ISSN:0003-2654    

DOI:10.1039/AN98308FX037

出版商:RSC    

年代:1983

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN98308BX039

出版商:RSC    

年代:1983

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN98308FP101

出版商:RSC    

年代:1983

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN98308BP105

出版商:RSC    

年代:1983

数据来源: RSC

摘要:

OCTOBER 1983 Vol. 108 No. 1291 Piezoelectric Crystals for Mass and Chemical Measurements A Review John F. Alder and John J. McCallum Department of Instrumentation and Analytical Science University of Manchester Institute of Science and Technology P.O. Box 88 Manchestev M60 lQD Summary of Contents Introduction Historical Theory Quartz crystal microbalance Adsorption desorption and decomposition studies Aerosols and suspended particles Electrogravimetric analysis Piezoelectric crystal detectors for chromatography Gas detection Ammonia and other nitrogen compounds Aromatics Dissolved carbon dioxide Carbon monoxide Cyanide Explosives Hydrocarbons and halogenated hydrocarbons Hydrogen Hydrogen chloride Hydrogen sulphide Mercury Methane and other hydrocarbons Organophosphorus compounds Sulphur dioxide Toluene diisoc yana t e Water Bacterial and fungal growth Solution property measurement Trace metal studies Thermal analysis Conclusions Keywords Review ; fiiezoelectric crystals Introduction In the past 100 years or so piezoelectricity has gone from a scientific curiosity to a widely exploited phenomenon with applications in both science and engineering.Since 1977 Guilbault and co-workers have published four reviews on piezoelectric crystals applied to chemical analysis two of which are essentially reviews of his group’s work,lP2 the other two being more comprehensive containing 673 and 44 references4 Bearing in mind the previous reviews we have tried to view the subject from a slightly different aspect bringing out what we consider to be the important practical aspects of the use of piezoelectric crystals in mass and chemical measurements.116 1170 ALDER AND MCCALLUM PIEZOELECTRIC CRYSTALS Analyst vd. 108 Historical Cady,5 in his definitive work on piezoelectrics relates that Coulomb was the first to conjec-ture the possible production of electricity by the application of pressure on a suitable material. Hauy and later BecquereP performed some experiments in which certain crystals showed electrical effects when compressed. Their findings reported positive results with non-piezoelectric crystals such as calcite and led to the conclusion that what they had observed was “contact electricity.” The credit of being first to observe the phenomenon of piezoelectricity falls to the Curie brothers Pierre and Jacques in 1880.’ They showed that when some crystals were compressed in particular directions an electrical potential was produced between the deformed surfaces, this potential being proportional to the applied pressure.The converse effect unforeseen by the Curies was predicted by Lippmann.8 By the end of 1881 the Curies had verified this effect and showed that the piezoelectric coefficient of quartz had the same value for the direct and converse effects. LangevinQ employed quartz plates to serve as emitters and receivers of high-frequency waves under water and this led to the development of sonar. Cady,lo*ll Pierce12*13 and others produced crystal-controlled oscillators with high stability for use as tuning devices and crystal filters for com-munication lines and radio This technology is still in wide use today and is the reason for the availability of cheap very high quality crystals for analytical work.It was found that by altering the cut angle of the quartz crystal with respect to its optical axis the temperature coefficient of the crystal could be changed. This discovery led to a wide range of crystal cuts (the “T” series of Y-cuts) that gave rise to crystals with properties suitable for many applications. Piezoelectricity remained a curiosity until the start of the First World War. Theory Crystals possessing the property of piezoelectricity can be predicted from crystallographic studies. Piezoelectricity occurs in crystals that do not possess a centre of symmetry of which there are 21 cla~ses.1~ The piezoelectric effect arises when pressure on a dielectric material deforms the crystal lattice and causes a separation of the centres of gravity of oppositely charged species which gives rise to a dipole moment in each molecule.Early mathematical treatment of this effect can be found in works by Cadys and V ~ i g t . l ~ ~ l ~ If electrodes are applied to the faces of a thin slab or rod of this material and an external current sensing circuit is connected a current will be seen to flow through the external circuit when stress is applied to the crystal. Releasing the stress causes a transient current flow in the opposite direction. If the converse effect is used and an alternating potential difference applied mechanical oscillations occur within the crystal lattice.Stable oscillations only occur at the natural resonant frequency of the crystal and at that frequency the crystal presents a low impedance to the exciting voltage. If the crystal is incorporated into the feedback loop of an oscillating circuit it becomes the frequency determining element of the circuit as its Q (quality factor) is very high typically several thousand. Quartz crystals used commercially are usually made from plates that were cut from a single crystal at specific angles to the principal optical axis. These blanks are then ground to specification by etching and lapping techniques. The blanks are chosen to contain no left-handed or electrically twinned material. The first oscillators employed crystals cut in the y - z plane with the electric field applied along the x-axis as the frequency determining element.This resulted in longitudinal vibra-tion along the y-axis and consequently length was the frequency-determining parameter and meant that high frequencies could not be obtained as the lengths required were too small to be practical. Initial work with thickness as the frequency determining dimension was plagued by inter-ference from harmonics and overtones. To overcome this Y-cut crystals vibrating in the shear mode were emp1oyed.l’ These crystals gave good results with few interfering vibration modes but were very sensitive to external temperature variations. Lack et aZ.18 showed that the temperature coefficient and frequency constant varied with the angle of rotation.Two angles were shown to have zero temperature coefficients +35”15’ (AT) and -49’00’ (BT) October 1983 FOR MASS AND CHEMICAL MEASUREMENTS. A REVIEW 1171 For the majority of piezoelectric work in the analytical chemistry field AT-cut crystals have been used. Literature covering the response shape ageing and performance of these crystals is outside the scope of this work but may be found in the electrical engineering literat~re.19-~4 I t has been known that by adding mass to the surface of the crystal one obtains a lower oscillation frequency; indeed this effect has always been used by crystal manufacturers to adjust a crystal to the desired frequency. Sauerbre~~59~~ utilised this idea as the basis of a sensitive microbalance. He derived an expression relating the change in frquency to the mass of material deposited.This expression only applies to AT-cut crystals vibrating in the thickness shear mode the electric field being applied along they-axis. The frequency ( F ) can be expressed as F = - = - v t r N 2t t where Vtr is the velocity of propagation of a transverse wave in the plane of the crystal t is the thickness and N is the frequency constant. With suitable mathematical manipulations one obtains the equation in the form where M is the mass of the plate p is its density and A is the surface area. Sauerbrey assumed that if the plate was divided into an infinite number of parallel planes along the x - z axis only those planes close to the surface would affect the frequency through their mass and not through their elastic character.The frequency change resulting from the deposition of a thin uniform film of any foreign substance is equivalent to that of a layer of quartz of the same mass. Sauerbrey developed the above equation to give a more general form : For pure shear-mode vibrations the strains are all zero at the principal faces. d F dM,F -=-F APN where dM is the mass of the deposited material. Substituting for the various constants gives for quartz where d F is the change in frequency due to the coating (Hz) F is the frequency of the plate (MHz) dM is the mass of coating deposited (g) and A is the coated area (cm2). From this, one can predict a mass sensitivity of 10-9 g Hz-l for a 10-MHz crystal. Lostis2' and later Stockbridge28 also treated the piezoelectric effect mathematically with similar results.Quartz Crystal Microbalance Oberg and L i g e n s j ~ ~ ~ used thickness shear mode AT-cut 3.5-MHz crystals as a film thick-ness monitor claiming a 1 Hz A-1 thickness increment and a total thickness increment of 2 pm. Warner and Stockbridge30 used 2.5-MHz fifth-overtone and 5-MHz fundamental crystals to measure small masses of material adsorbing on to surfaces in a vacuum system. At 2.5 MHz they claimed practical sensitivities of &lo pg cm-2 with temperature stabilisa-tion to *0.01 "C. The effect of mass on the resonance frequency was examined by Stockbridge.28 The thick-ness shear AT-mode was considered in detail and a perturbation analysis was used to derive the relationship between the frequency shift AFIF and the mass added uniformly over the electrode(s) AMIA.In the context of biomedical applications of piezoelectric devices King31 suggested inserting a vibrating crystal into the biological fluid to be examined and allowing some material t 1172 ALDER AND MCCALLUM PIEZOELECTRIC CRYSTALS Analyst VoZ. 108 adsorb on to a coating on the resonator; a low-frequency vibrating crystal was suggested for use with liquids. Richard~on,~~ reviewing King’s paper,31 noted that this was a good idea in theory but for practical purposes difficult to implement for biological systems. The effects of stressing materials on a quartz crystal microbalance were examined by Ullevig et aZ.33 They found that the radial sensitivity function of a quartz microbalance was altered by the presence of a heavily stressed film on one face of the crystal.The sensitivity of the centre of the crystal was found to increase drastically while the peripheral area became insensitive to mass changes.26 The over-all (integral) sensitivity of the crystals to a mass change was shown to be unchanged. They emphasised the need for a uniform layer over the entire active area of the crystal for meaningful data. Adsorption Desorption and Decomposition Studies Haller and White34 investigated the kinetics of formation of a butadiene polymer film exposed to 250-eV electrons at pressures between 3 x lob4 and Torr these being the conditions occurring in electron microscopes. They reported that the rate of growth of the film was pro-portional to the square root of the current density.The rate was found to increase with in-creasing pressure becoming independent at higher values. The oxidation stability of elastomers was studied by Fischer and King.35 Thin films of the elastomers 2 4 p m thick (25pg) were deposited on the surface of 9-MHz quartz crystals. Changes in the mass of the rubber due to oxidation at 150 “C were detected and recorded as frequency changes over test times of 0.5-3.0 h. The effects on the oxidation of the elastomer with respect to filler type vulcanisation inhibitors temperature and ultraviolet radiation were explored and reported. Oxygen absorption and volatility properties of submicron films of asphalt were examined by King and Corbett .36 They showed that polar aromatics absorbed oxygen very quickly, whereas asphaltenes absorb at a slow but prolonged rate.Saturates and naphthene aromatics showed virtually no absorption of oxygen but were found to be volatile. These findings were consistent with studies using other techniques. H~sseiny~~ produced adsorption isotherms for various gases (carbon monoxide carbon dioxide oxygen and ethylene) on various manganese - phosphorus compounds coated on quartz piezoelectric crystals. The ability to make these sensitive mass measurements in enclosed spaces on a microscale is one of the great attractions of the quartz crystal micro-balance. The current interest in thin-film technology for a wide variety of applications offers great potential for the exploitation of this ability in studying simple chemical systems. Aerosols and Suspended Particles The detection quantification and identification of the components of aerosols and suspended particles in air streams have always been of some importance with respect to health and safety.Indeed as the properties of aerosols have become better understood and their role in industrial toxicology better appreciated this importance is growing. Chuan3* measured the mass of airborne particles (“micro-grit” aluminium oxide) impinging on a 10-MHz crystal coated with an adhesive layer. The device was reported to yield particu-late mass concentrations down to 1 pg m-3 and could resolve individual particulates to a mass of 10-l1g. The adhesion of smaller particles was measured by integrating the frequency change. Chuan estimated a detection limit of 0.1 pg m-3 down to a minimum dimension of 1 pm although “implicit in this estimate is the assumption that the impaction efficiency is unity for all particles captured by the crystal an assumption that is not valid for particles less than about 1 pm size.” The author used temperature compensation via a second crystal not exposed to the aerosol stream and having the same temperature coefficient.We have found in this laboratory that such temperature compensation is not easy over a wide temperature range at the level of precision required for very precise work. Careful ageing and matching of the crystal tempera-ture coefficients is required if the compensation circuit is to be kept simple; otherwise tempera-ture measurement and calculated corrections will need to be employed.That is not a serious problem but will increase the complexity of data recovery systems. Olin and co-workers39@ used an electrostatic precipitator in conjunction with the quartz crystals claiming that the aerosol concentration could be measured with 5% accuracy in abou October 1983 FOR MASS AND CHEMICAL MEASUREMENTS. A REVIEW 1173 10 s. These workers examined car particulate emissions cigarette smoke and laboratory and office aerosols. In a comprehensive and detailed paper,39 they reported mass sensitivities to tobacco smoke of 15.5 and 179 Hz pg-l for 1.5- and 5.0-MHz AT-cut fundamental-mode crystals close to the theoretically predicted sensitivities ; the aerosol average concentration ranged between 7 and 14 pg m-3. Both the 1.5- and 5.0-MHz crystals stopped oscillating when the mass loading reached about 400 pg corresponding to about 6 and 60 kHz frequency change respectively.The frequency change was only slightly non-linear with added mass over that range right up to the point of cessation of oscillation. Using a 1.6-MHz AT-cut crystalq0 they claimed a sensitivity of 5.5 Hz pg-l for tobacco smoke and room aerosol. A piezoelectric device was produced to measure the size concentration of particles produced in the cloud generated by an underground nuclear explosion with the formation of a crater.41 Measurable signals were obtained from 5-pm particles having masses of the order of 3 x 10-10 g ejected a t velocities of about 210 m s-l. An instrument for the determination of the particle size distribution of aerosols was produced by Carpenter and Bren~hley.~~ Particles were deposited on to 10-MHz AT-cut quartz crystals by inertial impaction.The impactor comprised of four stages with cut-off sizes for unit density particles of 2.5 6.3 12.6 and 18.9pm. A flow-rate of 0.51 min-l was used and mass sensitivities of between 140 and 2303 599 Hz pg-l were obtained. The theoretical value from Sauerbrey’s equation was 923 Hz pg-l. The performance of quartz crystals to determine aerosol mass concentration was investigated by Daley and Lundgremq3 They examined the influence of temperature humidity particle collection characteristics response linearity and mass sensitivity. They reported that the mass-sensing ability decreased for particle sizes beginning at ca. 2 pm diameter and reached essentially zero at 20 pm.Sensitiv-ity in the 2-20 pm range could be improved by use of a viscous coating. These workers give a refreshingly detailed consideration of the effects of relative humidity. Using 10-MHz AT-cut gold electrode crystals coated with silicone grease the maximum slope of the response curve [frequency change veysws relative humidity (RH) of a clean humid atmosphere] was -0.2 Hz (yo RH)-l. Uncoated 5-MHz AT-cut platinum electrode crystals showed variable response between +5 and -5Hz over the range 12-80% RH. They also measured the effect of relative humidity on crystals that had aerosol deposited on them. Some materials gave a gradual increase over the range 5-90yo RH [room aerosol road dust calcium sulphate, uranine (a dye) methylene blue - uranine mixture] whereas sodium chloride and ammonium sulphate gave little or no response ta humidity changes up to 60% RH (sodium chloride) and 70% (ammonium sulphate) and then responded catastrophically at RH values above this, obviously by uptake of water.A coating of dimethylpolysiloxane took up less than 3% of the coating mass from atmospheres ranging from 5 to 85% RH. These humidity response characteristics little discussed in the gas-sensing literature are of considerable importance in the design of piezoelectric crystal detectors. Sem and Tsur~bayashi~~ used an electrostatic precipitator to deposit dust particles on to a piezoelectric microbalance sensor. A portable system (commercially available) developed is reported to be able to measure mass concentrations in the 100 pg m-2 range of airborne dust particles smaller than 10 pm.The major attractions of aerosol measurement using quartz crystal microbalance techniques are well illustrated in the literature and are as follows high potential sensitivity; good agree-ment with theoretical response thus largely obviating the need for continuous mass calibra-tion; wide mass and frequency response range and compatibility with the filtration and precipitation methods employed. The low-cost availability of these precision-engineered components makes them very attractive particularly linked with the practicality of disposal rather than having to clean contaminated sensors. Developments in the electronic data processing and miniaturisation of all the electrical components associated with these measure-ments wilI ensure their continued application to aerosol monitoring.Electragravimetric Analysls AT-cut quartz crystals of 1.65,3,6 and 9 MHz have been used as the cathodes in an electro-chemical ce11.45946 A current was allowed to pass for a known period of time and the crystal was removed from the cell washed and dried. The mass increase was then determined from the change in frequency. Cadmium solutions over the range 5 x t o 5 x M were examined 1174 ALDER AND MCCALLUM PIEZOELECTRIC CRYSTALS Analyst VoZ. 108 Nomura and Mimatsu4' used crystals with silver on platinum-plated gold electrodes to determine the iodide in solution. The iodide was electrodeposited at -0.05 V in a sample solution containing M potassium chloride adjusted to pH 9.8 with M sodium tetra-borate(II1) - sodium hydroxide solution.The reagent blank was passed through the detector cell until constant frequency then the sample solution containing iodide was allowed to pass for 1 min (10-6-10-5 M) or 10 min (10-7-10-6 M). The crystal surface was cleaned of iodide by electrolysis at -0.4 V after each determination. Although electrogravimetry has always played an important role in assay and certain other analytical procedures it is debatable whether the use of quartz crystals provides any major advantage other than the important one of miniaturisation and micro-determination. Coulo-metry and stripping voltammetry equal or better this performance. Piezoelectric Crystal Detectors for Chromatography The quartz piezoelectric crystal microbalance has potential as a detector for chromatography owing to its mass sensitivity simplicity and ruggedness all of which recommend it for portable and on-site remote operation.King,48 in 1964 was the first to use quartz crystals in this fashion. He used gas chromato-graphy (GC) stationary phases as the crystal coating material to improve the selectivity for certain groups of compounds. Com-parative chromatograms of a series of hydrocarbons were produced, Suggested coatings and their use are shown in Table I. TABLE I COATINGS SUGGESTED BY KING48 Coating material Detector characteristic Squalane Silicone oil Apiezon grease Polyethylene glycol Sulpholane Dinonyl phthalate Aldol 40 Tide (alkyl sulphonate) Hydrocarbon detection non-selective to compound type Selective detection of polar molecules such as aromatic, oxygenated and unsaturated compounds Silica gel Water vapour Molecular sieve Alumina Hygroscopic polymers (e.g., natural resins glues cellulose derivatives and synthetic polymers) Lead acetate Hydrogen sulphide Metallic silver Metallic copper Anthraquinonedisulphonic acid A portable room-temperature gas chromatograph was produced by Karasek and Gibbin~.