. W. R.. in prepara- tion. Wehmeyer, K.R., Halsall. H. R.. Heineman. W. R.. Volle. C. P., and Chen, I. W.. Anal. Chem., 1986. 58. 135. Tang, H. T.. Lunte, C. E., Halsall. H. B., and Heineman. W. R.. Anal. Chim. Acta. in the press. Cornish-Bowden, A., "Fundamentals of Enzyme Kinetics." Butterworth, London. 1979. Sampling and Analysis of Organic Vapours in the Flue Gases from Pottery Kilns Naima Bradley and E. David Morgan Department of Chemistry, University of Keele, Staffordshire ST5 5BG The ceramic industry is making increasing use of organic solvents, adhesives, colour and transfer media, particularly in the application of decoration to pottery and china. When the ceramics are fired in the kiln, these materials volatilise or pyrolyse and are emitted in the flue gases at the cooler end of the kiln. In West Germany.' stringent regulations have been laid down for the concentrations of organic substances permitted in the air in workplaces and in flue emissions.Similar controls are likely to be adopted by the EEC and will consequently apply in Britain. An EEC directive has already been published on combating air pollution; it applies to plants engaged in the manufacture of coarse ceramics, refractory bricks, stoneware pipes, facing and floor bricks and roof tiles. In anticipation ofANALYTICAL PROCEEDINGS, JANUARY 1989, VOL 26 7 further legislation designed to control the quality of the environment, we have initiated a project to measure the amount of organic vapour in the flue gases of various types of kilns and for various products of the ceramic industry. Analysis Method In order to determine the very low concentrations of these organic components, it is necessary to undertake a pre- concentration phase.This is carried out by pumping a known volume of the flue gases through a tube filled with an adsorbent. The vapours are then desorbed on heating, separated by gas chromatography and analysed by mass spectrometry. Tenax (surface area about 20 rn' g-1) is a porous polymer based on 2,6-diphenyl-p-phenylene oxide (Fig. 1). Its most important properties are its high temperature stability (350 "C). which allows the elution of many different types of compounds having different relative molecular masses in a relatively short time, and its low affinity for water. r 1 L Fig. 1. Structural model for Tenax Tenax shows selectivity towards certain classes of com- pounds.Hence, polar compounds are more easily retained than non-polar compounds owing to the non-uniformity of its surface charge. Further, high relative molecular mass com- pounds are more easily retained than more volatile com- pounds. Therefore, if Tenax is to be used for quantitative analysis, its limits with respect to its trapping efficiency for the compounds to be sampled need to be established. Breakthrough Volumes A Tenax-filled adsorption tube can be regarded as a very short chromatographic column. The organic contaminants in the air are adsorbed on to the Tenax. travel through it and are eventually purged. A safe sampling or breakthrough volume can be defined as that volume of air containing a particular organic contaminant that can be sampled without a significant amount of contaminant remaining uncollected. The break- through volumes were determined by using the elution analysis technique.2 A gas chromatograph was modified to include a column inlet pressure gauge.A glass column (4 mm i.d.) was packed with 0.35 g of Tenax and connected to a flame ionisation detector. The flow-rate for these measurements was set to between 150 and 200 ml min-1, only half the sampling flow-rate. It has been shown that there is little or no change in the experimentally determined breakthrough volumes for flow-rates of between 100 and 1000 ml min-1 for a similar Tenax adsorption tube.' Breakthrough volumes can vary widely; for example, acetophenone3 has a breakthrough volume of 1258 1 g-1 at 2loC, whereas that of a-pinenej is 4 1 g-1 at 20°C.For this reason the volumes were measured at several higher temperatures and the breakthrough volume at the sampling temperature was obtained by extrapolation. because an approximately linear relationship exists between the logarithm of the breakthrough volume and the increase in the absolute column temperature.s.6 The breakthrough vol- umes for methyl methacrylate and butyl methacrylate were measured at temperatures between 200 and 70°C. At 34°C (maximum sampling temperature) the breakthrough volume for both compounds was found to be greater than 15 1 (the maximum sampling volume used) after extrapolation. Workplace Sampling There are several factors to be taken into account during sampling, such as the temperature of the system, the concen- tration of the contaminants, the flow-rate through the adsorp- tion tube, the sampling volume and, to a lesser degree, the humidity in the air to be sampled.The water content of the air sample is not a significant problem as Tenax is not affected by it. However, such a problem might arise in kilns where combustion gases and gases evolved during the firing process are mixed in the same exhaust pipe. In this instance the humidity can be such that large amounts of water vapour condense inside the Tenax tube and prevent any other compounds from being adsorbed. The optimum flow-rate range is wide for adsorption tubes of this type,h viz., 5-600 ml min-1. In this instance. a flow-rate of 30&400 ml min-1 was chosen because it gave a good compromise between the volume of air sampled and the time required for sampling.The temperature has a marked effect on the breakthrough volume. For methyl methacrylate3 an increase in the tempera- ture from 21.1 to 26.7 "C decreases the breakthrough volume from 144.5 to 95 1 g-1, i . e . , there is a drop in the breakthrough volume of 49.5 1 g-1 for a temperature rise of 5.6"C. Temperatures in kiln exhaust pipes can reach as high as 300 "C. To keep the sampling temperature low. it is therefore advisable to collect the sample as far as possible from the kiln. The adsorption tube is designed so as to minimise the effects of temperature on the Tenax. A glass tube (400 X 4 mm i.d.) is filled with 0.35 g of Tenax (35-60 mesh) held in place by two silanised glass wool plugs.The Tenax-filled portion occupies 130 mm of the adsorption tube. The empty portion is inserted inside the vent and the Tenax filled portion remains outside the kiln vent during sampling and does not come into contact with hot flue gases. Before use, the adsorbent is conditioned under nitrogen at 250°C for 3-6 h to remove all desorbable compounds. 