*~ They reported that the instrument response was rapid and proportional to the sample size.The sensitivity was found to be in the parts per million concentration range. Compounds in the boiling range 40-200 "C were separated on columns of less than 50 cm in length with short retention times. They reported that their system could be applied for the separation of com-pounds normally unstable above room temperature.A short review of ambient-temperature gas chromatography using the piezoelectric detector appeared in 1972.50 Karasek and co-workers produced two papers dealing with the performance of the crystal detector. In the first paper,51 the crystal was coated with the same material as the column stationary phase to compensate for any loss of coating from the crystal. They reported per-formance characteristics and minimum detectable concentrations for a number of material October 1983 FOR MASS AND CHEMICAL MEASUREMENTS. A REVIEW 1175 (alkanes aromatic hydrocarbons 2-alkyl ketones and alkyl esters) over the range 2 x to 8 x 10-gg. Their second paper52 examined the effect of temperature on the detector over the range 25-100 "C.They found that the response of a piezoelectric detector for a compound eluted from a GC column was where A is the peak-area response W the total mass of the eluent y the activity coefficient of the eluent in the crystal coating PO the vapour pressure of the eluent at the operating tempera-ture F the carrier gas flow-rate and C a constant characteristic of the detector temperature of the crystal and the liquid phase used to coat the crystal. The authors pointed out the obvious consequence that as the vapour pressure of eluted compounds decreases the detector sensitivity increases and yet an adequate component vapour pressure is a requisite for GC. This trade-off requirement will therefore result in a relative narrowing of the useful range over which GC can be used effectively with adequate detector sensitivity.It is worth mentioning that this is true for a reversible system where the adsorption - desorption is physical in nature (i.e. Van der Waal's forces are predominant). If the adsorption involves chemical reaction sensitivity will be increased for components with higher vapour pressure but at a certain loss of reversibility which is of course unacceptable in a GC application. This problem pervades the whole of coated piezoelectric crystal gas detection and monitoring. The temperature of the detector is also important as one would imagine even though the authors remarked that this effect was found not to be as pronounced as was predicted from theoretical considerations. Although the reason for this was not clear it could be due to the way the effluent gas impinged upon the crystal or to the variation of the response function used to predict behaviour with temperature.Janghorbani and F r e ~ n d ~ ~ in a carefully developed paper described the response charac-teristics of a coated quartz piezoelectric crystal in terms of a partition detector for vapours dissolved in a gas stream. In developing the theory of such a partition detector they refer to chromatography theory and relate the peak-area response of the detector to an imaginary plug of the gas mixture : where A is the area under the peak due to component Y m is the constant describing the frequency change (Af) of the crystal due to an incremental mass addition (AW) to the surface (Af= mAW) and KY,X is the partition coefficient of gas Y in liquid X describing the ratio where Wy,x is the mass of gas Y in a unit volume of crystal coating material X (at equilibrium) and W is the mass of gas Y in a unit volume of gas phase.V is the volume of the liquid coating X present on the efective surface area of the crystal W is the mass of gas contained within the detector volume at equilibrium with the liquid coating and F is the flow-rate of the gas phase. The authors presented data to show good linear relationships between A and injected volume of octane hexane and pentane into a gas stream passing over a squalane-coated crystal to support their predictions. They went on to discuss the theory of response time and made some theoretical predictions, concluding that sub-second response times can be obtained by using small detector volumes.The effect of temperature on sensitivity [S Hz(conc.)-l] was also derived theoretically resulting in an equatio 1176 ALDER AND MCCALLUM PIEZOELECTRIC CRYSTALS Analyst VoZ. 108 S = HT exp A / T where A is a constant in the Clausius - Clapeyron equation : A LnP = B - T where P is the vapour pressure of solute (analyte gas in flow stream) at temperature T B is a constant and where m is defined above R is the gas constant and V is the volume of liquid coating (defined earlier) characterised by density p and relative molecular mass M I in which the analyte gas has an activity coefficient y. These equations are of considerable consequence and it is perhaps regrettable that in the literature the coefficients As the authors mentioned KY,X is directly proportional to the (chromatographic) retention volume for a given solute - substrate partition system.The authors also referred to the obvious interference problems when two solutes are present in the same element of gas stream passing through the detector; interestingly they said that, contrary to theoretical prediction “the frequency response to composite samples is invariably greater than the sum of frequency shifts due to each component when introduced separately.” Edmonds and West54 also considered the coated piezoelectric crystal as a partition detector. Starting with much the same premises as Janghorbani and Freund,S3 they considered the coated crystal exposed to an atmosphere containing the analyte vapour as a “simple static two-phase system in which a series of successive equilibrations take place.” They went on to derive theoretical response curves for the leading edge of the response veysus time curve and showed that the theory closely related to practice thus supporting the findings of the previous who used Fick’s law to derive their relationships.The limiting forms of the response equation by the two sets of workers relating frequency change54 or integrated peak area (frequency change x time)53 to concentration of analyte gas in the vapour phase are the same. Theory was shown to describe practice reasonably for n-alkanes on squalene5* and chloroform on Carbowax 20M and dinonyl phthalate. Edmonds and West went on to illustrate the importance of keeping the detector cell volume low to maximise response speed and also the need to maintain the coating material only on the most sensitive area of the crystal near the centre.26 They also illustrated a very important facet of piezoelectric devices the ease with which multi-component mixtures may in principle, be analysed.Hexane and chloroform possess different response characteristics on squalane and Carbowax coatings. Using this property an unseparated mixture of the two compounds in air was analysed yielding results that were in agreement with the known values. Adsorption measurements using a crystal are more difficult to obtain in a liquid than in the gas phase as there are greater energy losses at the crystal - liquid interface. This makes it more difficult for the crystal to maintain a stable oscillation.Schulz and King55 and Konash and B a ~ t i a a n s ~ ~ described two ways around this problem. Schulz and King55 sprayed the effluent from the liquid chromatograph on to the crystal surface the solvent evaporated and the mass of any residual solute determined from the change in oscillation frequency; a complete cycle took 60 s. Using an ethylene - propylene terpolymer in hexane a sensitivity of 0.2 Hz p.p.m.-1 of solute in hexane was obtained. They chromato-graphed butyl rubber and a mixture of polystyrene by gel permeation. The results obtained by this method compared favourably with those from a refractive index detector and better base-line return was claimed for the crystal. Konash and Bastiaans56 produced a novel cell design in which the crystal used had a resonant frequency of 7 MHz in air at 25 “C and the method of mounting appears to have compensated for the increase in energy loss by allowing the liquid to come into contact with only one face of the crystal.The data they presented are scant and inconclusive and the cause of the observed frequency changes when various solutes are added is far from clear. and m are not better defined in many instances October 1983 FOR MASS AND CHEMICAL MEASUREMENTS. A REVIEW 1177 Gas Detection Kin@’-59 proposed the use of piezoelectric crystals as sorption detectors in which the crystal is coated with a substrate that will react with or adsorb the material of interest. This sorption is a mixture of chemisorption and physisorption.Ammonia and Other Nitrogen Compounds Guilbault and co-workers were able to detect ammonia and nitrogen dioxide in the parts per lo9 (p:p.b.) range using coatings of modified Ucon 75-H-90000 or Ucon LB-300X.60 These materials were activated by passing nitrogen dioxide over the coated crystal surface for about 5 min. Infrared spectra of the exposed coatings indicated that new compounds had been formed that contained covalent nitrite groups and possibly nitroso groups. The coatings also proved to be sensitive to moisture. Ascorbic acid ascorbic acid - silver nitrate and an extract of Capsicum annuurn pods were also tried.61 Three extracts of the pods were obtained in alcohol chloroform and toluene. The chloroform extract gave a frequency change of 45 Hz for 10 p.p.m.of ammonia and 15 Hz for 0.001 p.p.m. whereas the alcohol extract gave 120 and 50 Hz respectively for a 10-ml gas sample. The toluene extract proved to be inactive. Some silver nitrate was added to the alcohol extract and this gave frequency changes of 75 and 15 Hz respectively for 1-ml samples of ammonia. The authors suggested that the active component could have been ascorbic acid and there-fore it was tried as a coating material. This gave as a response a 160-Hz change for 10 p.p.m. and 70 Hz for 0.001 p.p.m. on a 1-ml ammonia sample. The addition of silver nitrate gave 320 and 100 Hz changes respectively and a detection limit of 1 p.p.b. of ammonia was quoted for this coating. Nickel dimethylglyoxime was used as the coating in an attempt to determine the concentra-tion of ammonia in solution.s2 The effects of moisture were minimised by the use of hydro-phobic membranes between the sample solution and the crystal.Alternatively the crystal was allowed to reach equilibrium above a sample of distilled water and then transferred into the sampling chamber. A concentration of 0.15 M of aqueous ammonia was reported to give a frequency change of 135 Hz. The calibration graph was reported to be linear up to 0.45 M ammonia. L-Glutamic acid hydrochloride and pyridoxine hydrochloride were reported to have greater sensitivity and better selectivity to ammonia in ambient air than previously (or subsequently) obtained.63 Response times for both coatings were less than 1 min with complete reversibility of response after 5 min.No significant interferences were reported from other gases as long as “the effect of moisture from air was eliminated using a gas-chromatographic pre-column packed with silica gel. ” “Using pyridoxine hydrochloride as the coating material a very high sensitivity of detection of ammonia was found even in the parts per trillion range. At 1 p.p.m. of ammonia a frequency change of 1190 Hz was observed. At 0.01 p.p.m. of ammonia the frequency change obtained was about 386 Hz.” A plot of log(frequency change) veysus log (concentration) was reported to be linear over the range from 0.01 p.p.b. to 1 p.p.m. of am-monia. I t is not clear how stable 0.01 p.p.b. ammonia solutions in even silica-gel dried air would be and the lack of reproducibility data in this work engenders some scepticism.Whereas the use of dried air to determine equipment response characteristics is acceptable one should beware of trying to extrapolate results so obtained to a constant humidity atmosphere and even more to a real atmosphere where humidity changes between 10 and 95% may occur over very short periods in portable applications. For many analyte gases particularly those susceptible to hydrolysis or those which are polar drying of samples by silica gel or molecular sieves results in major losses of analyte. Edmonds et ~ 1 . ~ ~ reported using polyvinylpyrrolidone as their coating. A frequency change of the order of 700 Hz was noted for 1 p.p.m. of ammonia. Moisture present was monitored using a resistive-type humidity probe and a silver chloride-coated piezoelectric crystal.Gas-phase reactions of mono- di- and trimethylamine with various metal salts using a piezoelectric crystal were reported by Guilbault et a1.G5 The coating materials tested included iron( 111) chloride zinc chloride mercury(I1) bromide cobalt (11) chloride and zinc iodide. These workers found that for all three amines the iron salt was the most sensitive. Isotherms covering the range 20-100 mmHg pressure of the amine were drawn 1178 ALDER AND MCCALLUM PIEZOELECTRIC CRYSTALS Analyst VoZ. 108 Aromatics A Nujol mixture of trans-chlorocarbonylbis(triphenylphosphine)iridium(I) [trans-IrCl(C0) -(PPh,),] was used by Karmarker and Guilbault.66 This coating was found to be reactive to aromatic hydrocarbons but not as sensitive to aliphatics.Hence aromatics such as xylenes, benzaldehyde 1,3,5-trimethylbenzene anisole and butylbenzene could be detected whereas compounds such as hexane heptane octane and cyclohexane could be detected only at high concentrations. The effect of atmospheric moisture was reported to be nil; it is unfortunate that data to show this were not included in the paper. Toluene in ambient air was monitored using a Carbowax 550-coated crystala7 installed in a portable device. This coating gave a linear response over the range 30-300 p.p.m. No inter-ferences were observed from inorganic gases such as carbon monoxide sulphur dioxide, ammonia or nitrogen dioxide at 1000 pap.m. Organic vapours gave some interference but were insignificant at the 5% V/V level. Interference from moisture was eliminated using Nafion tubing (Du Pont Type 811).As the water is adsorbed and permeates across the walls of the tubing it is removed either by a counter-current flow of dry nitrogen sweeping the tubing or by use of a desiccant. Dissolved Carbon Dioxide Fogleman and Shumanaa examined the possibility of continuously monitoring the concentra-tion of carbon dioxide in water. AT-cut 9-or 15-MHz crystals with gold electrodes were coated with didodecylamine (DDDA) and dioctadecylamine (DODA). The coated crystal was separated from the solution by a membrane consisting of Teflon PVC or a Fluoropore filter. They found that DDDA was at least 33 times more sensitive to water than to dry carbon dioxide that 100% sulphur dioxide gave a frequency change of 84 Hz and 100yo RH (25-30 "C) gave a change of 1770 Hz in 90 min and 2736 Hz in 6 h.They reported that exposure to moisture had no effect on the sensitivity of the coating to carbon dioxide. Carbon Monoxide Ho et aLa9 used a quartz piezoelectric crystal to detect carbon monoxide concentrations in the p.p.b. and p.p.m. ranges depending on the sample size. The test sample containing the carbon monoxide was reacted with mercury(I1) oxide at 210 "C to produce mercury vapour. The mercury vapour liberated was adsorbed on the gold electrodes of the crystal (see the section on mercury). A background reference stream was generated by allowing the sample to pass through a column containing granular silver oxide to oxidise the carbon monoxide quantitatively. As there was no carbon monoxide in this stream any response was due to thermal decomposition.This also allowed the removal of any interference effect arising from the presence of hydrogen, as it was included in the decomposition background. Organics that may cause interferences were reported to be removed by either activated charcoal or molecular sieve 5A. Inorganic vapours were reported not to interfere except for sulphur dioxide which was reported to be "easily removed by a molecular sieve pre-column." The effect of moisture was studied. Water vapour was thought to affect the thermal decomposition of mercury(I1) oxide rather than by direct reaction and is known to deactivate silver oxide. These workersag removed moisture by the use of a desiccant. Calcium chloride and phosphorus(V) oxide were found to be most efficient in removing water vapour without affecting the concentration of the carbon monoxide in the air stream.This system would have practical use more as a fixed-site monitor than a portable system owing to the power requirements for the heating element around the mercury(I1) oxide cell. Cyanide Nomura and c o - ~ o r k e r s ~ ~ ~ ~ have studied the use of the piezoelectric crystal to determine cyanide in solution. AT-cut 9-MHz crystals with silver-plated gold electrodes were used. Initially,'O a constant volume of the sample or standard solution was adjusted to pH 9.6 with either a dihydrogen phosphate - borate or borate - hydroxide buffer and kept at 25 "C in a water-bath. The solution was stirred at 430 rev. min-1 and the crystal whose frequency had been previously determined was immersed and left for 15 min.The crystal was remove October 1983 FOR MASS AND CHEMICAL MEASUREMENTS. A REVIEW 1179 from the solution and washed with water then acetone. The crystal was then put into the oscillator set in an air-bath kept at 30 “C. The new frequency was determined after 1 min. A linear range of 10-7-10-5~ was quoted. EDTA was found to mask interference effects completely from cations forming cyanide complexes. Further studies were carried out and a paper appeared in the Japanese literature71 in which the frequency change obtained when the solution was allowed to pass across one side of the crystal was found to depend on density (d g cm-3) and the specific conductivity (K i2-l cm-1) and the following equation was produced : Cyanide solutions over the range 10-6-5 x One drop (5 p1) of the sample solution (adjusted to pH 10.4) was placed on the electrode of a horizontal crystal in an air-bath.72 The frequency was measured just after the drop had been applied and again 4 min later.A linear range of 10-44 x 1 0 - 3 ~ was quoted. The only interferents reported were silver(1) and mercury(I1) in the presence of EDTA. M were determined. Explosives The airborne concentration of explosive materials in production plant and airport surveil-lance is of some importance. Tomita et ~ 1 . ~ ~ used Carbowax 1000-coated 9-MHz crystals to monitor the level of mononitrotoluenes (MNT) in air to which the crystal shows greater sensi-tivity at low concentrations and low sensitivity at high concentrations.A 10-ml volume of air was injected into a nitrogen carrier gas stream flowing at 30 ml min-1. The detector cell was-kept at 50 “C and higher temperatures were reported to decrease the sensitivity markedly. The frequency change due to 7.5 p.p.m. of MNT was 74 Hz at 80 “C and 186 Hz at 50 “C. The only interferences reported were some perfumes and high concentra-tions of organic solvents for chloroform due to “dissolution of the coating.” There was some response to atmospheric humidity. It was assumed however that as the humidity of “room air is relatively constant during measurement this interference can be compensated by using air of the same humidity as the carrier gas. Actually no interference from humidity and no significant change in the sensitivity were observed when room air was used as a carrier gas and also as a diluent for MNT vapour.” It was accepted that less stability in the base line due to humidity changes might become a problem in long-term continuous measurement.Hydrocarbons and Halogenated Hydrocarbons Kindlund and Lundstrom74 used two crystals in their experiments one coated and the other an uncoated reference. The two frequencies around 12 MHz were mixed and the difference frequency was converted into a voltage and monitored by a strip-chart recorder. In their initial studies they examined fatty acids alkanes lecithins and silicone oils (DC 190). DC 190 gave the best response. These workers examined the effect of the thickness of the coating on the response using 0.5% halothane (CF,CHClBr) the effect of temperature on the solubility of some common solvents over the range -30 to +50 “C and the interference effects of moisture DC 190 did not show selectivity for halogenated hydrocarbons but appeared to discriminate against water-soluble solvents.This discrimination was probably due to a difference in the number of adsorption sites and/or (for acetone) in the heat of adsorption. Edmonds and West54 examined the behaviour of variously coated 9-MHz AT-cut crystals to chloroform and toluene with respect to detector cell volume gas flow-rate analyte concentra-tion and size of detector coating. They also reported that Pluronics 64 was the most sensitive coating for ethylbenzene o-xylene and hexane Carbowax 20RI for acetone and chloroform and squalane for cyclohexane.Hydrogen Bucur75 utilised a quartz crystal microbalance to detect hydrogen (deuterium) in an inert gas and for the determination of the deuterium content in a gaseous hydrogen - deuterium mixture. He also examined the hydrogen (deuterium) interaction with thin palladium films. The sensing elements were two AT-cut crystals coated on both sides with silver electrodes and 1180 ALDER AND MCCALLUM PIEZOELECTRIC CRYSTALS Analyst Vol. 108 thin palladium film (BT-cut crystals were also studied). One crystal was operated at a reson-ance frequency of 5.25MHz and the other at 8.35 MHz. Hydrogen was determined in nitrogen up to a 0.1 molar fraction and deuterium up to a 0.25 molar fraction the frequency change reported being 150 Hz in each instance.Using the relationship that the mass of analyte adsorbed on to the palladium was proportional to the square of the partial pressure, a plot of Af2 against molar fraction was linear up to the values noted above. He quoted a detection limit of 0.01% for hydrogen and 0.03% for deuterium. Deuterium in the mixture was determined by measuring the difference between the palladium film being saturated with hydrogen and the film being exposed to the hydrogen - deuterium mixture the extent of exchange between the two isotopes being proportional to the partial pressure of deuterium. Hydrogen Chloride Hydrogen chloride in the p.p.b. and p.p.m. concentration ranges was selectively detected in ambient air76 using either triphenylamine (TPA) or trimethylammonium chloride (TMA-HCl) coated crystals.These coatings were reported to have response times of less than 30 s. Ammonia and moisture were found to cause interference problems when the TMA-HCl coating was used. The use of a gas-chromatographic column packed with silica gel was reported to have eliminated the effect of moisture. Frequency changes quoted for TPA were 154 Hz (100 p.p.m.) 52 Hz (1 p.p.m.) 40 Hz (0.1 p.p.m.) 26 Hz (0.01 p.p.m.) and 16 Hz (0.001 p.p.m.) and for TMA-HC1 changes of ca. 400 Hz (100 p.p.m.) and ca. 180 Hz (0.001 p.p.m.). Hydrogen Sulphide Silver- copper- or lead acetate-coated crystals were suggested by King77 for the determination of hydrogen sulphide. Webber et aL7* examined the acetone extracts of a number of organic soots for sensitivity to this compound.The soots were prepared by burning the substance in air over a Bunsen burner flame and collecting the residue which was then extracted in acetone and the extract evaporated from the crystal. Table I1 shows the responses obtained; a linear relationship was reported over the range 1-50 p.p.m. Chlorobenzoic acid was used also to detect hydrogen sulphide in solution.79 In this instance a membrane protected the crystal from the effects of moisture. TABLE I1 RESPONSE OF SOOT EXTRACTS TO 10 P.P.m. OF HYDROGEN SULPHIDE7* Soot extract Frequency change/Hz Benzene . . Toluene Carbon tetrachloride Chloroform : Chloroform extract Acetone extract . . Chlorobenzoic acid Benzyl chloride . . ChIoroaniline Benzoyl chloride . . Chloroacetic acid . . Xylene .. 0 . . 0 0 I . 0 . . 10 30 35 8 . . 24 . . 0 ,. 70 Mercury He noted a decrease in the afinity for mercury at surface densities in an excess of 0.15 pg cm-2J which represents a total adsorbed mass of the order of 60 ng for the crystal used. A detection limit of 5 ng 1-1 for mercury vapour in a continuously flowing air stream and 0.7 ng for the total mass of vapour in a gaseous sampIe drawn in say soil gas measurements were quoted The gold electrodes were cleaned by heating to 400 “C. The use of gold-coated crystals as part of an air monitoring system was further developed by Scheide and co-workerssl-84 over the period 1974-81. In a11 of these papers gold-coated Bristow,so in 1972 used a gold-plated quartz crystal as a “sniffer” for mercury in soil October 1983 FOR MASS AND CHEMICAL MEASUREMENTS.A REVIEW 1181 quartz crystals were used the choice of fundamental frequency being either 5 9 or 15 MHz. Table I11 lists the responses obtained from these crystals for 50 ng 1-1 of mercury. I t was also noted that the9-MHz crystal gave a frequency change of 24 Hz for 100% relative humidity (RH) 2 Hz for 82% RH and 0 Hz for 67% RH at 25 “C. These figures appear to be low if one compares them with the values reported by Fogleman and Schuman.85 A portable system was developed and reported in 1978.84 It was designed to be used as a site monitor or as a personal dosimeter with an 8-h operation period. The accumulated dosage could be displayed by pressing the ON-display button and the average exposure obtained by a simple calculation.The gold surface could be regenerated by placing the crystal in an oven at 150 “C with hot clean air passing over the crystal. TABLE I11 RESPONSE OF GOLD-PLATED CRYSTALS TO 50 ng 1-1 OF MERCURY~~ Fundamental frequency/MHz Change in frequency/Hz 5 20 9 30-115 15 300-330 Methane and Other Hydrocarbons The level of methane and other combustible gases is of paramount importance in mine safety. Current methods of measuring methane involve the use of a flame-ionisation detector or by the use of a selective catalytic combuster (pellistor). King6 used a coated piezoelectric crystal to detect moisture arising from the selective com-bustion of methane and other hydrocarbons. The crystal was set in line after a dryer and combuster (heated filament) and the hydrogen was measured after catalytic oxidation over platinum at 150 “C.Some differentiation was obtained between ethane and methane and the alkenes ethylene and propylene by control of the filament temperature. In summing up, King pointed out the features of hydrocarbon determination as 5-min response time 10 p.p.b. detection limit reactive hydrocarbons determined directly other hydrocarbons and methane by difference (after total combustion). As the detector responds to the hydrogen combustion product (water) calibration can be easily achieved by internal electrolysis of water. Organophosphorus Compounds Guilbault and co-workers have published a number of papers dealing with the detection of these corn pound^.^^-^^ A series of inorganic salts were examined as potential coatings87 with mercury( 11) bromide examined in detail.Diisopropylmethyl phosphonate (DIMP) was used as the model compound. The crystals were coated by dipping into a 0.01 M solution of the coating material. Table IV summarises their results obtained on a vacuum line. Fair linearity is reported to exist with a vapour pressure detection limit of 10-4mmHg of DIMP. With respect to the response of the crystal to moisture, Guilbault noted “A total frequency of 400 cycles resulted from the application of 0.7 mm of the phosphonate molecules. Large amounts of air and oxygen (25 mm) caused little change in the frequency of the crystal (20 Hz). Large amounts of water vapour (25 mm) caused a 100 cycle change in the frequency of oscillation.’’ Upon application of a (high) vacuum water oxygen or air were quickly removed from the AT-cut 9- or 14-MHz crystals were used.TABLE IV RESULTS OBTAINED BY GUILBAULT FOR MAGNESIUM BROMIDE-COATED CRYSTALS87 Coating None . . . . None . . Magnesium bromide Magnesium bromide Magnesium bromide Magnesium bromide Magnesium bromide Crystal frequency/MHz * . 14 14 14 9 14 . . 14 14 Exposed to pressure of AF/Hz 3 mmHg water 25 0.7 mmHg DIMP 400 0.7 mmHg DIMP 250 2 mmHg DIMP 75 25 mmHg air 20 25 mmHg oxygen 20 25 mmHg water 10 1182 ALDER AND MCCALLUM PIEZOELECTRIC CRYSTALS Analyst VoZ. 108 crystal indicating physisorption. DIMP was removed with difficulty and the initial fre-quency was never re-attained indicating chemisorption of the DIMP.The slopes for the various coating salts examined were iron(II1) chloride 4 copper( 11) chloride 2 nickel chloride 1.1 and cadmium chloride 1 Hz p.p.m.-l; all were linear to at least 400 p.p.m. of DIMP. “For the most sensitive substrate Fe(DIMP),Cl, a sensitivity below 10 p.p.m. was attained.’’ For paraoxon (0,O-diethyl-0-9-nitrophenyl phosphate) an iron( 111) chloride - paraoxon-coated crystal reportedly gave a frequency change of 44 Hz for 100 p.p.b. (should this have been 100 p.p.m.?) (Table V) or 10 Hz for 35 p.p.m. The straight-line response was about 0.3 Hz p.p.m.-l up to 65 p.p.m. with a detection limit of 10 p.p.m. of paraoxon. These workers showed that better sensitivity was obtained if both sides of the crystal were coated.88 TABLE V RESPONSE TO PARAOXON DETECTOR FOR VARIOUS INTERFERENCES AND PARAOXONs8 Interference Dry air .. * . Laboratory air . . NO2 CO,. . Water . . so,. . Paraoxon . . A F due t o 100 p.p.b./Hz 0 1 3 2 0 8 44 Two oximes 2-pyridylaldoxime methiodide (2-PAM) and isonitrobenzoylacetone (IBA) , were found to be active towards organophosphorus compoundssg and were tried as coating materials. It was found that 2-PAM was too volatile for use in a flowing stream detector. IBA was found to be stable selective and sensitive but the reaction on the substrate surface was found to be irreversible and the sensitivity decreased with each injection of DIMP or DDVP (dimethyldichlorovinyl phosphonate) . To remedy this the cobalt complex of IBA was prepared and applied to the crystal.The resultant detector was reported to be stable, selective sensitive and completely reversible. This coating was further improved by the addition of a small amount of paraoxon to the solution containing the coating. The responses to parathion DDVP and DIMP were found to be 18 Hz (5 p.p.b.) 9 Hz (50 p.p.b.) and 14 Hz (20 p.p.b.) respectively. Interferences were only significant at levels greater than or equal to 100 p.p.m. Copper complexes bound to an experimental resin polymer (XAD-4) and a mixture of 1-dodecyl-3-hydroxyiminomethylpyridinium iodide (3-PAD) with Triton X-100 and sodium hydroxide were examined.g0 Again DIMP was used as the model compound. Table VI shows adsorption values for these coatings compared with SE-30. TABLE VI ADSORPTION OF DIMP ON TEST MATERIALS” Substance [AF(Hz)/p.p.b.DIMP]/pg coating XAD-4 - Cu2+ diamine 2.6 3-PAD 1.9 SE-30 0.4 A second paper dealing with the response of 3-PAD Triton X-100 - sodium hydroxide and other coated crystals to a number of organophosphorus compounds was published in 1981.91 AT-cut 9-MHz crystals in HC-25/U mounts with silver-plated electrodes were used. Tables VII and VIII show the responses of these coatings to the test compounds and interferences. The use of sodium hydroxide that “had a high deliquenscence which was desirable for the coating” (to permit hydrolysis of the organophosphorus compound) and which also adsorbs carbon dioxide from the atmosphere must come as a surprise to anyone working with the mass-sensitive piezoelectric crystal detectors for monitoring trace levels of organic compounds.Nonetheless the authors reported an improved sensitivity with the ternary mixture (56 October 1983 FOR MASS AND CHEMICAL MEASUREMENTS. A REVIEW 1183 TABLE VII RESPONSE OF COATINGS TO ORGANOPHOSPHORUS COMPOUNDS” AFIHz ‘ DIMP Malathion Parathiok Coating (15p.p.m.) (1 p.p.m.) (1.5p.p.m.) L-Histidine hydrochloride 30 1414 126 DL-Histidine hydrochloride . . - 474 23 Succinylcholine chloride . . 55 519 76 2-PAD 290 3-PAD . . * . 403 64 24 Succinylcholine iodide . . 41 490 44 - -Triton X-100 - 13% sodium hydroxide - 31% 3-PAD) over previously tried mixtures. They construed that the detector response must “be due to the physical and chemical adsorptions, dissolutions and associations catalysed by moisture and surfactant.” Temperature studies indicated that lower temperatures are optimal for sensitivity and higher for response time.The sensitivity was half the 22 “C value at 45 “C and the recovery time improved from 4 min at 20 “C to 1 min at 55 “C after exposure to 2 ml of 15 p.p.m. DIMP in a 30 ml min-l “room air” stream. In testing for potential interferences the ternary mixture coating was exposed to some “other common atmospheric constituents and pollutants,” voiz. ammonia carbon monoxide benzene toluene ethanol chloroform and sulphur dioxide (Table VIII) . Sulphur dioxide reacted with the sodium hydroxide and as a consequence interference for sulphur dioxide could be eliminated by using a binary coating mixture of 3-PAD and Triton X-100 when monitoring air samples containing high concentrations of SO,.Surprisingly two very common atmospheric constituents carbon dioxide and water vapour were not tested for their response which seems a regrettable omission. TABLE VIII RESPONSE OF 3-PAD MIXTURE TO INTERFERENCES” The response to 15 p.p.m. of DIMP was 610 Hz. Interference Concentration p.p.m. Ammonia Ammonia Carbon monoxide Benzene Toluene . . Ethanol . . Chloroform . . Sulphur dioxide Sulphur dioxide 1000 100 1000 . . 100 100 * . 100 100 1000 100 AFIHz 15 4 2 11 25 9 71 70 42 Sulphur Dioxide Guilbault and L o p e z - R ~ m a n ~ ~ ~ ~ reported the responses they obtained from a number of coatings. Their first paper compared sodium tetrachloromercuriate with Apiezon N SE-30, QF-1 Carbowax 20M and Versamid 900.They also examined the effect of supply voltage and ambient temperature on 9- and 14-MHz coated crystals. Table IX shows the amount of sulphur dioxide adsorbed with time. These workers chose the Carbowax 2OM-coated 9-MHz crystals as the detector for a general-purpose gas ~hromatograph.9~ The detector was found to be linear over the range 1-100 p.p.m. of sulphur dioxide had a response time of 5 s and a recovery time of about 1 min. Table X lists the responses obtained in their interference study. Frechette and C O - W O ~ ~ ~ ~ S ~ ~ ~ ~ ~ examined 14 coatings; Table XI shows the relative responses obtained. Nitrogen dioxide was found to be a common interferent. Two other coatings, tridodecylamine and tripropylamine were found to be the most sensitive to sulphur dioxide but were rejected because they had a high bleed rate and showed some irreversibility.These workers chose SDM polymer (styrene - dimethylaminopropylmaleimide 1 1 copolymer) fo 1184 ALDER AND MCCALLUM PIEZOELECTRIC CRYSTALS Analyst ‘Vd. 108 TABLE IX AMOUNT OF SULPHUR DIOXIDE ADSORBED WITH TIMEg2 SO adsorbed using the coating/pg Blank Timelmin (no coating) 0.10 -0.20 -0.30 -1 2 3 0.004 4 0.021 5 0.048 10 0.075 15 0.081 --HgCl,2-0.181 0.390 0.612 0.805 1.101 1.433 2.051 2.322 2.980 4.01 1 Apiezon N 0.126 0.148 0.172 0.182 0.188 0.210 0.235 0.281 0.298 0.340 Silicone 0.082 0.089 0.097 0.097 0.103 0.111 0.114 0.141 0.149 0.194 SE-30 Silicone 0.070 0.088 0.122 0.149 0.168 0.265 0.284 0.330 0.350 0.494 QF-1 Carbowax Versamid’ 20M 900 0.030 0.063 0.041 0.125 0.082 0.142 0.102 0.195 0.113 0.208 0.118 0.231 0.151 0.252 0.162 0.301 0.184 0.352 0.201 0.381 further study.Sample cell volumes of 2 and 0.01 cm3 were tested and it was found that the cell having the lower cell volume gave the highest sensitivity. They produced a probe with which they evaluated the detector response against substrate mass sample size and concentra-tion. It was suggested that there was the possibility of the coating becoming deactivated in the presence of large amounts of moisture. The sulphur dioxide was reported to dissolve to form sulphurous acid which would attack the coating and give a salt.An infrared study was carried out and this produced evidence supporting the hypothesis. TABLE X RESPONSES OBTAINED BY GUILBAULT AND LOPEZ-ROMANs3 FOR THEIR CARBOWAX 20M COATED CRYSTALS Gas injected Concentration of gas so Water vapour . . Air . . . . co . . k3 :: N,O . . NO2 NH 30 p.p.m. Saturated 99% 99 % 99% 99 % 99 % 99 % 99 % Response, arbitrary units 46 3 2 2 3 8 2 4 2 Guilbault and co-workersg6-S8 studied a number of amines as potential coatings including P-toluidine Amine 220 triethanolamine (TEA) Armeen 2s and quadrol [NNN’N’-tetrabis(2-hydroxypropyl)ethylenediamine] with greatest emphasis on quadrol. They reported favour-able responses with all of these compoundsg6 but found some problems P-toluidine was found to be too volatile for prolonged use and Amine 220 and Armeen 2s were found to give a curved calibration graph depending on sample volume and concentration.TEA and quadrol were reported to give linear responses over the range 10 p.p.b. to 30 p.p.m. It was reported that TABLE XI RELATIVE RESPONSE TO SULPHUR DIOXIDEs4 Compound Tridodecylamine . . Melamine . . Diallyl melamine . . Igepal CO-880 . . Cellulose nitrate . . Phenyldiethanolamine Diallylamine . . Relative response 160 4 . . 