5 2 I I Vent insulation 1;:: gases Fig. 2. Adsorption train. ( 1 ) Adsorption tube; (2) diaphragm pump; (3) digital thermometer; (4) platinum resistance; and ( 5 ) total flow counter The sampling point was selected with the help of the kiln manager and an employee of British Ceramic Research. The equipment that must be transported to the sampling site is shown in Fig. 2. Air is drawn through the adsorption tube ( 1 ) at a rate of 300-600 ml min-I by a diaphragm pump (2).The temperature is recorded with a digital thermometer (3) using a platinum resistance (4) placed inside a glass tube in series with the adsorption tube. The total volume sampled is measured with a total flow counter ( 5 ) . The volume sampled varies between 1 and 15 1 depending on the temperature and the type of substances encountered. After sampling, the tubes are sealed inside a glass container. The duration of the sampling period is 35-45 min. Analysis The organic vapours are desorbed thermally by placing the8 ANALYTICAL PROCEEDINGS. JANUARY 1989. VOL 26 sampling tube (2) (see Fig. 3) inside a tubular oven ( l ) , which has a controllable temperature range of 25-350°C. The adsorption tube is heated to 250 “C for 20 min while a stream of helium (15 ml min-1) is passed through it.The desorbed vapours are then concentrated in a glass-lined U-tube (3) cooled with liquid nitrogen in a Dewar flask (4). When all the material has been desorbed from the Tenax to the U-tube, the U-tube trap is flash heated (in less than 10 s) to 250°C by passing a direct current through the metal covering and the plug of organic vapours is swept on to the chromatography column and separated by means of a temperature programme using a Hewlett-Packard 5890 gas chromatograph coupled to a Hewlett-Packard 5970B mass selective detector with an HP 59970C ChemStation. A fused silica capillary column (12 m x 0.2 mm i.d.) coated with HP-1 (a cross-linked methylsilicone gum), 0.33 pm film thickness, is used.For the first 2 min the chromatographic separation is performed at 30 “C; the column temperature is then programmed to 160°C (at 3 “C min-1). On completion of the chromatographic separation, the compounds are identified using mass spectral libraries. I TO GC - MS Fig. 3. (3) glass-lined U-tube; and (4) Dewar flask Desorption apparatus. (1) Tubular oven; (2) sampling tube; Discussion Two types of continuous feed kiln were investigated. The first was an electrically fired kiln where the ware is placed on a moving belt. The ware travels through the kiln where it is fired at 860-900°C. The material given off as the ware is fired is withdrawn by a powerful extractor fan which, as both ends of the kiln are open, also draws air from the surrounding atmosphere, thereby cooling and diluting the flue gases.The temperature at the sampling point was 34 “C and the flow-rate was 0.62 m3 s-1. Two types of decorative process were used for the ware fired in this kiln: transfers and hand decoration. The compounds identified were butyl and methyl methacrylate from the transfers. In this instance the printing is carried out on a film made from poly(buty1 methacrylate) and poly(methy1 methacrylate). These polymers depolymerise when the ware is fired. In hand decoration a mixture of monoterpenes was identified, viz., cx-pinene, camphor, limonene and carene, which arise from the solvent used. The second type of continuous kiln investigated was a twin tunnel gas fired kiln consisting of two long straight tunnels. The combustion chambers are separated from the ware tunnels by refractory walls,’ the heat being transmitted mainly by conduction through the refractory material. The combustion chambers and the ware tunnels have different exhaust systems. The ware is stacked on to cars and pushed through the kiln on a rail system; most of the ware fired in this kiln is decorated with transfers.The two main compounds identified were butyl and methyl methacrylate. These and other substances are present at levels well below the limits permitted by present legislation and do not make a significant contribution to the level of organic contaminants. We thank the Trustees of the Analytical Chemistry Trust Fund of the Royal Society of Chemistry for the award of an SAC Research Studentship.We gratefully acknowledge the help and guidance of Mr. W. H. Holmes, D. L. Salt and E. Davies of the Whitewares Division of British Ceramic Research, Stoke-on-Trent. E. D. M. thanks the SERC for a grant for the purchase of equipment. References 1. “Bundesministerium des Innern. Technische Anleitung zur Reinhaltung der Luft.” Bundesministerium des Innern. Bonn. FRG. August 1974. pp. 1620. Modification 1986. pp. 12-13. Gallant, R. F.. King, J . W . . Levins. P. L.. and Piecewics. J . F.. Eur. Pat. A p p l . . 60017-781054. March 1978. Poole. C. F.. and Schuette. S . A.. ”Contemporary Practice of Chromatography.’’ Elsevier. Amsterdam. 1984. p. 478. Riba. M. L.. Randrianalimanana. E.. Mathieu. J . . andTorres. L., In[. J . Envirori. Anal. Cherii..1985. 19. 133. Tanaka. T., J . Chromcitogr.. 1978, 153. 7. Brown. R. H., and Purnell. C. J . . J . Chromutogr., 1Y79.178.79. Singer. F., and Singer, S. S . . “Industrial Ceramics.” Chapman and Hall. London. 1963. pp. 1002-1012. 2. 3 . 4. 5 . 6. 7. Potentiometric Determination of Sodium and Potassium in Blood Serum: an Assessment of the Use of Bis(crown ether)-based Ion-selective Electrodes G. J. Moody, Bahruddin B. Saad and J. D. R. Thomas School of Chemistry and Applied Chemistry, University of Wales College of Cardiff, P.O. Box 972, Cardiff CF7 3TB Ion-selective electrodes (ISEs) are steadily replacing flame photometry for the determination of sodium and potassium in body fluids. 