24 . . 69 . . 35 . . . . 70 150 Compound UC-W98 . . . . . . Versamid 900 PP-2040 . . . . * . . . PE-100 . . 2,2-(m-tolylin1ino)diethanol .. Tripropylamine . . . . SDM polymer . . . . Relative response 45 50 190 68 225 3 19 October 1983 FOR MASS AND CHEMICAL MEASUREMENTS. A REVIEW 1185 “moisture physically condenses on the crystal surface thereby giving a strong response, Nitrogen dioxide and moisture are the major interferences. For upper atmospheric clean air studies these interferences will not have any effect on the detection of sulphur dioxide. The use of a GC column or drier to separate sulphur dioxide nitrogen dioxide or moisture followed by separate detection will solve this problem.’’ A hydrophobic membrane filter was later added to reduce the level of moisture reaching a quadrol coated detector crystal.97 The response due to “laboratory air” was reduced from 300 to 70 Hz using a 0.45-pm pore size Acropor filter membrane.Further development of their quadrol-coated crystals led to a portable system designed to monitor car exhaust fumes and refinery stack gases.98 Four hydrophobic membrane layers were used to reduce the moisture reaching the detector. Cheney and c o - w o r k e r ~ ~ ~ J ~ examined TEA as a coating suitable for sulphur dioxide. They reported the 90,95 and 99% response times which at 25 p.p.m. were between 9.25 and 11 min and at 761 p.p.m. between 1 and 5.25 min. A wide range of sensitivities and temperature variations of sensitivity for the coating towards sulphur dioxide were reported. The authors commented that the slow response below 25 p.p.m. rendered the study of response below this level impractical. The TEA coating was modified with triisopropanolamine (TIP) on Teflon to reduce the volatility that was achieved but at the expense of sensitivity.Ethylenedinitrilotetraethanollol appeared to offer better sulphur dioxide sensitivity at higher temperatures and flow-rates. Toluene Diisocyanate Alder and Isaac102J*3 reported some studies on the use of piezoelectric crystals as part of a portable instrument for personal monitoring with emphasis on shipboard use. They outlined the requirements desired for such a use and examined a number of coatings with emphasis on polyethylene glycol 400 as a model compound for development studies. This coating was found to be unsuitable for toluene diisocyanate (TDI) because of water sensitivity and irrevers-ible TDI adsorption. Using statistical analysis of the data an extrapolated limit of detection of the order of 6 p.p.b.of toluene diisocyanate in dry air was predicted. These authors suggested that the irreversibility of the coating may be exploited to give an indication of the time-weighted average (TWA) exposure of the operator to the hazardous material. Water Gjessing et al.lo4 used two gold-plated 8-MHz quartz crystals one coated with a hygroscopic material with the other left sealed and uncoated. The materials tested included sodium fluoride barium fluoride silicon oxides magnesium fluoride and some types of glass. A hygroscopic film of SiO showed promise for water with a sensitivity of 6.5 Hz %(RH)-l a t 20 “C. A patent covering the use of coated piezoelectric crystals as a moisture analyser was filed by Kingf05 in 1969.In this patent the halides of lithium and calcium are specifically mentioned and emphasised in the claims athough a number of other salts are listed. A number of coatings were examined by Lee et a1.lo6 and gelatine was found to be the most suitable. They stated that the sensitivity of this coating was 3.8 Hz p.p.m.-l (V/V) of water with 150pg of coating. This coated crystal was used to determine the concentration of moisture in a number of different commercial gases. The results obtained were compared with two recognised techniques gravimetry and the use of the Du Pont 303 moisture analyser. The results compared favourably. Bacterial and Fungal Growth A preliminary study into the use of quartz crystals to measure the growth rate of bacteria and fungi was carried out by Downes.lO7 He was unsuccessful in measuring any mass change caused by growth of these living systems.This was ascribed to the growth rate for the cells studied which was very slow and the possibility that the cell membranes could be ruptured by the high oscillation frequency. Solution Property Measurement Nomura and co-workers examined the change in frequency obtained when a quartz crystal was immersed in organic solution^.^^*^^^^ They found that the change in oscillation frequenc 1186 ALDER AND MCCALLUM PIEZOELECTRIC CRYSTALS Analyst VoZ. 108 depended on the specific gravity and viscosity of the solvents. The frequency was shown to decrease with increasing electrolyte concentration added depending on the increase in specific conductivity.They reported that AF = ad* + b$ - c where a b and c are constants determined by the crystal d is the density and r] is the viscosity of the solvent .lo9 Trace Metal Studies In another paper Nomura et aZ. reported the extraction of the 8-quinolinate chelate of lead into chloroform.l1° This solution was then transferred into a 10-ml beaker and a platinum-plated crystal immersed in the solution. The frequency was noted after 2 min and the pro-cedure repeated with a reagent blank after thorough washing and drying. Lead concentrations over the range 3 x lo-% x 1 0 A 5 ~ in the aqueous solution were examined. Interferences from iron(III) nickel cobalt(II) zinc cadmium and silver could be masked using L-ascorbic acid and cyanide. Nomura and Maruyamalll published a curious paper reporting the examination of the stability of aqueous solutions of a number of metal ions and the selective determination of iron(II1) as its phosphate.They found that with a standard crystal the frequency change was directly proportional to the specific conductivity for up to 2 mM where deviations due to viscosity and density were reported to occur. At concentrations above 20 mM abrupt fre-quency changes were found to occur as a result of the solution being able to short-circuit the quartz. Under these conditions they reported the electrodeposition of metal ions such as Mn2+ Ni2+ Co2+ Zn2+ Cd2+ Ag+ Cu2+ and Pb2+. With aluminium and iron adsorption of some salt was thought to cause the frequency change. To investigate this in more detail and to determine iron(III) they modified a standard crystal by covering one surface with a quartz plate separated from the crystal surface by “four slim hairs laid on each edge of the quartz plate.” The lead wires of the crystal holder were coated with epoxy resin to avoid electrolysis problems.A calibration graph of AF versm iron(II1) was produced and shown to be linear over the range 1 x 10-5-1 x 1 0 - 4 ~ . Some interference studies were carried out on a 5 x 1 0 - 5 ~ iron(II1) solution. Ten-fold molar amounts were thought to interfere if changes of greater than 15% were obtained. They reported that this occurred with lead aluminium bismuth, sulphide and thiosulphite. These were fused with epoxy resin. Thermal Analysis King et aZ.112 utilised piezoelectric crystals as supports for thin film thermocouples for differ-ential thermal analysis over the temperature range - 125 to 500 “C.Their system was reported to give calorimetric data for indium that agreed well with the established literature. A quartz cryst a1 t hermogravimet ric analyser for t emperature-programmed analysis of deposited films was described by Henderson et aZ.l13 They showed that their technique was applicable at temperatures up to 570 “C but suggested that temperatures in excess of 1000 “C should be possible using lithium niobate piezoelectric crystals. Very fast heating rates are possible with this device as thermal equilibrium is reportedly easier to achieve during heating. It is suggested that this system could be applied to study the volatilisation of thin films and coatings the kinetics of surface phenomena and flammability studies of polymers as thin films rather than as the bulk properties required by conventional thermogravimetric instrumen-tation.Conclusions As part of a portable detector system quartz piezoelectric crystals have potential in the field of personal exposure monitoring. They have high mass sensitivity dependent only on the oscillation frequency of the crystal. The specificity of the crystal is determined by the in-genuity of the worker in choosing the coating. Interferences such as moisture may be removed by physical means (e.g. filters membranes October 1983 FOR MASS AND CHEMICAL MEASUREMENTS. A REVIEW 1187 pre-columns) by correction using either a second crystal or an alternative moisture sensitive device (e.g.a resistive probe). Alternatively moisture could be used as part of the chemical system and allowed to become involved in the surface reaction.g0 The frequency-change data obtained from the crystals on exposure to analyte vapour lend themselves very well to modern data processing techniques. The use of on-board microcomputers to output gas concentration information in real time will guarantee interest in these devices in the future development of portable monitoring equipment. It is a pity that all workers in this field do not pay more attention to reproducibility of results calibration and reproducibility both of coating the crystals for gas analysis and of response to the analyte and interferences. Accepting the wide variation of humidity in real situations the disregard paid to the response of detector crystals proposed for practical applications by many if not most workers is surprising.To show that a detector could still be sensitive to 10 p.p.m. of a gas over a range of humidity of 10-95% at 28 “C would not only impress workers in this but also in all the gas-sensing portable instrument fields. To show that the device still worked at the not uncommon ambient temperature of -5 “C would be even more impressive. Now that many of the limitations and advantages of the piezoelectric crystal detectors have been identified and the instrumentation and data manipulation side largely developed the time is ripe to choose those situations where the piezoelectric crystal detector is best able to succeed and to characterise its performance under truly practical conditions.Applications where only low selectivity are required for example in the monitoring of families of compounds, seem likely to succeed as the selectivity problem is undoubtedly the most limiting. The application to short-term (working day) rather than long-term monitoring with disposable crystals also seems attractive. The idea of using several sensor crystals with different responses to give data that would permit interference correction or a multi-component detection capability has crossed many people’s minds. The availability of the data handling packages to do this now renders it possible even on a portable system. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. References Webber L. M. Hlavay J. and Guilbault G. G. Mikrochim. Acta 1978 351. Guilbault G. G. Anal. Proc. 1982 19 68. Hlavay J. and Guilbault G. G. Anal. Chem. 1977 49 1890. Guilbault G. G. Ion. Sel. Electrode Rev. 1980 2 3. Cady W. G. “Piezoelectricity,” First Edition McGraw-Hill New York and London 1946. Becquerel A. C. Bull. Soc. Philomath. Paris 1820 7 149 (series 3). Curie J. and Curie P. Bull. Soc. Min. Paris 1880 3 90. Lippmann G. An. Chim. Phys. Ser. 5 1881 24 145. Langevin A. “Piezoelectricity,” First Edition McGraw-Hill New York and London 1946 p. 5. Cady W. G. Proc. Inst. Radio Eng. 1924 12 805. Cady W. G. J . Opt. SOG. Am. 1925 10 415.Pierce G. W. Proc. Am. Acad. Arts Sci. 1923 59 81. Pierce G. W. Proc. Am. Acad. Arts Sci. 1925 60 277. Phillips F. C. “An Introduction to Crystallography,” Third Edition Longmans London 1963, Voigt W. Abh. Ges. Wiss. Gottingen 1890 36 1. Voigt W. “Lehrbuch der Kristallphysik,” B. G. Teubner Leipzig First Edition 1910; Second Tillyer E. D. U.S. Pat. 1907 613 1933. Lack F. R. Willard G. W. and Fair I. E. Bell Syst. Tech. J. 1934 13 453. Bechmann R. J . Sci. Instrum. 1952 29 73. Cady W. G. Proc. Inst. Radio Eng. 1922 10 83. Warner A. W. Bell Syst. Tech. J. 1960 39 1193. Warner A. W. Fraser D. B. and Stockbridge C. D. IEEE Tram. Sonics Ultrason. 1965 SU-12 52. Spencer W. J. IEEE Trans. Sonics Ultrason. 1965 SU-12 1. Horton W. H. IEEE Trans. Sonics Ultrason.1966 SU-13 111. Sauerbrey G. Z. Phys. Verha. 1957 8 113. Sauerbrey G. Z. 2. Phys. 1959 155 206. Lostis M. Thesis University of Paris Faculty of Science 1958. Stockbridge C. D. Vac. Microbalance Tech. 1966 5 193. Oberg P. and Ligensjo J. Rev. Sci. Instrum. 1959 30 1053. Warner A. W. and Stockbridge C. D. Vac. Microbalance Tech. 1963 3 55. King W. H. Jr. Bull. N.Y. Acad. Med. 1972 48 459. p. 155; Fourth Edition Oliver and Boyd Edinburgh 1971 p. 167. Edition 1928 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82.83. 84. 85. 86. 87. 88. 89. 90. 91. 92. 93. 94. 95. 96. 97. 98. 99. 1188 ALDER AND MCCALLUM PIEZOELECTRIC CRYSTALS Analyst VoZ. 108 Richardson P. D. Bull. N . Y . Acad. Med. 1972 48 465. Ullevig D. M. Evans J. F. and Albrecht M. G. Anal. Chem. 1982 54 2341. Haller I. and White P. J. Phys. Chem. 1963 67 1784. Fischer W. F. and King W. H. Jr. Anal. Chem. 1967 39 1265. King W. H. Jr. and Corbett L. W. Anal. Chem. 1969 41 580. Hosseiny A. PhD Thesis University of Manchester 1980. Chuan R. L. J. Aerosol Sci. 1970 1 111. Olin J. G. Sem G. j. and Christenson D. L. Am. Ind. Hyg. Assoc. J. 1971 32 209. O h J . G. and Sem G. J. Atmos. Environ. 1971 5 653. Chabre A. “Study of the Impact of Fine Particles on a Piezoelectric Target in View of Determining their Mass and Concentration,” Aix-Marseilles University France 1972 (in French) ; Nucl.Sci. Abstr. 1973 37 ref. 22601. Carpenter T. E. and Brenchley D. L. Am. Ind. Hyg. Assoc. J. 1973 33 503. Daley P. S. and Lundgren D. A. Am. Ind. Hyg. Assoc. J. 1975 36 518. Sem G. J. and Tsurubayashi K. Am. Ind. Hyg. Assoc. J. 1975 36 791. Mieure J . P. and Jones J . L. Talanta 1969 16 149. Jones J. L. and Mieure J . P. Anal. Chem. 1969 41 484. Nomura T. and Mimatsu T. Anal. Chim. Acta 1982 143 237. King W. H. Jr. Anal. Chem. 1964 36 1735. Karasek F. W. and Gibbins K. R. J. Chromatogr. Sci. 1971 9 535. Smith H. R. Am. Lab. 1972 49 (Oct.) 49. Karasek F. W. and Tierney J. M. J. Chromatogr. 1974 89 31. Karasek F. W. Guy P. Hill H. H. Jr. and Tierney J. M.J. Chromatogr. 1976 124 179. Janghorbani M. and Freund H. Anal. Chem. 1973 45 325. Edmonds T. E. and West T. S. Anal. Chim. Acta 1980 117 147. Schulz W. W. and King W. H. Jr. J. Chromatogr. Sci. 1973 11 343. Konash P. L. and Bastiaans G. J. Anal. Chem. 1980 52 1929. King W. H. Jr. U.S. Pat. 3 164 004 1965. King W. H. Jr. Res. Dev. 1969 20(4) 28. King W. H. Jr. Res. Dev. 1969 20(5) 28. Karmarker K. H. and Guilbault G. G. Anal. Chim. Acta 1975 75 111. Webber L. M. and Guilbault G. G. Anal. Chem. 1976 48 2244, Webber L. M. and Guilbault G. G. Anal. Chim. Acta 1977 93 145. Hlavay J. and Guilbault G. G. Anal. Chem. 1978 50 1044. Edmonds T. E. Fraser S. M. and West T. S. Poster at Royal Society of Chemistry “International Conference on the Detection and Measurement of Hazardous Substances in the Atmosphere,” City University London 20-22 December 1982.Guilbault G. G. Lopez-Roman A. and Billedeau S. M. Anal. Chim. Acta 1972 58 421. Karmarker K. H. and Guilbault G. G. Environ. Lett. 1975 10 237. Ho M. H. Guilbault G. G. and Reitz B. Anal. Chem. 1980 52 1489. Fogleman W. W. and Shuman M. S. Anal. Lett. 1976 9 751. Ho M. H. Guilbault G. G. and Scheide E. P. Anal. Chem. 1982 54 1998. Nomura T. and Hattori O. Anal. Chim. Acta 1980 115 323. Nomura T. and Minemura A. Nippon Kagaku Kaishi 1980 1621. Nomura T. Anal. Chim. Acta 1981 124 81. Tomita Y. Ho M. H. and Guilbault G. G. Anal. Chem. 1979 51 1475. Kindlund A. and Lundstrom I. Sensors Actuators 198213 3 63. Bucur R. V. Rev. Roum. Phys. 1974 19 779. Hlavay J. and Guilbault G.G. Anal. Chem. 1978 50 965. King W. H. Jr. Anal. Chem. 1964 36 1735. Webber L. M. Karmarker K. H. and Guilbault G. G. Anal. Chim. Acta 1978 97 29. Webber L. M. and Guilbault G. G Anal. Chim. Acta 1977 93 145. Bristow Q. J. Geochim. Explor. 1972 1 55. Scheide E. P. and Taylor J. K. Am. Chem. SOC. Div. Environ. Chem. Prepr. 1974 14 329. Scheide E. P. and Taylor J. K. Am. Ind. Hyg. Assoc. J. 1975 36 897. Scheide E. P. and Taylor J. K. Environ. Scz. Technol. 1974 8 1097. Scheide E. P. and Warner R. B. j. Am. Ind. Hyg. Assoc. J - 1978 39 745. Fogleman W. W. and Shuman M. S. Anal. Lett. 1976 9 751. King W. H. Jr. Environ. Sci. Technol. 1970 4 1136. Guilbault G. G. Anal. Chim. Acta 1967 39 260. Scheide E. P. and Guilbault G. G. Anal. Chem. 1972 44 1764. Shackelford W.M. and Guilbault G. G. Anal. Chim. Ada 1974 73 383. Guilbault G. G. Affolter J. Tomita Y. and Kolesar E. S. Jr. Anal. Chem. 1981 53 2057. Guilbault G. G. Tomita Y. and Kolesar E. S. Jr. Sensors Actuators 1981/2 2 43. Guilbault G. G. and Lopez-Roman A. Environ. Lett. 1971 2 35. Lopez-Roman A. and Guilbault G. G. Anal. Lett. 1972 15 225. Frechette M. W. Fasching J. L. and Rosie D. M. Anal. Chem. 1973 45 1765. Frechette M. W. and Fasching J. L. Environ. Sci. Technol. 1973 7 1135. Karmarker K. H. and Guilbault G. G. Anal. Chim. Ada 1974 71 419. Karmarker K. H. Webber L. M. and Guilbault G. G. Environ. Lett. 1975 8 345. Karmarker K. H. Webber L. M. and Guilbault G. G. Anal. Chim. Acta 1976 81 265. Cheney J. L. and Homolya J. B. Anal. Lett. 1975 8 175 October 1983 FOR MASS AND CHEMICAL MEASUREMENTS. A REVIEW 100. 101. 102. 103. 104. 105. 106. 107. 108. 109. 110. 111. 112. 113. 1189 Cheney J. L. and Homolya J. B. Sci. Total Environ. 1976 5 69. Cheney J. L. Norwood T. and Homolya J. Anal. Lett. 1976 9 361. Alder J. F. and Isaac C. A. Anal. Chim. Acta 1981 129 163. Alder J. F. and Isaac C . A. Anal. Chim. Ada 1981 129 175. Gjessing D. T. Holm C. and Lanes T. Electron. Lett. 1967 3 156. King W. H. Jr. U.S. Pat. 3 427 864 1969. Lee C. W. Fung Y . S. and Fung K. W. Anal. Chim. Acta 1982 135 277. Downes J . Internal Report Imperial College London 1979. Nomura T. Okuhara M. Murata K. and Hattori O. Bunseki Kagaku 1981 30 417 (in Japanese Nomura T. and Okuhara M. Anal. Chim. Acta 1982 142 281. Nomura T. Yamashita T. and West T. S. Anal. Chim. Acta 1982 143 243. Nomura T. and Maruyama M. Anal. Chim. Acta 1983 147 365. King W. H. Jr. Camili C. T. and Findeis A. F. Anal. Chenz. 1968 40 1330. Henderson D. E. Di Tarantor M. B. Tinkin W. G. Ahlgren D. J. Gatenby D. A. and Shun, Received March lst 1983 Accepted May loth 1983 with English abstract). T. W. Anal. Chem. 1982 54 2067

ISSN:0003-2654    

DOI:10.1039/AN9830801169

出版商:RSC    

年代:1983

数据来源: RSC

摘要:

1190 Analyst October 1983 Vol. 108 $9. 1190-1194 Limits of Detection of Trace Elements in Biological Materials Analysed by Instrumental Neutron Activation Analysis Using X-ray Spectrometry and Magnetic Deflection of P-Rays Mariana Mantel Soreq Nuclear Research Centre Yavne Israel The limits of detection of 18 trace elements in blood urine and biological standards (from the International Atomic Energy Agency and the National Bureau of Standards) analysed by neutron activation followed by X-ray spectrometry and magnetic deflection were calculated. In addition to the usual parameters that affect the limits of detection obtainable by X-ray spectrometry the influence of the spectral interference (background) resulting from the complex matrices was also taken into account.The method is especially suitable for elements which after neutron activation emit X-rays with energies below 16 keV. Keywords Limits of detection of trace elements ; biological materials ; instru-mental neutron activation ; X-ray spectrometry ; magnetic deflection of 8-rays The application of X-ray spectrometry to instrumental neutron activation (INA) has been intensively studied in our 1aboratorylJ and its advantages and shortcomings pointed out. However in order to apply this technique to complex matrices it is necessary to suppress the background from the irradiated matrix. Because Si(Li) detectors used for the measurement of X-rays have a low sensitivity to photons with energies higher than 80 keV the interference is entirely due to P-rays. The latter produce a high background that in most instances, completely obscures the X-ray peaks.To overcome this interference we developed3 a method of background reduction by magnetic deflection of P-rays based on the fact that magnetic fields deflect the electrons of /3-rays but have no influence on X-rays. The sample is placed between the poles of a magnet located above the detector. The X-rays reach the detector unaltered whereas the /&rays are deflected according to their energy and to the intensity of the niagnetic field. A systematic study4J was carried out and the optimum conditions for the reduction in background by the removal of /3-particles established. Practical applications, such as the determination of bromine in blood serum6 and the determination of niobium in steel.^,^ were developed.In this work we calculated the limits of detection of 18 trace elements in various biological matrices blood urine and different biological standards provided by the International Atomic Energy Agency (IAEA) and the National Bureau of Standards (NBS). In particular, the influence of the background resulting from these complex matrices was taken into account as the effect on the limit of detection is considerable. The elements were determined by the method described. Experimental Materials Standards were prepared from high purity (Fluka or Johnson Matthey) compounds and the following biological standards (except blood and urine) provided by the IAEA and the NBS, were used. Milk powder. Animal mzlscle. IAEA reference material for multi-element analysis H-4.Bovine liver. NBS standard reference material 1577. Orchard Eeaves. NBS standard reference material 1571. Bowen’s kale. Blood and wine. Obtained from healthy volunteers. IAEA intercomparison sample for trace element analysis A-1 1. Provided by the IAEA MANTEL 1191 Apparatus The experimental set-up,6 shown schematically in Fig. 1 consists of (A) a Si(Li) diode (Seforad Israel) of 100 mm2 area 4 mm depletion depth and having a 1.0 mm thick beryllium window coupled through a pre-amplifier and an Ortec Model 485 linear amplifier to a 4096 channel analyser (Promeda Elscint Israel). The resolution of this system for 6.4 keV Fe K X-rays is 400 eV. B is an electromagnet that permits magnetic fields of up to 1.3 T (18-mm gap between poles) to be obtained.The magnet was designed in such a way as to permit positioning of the detector as close as 22 nim from the poles of the magnet. The smallest possible distance from the sample to the surface of the detector is 27 mm. Fig. 1. Schematic representa-tion of the experimental apparatus. A Be window; B sample holder; D detector; E electromagnet; and S sample. Sample Preparation Samples were prepared in special small polyethylene irradiation vials (1 cm id.) and sealed with polyethylene stoppers. The following procedures were used. Standards Soluble compounds. Aliquots of dilute solutions (a few micrograms per millilitre) in water, nitric acid or any other solvent that would not interfere with the irradiation were introduced into the vial and evaporated to dryness under an infrared lamp.Insoluble compounds. A few milligrams of metal powder or oxide were weighed into the vial. Biological standards. About 50 mg were weighed into the vial. Blood. Blood serum (0.5 ml) was introduced into the vial and evaporated to dryness under an infrared lamp. Urine. Urine (0.5 ml) was introduced into the vial and evaporated to dryness. A second aliquot of 0.5 ml was added and the procedure repeated so as to arrive at a total volume of 1 ml. Irradiation The irradiations were carried out in the pneumatic tube of the IRR-1 reactor at a thermal neutron flux of 1 x 1013 n cm2 s-l. For isotopes with half-lives of up to 12 h the samples were irradiated for a length of time equal to one half-life of the isotope obtained from the trace element to be analysed and counted for one half-life after one half-life cooling time.For half-lives longer than 12 h the maximum irradiation and cooling time was 12 h. Procedwe The standard samples were irradiated and counted using the experimental set-up shown in Fig. 1 and the activity obtained from 1 pg of the element (counts per microgram) was deter-mined. Blood urine and biological standards were irradiated and the background was measured in the same way. From these two values the limit of detection of each of the elements studied in every biological matrix was calculated 1192 MANTEL TRACE ELEMENTS IN BIOLOGICAL MATERIALS BY INAA Analyst VoZ. 108 Results The background obtained from the seven biological matrices studied was measured over an energy range of 2-20 keV.The values obtained were compared with those calculated based on the composition of the matrix the irradiation time and the reduction in background due to the magnetic field. Very good agreement was found between the measured and calculated values. Using pure compounds (standards) of each of the trace elements the activity per microgram of element obtainable by the present method without the interference of the back-ground was measured. These values (corrected for absorption in the matrix for up to 9 keV), together with the background values were used to calculate the limits of detection of 18 trace elements in seven biological matrices. The limit of detection was taken to be the smallest amount of the element that permits the attainment of a signal-to-noise ratio of 3dg(B is the background).The results obtained are shown in Table I. Where not otherwise stated the limit of detection refers to all seven matrices. The following conclusions may be drawn. In spite of the variations in composition the limits of detection for short-lived nuclides are practically the same in all the matrices studied. For nuclides with half-lives longer than 3 h the sensitivity will be higher in such matrices as orchard leaves which have a relatively low sodium content and lower in animal muscle with a high sodium content (values indicated by their respective footnotes in Table I). For longer half-lives the sensitivity will depend only on the concentration of phosphorus in the matrix. Only those trace elements that following neutron activation produce radioisotopes that decay by the emission of K or L X-rays with energies up to 16 keV were studied.For higher X-ray energies the background can be reduced with the same efficiency by plastic absorbers and the additional cost of the magnet is not necessary. Discussion One of the major problems associated with the application of instrumental neutron activa-tion and X-ray spectrometry to biological materials is the interference of #%particles emitted TABLE I LIMITS OF DETECTION OF TRACE ELEMENTS IN BIOLOGICAL MATRICES MAGNETIC DEFLECTION DETERMINED BY INA USING X-RAY SPECTROMETRY AND Samples of 50 mg (50 pl for blood) were irradiated for one half-life and counted for one half-life after one half-life cooling time. For half-lives longer than 12 h the samples were irradiated and counted for 12 h after 12 h cooling time .X-ray X-ray energy/ Limit of detection,* Element Isotope 4 measured keV Scandium Chromium Cobalt . . Copper . . Zinc . . Germanium Selenium Bromine Rubidium Strontium Yttrium. . Niobium Uranium Thorium Mercury Platinum Iridium . . Osmium ,. ,. . 20 s 27.8 d 10.5 min 12.8 h 13.8 h 46 s 3.9 min 6.2 min 1.06 min 2.8 h 3.1 h 6.3 min 23.5 rnin 22.4 rnin 65 h 80 rnin 14 h 1.45 rnin Sc Ka V Ka Co Ka Ni Ka Zn Ka Ge Ka Se Ka Br Ka Rb Ka Sr Ka Y Ka Nb Ka NP La1 Pa Lal Au La, Pt La, In La, 0 s LUl 4.5 5.4 6.9 7.5 8.6 9.9 11.2 11.9 13.4 14.2 14.9 16.6 13.9 13.3 9.7 9.7 9.1 8.9 p.p.m.5 x 10-2 3.4 x 10-1,t 1.1 x 10-1: 7 x 10-3 1.8 x lO-l,t 7 x 12.6,t 5.0: 2.8 x 1.5 x 1.4 x 1.0 1.3 x lo2 8.1 2 x 10-2 1 x 10-1 3 x 10-1 4.2,t 2.8$ 2.0 6 x 7 x 10-1,t 3 x 10-1: * Average of the results (range f40%) obtained for all seven matrices. t Limit of detection in animal muscle. Limit of detection in orchard leaves October 1983 USING X-RAY SPECTROMETRY AND MAGNETIC DEFLECTION OF P-RAYS 1193 from the irradiated sample.lP2 These /I-particles increase the dead-time of the Si(Li) detector, upset its resolution and produce a high background which may completely obscure the X-ray peaks. The interference is especially high in biological matrices where the major inorganic components are sodium potassium chlorine and phosphorus all strong /3-emitters following neutron activation.It follows that the possibility of non-destructively determining trace elements in these matrices will depend on the extent to which the P-ray interference can be reduced. As shown previously,6 the application of magnetic fields makes possible the non-destructive measurement of X-rays in neutron activated complex matrices. Theoretical calculations showed' that the intensity of the magnetic field necessary to deflect /I-particles depends on their energy and on the source - detector distance. For example at a distance of 2.7 em (the sample - detector distance in our set-up) see Fig. 1 a 0.2-T magnet is necessary to remove 91% of the /3-particles emitted by 32P (1.7 MeV) and a 0.5-T magnet is needed for a similar reduction of those emitted by 35Cl (4.92 MeV).TABLE I1 BACKGROUND OBTAINED WITH A 1.3-T ELECTROMAGNET Radionuclide . . . . 38P 48K 38Cl 24Na Background,* yo . . 0.8 3.6 5.6 15.8 * Average of values obtained for different energies (up to 16 keV) expressed as a percentage of the background obtained without the magnet. The background obtained from each of the four principal radioactive isotopes (24Na 42K, 38Cl and 32P) produced in an irradiated biological matrix was measured both with and without a magnet. The results showed that the decrease in background is not the same for all the nuclides and varies with energy for the same n~clide.~ Table I1 shows the background (expressed as a percentage of the background obtained without a magnet) obtained for each of TABLE I11 PERCENTAGE CONTRIBUTION OF THE MAJOR INORGANIC COMPONENTS TO THE TOTAL ACTIVITY (MEASURED WITHOUT THE MAGNET) OF THE IRRADIATED MATRIX [ISOTOPES OBTAINED BY THE (n,~) REACTION 24Na 42K Wl, 27Mg 32P and 45Ca] Matrix Animal muscleD .. . . Milk powderlo . . . . Bovine liver11 . . . . Orchard leaveslS . . Bowen's kale" . . . . Urine" 5 . . . . ~iood14 . . Matrix Animal muscleo . . . . Milk powderlo Bovine liver" . . . . Orchard leaves" . . Bowen's kalela . . . . UrineI4 f . . . . ~iood14 Element A Na K c1 I \ I A A I r 1 r -t % % % & & A-p.p.m. A* Bf C$ p.p.m. A B C p.p.m. A B C 2.1 x 10' 46 59 6 1.6 x lo4 28 37 1 1.9 x 10' 13 0.2 -4.4 x 10' 15 71 6 1.7 x lo4 5.2 22 1 9.1 x 10; 78 5 -8.2 x 10 0.8 5.5 0.1 1.5 x lo4 12 88 0.6 2.0 x 10' 58 4.7 -2.5 x loa 18 53 1.5 2.5 x lo4 13 43 0.5 3.4 x los 64 2.4 -4.0 17 88 71 3.0 1 5.3 1.5 7.5 2.4 x loa 23 70 6.5 9.7 x los 7 23 0.5 2.7 X lo* 67 2.6 -3.3 x 10' 24 95 81 1.7 x 10' 0.1 0.4 0.1 3.8 X 10' 76 4.0 -82 6.5 -Element MI? P Ca % & p.p.m.A B C 1.0 X 10' 10.3 - -1.1 X 10' 1.4 - -6.0 X 10' 2 - -6.2 X 10' 29 - -1.6 X 10' 4.6 - -2.2 x 10 - - -0.1 - - -- -7 p.p.m. A B C 9.5 x 10' 2.7 3.8 77 9.1 X 10' 0.4 2 38 1.1 x 104 1 4.4 92 2.1 x los 0.2 1.8 8.3 4.5 X los 0.4 1.6 9 1.4 x 10' - 0.6 10.5 0.5 - 0.2 27.2 , p.p.m. 1.9 x 10' 1.3 x 101 1.2 x 10' 1 x 10% 6 x lo-* 2.1 x 1 0 4 4.1 x 1 0 4 * A 5 min irradiation and 6 min cooling time.f B 3 h irradiation and 3 h cooling time. 3 C 4 d irradiation and 4 d cooling time. $Grams per 24 h average value 1194 MANTEL the four nuclides with a 1 -3-T electromagnet. The greatest reduction in background is seen to be due to 32P. This may be because for this nuclide which is a pure /3-emitter the background is entirely due to the effect of /3-particles that are efficiently deflected by the magnet. For the other nuclides which are /3- and y-emitters the Compton peaks of the y-rays will also contri-bute to the background. The smallest decrease in background is obtained for Z4Na which at the same time has the highest abundance (100%) of gamma rays.* From the results shown in Table 11 it follows that the reduction in background obtained with a magnet for a given matrix depends to a great extent on its composition.Thus the limiting factors in the limit of detection obtainable by the present method will be the composition of the matrix and the intensity of the magnetic field. In considering the influence of the background on the limits of detection of trace elements in biological materials one may group the trace elements according to the half-lives of the radio-isotopes used for their determination i.e. those with half-lives of a few minutes to 3 h 3 h 4 d, or more than 4 d. The amounts of the major inorganic components of the matrices studied in this work and the contribution of each to the total activity (measured without a magnet) of the irradiated sample, after (A) 5 min (B) 3 h and (C) 4 d of irradiation and equal cooling time are given in Table 111.In the first instance all the major components of the matrix contribute to the background the highest contribution being due to chlorine (except in animal muscle). In the second instance the background is essentially due to sodium and potassium and in the third instance to phosphorus and calcium. Conclusion X-ray spectrometry and magnetic deflection of p-rays were applied to instrumental neutron activation of seven biological matrices. The limits of detection of 18 trace elements were calculated taking into account the influence of the background which is specific for every matrix. These limits of detection are based on the experimental set-up available in our laboratory and are thus only a guide line for other workers interested in applying this method under their specific conditions.This work was supported by the IAEA Vienna. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. References Mantel M. and Amiel S. Anal. Chem. 1972 44 548. Mantel M. and Amiel S. in Amiel S . Editor “Nondestructive Activation Analysis,” Elsevier, Amiel S. Mantel M. and Alfassi 2. B. J . Radioanal. Chem. 1977 37 189. Mantel M. Alfassi 2. B. and Amiel S. Anal. Chem. 1978 50 441. Alfassi 2. B. Biran-Izak T. and Mantel M. Nucl. Instrum. Methods 1979 17 227. Rapaport M. S Mantel M. and Nothmann R. Anal. Chem. 1979 51 1356. Rapaport S. Mantel M. and Shenberg C. J . Radioanal. Chem. 1982 75 145. Lederer M. Hollander J. M. and Perlmann I. “Table of Isotopes,” Sixth Edition John Wiley New Parr R. M. “Intercomparison of Minor and Trace Elements in IAEA Animal Muscle (H-4),” IAEA/ Dybczynsky R. Veglia A. and Sushny O. “Report on the Intercomparison Run A-1 for the Deter-“Certificate of Analysis Standard Reference Material 1577 Bovine Liver,” National Bureau of Stan-“Certificate of Analysis Standard Reference Material 1571 Orchard Leaves,” National Bureau of Bowen H. J. M. J . Radioanal. Chem. 1974 19(2) 215. “Report of the Task Group on Reference Man,” International Commission on Radiological Protection, Received February 2nd 1983 Accepted April 21st 1983 Amsterdam 1981 pp. 25-41. York 1967. RL/69 Report No. 2 October 1980. mination of Inorganic Constituents of Milk Powder,” IAEA/RL/68 July 1980. dards Washington DC June 1977. Standards Washington DC January 1971. No,. 23 1974 p. 290

ISSN:0003-2654    

DOI:10.1039/AN9830801190

出版商:RSC    

年代:1983

数据来源: RSC

摘要:

Analyst October 1983 Vol. 108 pp. 1195-1208 1195 Discrimination Against Atomic-emission Spectral Interferences in Wavelength-modulated Continuum Source Excited Flame Atomic-fluorescence Spectrometry* John T. McCaffrey Man-Li Wang Wu and R. G. Michelt Defiartment of Chemistry University of Connecticut Storm CT 06268 USA The extent of spectral interferences at commonly used analytical atomic-fluorescence lines was demonstrated to be severe enough to necessitate the exploration of possible instrumental approaches to discriminate against the more serious type of spectral interference namely the atomic emission of concomitant metals in samples. Two novel methods of double modulation were incorporated into the instrumentation in order to allow such discrimina-tion. The two methods were evaluated with respect to their effect on detection limits and their effect on the particularly severe interference of potassium emission at 404.4 nm on lead fluorescence at 405.8 nm.Keywords A tomic-jluorescence spectrometry ; double modulation ; spectral interferences ; wavelength modulation ; continuum source Introduction Continuum source excited atomic-fluorescence spectrometry (AFC) has been proposed to offer many advantages in multi-element analysis.192 Amongst them are good detection limits in a system capable of determining quantitatively a large number of elements in samples using only one light source. Wavelength-modulated AFC permits the measurement of a combined emission and fluorescence signal hence permitting the determination of elements such as sodium and potassium which are more sensitively determined by atomic emission than by atomic fluorescence.The ability to measure both emission and fluorescence as combined signals has been found to be useful in the analysis of real samples. However atomic-emission measurements of any kind usually require the use of a narrow spectral band pass for the mono-chromator in order to discriminate against spectral interferences. In contrast fluorescence measurements particularly with line-source excitation exhibit few spectral interferences. Thus it is possible to use a large spectral band pass i.e. wide slit widths and a monochromator with a small f number in order to improve sensitivity by greater light throughput. The combination of emission and fluorescence in wavelength-modulated AFC must therefore, result in a number of spectral interferences unless the spectral band pass is made narrow.This would inevitably reduce sensitivities. Limited data presented in the literature2 and extensive data presented here demonstrate conclusively that the extent of spectral interferences that affect wavelength-modulated AFC is severe enough to necessitate the exploration of possible instrumental approaches to discriminating against such interferences. Instrumentation for AFC has been described el~ewhere.l-~ In such instrumentation the use of source intensity modulation alone allows for the discrimination against spectral inter-ferences caused by atomic emission of concomitant elements in the matrix but cannot dis-criminate against signals caused by the scatter of incident radiation from incompletely vaporised species in the flame.Conversely wavelength modulation can discriminate against ~ c a t t e r ~ - ~ but cannot discriminate against spectral interferences caused by atomic emission of concomitant elements. Neither approach will discriminate against spectral interferences caused by atomic fluorescence of concomitant elements. * Portions of this paper were presented at FACSS VIII Philadelphia PA on September 20 1981 and FACSS IX Philadelphia PA on September 20 1982. t To whom correspondence should be addressed 1196 MCCAFFREY et al. DISCRIMINATION AGAINST AES Analyst VoZ. 108 The combination of both source intensity modulation and wavelength modulation is called double modulation and discriminates against both scatter and spectral interferences caused by atomic emission of concomitant elements.Elser and Winefordners have described a double-modulation system that was used for both continuum source excited atomic absorption6 and atomic fl~orescence.~ The atomic-fluorescence instrument suffered from poor sensitivity. In this paper we describe two instrumental systems that achieve double modulation for AFC and give detection limits that closely approach those obtainable with the best source intensity or wavelength-modulated AFC instrumentation. The two novel methods of double modulation described here are called fixed-phase double modulation (FPDM) and dual lock-in double modulation (DLDM) and these were evaluated with respect to their effect on the particularly severe interference of potassium emission at 404.4 nm on lead fluorescence at 405.8 nm.Experimental Spectral Interference Studies The instrumentation used for the spectral interference studies of wavelength-modulated AFC was similar to that described pre~iously.~ The performances of the two instruments were identical in all respects. The instrumental conditions used for the interference studies are given in Table I. Wavelength modulation was accomplished with a rotating four-quadrant quartz chopper,3 also called a sectored wheel. The wheel was placed behind the middle slit of a double mono-chromator as described previ~usly.~ TABLE I GENERAL EXPERIMENTAL CONDITIONS Monochromator*-Entrancelexit slits . . . . 0.25 mm Middle slit . . .. 2.0mm Spectral band pass . . . . 0.5 nm Source*-Current . . 17A Power . . 300W Wavelength modulation frequency . . . . 30Hz Angle of incidence of light in monochromator Flame . . . . Nitrogen-separated air - acetylene operated as on wavelength modulation wheel . . 45" a stoicheiometric mixture * See Table I11 for further details. Table I1 lists the commonly used analytical atomic-fluorescence lines that were tested for spectral interferences together with those metals which have experimentally observed atomic-absorption -emission or -fluorescence lines (from reference 7) within 2 nm of the analytical fluorescence lines and which were tested as possible interferences. Lines within 2 nm were chosen because this allowed for interferences that were likely to occur on our instrument.This had a nominal spectral band pass of 0.5 nm that when using wavelength modulation, gave an effective spectral band pass of about 1.5 nm. Nominal band passes of about 0.5 nm are typical of some AFC instruments1s2 although not all such instrument^.^ Clearly those instruments with smaller band passes will not be as subject to interferences although in all probability they will not have as much light gathering power. Single element solutions of the analyte were used to obtain calibration graphs at the analyte wavelength. At the same wavelength calibration graphs for the interferents were obtained. An interference was deemed negligible if a 1000 pg ml-l interferent solution resulted in no detectable signal at the analyte wavelength.Double Modulation Studies The components common to both methods of double modulation are listed in Table 111 October 1983 INTERFERENCES IN CONTINUUM SOURCE EXCITED FLAME AFS TABLE I1 TESTED ANALYTICAL FLUORESCENCE LINES AND INTERFERENCES 1197 Fixed-phase Double Modulation Analyte Cd Ca Cr c o c u In In Fe Pb Mg . . Mn . . Analytical linelnm 228.80 422.67 357.87 369.35 240.73 242.49 252.14 324.75 327.40 303.94 410.18 451.13 248.33 283.31 405.78 285.21 279.48 279.83 280.11 Possible interferents Bi As Co Ni Sr Co Fe Mn Ni Cr Co Fe Mn Ni Pd Cr Ca Co Co Pt Sr Sn Co Fe Cd In Pd Cd In Ag Co Fe MG Ni Pt Sn Bi Co Cu. Pd. Fe Mg Pti T1 Sn K Sr ~ b Sn Bi Pb Mg Analyte Mn .. Ni Pd Pt Ag Na . . Sr . . T1 Sn ,. Zn Analytical linelnm 403.08 403.31 403.45 232.00 352.45 343.46 360.96 363.47 265.95 328.07 338.29 589.59 460.73 377.57 535.05 286.33 303.41 317.51 213.86 Possible inferents K Co Ni Bi Co Tl Pd Ni Co Fe Ni Pd Cr Co Mg Ni Pb Mg Ni Pt Cd Cu Bi Ni Mg Co In Fe Mn Ni Pt Pt Fixed-phase double modulation (FPDM) is a special case of the general method used by Elser and WinefordnerJ6 who used two modulation frequencies that had random frequency and phase relationships and required tuned filters to derive a reference signal in order to extract the fluorescence signal corrected for background scatter and structured emission.With FPDM two square-wave modulation frequencies which have the phase and frequency relationships shown in Fig. 1 allow the background-corrected fluorescence signal to be extracted with a conventional lock-in amplifier or a synchronous photon counter. FPDM depends on modulat-ing the source intensity at exactly double the frequency and 90" out of phase with the wave-length modulation. The required atomic-fluorescence signal is then detected at the wavelength modulation frequency but 90" out of phase with the wavelength modulation. For clarity the signal processing shown in Fig. 1 is in terms of synchronous detection of the type normally found in photon counters i.e. the signal plus background ( A ) and background (B) are routed TABLE I11 INSTRUMENTAL COMPONENTS COMMON TO BOTH DOUBLE MODULATION METHODS Component Type and/or description Source .. . . 300 W xenon arc (Model VIX300 Xenon arc power supply . . . . Model PS300-1 UV) Monochromator . Model DH-20A UV - visible 0.2 m double monochromator fl4.2 aperture Lenses . . . . Silica lenses 2 in diameter bi-convex focal length 50 mm Pre-mix burner chamber . . Burner head and nitrogen separator Wavelength modulation chopper . . Sectored wheel.3 30 Hz modula-tion frequency (rotating quartz chopper) Photomultiplier tube . . . . 9893QB/350 Supplier Varian Eimac Division, San Carlos CA Varian Eimac Division, San Carlos CA Instruments SA Inc., Metuchen N J Esco Products Oak Ridge, Perkin-Elmer Corp., Laboratory constructed NJ Norwalk CT Laboratory constructed EMI-Gencom Inc., Plainview NY Betram Assoc.Syosset, NY Photomultiplier power supply . . Model 21 1198 MCCAFFREY et al. DISCRIMINATION AGAINST AES Analyst Vol. 108 I I I S time I Source i intensity I modulation Fig. 1. Fixed-phase double modulation waveforms. FB = flame background emission; S = scatter of source radiation; AE = atomic emission from analyte; AE' = matrix emission a t a different wavelength to analyte A E ; AE" = matrix emission at same or similar wavelength to analyte A E ; and AF = wanted atomic fluorescence. The frequency relationships together with the 90" phase shifts place half of the various possible interfering signals (FB S AE AE' AE") into each of the background (B) and signal ( A ) channels.The subtraction (A - B) gives the wanted AF signal as follows ( A - B) = AF = (FB + FB + AE + AE'+ AE" + S + AF) - (FB + FB + AE + AE'+ AE" + S ) . into two separate counters. B is then subtracted from A to give the required signal. The discrimination against scatter and atomic emission is illustrated by the equation in the caption to Fig. 1. FPDM can be treated mathematically in a similar fashion to the treatment of Elser and WinefordneP and corresponds to their second-derivative difference mode of detection. Those additional components necessary for FPDM are listed in Table IV and a block diagram of the instrumentation is shown in Fig. 2. TABLE IV ADDITIONAL COMPONENTS NECESSARY FOR FIXED-PHASE DOUBLE MODULATION Component Type and/or description Source modulation motor .. . . Model BSMX 2038 115 VAC, 50-60 Hz polarised synchronous motor 1800 rev min-l Photon counter* . . . . Model 11 12 photon counter/ processor (in synchronous counting mode) Amplifier/discriminator . . . . Model 1120 Power amplifier to drive synchronous motor . . . . . . Two Model W5M 10 W audio amplifiers operated in parallel Supplier Elinco Inc. Norwalk CT Princeton Applied Research Princeton N J Princeton Applied Research Princeton N J Heathkit Benton Harbor MI Frequency doubler . . . . 30-60 Hz frequency doubler Laboratory constructed Digital phase shifters . . . . 60 and 30 Hz TTL phase shifters Laboratory constructed Chopper for source modulation . . Two-blade chopper Laboratory constructed * Occasionally the PAR lock-in amplifier (Table V) was used instead of the photon counter.FPDM required that source modulation occurred at exactly double the frequency of wave-length modulation. The synchronous motor was driven with a 60-Hz frequency doubled signal derived from the 30-H~ wavelength modulation reference signal. It was necessary to operate the system at these frequencies because the synchronous motor was designed only for 50-60-Hz operation. Work is progressing on replacing the synchronous motor with a high power bipolar stepper motor and this should allow FPDM over a much larger range of modula-tion frequencies October 1983 INTERFERENCES IN CONTINUUM SOURCE EXCITED FLAME AFS 1199 It can be seen from the phase relationships in Fig. 1 that FPDM requires accurate control of the phase of both wavelength modulation and source intensity modulation.This was achieved by separate control over the phase of the wavelength modulation reference signal and the phase of the source intensity modulation with respect to the wavelength modulation. The synchronous motor was of a polarised design to ensure that the source intensity chopper always synchronised in a particular position relative to the driving 60-HZ waveform. This was in an attempt to ensure that the phase relationships of the instrument would stay constant whenever the instrument was switched off and on again. ynchronous motor Double monochromator Fig. 2. Instrumental arrangement for fixed-phase double modulation. Experimental procedzcre for FPDM Firstly the correct wavelength was obtained by observing the output signal under wavelength modulation condi-tions.The reference signal was then phase shifted to place the detection phase 90" out of phase with the analytical signal by aspiration of an analyte solution and zeroing of the signal. Then the source modulation was switched on. The signal from the photomultiplier tube was then observed on the oscilloscope while phase shifting the source modulation signal with respect to the wavelength modulation signal until the waveform of Fig. 1 was obtained. Fine adjust-ment of the source-intensity modulation phase was possible by observing the output signal of the photon counter or lock-in amplifier because the proper phase corresponded to the maximum net signal when aspirating an analyte solution.The adjustment of the source modulation phase does not affect the wavelength modulation phase. An oscilloscope was necessary to obtain the desired phase relationships. Dual Lock-in Double Modulation Dual lock-in double modulation (DLDM) is similar to the method of Koizumi et aZ.* who used polarisation modulation of the light source (Zeeman effect background correction) combined with source intensity modulation in order to achieve background correction for graphite furnace atomic-absorption spectrometry. They used two widely separated modulation fre-quencies and discriminated between the two frequencies by the use of a filter tuned to the source intensity modulation frequency. The output of the tuned filter was then fed to a lock-in amplifier tuned to the Zeeman modulation frequency.The band pass filter discriminated against all signals not source intensity modulated and therefore did not respond to structured emission signals. Synchronous detection at the Zeeman modulation frequency served to discriminate against all remaining signals not Zeeman modulated i.e. molecular absorption and scattered light. The combination of the two modulation techniques allowed discrimination against both continuum-background and atomic-emission signals. In our work a sixth-orde 1200 MCCAFFREY et aE. DISCRIMINATION AGAINST AES Analyst Vol. 108 active filter was used with a lock-in amplifier but it was found impossible to resolve completely between source-intensity modulation at 350 Hz (the maximum frequency possible with our laboratory-built mechanical chopper) and wavelength modulation at 30 Hz.This was due to the closeness of our two modulation frequencies. Instead a second lock-in amplifier was used to resolve the two frequencies and directly replaced the tuned filter. The approximate wave-forms for DLDM are shown in Fig. 3. Source intensity modulation cycle + I + I I ! FB ! FB ! FB I I fT L. Wavelength modulation4 Time or wavelength cycle - 5 Fig. 3. Dual lock-in double modulation waveforms (for acronyms see Fig. 1). The first lock-in amplifier tuned to the source intensity modula-tion frequency discriminates against all signals not source intensity modulated (AE AE' AE" FB). The second lock-in amplifier tuned to the wavelength modulation frequency discriminates against all remaining signals not wavelength modulated ( S ) .For clarity the relative frequency relationships (30 Hz wavelength modulation and 350 Hz source modula-tion) have not been represented exactly in this diagram. The components that were necessary for DLDM in addition to those in Table I11 are given in Table V. Simultaneous source modulation at about 350 Hz and asynchronous wavelength modulation at about 30 Hz were used. The commercial lock-in amplifier set a t minimum output low-pass filtering was used for synchronous detection at the source modulation fre-quency. The output of this lock-in was then fed to a second laboratory-constructed lock-in amplifier tuned to the wavelength modulation frequency. This instrumental arrangement is shown in Fig. 4. The first lock-in amplifier discriminated against all signals that were not source modulated (e.g.structured background and atomic emission). The second lock-in amplifier discriminated against those signals that were not wavelength modulated (e.g. scatter of incident radiation) as shown in Fig. 3. TABLE V ADDITIONAL COMPONENTS NECESSARY FOR DUAL LOCK-IN DOUBLE MODULATION Component Type and/or description Supplier Lock-in amplifier . . . . PAR Model 5204 lock-in Princeton Applied Lock-in amplifier . . . . X Y multiplier based lock-in Laboratory constructed Source modulation chopper . . . . Five-blade chopper Laboratory constructed Source modulation motor . . . . Variable speed 0-20 V d.c. motor TRW-Globe Dayton OH Variable d.c. power supply . . . . 0-20V d.c. Laboratory constructed analyser Research Princeton N J amplifie October 1983 INTERFERENCES IN CONTINUUM SOURCE EXCITED FLAME AFS Lock-in Recorder 1201 \ J ' I 7 : Lock-in In amplifier (2) Fig.4. Instrumental arrangement for dual lock-in double modulation. \ 7 t +j Lens Experimental procedzlre for DLDM The first lock-in amplifier was tuned using source modulation alone. The second lock-in amplifier was then tuned when using simultaneous source intensity modulation and wavelength modulation. The wavelength of interest was obtained using wavelength modulation alone. Reference Square Modulation Waveforms For both methods of double modulation square modulation waveforms were used to ensure maximum efficiency of detection of the fluorescence signals.It has been showns that square-wave modulation has a signal to noise ratio (SNR) advantage over sinusoidal modulation when using synchronous detection. The wavelength modulation system3 that was used in this work inherently gives square-waveform wavelength modulation and it is simple to achieve square-waveform source intensity modulation by the use of large mechanical chopper blades. These square waveforms are particularly important for double modulation because of the expected loss of signal inherent in using modulation techniques i e . a factor of two for any sort of 50% duty cycle modulation over direct current detection and a further factor of two after combining two types of modulation. Hence by using efficient square-wave modulation systems the loss in signal caused by double-modulation over single-modulation techniques can be restricted to the theoretical factor of two, The signal processing system of Koizumi et aZ.8 was inherently sinusoidal because the shape of the analytical signal was transformed by the tuned filter.The DLDM modification described allowed for square-wave signal processing. \ I Chopper General Experimental Conditions All measurements were made under the conditions given in Table I except where indicated in the text. Single element aqueous metal solutions were prepared in 0.04 M hydrochloric acid (to ensure maximum stability) from 1000 pg ml-l stock solutions made with analytical-reagent grade salts. / / 4 i NBS sample preparation Dried National Bureau of Standards (NBS) tomato leaves (SRM 1573) were weighed (0.1-0.4 g).Known amounts of aqueous standards were added to each sample to allow the con-struction of a calibration graph for analysis by the method of standard additions and then th 1202 MCCAFFREY et aZ. DISCRIMINATION AGAINST AES Analyst VoZ. 108 samples were digested in 10 ml of nitric acid for 4-8 h dpending upon the mass of the sample. The solutions were heated until dry 10 ml of aqua regia were added to each sample to continue the digestion and then the digests were brought to near dryness (approximately 1 ml). The residues were diluted with distilled water to 100 ml in calibrated flasks. A blank solution consisting of only the acids was prepared by the same digestion procedure. The samples were then analysed using the techniques of wavelength modulation and FPDM as described.Four samples were analysed by each method to obtain a determination of the precision of the analysis. Digested acid blanks were subtracted in all instances. All the standard additions graphs had a slope of approximately 1 indicating minimum chemical interference. Results and Discussion Spectral Interference Studies The interferences that were observed when using wavelength-modulated AFC are given in Table VI. The slope is a real slope in units of counts s-l per pg ml-l. This slope therefore could be viewed as the number of counts s-l obtained on the photon counter for a 1 pg ml-1 