1-3 Such determinations are normally conducted with the sodium glass membrane electrode and the valinomycin PVC matrix membrane potassium IS€.However. the use of glass membranes poses various difficulties, such as contamina- tion of the glass membrane surface by proteins, high resistance and interferences from hydrogen ions. Further, there is a continuing search for alternative and improved sensors. The observation that potassium forms a 1 : 2 complex with benzo-15-crown-5 has led to the synthesis of bis(crown ethers) where the two monomeric crown units are linked together by a single bridge.j.5 It has been found4 that the co-operative effect of the two crown ether rings in the bis(crown ethers) gives remarkable selectivity for alkali metal ions through the formation of 1 : 1 cation - bis(crown ether) intramolecular sandwich complexes. Therefore.this study concerns optimisa- tion studies on a bis( 12-crown-4) ( I ) and a bis(benzo-15-crown- 5 ) (11) sensor (Fig. 1) with plasticising solvent mediators in PVC, in the presence and absence of an anion excluder. The optimised system has been utilised in the potentiometric sensing of sodium and potassium ions in serum using analateANALYTICAL PROCEEDINGS, JANUARY 1989. VOL 26 9 ~ Table 1 . Some characteristics of sodium bis( 12-crown-4) (sensor I ) electrodes compared with a commercial (EIL) glass membrane sodium electrode Electrode No. 1 3 4 5 6 7 L Solvent Mole mediator KTCIPB BBPA 0 BEHA 0 DOS 0 NPOE 0 NPOE 5 0 (EIL glass membrane elect rode) Slope/ mV decade I 53.0 52.0 60.0 60.8 61 .o 6O. 5 Detect ion limit/ 4.0 1 .8 1.3 6.3 6.0 3.2 10 - '' hl Log k c l , B [separate solution method (10 2 M)] Resist ancei MR B = K B = Li B = C a B = M g 23 - 1.43 Not evaluated owing to 44 -0.81 poor slopes 60 - 1.38 -2.93 -4.06 -3.96 4 - 1.74 -2.40 -3.88 -3.94 0.08 - 1 .x5 - 1.80 -3.68 -3.51 - - 1 .58 - 1.41 -4.17 -4.21 * BBPA = bis( 1-butylpentyl adipate); BEHA = bis(2-ethylhexql) adipate: DOS = dioctyl sebacate; NPOE = 2-nitrophenyl octyl ether addition and flow injection analysis (FIA) techniques. Opt imisa t ion of Electrodes PVC matrix membranes were cast from mixtures of plasticising solvent mediator (360 mg), PVC (170 mg) and sensor (5 mg) and ISEs were assembled according to established proce- dures.6 In some instances, a 50% molar ratio of tetraphenyl- borate anion excluder relative to sensor was also added.The characteristics of the resulting conventional-type electrodes are summarised in Tables 1 and 2.The separate solution method at a 10-2, concentration of cations was used to evaluate the selectivity coefficients. The best sodium bis( 12-crown-4) and potassium bis(benzo- 15-crown-5) ISE membranes contain NPOE and anion excluder (electrodes 5 and 13. Tables 1 and 2). The best sodium bis( 12-crown-4) electrode ( 5 ) is not only superior to the EIL glass membrane electrode (6) with respect to selectivity over physiologically important cations, but also with respect to response times and pH interferences. The best bis(benzo-15- crown-5) electrode (13) has characteristics similar to the valinomycin electrode (13). The electrodes were tested for possible interferences from biochemicals normally present in blood serum.Standard amounts of biochemicals at the upper range (Table 3). e.g., 0.03O/O urea, were dissolved in 5 mM sodium chloride and potassium chloride, respectively, in electrolytes A and B (that is. mock serum A and B as described bclow). and injected into the FIA stream while running a carrier stream of electrolyte A or B, as appropriate. The peak heights corresponding to five injections of each of the same samples containing the same concentration of sodium or potassium (5 mM) with and without biochemicals were compared. The e.m.f. differences ( A E ) are summarised in Table 3. The studies of the interferences of the biochemicals on the sodium bis( 12-crown-4) electrode are inconclusive owing to the presence of sodium (typically 2-6%) in the Sigma formula- tions, but there is negligible interference on the potassium bis( benzo- 15-crown-5) electrode.Serum Measurements For studies relating the serum measurements, all standard sodium and potassium solutions were prepared in mock serum II Fig. 1. a bis( 13-croivn-4): sensor I1 = a his( benzo-15-crown-5) Structures of bis(crown ether) compounds studied. Sensor I = Table 2. Some characteristics of potassium bis(benzo- 15-crown-5) and \ alinomycin potassium electrodes Lop kK:h Slope' Detection (separate solution method ( 1 0 2 M)] Electrode Solvent Mole % mV limit/ Resistance: No. mediator* KTCIPB decade I 10 51 MR B = Na B = Li B = C a B = M g 7 BBPA 8 BEHA 9 DOS 10 NPOE 11 BEHA 12 DOS 13 NPOE 14 NPOE (Valinomqcin) * Abbreviations as in Table 1 .0 0 0 0 50 50 50 0 52.0 60.0 60.5 61 .o 45.5 57.5 59.2 59.6 7.6 7.5 2.5 3.2 3.5 7.5 8.0 _ _ 3.3 34 43 110 5 3 7 1 0 - - - -3.16 -2.72 -3.23 -4.21 -4.18 -2.53 -3.25 -4.20 -4.08 -1 -._ 58 -3.38 -4.00 -4.04 -2.67 - -3.05 - 3 . I6 -3.94 -4.09 -3.08 -3.14 -3.88 -3.92 -3.02 -2.88 -3.80 -3.96 - -10 ANALYTICAL PROCEEDINGS. JANUARY 1989. VOL 26 Table 3. Effect of biochemicals on bis(crown ether) electrodes AE*lmV r = 0.68 Relative molecular Constituent mass Urea . . . . . . 60 Glucose . . . . 180 Albumin . . . . 69000 a-Globulin . . . . 41 OOG54000 (3-Globulin . . . . 5 x 10h-20 x loh y-Globulin . . . . 1.50000 Mixture of the above sixcomponents . . - r = 0.012 Normal range in human plasma (g per 100 cm3) 0.02-0.03 0.06s-0.09 2.8-4.5 0.3-0.6 0.6-1.1 0.7-1.5 ~ Bis( 12C4) Bis( 15CS) (electrode 5 ) (electrode 13) +0.5 (0.02) 0 (0.02) 0 (0.005) 0 (0.004) +9.0 (0.7) -3.0 (0.4) + 10.2 (0.76) 0 (0.006) -0.5 (0.01) 0 ( 0 ) +8.0 ( 1.2) +3.0 (0.8) + 10.0 (1.6) -2.2 (0.05) * A€ = differences in e.m.f.of a 5 mM solution of the primary ions with and without the biochemicals (s.d. in parentheses. 