solution. This aids in interpreting the magnitude of an interference. For example for cadmium at 228.8 nm (the first element in Table VI) 506 counts s-l could be obtained for a 1 pg ml-l solution assuming that the calibration graph is linear at that concentration.For the same concentration of nickel at 228.8 nm 1.1 counts s-l could be obtained. Hence at the same concentration the interference of nickel on cadmium could be considered insignificant but assuming the linearity of the calibration graphs a &fold excess of nickel would give an error of about 1%. TABLE VI OBSERVED* SPECTRAL INTERFERENCES IN WAVELENGTH-MODULATED AFC Cd Cr Cr c u c u In Fe Pb Pb Mg Analyte * . Analyte wavelength/ nm 228.8 357.9 359.3 324.8 327.4 410.2 248.3 283.3 405.8 285.2 Slopet /counts per pgml-l 506 1260 1.1 7.5 - 1.4 - 15 1060 - 15 2.8 5.3 -0.7 3.7 - 0.7 -0.85 1370 - 38 850 - 860 2 900 242 -2.8 -0.7 265 2.8 - 6.3 990 - 180 4.9 x 104 -0.6 - 5.2 Limit of detection / pg ml-l 0.05 12 0.05 5 10 5 0.1 5 35 15 0.1 100 1 .o 10 0.1 30 125 0.1 0.003 0.2 1.0 50 50 100 26 0.15 1.3 0.001 175 25 Interferents and wavelengthslnm Ni 229.0 [AF] Co 357.9 357.8 Fe 357.0 358.6 Ni 357.2 Fe 358.1 358.7 [AE] Co 359.5 [AE] Ni 359.8 Cd 326.1 [AE] [AF] In 325.6 [AE] Pd 324.3 325.2 In 325.9 [AE] Cd 326.1 [AE] [AF] Ag 328.1 (AE) (AF) Co 409.2 Pd 247.6 [AF] Pt 283.0 [AF] Sn 284.0 [AE] [AF] K 404.4 404.7 CAE] Pb 286.4 Sn 286.3 284.0 [AE] [AF October 1983 INTERFERENCES IN CONTINUUM SOURCE EXCITED FLAME AFS 1203 Mn Mn Ni Ni Pd Pd Pd Ag Sn Sn Analyte * .. . Analyte wavelength/ nm 279.5 279.8 280.1 403.1 403.3 403.4 232.0 352.5 343.5 361.0 363.5 328.1 286.3 303.4 TABLE VI-continued Slope? /counts per pg ml-l 7 200 -0.5 12 3 020 - 65 2.2 - 0.8 290 14 - 8.9 15.5 28 35 - 5.7 1 50 - 39 - 3.7 68.5 0.4 190 - 1.4 2 640 - 540 17 -2.65 x 104 16.9 - 22 - 23 55 3.1 - 1.2 Limit of detection$/ pg ml-1 0.008 160 0.5 0.1 8 0.5 10 0.05 2 20 1.3 5 1.3 0.5 1 .o 5 100 3 > 100 0.5 100 0.05 0.1 1.5 0.001 2 3 2 1 10 350 Interferentss and wavelength§/nm Pb 280.2 [AF] Mg 279.6 280.3 K 404.4 [AE] Co 230.9 Co 352.7 (AE) Pd 351.7 [AE], T1 352.9 Co 343.2 343.3 [AE] Ni 344.6 [AE] [AF] Ni 361.9 [AE] Co 360.2 Cr 360.5 MgOH 362.4 [AE] Pb 364.0 [AE] [AF] Cu 327.4 (AE) (AF) Mg 285.2 (AF) Co 304.4 Fe 302.1 [AF] In 303.9 [AF] Ni 303.2 Pt 304.3 [AF] * For experimental conditions see text and Table I.t Slope of linear region of log - log plot of signal us. concentration. $ Approximate “limit of detection” corresponding to the concentration a t which the calibration graph intercepts the background level. This is within a factor of 2 of the calculated detection limit based on SNR - 2. Count time 1 s; and lamp current 17 A. The symbols (AE) and (AF) indicate that the wavelength is an analytical line that is used routinely for atomic emission or atomic fluorescence respectively The symbols [AE] and [AF] indicate that AE or AF has been observed at the indicated wavelengths but that they are not usual analytical lines.Element and most likely wavelength(s) in nanometres causing an interference. The sign of the slope indicates the nature of the spectral interference. A positive slope means that the interfering spectral line was within the 0.5 nm nominal spectral band pass of the monochromator and added to the signal. A negative slope means that the interfering line occurred during the background measuring part of the wavelength modulation cycle. There-fore the interfering line occurred near the nominal analyte wavelength but outside the 0.5 nm nominal band pass of the data measuring part of the wavelength modulation cycle and thus was subtracted from the signal.This resulted in the interferent signal becoming more nega-tive with increasing concentration hence the assigned negative slope. In the real negative slope situation which was tested here for the interference of potassium on lead at 405.8 nm the net analyte signal decreased with increasing interferent concentration. The magnitude of the interference clearly depends on interferent concentration. However, at equal concentrations of analyte and interferent three quarters of the analyte metals listed were affected by at least one interferent with a signal change of more than 1%. The limits o 1204 MCCAFFREY et al. DISCRIMINATION AGAINST AES Analyst VoZ.108 detection give a further indication of the magnitude of interference. For example a small number indicates a worse interference than a large number and any concentration of inter-ferent below its limit of detection does not cause an observable interference. Table VI only serves as a guide to the interferences likely to be encountered in wavelength-modulated AFC. This is particularly so in view of the usual difficulty in reproducing flame and instrumental conditions. Flame conditions will of course significantly alter the relative magnitudes of the atomic signals from all metals. Nature of the Spectral Interference The interferences in Table VI could be due to either atomic emission (AE) of the interferent or atomic fluorescence (AF) of the interferent excited by the continuum light source.These were not differentiated in this work. However an interference due to atomic emission can be distinguished from atomic fluorescence by using source intensity modulation alone or by making a blank measurement after blocking the light source. An indication of the sources of the inter-ferences (AE or AF) is given in Table VI. The symbols (AE) and (AF) indicate that the wavelength in question is an analytical line used routinely for AE or AF respectively. The symbols [AE] and [AF] indicate that AE or AF have been observed at the indicated wave-lengths but that they are not the usual analytical lines. Two symbols together indicate that the interference is probably a combination of both AE and AF. One symbol indicates that the interference is probably dominated or entirely due to the indicated process.The relative magnitudes of the contributions of the two processes to the interferences will depend on the intensity and age of the light source as well as the character of the lines in question. The potassium interference on lead at 405.8 nm was verified on our system to be entirely due to potassium atomic emission at a 1000 pg ml-l potassium concentration. Double Modulation The merits of the two methods of double modulation were evaluated by studying the spectral interference of potassium emission at 404.4 nm on the measurement of lead fluorescence at 405.8 nm when using wavelength-modulated AFC. Wavelength modulation alone was used and signals were measured at 405.8 nm while varying in separate experiments the concentra-tion of lead or potassium.Lead produced the expected linear calibration graph with increas-ing positive signals (Fig. 5). Potassium also gave a linear calibration graph but the signals decreased with increasing concentration (Fig. 5). This was a result of the potassium signal appearing in the background part of the wavelength modulation cycle because the potassium 1 0s c 'a 105 2 100 1 03 U 3 C 0 m .-7 lo4 cn 3 U 3 z S 103 1 o2 1 02 0.1 1.0 10 100 1000 104 Fig. Concentration/pg ml- ' 0 10' 1 02 1 03 Lead concentration/pg ml-' 6. Fixed-phase double modula-tion calibration graph showing the elirni-Fig. 5. Spectral interference nation of the potassium interference at the lead 405.8 nm line.Both the (0) aqueous lead data and the (0) data obtained with 100 pg ml-l potassium added are shown. The solid line corresponds to the least-squares log - log regression line for the points with (0) 100 pg ml-I of potassium added. from potassium at the lead 405.8-nm line. Potassium (A) and lead (B) calibration graphs a t 405.8 nm. The broken line indicates an increasingly negative signal October 1983 INTERFERENCES IN CONTINUUM SOURCE EXCITED FLAME AFS 1205 wavelength was to one side of the lead line. A concentration of 1000 pg ml-l of potassium gave a signal count equivalent to -200 pg ml-l of lead. A calibration graph for lead at 405.8 nm was obtained with and without the addition of 100 pg ml-1 potassium using FPDM (Fig. 6).Similar data were also obtained for FPDM at the 1000 pg ml-1 level (Fig. 7). The results indicated that the system did not respond to potas-sium because the calibration graphs did not change after addition of potassium. Similar 0 I I 10’ 1 o2 103 Lead concentration/pg ml-’ Fig. 7. Fixed-phase double modula-tion calibration graph showing the elimi-nation of the potassium interference at the lead 405.8nm line. Both (0) the aqueous calibration data for lead and (0) the data obtained with 1000 pg ml-l of potassium added are shown. The solid line corresponds to the least-squares log -log regression line for the points with (0) 1000 pg ml-l of potassium added. behaviour was observed for DLDM. However the experiments with varying concentrations of potassium demonstrated as expected that the noise on the lead signal was increased in the presence of potassium.This is because the techniques of double modulation do discriminate completely against the interfering emission signal but do not discriminate against the accomp-anying noise. This is evident in Fig. 7 from the large scatter of the points about the calibra-tion graph when 1000 pg ml-l of potassium are present relative to the scatter observed when 100 pg ml-l of potassium are present (Fig. 6 ) . Table VII gives the log - log regression equations for the lead calibration graphs with and without potassium present. The standard deviation about the regression line for the lead calibration graph almost doubled when 100 pg ml-l of potassium were present relative to the calibration graph when no potassium was present.TABLE VII EFFECT OF ADDED POTASSIUM ON THE DETERMINATION OF LEAD AT 405.8 nm Potassium concentration*/ pg nil-’ s,.,.t y s Regression equations 0 0.06 0.996 y = 0.84(f0.07)~ + 1.9 100 0.10 0.992 y = 0.87(&0.11)x + 1.9 1000 0.38 0.894 y = 0.83(&0.42)~ + 2.0 * Concentration of potassium added to aqueous soh tions of lead. t Standard deviation about the regression line; y = fluorescence signal; x = $ Correlation coefficient of the regression equation. 5 Log - log regression equation with the 95% confidence limit for the slope given All equations are based on a total of eight lead concentrations, lead concentration in micrograms per millilitre. in parentheses. varying from 5 to 1000 pg ml-1. See discussion in text 1206 MCCAFFREY et al.DISCRIMINATION AGAINST AES Analyst VoZ. 108 Further when 1000 pg ml-l of potassium were present the standard deviation about the regression line increased 6-fold relative to the aqueous lead calibration graph without potas-sium added and lead detection limits were degraded to approximately 50 pg ml-1 when 1000 pg ml-l of potassium were present. The points below 50 pg ml-l in Fig. 7 were not significantly different from the blank but were included in Fig. 7 and in the regression analysis (Table VII) in order that a direct com-parison of the decreased precision resulting from high potassium concentrations could be made. The slope of the lead calibration graph (Table VII) is less than one because the poor sensitivity shown by lead results in a linear range of only about two orders of magnitude.For metals with better sensitivity than lead the linear range will be longer. We are currently carrying out detailed studies concerned with the effect of spectral interferences on the signal to noise ratio of double-modulated AFC during the analysis of various real samples (standard reference materials). However it is clear from Fig. 7 and from Table VII that the determination of lead at 405.8 nm in the presence of concentrations of potassium above about 100 pg ml-l would be affected by decreases in precision. The lead line at 283.3 nm is not affected by potassium atomic emission and would in fact be the wavelength of choice for lead determina-tion. The 405.8 nm lead line was chosen here only to demonstrate the effectiveness of double modulation in discriminating against spectral interferences of which the lead/potassium situa-tion is probably an example of the worst that such an interference could be in samples other than those in which the major constituent results in a spectral interference.Detection Limits for the Double Modulation Systems The detection limits given in Table VI were not obtained under optimum conditions and are consistent only in the relative magnitudes of each associated interferent and analyte detection limit and only indicate the severity of a spectral interference on a particular metal. Detection limits obtained under conditions close to the optimum are given in Table VIII for source intensity wavelength and double modulation as described and for the double modulation method of Elser and WinefordneF as applied to AFC.* It can be seen that the double modula-tion methods described here gave detection limits that were improved over the previous double modulation AFC figures.6 Double modulation halves the amount of time spent measuring the analyte fluorescence compared with single modulation.Therefore double modulation would be expected to produce detection limits about a factor of 2 worse than source modulation alone or wavelength modulation alone. Only FPDM achieved this goal being at most a factor of 3 worse than either method of single modulation. The calculated detection limits in Table VIII were based on measurements of aqueous solu-tions of pure metal salts with no interferents present and on the assumption that the noise was TABLE VIII LIMITS OF DETECTION USING SOURCE INTENSITY WAVELENGTR AND DOUBLE MODULATION FOR CONTINUUM SOURCE EXCITED ATOMIC-FLUORESCENCE MEASUREMENTS Wavelength/ Element nm Zn .. . . 213.9 co . . . . 240.7 Pb . . . . 283.3 Mg . . . . 285.2 Cr . . . . 357.9 Pb . . . . 405.8 Sr . . 460.7 7 SMt 9 20 200 0.4 4 200 2 Limit of detection*/pg 1-I A 1 WM FPDMS DLDMT DMAFFSII - 10 20 50 30 30 100 -200 500 700 -0.6 0.9 0.9 30 4 10 20 600 300 500 2 000 5 000 1 3 20 -* A 300-W xenon arc operated at 20 A and 10 s count time was used in this work for all experiments except SM DMAFFS when a 900-W xenon arc was used and the data were taken from reference 4. t SM source intensity modulation. WM wavelength modulation.FPDM fixed-phase double modulation. 7 DLDM dual lock-in double modulation. 11 Double modulation by the method of reference 4. DMAFFS is double modulation atomic-fluorescence flame spectrometry October 1983 INTERFERENCES IN CONTINUUM SOURCE EXCITED FLAME AFS 1207 white (shot noise) and could be calculated by taking the square root of the background. The detection limits were demonstrated to be shot noise limited by standard deviation measure-ments of example metals at concentrations close to the detection limit. The detection limits for DLDM were up to eight times lower than those for FPDM. How-ever they were still an improvement over previously published double modulation results. The reasons for the low detection limits for DLDM were not clear but it was thought that they were due to the use of the laboratory-constructed lock-in amplifier which may have been less efficient at filtering noise than a commercial instrument.Also a photon counter was used for FPDM which in our experience is usually more sensitive than a lock-in amplifier for flame spectroscopic measurements. In the low background regions of the flame this sensitivity improvement can be a factor of 3-5. Real Sample Analysis Table IX shows the results obtained for the analysis of NBS tomato leaves. The results are in good agreement with the NBS certified values in all instances except for the determination of iron by wavelength modulation at 248.3 nm. The low value obtained for iron by wave-length modulation indicates that a spectral interference occurred during the background measuring part of the wavelength modulation cycle.The interference was corrected by FPDM because the FPDM result was in agreement with the NBS certified value. The exact nature of the interference is unknown and does not correspond to any of the atomic line inter-ferences shown in Table VI. Based on the number and severity of the spectral interferences shown in Table VI it may appear that the determination of any metal in a real sample would be impossible by wavelength modulation. However as shown in Table IX it is often possible to determine a metal in a complex sample using wavelength modulation alone but the technique of double modulation may be necessary to ensure accuracy when determining a large number of metals in a complex sample.The ability to carry out both wavelength modulation and double modulation in a single instrument is advantageous for real sample analysis. Wavelength modulation can exhibit higher sensitivity by providing a combined emission and fluorescence signal which is useful for those elements which are more sensitively determined by atomic emission than by atomic fluorescence. However as shown here for the determination of iron in tomato leaves double modulation is required in certain situations in order to obtain accurate results. TABLE IX ANALYSIS OF NBS TOMATO LEAVES (SRM 1573) BY WAVELENGTH MODULATION AND FPDM Contentlpg g-l Wavelength/ - Element nm NBS value* WMt FPDMS Zn . . . . 213.9 62 ( f 6 ) 60 (*2) 58 ( f 4 ) Mn . . . . 279.5 238 ( f 7 ) 230 ( f 9 ) 227 (f7) cu .. 324.8 11 (&I) 11 ( f 2 ) 11 (f1) Fe . . . . 248.3 690 (f25) 614 (f14) 685 (h50) * Certified value. t WM wavelength modulation f (standard deviation of four samples) FPDM fixed-phase double modulation f (standard deviation of four samples). Conclusion The extent of spectral interferences listed in Table VI indicates that wavelength-modulated AFC is unlikely to be a technique of choice for a wide variety of analyses. The removal of the potassium atomic emission interference on lead determinations at 405.8 nm by the application of either of the double modulation techniques indicates that it is possible instrumentally to discriminate against all spectral interferences caused by atomic emission of concomitant ele-ments in any matrix.This is because there is unlikely to be a worse example of atomic emission interference than the potassium in lead situation except where a major element cause 1208 MCCAFFREY WU AND MICHEL a spectral interference. Only in the latter instance is the decrease in precision likely to make the technique unusable for trace analysis. Double-modulated AFC is therefore more likely to be of general use than wavelength-modulated AFC. The fixed-phase version is more suit-able for routine analyses because of its sensitivity advantage over dual lock-in double modula-tion although the reason for this advantage is not clear. The general advantages of AFC lie primarily in the ability to carry out sequential multi-element analyses with the use of only one light source.ly2 Various tables of detection limits in the literat~rel-~ define the typical range of sensitivity to be expected of AFC.The example detection limits in Table VIII indicate that double-modulated AFC (FPDM) falls mostly within that range. One reason for the favourable sensitivity of the double modulation techniques described probably lies in the use of efficient square waveforms for the modulation devices. However a monochromator with a small f number a modern design of xenon arc and for FPDM photon counting probably also made a contribution to the high sensitivities that were achieved relative to previously published4 work on double modulation. There is no question that there are more sensitive techniques available to the analyst. However where sensitivity is adequate the sequential multi-element capability of AFC will have advantages for some analyses over single element techniques such as line source excited atomic absorption or fluorescence.Although double modulation does discriminate against atomic emission spectral interferences, there remains the problem of the noise associated with the spectral interferences. We are currently involved in studies to characterise more fully the seriousness of the decrease in pre-cision caused by this noise and by scatter noise present during the analysis of real samples and at varying concentrations of analyte and interferent in aqueous solutions. We are also in the process of evaluating the importance of spectral interferences caused by atomic fluorescence of concomitant elements in the matrix. However Table VI does give some indication that this is much less of a potential problem than atomic emission spectral interferences. Finally there is the possibility of improved sensitivity in AFC by the replacement of the flame with a furnace and by the use of more powerful xenon arcs. The authors express their appreciation to John E. Gammerino in the Chemistry Department Instrument Shop for construction of the wavelength modulation unit. Acknowledgement is made to the donors of the Petroleum Research Fund administered by the American Chemical Society for the support of this research. Acknowledgement is also made to Jenny Jones for assistance during the digestion of the Standard Reference Materials. 1. 2. 3. 4. 5. 6. 7. 8. 9. References Johnson D. J. Plankey F. W. and Winefordner J. D. Anal. Chem. 1975 47 1739. Ullman A. H. Pollard B. D. Boutilier G. D. Bateh R. P. Hanley P. and Winefordner J . D., Michel R. G. Sneddon J . Hunter J . K. Ottaway J . M. and Fell G. S. Analyst 1981 106 288. Fowler W. K. Knapp D. O. and Winefordner J . D. Anal. Chem. 1974 46 601. Lipari F. and Plankey F. W. Anal. Chem. 1978 50 386. Elser R. C. and Winefordner J. D. Anal. Chem. 1972 44 698. Parsons M. L. Smith B. W. and Bentley G. E. “Handbook of Flame Spectroscopy,” First Edition, Koizumi H. Yasuda K. and Katayama M. Anal. Chem. 1977 49 1106. O’Haver T. C. Epstein M. S. and Zander A. T. Anal. Chem. 1977 49 458. Anal. Chem. 1979 51 2382. Plenum New York 1975 Chapter 2. Received December 31st. 1982 Accepted May loth 198