11 = 5 ) . A (consisting of 5 mM KCI + 1 mM CaC12 + 1 mM MgCI? in 0.05 M Trizma base buffer) and B (consisting of 140 mM NaCl + 1 mM CaCI: + 1 mM MgC12 in 0.05 M Trizma base buffer), respectively, with the pH adjusted to 7.5 using concentrated hydrochloric acid. A direct potentiometric approach ( i . e . , without dilution) for the determinations of sodium and potassium levels in serum using the analate addition technique was used.The best sodium bis( 12-crown-4) electrode, namely electrode 5, in conjunction with a reference electrode, was placed in 1 mM sodium chloride (15 cm-7) in electrolyte A, and spiked with undiluted serum (0.15 cm-7). For the potassium determinations in serum, both the valinomycin and bis(benzo-15-crown-5) macro-ISE (elec- trode 13) were immersed in 0.5 mM potassium in electrolyte B (15 cm-') and spiked with serum (0.15 cm-') and the mV differences were noted. The analate addition was based on where A E is the e.m.f. difference on addition of analyte of unknown concentration C., and volume V,, to a standard solution of analyte of concentration and volume Co and Vo, respectively; S is the slope of electrode. A flow-through sandwich potentiometric detector for FIA in which a PVC sensor cocktail was applied on a conductive epoxy support7 was also used for serum analysis.I I I 0 0 I 120 120 130 140 150120 130 140 150 [ N a ] / m ~ flame photometry Fig. 2. Correlation of potentiometric and flame photometric tech- niques for the determination of sodium, using ISEs ( 5 ) of sensor I [bis( 12-crown-4)) in serum using the analate addition: ( a ) direct potentiometry and FIA; ( b ) indirect potentiometry The blood serum measurements were made on samples obtained from the University Hospital of Wales, Cardiff, which were stored at 4 "C. The determinations were carried out on the same day. before which the samples were allowed to reach room temperature over about 1 h. With regard to electrode quality during use and blood serum measurements.the optimised potassium bis( benzo-15-crown- 5) electrode ( 13) possessed over-all characteristics comparable to those of the valinomycin electrode (14). However. its lifetime is inferior to both the bis( 12-crown-4) electrode (5) and valinomycin electrode (14), as the slopes decreased and the resistance increased after 70 exposures to serum in a macro- ISE mode. The bis(12-crown-4) electrode (8). on the other hand, showed no significant differences in slope and resistance until after 89 contact periods in serum and also functioned well 1 month later. The sodium bis( 12-crown-4) electrode ( 5 ) , although superior to an EIL sodium glass electrode (6), as noted above, does not. however, give a good correlation with flame photometry (Fig. As can be seen from Fig.3, there is a more reasonable correlation between the potassium ion levels in serum deter- mined using the bis(crown ether) electrode (13) and flame photometry. The more laborious analate addition technique seems to yield better correlations than the faster FIA method. 2). 5 0' '1 , , , , ( a ) 0 2 3 4 5 6 7 2 3 4 5 6 7 2 3 4 5 6 7 [ K l l m ~ flame photometry Fig. 3. Correlation of potentiometry and flame photometric tech- niques for the determination of potassium in serum using ISEs (13) of sensor I1 [bis(benzo-lS-crown-5)] ( a ) and (14) valinomycin ( h ) . each using analate addition. and FIA method ( c ) using sensor I1 Conclusion The optimisation of bis( 12-crown-4) and bis( benzo- 15-crown- 5) sensors in association with plasticising solvent mediators in PVC showed that the best electrodes were based on NPOE plasticising solvent mediator and a 50% molar ratio of anion excluder relative to sensor.With regard to the use of these electrodes for serum analysis, the bis( benzo- 15-crown-5) potassium electrodes gave better correlations with flame photometry than the bis( 12-crown-4) sodium electrodes for potassium and sodium determinations, respectively. Financial support and leave of absence from the Universiti Sains Malaysia to one of us (B.B.S.) is gratefully acknowl- edged. Also, Dr. Keith Davies, University Hospital of Wales. is thanked for providing serum samples. References 1. Savory, J . , Bertholf. R. L.. Boyd. C. J . . Bruns. D. E.. Filder. R . A , . Lovell. M., Snipe. J. R.. Wills. M. R.. Czaban.J . D..ANALYTICAL PROCEEDINGS. JANUARY 1989. VOL 26 11 Coffe!. K. F.. and O'Connell. K. M.. And. Cliim. A m . 1986. 180. 99. 2. Oesch. U.. Ammann. D.. and Simon. W.. Cliti. Cheni.. 1986. 32. 1448. Kimura. K.. and Shono. T.. And. Cherii. S ) * t i i p . Ser.. 1985. 22. 155. 2 . 6 . 3 . Worth. H. G . J . . Ancily.sr. 1988. 113. 373. 7. 1. Mallinson. P. R.. and Truter. M. R.. J . Clietn. SOC., Perkiri Trrrns. 2, 1973. 1818. Craggs. A . . Moody. G. J.. and Thomas. J . D. R.. J . Chem. Editc.. 1973, 51. 541. Alegret. S.. Alonso. J . . Bartroli. J.. Lima, J . 1. F. C.. Machado. A. A. S. C.. and Paulis. J . M.. Atid. Leu.. 19x5. IS. 2391. Spectrophotometric Determination of Manganese in Steel After Sol id- p hase Extraction of Tri met h y lene bis( trip hen y I p hosp hon i u m ) Permanganate with Microcrystalline Naphthalene D.Thorburn Burns, D. Chimpalee and N. Chimpalee Department of Analytical Chemistry, The Queen's University of Belfast, Belfast BT9 5AG The permanganate ion forms readily liquid - liquid extractable ion pairs with onium cations that can be used in the spectrophotometric determination of manganese. 1 The first solid - liquid extraction of permanganate using microcrystalline naphthalene is now reported. based on an earlier liquid - liquid extraction using the ethylenebis(tripheny1phosphonium) cation.: The interferences and calibration range were found to be similar for both systems. The optimised procedure for the analysis of steel samples is as follows. Procedure for Steel Samples Reagent Solutions Siilphiiric ncid - phosphoric acid solittion Prepare by mixing 150 ml of concentrated sulphuric acid with 150 ml of 85% orthophosphoric acid and carefully adding the mixture to 600 ml of water.diluting to 1 1 with distilled water. Potossiuni periodate solutiori. 5 Yo m/V Prepare by dissolving 25 g of potassium periodate in a mixture of 300 ml of water and 100 ml of concentrated nitric acid, gently warming to aid dissolution, cooling and diluting to 500 ml with distilled water. p H 6 buffer Prepare by mixing 61.5 ml of 0 . 2 ~ disodium hydrogen phosphate with 438.5 ml of 0.2 M sodium dihydrogen phosphate and diluting to 1 1 with distilled water. Procedure For samples containing 0.2-2.0% of manganese, dissolve accurately weighed 0.3-g samples in 35 ml of the sulphuric acid - phosphoric acid mixture in 250-ml conical flasks.Oxidise with concentrated nitric acid (2 ml) and boil to expel nitrous fumes. If any carbides remain. evaporate the solution to fumes and cool. Add 50 ml of water and warm to dissohre the soluble salts. Cool the flask and, if necessary. filter the contents into a 250-ml beaker. Dilute to 70 ml with distilled water and add 10 ml of concentrated nitric acid. Boil the mixture for 2 min, add 10 ml of 5% potassium periodate solution and boil for a further 4 min. Cool. transfer into a 100-ml calibrated flask and dilute to volume with distilled water.3 Place a 3-ml aliquot of this solution in a 50-ml beaker and add 5 ml of 0.5% potassium periodate solution and 5 ml of 10(% ni/V ammonium fluoride solution. Adjust the pH to 6.0 by careful addition of 2~ ammonia solution or 2~ hydrochloric acid with stirring.Transfer the solution to a ground-glass stoppered conical flask and add 5 ml of pH 6 buffer solution and 5 ml of 1 YO m/V trimethylenebis( triphenylphosphonium) bromide solution. Swirl to mix. add 1 ml of 20% naphthalene in acetone solution and shake vigorously for 30 s. Filter the separated pink solid through a sintered-glass filter (porosity No. 3). Wash with water and drain or suck dry. Dissolve the solid in chloroform and dilute to volume in a 10-ml calibrated flask. Remove any residual water by addition of 0.2 g of anhydrous sodium sulphate and cover the flask with aluminium foil to protect the contents from daylight. Measure the absorbance of the final solution at 538 nm immediately if the steel contains large amounts of chromium.In the absence of chromium extracts are stable for at least 2 h . Table 1. Determination of manganese in certified steels Manganese content. "/o mlm BCS steel Found? No. Certified' 1 101 1 0.22 (021-0.23) 0.210 ? 0.01 1 4571 1 0.30 (0.29-0.30) 0.295 * 0.009 485 0.50 (0.49-0.53) 0.490 k 0.030 2561 1 1.02 ( 1.00-1.03) 1 .oo ? 0.03 214'2 1.61 ( 1 .59-1.62) 1.60 ? 0.03 456 0.17 (0.174.18) 0.163 k 0.010 46W 1 0.67 (0.6&0.68) 0.671 ? 0.009 * Certified range in parentheses. + Mean k9S0/' confidence limits f o r five replicates. Prepare a calibration graph over the range &120 ug of manganese using 0.3 g of high-purity iron with aliquots of standard manganese( 11) solution and proceeding as for the steel samples.Results The results (Table 1 ) for the determination of manganese in a range of British Chemical Standard steels were in good agreement with the certificate values. References 1 . 2. 3. Bowd. A. J . . Thorburn Burn\. D.. and Fogg. A. G.. Tularirri. 1969. 16. 719. Thorburn Burns. D.. and Chimpalee. D.. AnfJI. Chirn. Acru. 19x7. 199, 241. Analytical Panel. *'Methods of Chemical Analysis of I r o n and Steel." British Steel Corporation. Shcffield. 197.1. pp. 82-83.12 ANALYTICAL PROCEEDINGS, JANUARY 1989. VOL 26 Studies on Two Epoxyoctacosahydro[ 12]cyclacene Derivatives as Sensor Coatings on Quartz Piezoelectric Crystals for Detecting Aromatic Vapours M. A. F. Elmosalamy, G. J. Moodyand J. D. R. Thomas School of Chemistry and Applied Chemistry, University of Wales College of Cardiff, P.O.Box 972, Cardiff CF7 3TB F. A. Kohnke" and J. F. Stoddart Department of Chemistry, The University, Sheffield S3 7HF Coated piezoelectric quartz crystals have recently been devel- oped'-4 into a sensitive and selective technique for detecting various air and gaseous pollutants. The basis of the method has been ascribed to the relationship between the mass of coating materials deposited on the crystal surface and the change in frequency according to Sauerbrey's basic equationW A F = -2.3 X 10hPAMIA . . . . (1) where A F is the frequency change (Hz), F is the basic frequency (MHz) of the quartz plate, AM is the mass (g) of the deposited material and A is the area (cm') coated. Equation (1) can be simplified to A F = KC .. . . . . * * (2) where C is the concentration (e.g., mg m-3) of sample gas and K is a constant which refers to the basic frequency of the quartz plate, the area coated and a factor to convert the mass (g) of injected gas data.7.x However, the observed decreases in frequency on applying the logarithmic form of equation (2) yield slopes of less than 1 for plots of logAF versiis log C, that is, the sensing ranges extend to greater dilutions than indicated by equation ( l ) . 7 . 8 Nevertheless, the log - log slopes are increased by attentions to the sampling procedure, but they still fall short of 1. The piezoelectric quartz crystal detection method for gases and vapours has been explored for aromatic hydrocarbons. Thus, a mixture of nujol with trans-chlorocarbonyl bis( tri- phenylphosphine)iridium( I) [rrans-IrCl( CO)( PPh3)?] was used as a detector of aromatic hydrocarbons by Karmarker and "Present address: Dipartimento di Chimica Organica e Biologica.Universita di Messina. Salita Sperone. 31. 98166 Messina. Italy. Guilbault.9 This coating was found to be reactive towards aromatic hydrocarbons but not as sensitive towards aliphatic materials. Toluene in ambient air was monitored using a Carbowax 550-coated crystall" installed in a portable device. This coating gave a linear response over a long range with a reproducibility of better than 4%. No interferences were observed from inorganic gases. such as carbon monoxide, 1 2 Fig. 1. tive (1) and tetraepoxy analogue (2) Formulae of hexaepoxyoctacosahydro[ 12lcyclacene deriva- sulphur dioxide, ammonia or nitrogen dioxide, at the level studied.1 0 Organic vapours gave some interference but were insignificant at the 5% VIV level. Carbowax 1000 has been investigated for 2-nitrotoluene. 1 1 Edmonds and West" examined the behaviour of various coatings on a 9-MHz AT-cut quartz crystal towards toluene and chloroform with regard to various parameters and reported that Pluronics 64 (a GLC stationary phase material) is the most sensitive coating for ethylbenzene, 2-methyltoluene and hex- ane. Table 1. Response (AFIHz) of coated quartz crystal to various dilutions of headspace nitrobenzene vapour (sensor 1) AFIHz for nitrobenzene for 2.42 factors of dilution Day 3 7 7 - * * Mean 8 10 15 20 33 25 27 29 32 35 38 58 7 Headspace v a po u r 1 1 1 109 113 111 110 110 90 88 73 80 85 88 81 9( 1 74 70 73 1 x 2.42 59 6( 1 61 60 62 58 52 48 36 43 53 50 46 48 44 40 50 3 x 2.42 38 38 3Y 38 43 31 28 34 '4 25 37 29 37 28 76 '4 35 3 x 2.42 25 '4 26 25 25 20 16 16 19 1Y 20 19 18 18 26 19 -- 97 -- 4 x 2.42 16 16 17 16 17 13 1 1 14 12 12 13 12 13 13 1.3 13 16 5 x 2.43 10 11 9 10 12 10 9 10 10 Y 9 9 8 10 9 9 11ANALYTICAL PROCEEDINGS.JANUARY 1989. VOL 26 13 More recently, in a study of the roles of some chemically modified cyclodextrins. 2.6-per-O-(rerr-butyldimethylsilyl)-~- cyclodextrin (DSKD) was found to be the best sensor for the range from about 80 to about 4 x 105 mg m-3 benzene vapour in air13 with good selectivity, toluene being the most serious interferent .I3 Charcoal, PEG-750. PEG-400 and other materials have been assessed as coatings for the piezoelectric quartz crystal detection of nitrobenzene.IJ The sorbents were tested to fill equilibration for 35 mg m-3 of nitrobenzene.The toxicity of nitrobenzene (the maximum allowable concentration is 5 mg m-3) presents a need for such sensitive detectors. A further prospect for a piezoelectric quartz crystal coating for this purpose is offered by the recently synthesised".lh hexa- epoxyoctacosahydro[ 12lcyclacene derivative (1) which is eval- uated here. Also examined was the tetraepoxy analogue (2) for its prospects as a possible sensor for toluene vapour. The structures of 1 and 2 are shown in Fig. 1. Experimental The apparatus and detector cell used are as described previously.8 measurements being made at 25 ? 0.1 "C.The AT-cut quartz crystal with gold-plated electrodes on each side (Quartz Crystal, Wellington Crescent, New Malden, Surrey, but now available from Webster Electronics, Rosemills. Hartbridge. Ilminster. Somerset) had a resonant frequency of 9 MHz. The synthesis of the epoxyoctacosahydro[ 12lcyclacene derivatives 1 and 2 have been described in detail elsewhere. l i . l h Test Samples Nitrobenzene vapour samples were obtained with a previously flushed out 10-cm3 syringe from the headspace of its liquid equilibrated under dry air in a thermostat at 25°C. The concentration was calculated to be 1.84 x 103 mg m-3 from the quoted17 equilibrium vapour pressure of 0.277 mm Hg. However, there are other vapour pressures in the literature. namely. 0.31018 and 0.246 mm€-lg,l(~ corresponding to 2.24 X 103 and 1.63 x 103 mg m-3 of nitrobenzene.respectively. The samples of toluene and other vapours were obtained similarly. 17-19 Serial dilutions of the headspace samples were prepared by the syringe dilution method.9 modified as described pre- vious1y.X 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. Ammonia test samples were taken from the headspace of 2~ ammonia solution as previously described.8 Method of Coating Compound 1 or 2, as appropriate. dissolved in chloroform (0.6% mlV), was brush-coated on the surface of the electrode on each side of the crystal. After drying, the coated crystal was fitted into the detector compartment of the apparatus assembly.8 In each instance, the coating applied caused a decrease of ca.12 kHz in the frequency of the crystal. The sensor compounds could be removed from the crystal by stripping with chloroform and air dried ready for reloading. Operation of Piezoelectric Quartz Crystal Detection Apparatus The responses of coated piezoelectric quartz crystals were tested with nitrobenzene ( 5 cm3 injected at the rate of 8 s cm-?) of six different dilutions of the headspace vapour and the mean decrease in frequency for replicate samples ( n = 3) was calculated. All samples were injected into a carrier air stream provided with a Pitman Instruments Model 7069 air sampler pump and dried by passing through silica gel before reaching the quartz crystal cell.The air stream rate was 20 cm3 min-1. A typical recorder trace (Fig. 2) illustrates the response of this detection system which corresponds to a flow injection analysis procedure. Results and Discussion Sensor 1 Calibration data for many nitrobenzene runs with coating of sensor 1 are shown in Table 1 and Fig. 3. The frequency decreases 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 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). Taking account of the headspace vapour containing Table 2. Response of a piezoelectric quartz crystal (gold electrode) to various vapours for coatings of components 1 and 2 Compound 1 Compound 2 : AFiHz.Response. ARHz No. o f calibrations response. Interferent 10T mg m- 1st coating 2nd coating 1st coating Zndcoating 1st coating Concent ration ' 3 3 - 3 - Toluene . . . . . . 30.0 19 '0 6 L 86 6.6 13 13 6 L 1.5 9 10 6 2-Nitrotoluene . . . . 1.40 48 46 6 0.31 25 23 6 - 0.07 16 15 6 - 0.33 10 30 6 0.07 1 0 11 6 2 3.37 1 1 12 6 - 0.74 8 9 6 - 17 18 5 0.02 - 0.30 10 12 5 L 4.2 x 9 6 - 6 7 A 0.9 - Ammonia . . . . . . 3.90 40 42 3 1 0.48 19 19 3 1 0.06 10 1 1 3 1 3 136 7 - ? - 3-Nitrotoluene . . . . 1.50 38 37 6 - 85 ? - - 7 7 - 7 3 3 - 7 - 3 1 7 - Chlorobenzene . . . . 15.4 16 18 6 L 81 - Bromobenzene . . . . 4.18 30 31 5 L 75 Benzene . . . . . . 19.2 10 12 6 - 125 - - - - Nitrobenzene .. . . 1 .84 111 109 3 3 121 Calculated from vapour pressure data. except for ammonia. which was analysed ;is discussed14 ANALYTICAL PROCEEDINGS. JANUARY 1989. VOL 36 about 1.84 x 1 0 3 mg m-3 of nitrobenzene (calculated for 0.277 mmHg vapour pressure), the decreases in frequency are less than the 136, 53. 86 and 48 Hz observed by Sanchez-Pedreno et a1.14 for 35 mg m-3 of nitrobenzene by charcoal, PEG-400, PEG-750 and Quadrol. respectively. However, these other data14 are for full equilibration, whereas the data presented here are for successive injections of samples. t 8993.120 I i C 0) 3 0- L L 2 11 Hz /- 320 Headspace 1 vapour 1840 mg m - 3 J Time - Fig. 2. using a quartz crystal (gold electrode) coated with compound 1 Typical recorder trace of a calibration of' nitrobenzene vapour Clieriiical interferences Nitrobenzene is a toxic substance and the threshold limit value is 5 mg m-3 (corresponding to 10-6 mol mol-1). Therefore, various organic interferences were tested and the data are summarized in Table 2.Except for 2- and 3-nitrotoluene, there were no significant interferences from a wide range of aromatic vapours, namely. toluene. chlorobenzene. bromobenzene and benzene, but ammonia is an interferent. While the concentration range studied by Sanchez-Pedreno et ~ 1 . 1 4 (10-50 mg m-3) was at the lower end of the range for this work, their turn-round time between samples was long. ranging from 10.2 to 29.2 min, according to sorbent. Also, the sorbents used are not selective, so that much of the sensitivity advantage is lost. Lifetime of the detector The coated quartz crystal detector for nitrobenzene functioned for 9 weeks (Fig.3). although the sensitivity (Table 1) declined. During this time, the coated crystal was also exposed on many occasions to seven different interferences but with no lasting deleterious effect (Table 2). After evaluation for nine weeks. the sensor coating was removed with chloroform and fresh hexaepoxyoctacosahydro- [ 12)cyclacene sensor I applied. The newly loaded sensor behaved similarly towards the various compounds previously investigated (Table 2). Sensor 2 Sensor 2 does not show the degree of selectivity shown by sensor 1 towards nitrobenzene towards any of the aromatic vapours studied (Table 2, last column), and ranks as a more universal type of sensor.However. the response towards 2-nitrotoluene equals, and even slightly excels. that towards nitrobenzene. Mode of Detection X-ray crystallography has shownIi.lh that compound 1 supports a rigid cavity wherein the two benzene rings are parallel with an interplanar ring into the cavity such that it assumes an orthogonal relationship with respect to the two benzene rings. Particularly for a phenyl group carrying an electron-with- drawing substituent. the resulting edge-to-face interaction'(c 2i could be sufficiently stabilising electrostatically to allow nitrobenzene to enter the cavity of 1. Equally well. however. the nitrobenzene could be trapped temporarily within inter- stitial space between the doughnut-shaped molecules. Further experimentation in progress will, it is hoped, resolve this dilemma and provide the basis for explaining the relatiire lack of selectivity exhibited by compound 2.2.0 1.6 - N I Li m 1 a - 1.2 0.8 - 2 - 1 0 Log [nitrobenzene] normalised to [headspace] = 1 Fig. 3. Calibration of a piezoelectric quartz crystal coated with compound 1 for S-cm3 samples of nitrobenzene headspace vapour and successive dilutions ( ~ 3 . 4 3 ) thereof. Time: 0.3: A. 15: C. 25: 0. 39: A. 38: and .. 59 d Conclusion Quartz crystals coated with a hexaepoxyoctacosahydro[ 121- cyclacene derivative (1) facilitate the detection of nitrobenzene vapour. Except for ammonia and 2- and 3-nitrotoluene. interference from some common aromatic compounds is minimal, and the device has a long operational lifetime.Coatings of a corresponding tetraepoxy derkrative (2) are of more universal sensing ability. The authors are grateful for the award of a Leverhulme Research Fellowship (to J.F.S.). The joint research pro- gramme was made possible as a result of generous support from the SERC Chemistry Committee under the auspices of their initiative on chemical sensors. Zagazig University. Egypt. is also thanked for leave of absence and financial assistance (to M. A.F.E. ). References 1. 3. 3 . Hlavay. J . . and Guilbault. G . G.. A t i d Climz.. 1977.49. 1890. Guilbault. G. G.. foii-Selecii\x> Electrode Re\. .. 19x0. 2. 3 . Guilbault. G . G.. and Ngeh-Ngwainbi. J . . in Guilbault. G. G.. and Mascini. M.. Editors. "Analytical Uses of Immobilised Biological Compounds for Detection.Medical and Industrial Uses." NATO AS1 Series. Series C : Mathematical and Physical Sciences. Vol. 336. Reidel. Lancaster. 198X. p. 187. Alder. J . F.. and McCallum. J . J . . Atzulj-st. 1983. 108. 1169. Sauerbrey. G. Z.. Z . P l i j ~ . . 1959. 155. 306. Sauerbrey, G. Z . , Z . P1ij.s.. 1964. 178. 457. 4. 5 . 6 .ANALYTICAL PROCEEDINGS, JANUARY 1989. VOL 26 1s 7. 8. 9. 10. 11. 12. 13. 11. 15. 16. Beitnes. H.. and Schrsder. K.. A n d . Chirn. Acrri. 1984. 158. 57. Lai. C. S . - I . . Moody. G. J . . and Thoma\. J . D. R.. A n a l w . 1986. 111. 51 1 . Karmarkar. K. H.. and Guilbault. G . G.. Etzl-iron. Lett.. 1975. 10. 237. Ho. M. H.. Guilbault. G. G.. and Reitz. B.. Anal. Chetn.. 1980. 52. 1489. Tomita. Y.. Ho. M. H.. and Guilbault. G. G.. Anal.Chetn., 1979. 51. 1475. Edmonds. T. E.. and West. T. S . . Anal. Chini. Acra. 1980. 117. 147. Lai. C. S.-I.. Moody. G . J . . Thomas. J . D. R.. Mulligan. D. C., Stoddart. J . F.. and Zarzycki. R.. J . Chem. Soc., Perkiri Trans. 2. 1980. 319. Sanchez-Pedreno. J . A. 0.. Drew. P. K. P.. and Alder, J . F . . Anal. Chim. A m . 1986. 182. 285. Kohnke. F. H.. Slawin. A. M. Z.. Stoddart. J . F.. and Williams. D. J.. Ange14,. Chem.. In[. Ed. Engl.. 1987, 26. 897. Ashton. P. R.. Isaacs, N . S.. Kohnke. F. H.. Slawin, A. M. Z . . Spencer. C. M.. Stoddart. J . F.. and Williams. D. J . , Hrl\j. Chim. Acfa, in preparation. 17. 18. 19. 20. 21. 22. 23. 21. 25. Kirk. R. E . . and Othrner. D. F.. "Encyclopedia of Chemical Technology." Volume 21. Third Edition. Wiley. New York and Chichester, 1983, p.387. West, R. C., and Astle. M. J . , Edirors, "Handbook of Chemistry and Physics," Sixty-fifth Edition. CRC Press, Boca Raton, FL, 1985. pp. D204 and D206. Dean. J . A.. Ediror. "Lange's Handbook of Chemistry." Twelfth Edition, McGraw-Hill. New York. 1979, pp. l(k-52. Gould. R . 0.. Gray. A. M., Taylor. P.. and Walkinshaw. M. D.. 1. Am. Chem. Soc.. 1985. 107, 5921. Burley. S. K.. and Petsko. G. A.. Science. 1985. 229. 2 3 . Burley, S . K . . and Petsko. G . A.. J . Am. Chem. Soc.. 1986. 108. 799s. Slawin. A. M. Z . . Spencer. N.. Stoddart, J . F . . and Williams. D. J . , J . Chem. Soc., Chem. Cornmun.. 1987. 1070. Alston. D. R.. Slawin, A. M. Z . . Stoddart. J . F., Williams. D. J . , and Zarzycki. R., Angerr,. Chern., Inr. Ed. Engl.. 1987.26. 693. Moody, G. J . , Owusu. R. K., Slawin, A. M. Z . . Spencer. N . . Stoddart. J . F.. Thomas. J . D. R.. and Williams, D. J . , Angerr.. Chern., lnt. Ed. Engl.. 1971. 26. 390. Some Analytical Applications of Polymer Modified Electrodes Mary Meaney, Johannes G. Vos and Malcolm R. Smyth School of Chemical Sciences, NlHE Dublin, Glasnevin, Dublin 9, Ireland Gordon G. Wallace Department of Chemistry, University of Wollongong, Wollongong, New South Wales 2500, Australia The design of chemically modified electrodes (CMEs) for electroanalysis has been the subject of much research in the last few years.1-1') In attempts to enhance the sensitivity of voltammetric methods. the use of CMEs has three major advantages. The first is that the coating employed can be used to provide a surface that is capable of efficient pre-concentra- tion.The second advantage can be seen in terms of the facilitation of faster electron transfer reactions and the third in better surface protection, resulting in decreased memory effects. A variety of different approaches to electrode modifi- cation have been reported. One of the first approaches was introduced by Brown er ul. .? whose method involved modification of an electrode surface through the irreversible adsorption of selected aromatic hydrocarbons. Using this approach, Landum er ul. 3 described the electrochemical characteristics of methyl viologen on gold. whereas Davis and Murray4 studied the behaviour of iron porphyrins on Sn02. A second approach has been to modify the electrode surface through the covalent attachment of electroactive groups via silanisation reactions on oxide surfaces.Hence Abruna et ul. have described the use of trichlorosilane for this purpose, whereas Wrighton and co-workers have used trichlorosilyl- ferrocene to modify p1atinum.h gold7 and germanium8 elec- trodes. In our laboratories. the main interest in this area of research has been to investigate the use of polymer modified electrodes for analytical purposes.y-12 The electrodes can be coated either with electrochemically generated polymer layers, such as polypyrrole,Y.l3 or with chemically generated redox polymers. such as those containing ruthenium( 11) - ruthenium(II1) active sites.ll.12.14 The former type of polymer modified electrode has been shown to have the capability to pre-concentrate analytes at the electrode surface, whereas the use of ruthenium- containing polymers has been found to enhance the electron transfer characteristics of some sluggish electrode processes.This enhancement is thought to be due to the electrocatalytic properties of the ruthenium sites in the polymer, through which rapid charge transfer can occur, possibly via a charge hopping Drocess. These redox polymers (dissolved in an appropriate low- boiling organic solvent) are usually applied directly on to the electrode surface using a pipette and the solvent is allowed to evaporate. Other methods of introducing ruthenium on to an electrode surface have been reported based on electrostatic interactions between polyelectrolytes,15 incorporation of ruthenium oxide in polypyrrole,lh formation of a bilayer electrode such as platinum - ruthenium polymer - polypyrrole17 and attachment of ruthenium(I1) to amino functional groups on graphite electrode surfaces.18 In a stationary solution, we have shown that by using an [ Ru( bpy)?( PVP)5Cl]Cl modified electrode (bpy = bipyridyl; PVP = polyvinylpyridine), it is possible to increase the sensitivity in the determination of analytes such as Fell, nitrite and nickel bis(2-hydroxyethy1)dithiocarbamate [Ni(HDTC)?] by a factor of at least two compared with an uncoated glassy carbon electrode.'(' Chemical modification of this kind also resulted in better reproducibility, as no adsorption due to the oxidation of HDTC-, for instance, was apparent. However. the major drawback of using electrocatalytic polymers in this way is that all the analytes give rise to responses that are at and above the level of the background signal owing to the oxidation of Ru" to RuIII.This problem can be overcome if the analysis is carried out in a flowing solution, because if the electrode is held at a fixed potential, the signal due to the Ru" - RuI" couple in the polymer layer then becomes part of the background signal. We have demonstrated using flow injection analysis (FIA) that the use of such a polymer modified electrode (incorporated as the working electrode in an electrochemical detector) can result in a 3-6 fold increase in the peak height for selected metal - HDTC complexes compared with an uncoated glassy carbon electrode. 19 However, problems were encountered due to loss of the polymer layer on continuous use. This loss can be attributed to the fact that the polymer layer is only weakly bound to the surface of the carbon substrate by chemisorption. We have therefore investigated several means of improving the stability of the polymer layer, which have involved treatment with ultraviolet light to induce greater cross-linking of the polymer layer, and formation of bilayer electrodes incorporat-16 ANALYTICAL PROCEEDINGS. JANUARY 1989. VOL 26 ing the ruthenium polymer coated with either a conductive polymer [such as poly(3-methylthiophene)] or a non-conduc- tive polymer [such as poly(N-ethyItyramine)].2(’ These stabil- isation procedures resulted in a slight loss of sensitivity, but produced electrodes that had half-lives more than six times longer than those of the non-stabilised electrodes. It is anticipated that these polymer modified electrodes will eventually find further use in the quantification of inorganic species using FIA and HPLC techniques. 1. 2. 3. 4. 5 . 6. 7. References Wallace. G . G.. in Edrnonds. T. E . , Editor, “Chemical Sensors” Blackie, Glasgow and London, 1988, pp. 132-154. Brown, A. P.. Koval, C.. and Anson. F. C.. J. Electroanal. Chem., 1976. 72. 379. Landum. H . L., Salmon, R. T., and Hawkridge. F. M..J. Am. Chem. Soc.. 1977, 99. 3154. Davis, D. G., and Murray, R. W., Anal. Chem.. 1977.49, 194. Abruna. H. D.. Meyer. T. J.. and Murray, R. W.. Inorg. Chem.. 1970, 18. 3233. Wrighton. M. S . , Austin, R . G., Bocarsly, A. B., Bolks, J . M.. Haas, 0.. Legg, K. D., Nadjo. L.. and Palezzolto, M. C.. J . Electroanal. Chem.. 1978, 87, 429. Wrighton, M. S . . Palezzolto. M. C.. Bocarsly, A. B.. Fischer. A. B.. and Nadjo. L.. J . Am. Chem. SOC.. 1978, 100. 7262. 8. 9. 10. Bolts, J . M., and Wrighton. M. S . , J . Am. Chem. SOC.. 1978. 100. 5257. O’Riordan, D. M. T., and Wallace. G. G., Anal. Chem.. 1986, 58, 128. Irnisides, M. D.. O’Riordan. D. M. T., and Wallace, G. G.. in Smyth, M. R.. and Vos. J. G.. Editors. ”Electrochemistry. Sensors and Analysis.“ Elsevier. Amsterdam, 1986. pp. 293-302. Haas. 0.. and Vos. J . G., J. Electroanal. Chem.. 1980. 113. 139. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. Geraty, S. M.. Arrigan. D. W. M., and Vos. J . G., in Smyth. M. R . , and Vos. J . G.. Editors. “Electrochemistry. Sensors and Analysis,” Elsevier, Amsterdam. 1986, pp. 303-308. Bull, R. A., Fan, F. R.. and Bard. A. J . . J . Electrochem. Soc.. 1982, 129. 1009. Andrieux, C. P., Haas, 0.. and Saveant, J. M., J. Am. Chem. SOC., 1986, 108, 8175. Oyarna, N.. and Anson, F. C.. Anal. Chem., 1980. 52. 1192. Noufi. R., J. Electrochem. SOC.. 1983. 130. 2126. Murao, K.. and Suzuki, K., Polym. Pre.. Am. Chem. SOC. Div. Polym. Chem., 1984, 25. 260. Oyama. N.. Brown, A. P.. and Anson. F. C.. J. Electroanal. Chem., 1978. 87. 435. Barisci. N.. Wilke, E . . Wallace. G. G.. Meaney, M.. Smyth, M. R . . and Vos, J . G.. Elecrroanal~sis, submitted for publication. Meaney. M.. Smyth, M. R.. Vos, J . G.. and Wallace. G. G.. Electroanalysis, in the press.