ISSN:0003-2654    

DOI:10.1039/AN9830801195

出版商:RSC    

年代:1983

数据来源: RSC

摘要:

Analyst October 1983 VoZ. 108 pp. 1209-1220 1209 Sulphide Ion-selective Electrode Studies Concerning Desdfovibrio Species of Sulphate-reducing Bacteria lsmail K. Al-Hitti Gwilym J. Moody and J. D. R. Thomas Department of Applied Chemistry Redwood Building U WIST Cardifl CF1 3XA The sulphide ion-selective electrode has been used for further studies on the Desulfovibrio species of sulphate-reducing bacteria. Emphasis has been placed on D. vulgaris which thrives in sodium dithionite metabolic medium, which provides an alternative sulphur source to sulphate. The bacterium also grows well in media containing certain organic materials namely, cysteine cystine and glutathione as alternative sulphur sources to sulphate. The bacterium does not grow with methionine as the sulphur source and this is attributed to the relative stability of the C-S-C bonds which are not adjacent to the amino group.The growth of D. vulgaris is retarded with barium sulphate or elemental sulphur as sulphur sources. However under certain circumstances there is less retardation with flowers of sulphur. More general parameters have also been studied including age of starter culture inoculum size poising agents pH and yeast stimulator. The active life of D. gigas was significantly less than for D . desulfuricans and D. vulgaris. Growth was more vigorous as the inoculum size was increased, while the optimum pH was between 7 and 8. Poising agents are less essential for sub-culturing from fresh starter cultures than for older ones while yeast extract accelerates the growth of D.vulgaris. Acetate is confirmed to be inappropriate as an organic carbon nutrient. D. vulgaris is considerably less sensitive to the salinity of the culture media (up to 3% m/V sodium chloride) than D. desulfuricans and D. gigas which were generally inactive even with 1% wz/V sodium chloride. Finally sodium tetraborate(II1) (2% m/ V ) and 2,kdinitrophenol (2% m/V) are confirmed as effective bactericides for D. vulgaris which is the most robust of the three species (D. desulfuricans D. gigas and D. vulgaris) studied in this work. Keywords Sulphate-reducing bacteria Desulfovibrio bacteria ; sulphide ion-selective electrode Various potentiometric methods have been employed for monitoring bacterial growth. Thus, Hussein and Guilbault1.s2 replaced the conventional but tedious turbidimetric monitoring method by nitrate and ammonium ion-selective electrodes for following the decrease in nitrate concentration and increase in ammonium concentration respectively in the presence of E.coZi. The resulting growth curves based on e.m.f. measurements were used to determine the best time for harvesting the bacterial cultures for enzyme extraction. Ion-selective electrodes and other potentiometric probes can provide useful metabolic information concerning pH carbon dioxide and ammonia species for proteus culture^.^ In contrast to an autoanalyser pr~cedure,~ such in siiu methods have the advantage of not con-suming any culture. Sulphate-reducing bacteria derive their energy from the anaerobic reduction of sulphate and the idea of employing potentiometric techniques to monitor their growth was first used by Alico and Liegey.5 This study of the growth pattern of Desd'jovibrio desulfiricarts involved measurements of the redox potentials with a platinum electrode coupled to a saturated calomel electrode alongside turbidimetric measurements of sulphate substrate consumption during growth.The liberation of hydrogen sulphide is the best indication of thriving and multiplication of sulphate-reducing bacteria. This lends itself to monitoring by the sulphide ion-selective electrode and the Orion 94-16A electrode has been used for studying the production of sulphide in cultures of D. desuZ+uricans6 and slurries of estuarine marshland that have been collected with sterilised syringes.' These monitoring modes of sulphide ion-selective electrodes would have been helpful in earlier work e.g.in the failure of growth of sulphate-reducing bacteri 1210 AL-HITTI ef al. SULPHIDE ISE STUDIES CONCERNING Analyst Vol. 108 populations in solid media and where reliable pictures of the activity of the organisms in different samples required comparison of the rates of sulphide formation in liquid media.8 The electrode would also have helped in determining numbers of these bacteria in samples of oil well waters when the rate of hydrogen sulphide formation after inoculation into a culture medium was used.9~10 A detailed study of the role of sulphide ion-selective electrodes for monitoring the growth of various Desulfovibrio species of sulphate-reducing bacteria has recently been reported using both indirect and direct methods.l1,l2 Three species of Desuulfovibrio were studied namely, D.desuZ+uricans D. gigas and D. vulgaris. The sulphide determined by both modes matched the amounts expected from the nutrient sulphate initially present in the culture media and also the sulphide recovered gravimetrically.11 All three species of the bacteria were shown to thrive on the metabolic intermediates sulphite thiosulphate and metabisulphite but they did not survive in the more stable dithionate or in the absence of inorganic sulphur. This paper describes a study of other parameters affecting the growth of sulphate-reducing bacteria namely redox poising agents pH inoculum size stimulators age of starter culture, elemental sulphur insoluble sulphate and inhibitors.Attention has also been given t o their viability in the presence of certain organic sulphur sources other than thioglycollate. Experimental Micro-organisms and Reagents All reagents were of analytical-reagent grade unless otherwise specified. Most of this work was carried out with Desulfovibrio vulgaris Strain No. NClB 8457 1975. Additionally Desulfovibrio desulfiricans Strain No. NClB 8307 1979 and Desulfovibrio gigas, Strain No. NClB 9332 1979 were used for the purposes of comparison. Refrigerated freeze-dried cultures were obtained from the National Collection of Industrial Bacteria (NCIB), Aberdeen. Preparation of Starter Cultures and Sub-cultures of Desulfovibrio Species Cultures of the three Desulfovibrio species were revived13 in a slightly modified version of Postgate's medium for sulphate reducers14J5 as previously described11 in order to provide starter cultures from which the inocula for sub-cultures were taken.Thus the basal medium for growing starter cultures contained K,HPO, 0.25 g; NH,C1 0.50 g; yeast extract (Oxoid Code L21) 0.50 g; MgSO4.7H,O 1.00 g; Na,SO (anhydrous) 0.50 g ; CaC12.2H,0 0.05 g; sodium lactate (70% m/m solution) 2.50 cm3; and doubly de-ionised water 500 cm3. The pH of the medium was adjusted to 7.40 with 5 M sodium hydroxide solution and autoclaved at 125 "C for 30 min. The culture was then cooled and the preparation completed by adding 0.50 g of sodium thioglycollate and 0.50 g of sodium ascorbate as poising agents in order to lower the oxidation - reduction potential of the medium for initiating growth of the sulphate-reducing bacteria.The pH was finally adjusted to 7.80 and the culture was autoclaved at 125 "C for 30 min. The medium was then cooled rapidly and anaerobicity was assured by the vigorous bubbling of white-spot nitrogen for at least 30 min prior to inoculation. The basal medium was then inoculated with the freeze-dried Desulfovibrio species. Any remaining air above the medium was flushed out with nitrogen. The sealed container was incubated at 30 "C and bacterial growth became evident after 2-3 d. This medium called the "starter culture," was used as the source for the sub-cultures. The sub-cultures were set up in basal media as detailed above except that magnesium chloride (0.83 g) was used instead of magnesium sulphate.Appropriate variations in content were made for parameter studies on the sulphur species the organic carbon source pH etc. as described in the text and in Tables I I1 and 111. The fresh basal medium of the sub-culture was aseptically charged with an aliquot of the starter culture (normally 2 cm3). The pH of the sub-cultures was strictly adjusted to the optimum growth range of 7.0-7.5 unless otherwise indicated. Monitoring of Sulphide Produced by Desulfovibrio Bacteria duction namely an indirect method and a direct method. claved prior to inoculating with the starter culture. Two main modes previously described,ll were employed for monitoring the sulphide pro-The sub-culture flasks were auto October 1983 DES ULFOVIBRIO SPECIES OF SULPHATE-REDUCING BACTERIA 121 1 Indiyect method Hydrogen sulphide produced by bacterial action was swept with white-spot nitogen from the culture flask which contained basal medium inoculated with 2 cm3 of starter culture into a monitoring flask fitted with an Orion Model 94-16A sulphide ion-selective electrode and a Corning Model 476002 double-junction reference electrode with an outer 10% potassium nitrate filler solution.Both the culture flask and remote monitoring flask were thermostated at 30 "C. Direct method The culture flask with basal medium after sterilisation and flushing out with white-spot nitrogen was charged with 2 cm3 of starter culture and fitted with an Orion Model 94-16A, sulphide ion-selective electrode a Corning Model 476002 double-junction reference electrode with an outer 10% potassium solution and a combination K E N 4 pH glass electrode.The flask was thermostated at 30 "C and the medium was stirred gently by a magnetic stirrer. During growth the pH was monitored as well as the e.m.f. of the sulphide ion-selective reference electrode pair. Potentiometric Measurements E.m.f. and pH measurements were recorded with a Beckman Model 4500 digital pH-millivoltmeter in conjunction with a Servoscribe Model RE541 potentiometric recorder. Concentrations of sulphide monitored by direct and indirect methods were calculated as previously described.ll Gravimetric Determination of Sulphide Produced by Cultures the monitoring flask of the indirect type of experiments. charged with 0.5 M lead nitrate. weighed.The sulphide produced was determined gravimetrically by precipitation of the sulphide in For these the monitoring flasks were The filtered lead sulphide precipitate was dried at 110 "C and Results and Discussion Studies of Alternative Sulphur Sources Dithionite An earlier study11 showed that the three species of Desulfovibrio in the culture media with lactate as an electron and carbon donor thrived with sulphate as an electron acceptor and also with sulphite thiosulphate and metabisulphite intermediates instead of sulphate. In con-trast all the species failed to grow with the chemically stable dithionate (S,0,2-) in the basal medium and they also failed to grow in the absence of an inorganic sulphur source. Dithionite ions (S,0,2-) are also involved in the metabolic mechanism of inorganic sulphur compounds by sulphate-reducing bacteria (see scheme in reference 11).Experiments with sodium dithionite as the sulphur source indicated that D. vulgaris thrived with a sulphide yield (0.144 g) near to the theoretically expected value (0.139 g). The reaction is stoicheiometric and consistent with the following equation : 3CH,CHOHCOONa + Na,S,O -+ 3CH,COONa + Na,CO + 2C0 + 2H,S + H (1) This supports an earlier conclusion16 that dithionite could effectively replace sulphate for growing D. desulfiricans possibly because dithionite spontaneously decomposed to sulphate.ls Dithionite is a strong reducing agent but it has been shown to be itself reduced rapidly by cells and cell extracts of sulphate-reducing bacteria.17 Also with gaseous hydrogen as a reductant cytochrome C accelerated the reduction of dithionite ions.Thus although dithionite and dithionate differ by just two oxygen atoms only the former is reduced to sulphide by the organisms. Furthermore dithionate is the only 0x0 - sulphur species that is metabolically stable. This is compatible with the view that the dithionate ion is of a different structural type from the higher polythionates.l 1212 Analyst VoZ. 108 Organic szclphur sources Preliminary studies on organic sulphur sources in the culture media of sulphate-reducing bacteria have indicated that such compounds could serve as both organic nutrient and sulphur source.12 However it was emphasised that none of the three species of sulphate-reducing bacteria studied1l,l2 utilise the organic sulphur of thioglycollate (Table I).This confirms Pankhurst’s observation that the organic sulphur of thioglycollate is not ~ti1ised.l~ Several experiments were designed for cystine and cysteine as organic sulphur sources for D. vulgaris species employing the indirect monitoring method (Table I). The first run for each sulphur source (runs 2 and 5 respectively) contained sodium thioglycollate and sodium lactate while for the second runs (3 and 6) sodium thioglycollate was omitted. Both sodium thioglycollate and sodium lactate were omitted from the third runs (4 and 7). AL-HITTI et al. SULPHIDE ISE STUDIES CONCERNING TABLE I TOTAL SULPHIDE PRODUCTION BY D. vulgaris FROM VARIOUS ORGANIC SULPHUR SUBSTRATES INSTEAD OF INORGANIC SOURCES Experiment Organic sulphur source 1 Thioglycollate + lactate 2 3 Cystine without thioglycollate 4 Cystine with thioglycollate and lactate Cystine without thioglycollate and lactate 5 6 Cysteine without thioglycollate 7 8 9 Cysteine with thioglycollate and lactate Cysteine without thioglycollate and lactate Glutathione with thioglycollate and lactate Methionine with thioglycollate and lactate Mass of sulphur sourcelg 0.500 0.479 (0.500)t 0.479 0.479 0.480 (0.500)t 0.480 0.480 1.220 (0.500)t 0.592 (0.500) t Equivalent nutrient sulphur contentlg 0.140 0.127 (0.140) 0.127 0.127 0.127 (0.140) 0.127 0.127 0.127 (0.140) Sulphide content in monitoring flask/g electrode metrically 0.006 None* 0.134 0.151 Sulphide-0.134 0.139 0.120 -0.124 -0.124 0.121 0.116 0.105 0.124 0.128 0.112 -0.122 0.124 Monitoring method Indirect Indirect Indirect Indirect Direct Indirect Indirect Indirect Direct Indirect 0.127 0.004 0.002 Indirect (0.140) Age of starter cultureld 49 52 63 75 208 166 178 190 129 10 16 *Trace amounts only of black precipitate in gravimetry.t Mass of thioglycollate. For both cystine and cysteine the organisms entered their logarithmic phase within 24 h as verified by the rapid production of sulphide detected in the remote monitoring vessel while the growth of D. vdgaris occurred after only 3 d with inorganic sources under comparable circum-stances. The multiplication of D. vulgaris occurred with cystine and cysteine even in the absence of sodium lactate (runs 4 and 7) thus confirming that both cystine and cysteine can function as organic carbon and hydrogen sources.1g The growth of D.vuZgaris during direct monitoring (second runs 4 and 7) in the presence of cystine and cysteine respectively but in the absence of lactate eliminates the possibility that inorganic carbon or trace amounts of carbon dioxide in the nitrogen of the indirect method might have stimulated growth. The metabolic reduction of cystine and cysteine to sulphide was efficient in terms of mass balance and tests for cystine and cysteine in the terminal culture media proved negative. The degradation of cystine and cysteine to sulphide may respectively be represented by20 : HH H H I 1 I I I I I I 4 HOOC-C-C-S-S-C-C-C00H + 2H2O -+ CHSCOOH + HZS + NH3 + C02 + + H2 (2) H,NH HNH, H - - (3) I HS-CH2-C-COOH + 2H2O -+ CH3COOH + H2S + NH3 + C02 + H2 .. October 1983 DES ULFOVIBRIO SPECIES OF SULPHATE-REDUCING BACTERIA 1213 Two additional organic sulphur sources were examined namely glutathione and methionine. Glutathione was degraded as confirmed by the collected sulphide (run 8 Table I) and is consistent with the following equation : I I It I 1 H 0 H CH, I I HOOC-C-CH,CH,C-N-C-H + 6H,O + 4CH3COOH + 3NH + 2C0 + H,S + H (4) I NH2 c=o I HOOC-CH,-NH The low culture pH (7.0-7.3) measured after the experiment had progressed to the stationary stage may be related to the acetic acid which is produced in a relatively greater amount than ammonia [equation (a)].The media containing inorganic sodium salts as sulphur sources tended to become more basic (pH ca. 8.5). No growth of D. vulgaris is observed with methionine as indicated by the very low yield of sulphide obtained (run 9 Table I) The resistance of methionine to microbial degradation may be related to the relative stability of the C-S-C bonds that are remote from the amino group compared with the C-S-S-C bonds in cystine and the C-S-H bonds in cysteine and glutathione, each having an amino group attached to the penultimate carbon away from the sulphur atom. Elemental sulphur as a terminal electron acceptor Elemental sulphur is not included as an intermediate in the sulphur cycle as it has been shown not to be involved in either the reduction or oxidation of sulphide.21 For example, elemental sulphur purified by redistillation was not attacked while colloidal sulphur permitted only slow growth or slow hydrogen absorption when a resting cell suspension was used.16 Confirmatory experiments have been carried out in this study using elemental native sulphur as an alternative sulphur source for D.vulgaris. Direct monitoring experiments with native sulphur showed little growth only as indicated by the low yield of sulphide (0.040 g) compared with the expected value (0.127 g). The native sulphur was autoclaved within the culture flask. However when flowers of sulphur was used under direct and indirect monitoring conditions there was bacterial growth. The flowers of sulphur was either autoclaved within the culture medium or added to the culture medium immediately prior to inoculation with starter culture.In each instance there was bacterial growth and reasonable amounts of sulphide were collected (0.076 g) during indirect monitoring for the growth term of the D. vul-garis with autoclaved flowers of sulphur compared with the expected value (0.127 g). For the non-autoclaved flowers of sulphur experiment the sulphide produced (0.320 g) was also con-siderable in relation to that expected (0.500g) for indirect monitoring. The growth of D. vulgaris in the presence of flowers of sulphur was easier to follow by the indirect method than by the direct method as in the latter instance colloidal sulphur accumulated on the electrodes to give spurious e.m.f. measurements particularly towards the end of experiments.Autoclaved cultures with flowers of sulphur became yellow - blue and the flowers of sulphur powder became solid and settled to the bottom of the culture flask. Utilisation of flowers of sulphur by D. vulgaris may be ascribed to polymerisation of sulphur at the temperature of sterilisation. Polysulphides are more soluble than flowers of sulphur.22 For the non-autoclaved flowers of sulphur the growth of D. vuulgaris is also clearly possible and its utilisation may be due to the partial solubility of this form of sulphur owing to the small particle size.l6 Barium sulphate as a solid sulphur source Experiments were conducted by the indirect monitoring method to determine whether D. vzdgaris can thrive and survive with barium sulphate (0.926 g) as a sulphur source.The bacterium grew slowly and the culture became slightly cloudy and yellow in appearance. The solubility of barium sulphate (0.00246 g dm-3 at 25 "C) corresponds to 1.05 x 1214 AL-HITTI et al. SULPHXDE ISE STUDIES CONCERNING Analyst Vol. 108 concentration and 3.4 x lom4 g dm-3 of sulphur. In relation to this the amount of sulphide monitored by the sulphide ion-selective electrode corresponded to 0.026 g (0.048 g by the gravimetric method). These results indicate that soluble sulphate reduced to sulphide by the bacteria is not replenished sufficiently fast by further dissolution of barium sulphate. The eventual termination of growth may be attributed to insufficient soluble sulphate as an effective energy source. This indicates that barium sulphate is unsuitable as a terminal sulphur source for bacterial growth.General Parameters of Bacterial Growth Inoculum age Growth studies on Desulfovibrio species indicate that the age of starter culture has no effect on sulphide yield for D. vulgaris and D. desuZjuricans.ll However the lag phase lengthens with age. With D. gigas however the age of starter culture influences the activity of the organisms resulting in reduced yields of the sulphide end-product. This has been tested in this work by several experiments using the indirect monitoring method for D. gigas from different ages of starter cultures and using sulphite as inorganic sulphur source. For example, the amount of sulphide produced was 0.128 0.008 and 0.002 g respectively for experiments corresponding to starter cultures of ages 25,45 and 110 d (Fig.1) and confirming the effective lifetime of D. gigas to be shorter than those of D. vulgaris and D. desulfitricans. 10-3 H A 10-5 0 10-7 I O - ~ .c CT -lo-” 0 50 100 150 Time/h 200 250 Fig. 1. Effect of inoculum age on growth of D. gigas expressed as sulphide produced from a sulphite-based nutrient as monitored by the sulphide ion-selective electrode (indirect method). D. gigas is an exceptional species because of its large spirilloid cell and intracellular granule^.^^,^^ These unique characteristics probably enhance accumulation of the micro-organisms and they settle to the bottom of the starter culture. Their cultures always clarified after one month and large colonies were clearly seen in the bottom of the culture flask.In order to compare the robustness of the three species very old starter cultures of 9,5 and 8 months of D. desulfuricans D. gigas and D. vulgaris respectively were regrown using the direct monitoring method. The necessary culture nutrients were autoclaved and added to the old starter cultures of the appropriate bacteria. The pH of the cultures was adjusted to 7.0-7.5 by adding sterilised sodium hydroxide solution. Renewed activity of D. vztlgaris and D. desuZfwicans was observed as can be concluded from the characteristic growth phasell deduced from potential and pH measurements during the experiment (Fig. 2B and C). No growth activity could be detected for the D. gigas culture as indicated by the almost constant sulphide electrode potential and pH measurements during the 12 d of the experiment (Fig.2A). It is noted that the D. gigas system had a high sulphide content (Fig. 2A) which was un-affected by the renewed nutrient culture for reactivating bacterial activity October 1983 DESULFOVIBRI~ SPECIES OF SULPHATE-REDUCING BACTERIA 1215 .-0 50 100 150 200 240 Time/h Fig. 2. Regrowth attempts for starter cultures of three Desulfovibrio species monitored potentio-metrically (direct method). A D. gigas 5 months old (e.m.f. of sulphide ISE system); B D. vulgaris, 8 months old (e.m.f. of sulphide ISE system); C D . desulfuricans 9 months old (e.m.f. of sulphide ISE system) ; and A* B* and C* pH changes for A B and C systems respectively. Efect of inoculum size Indirect monitoring experiments were designed to study the effect of size of the starter culture sample on the growth of D.vzllgaris and on the rate of production of hydrogen sulphide. Three basal media of identical composition were used with inocula of the same age. The basal media were charged with 2 4 and 6 cm3 of starter culture and the growth was followed potentiometrically. As the sample size was increased the rate of production of hydrogen sulphide increased viz. 3.3 x 10-4 7 x 10-4 and 2.7 x mol dm-3 d-l averaged over a 3-d yield. Also the total yields of sulphide were 0.120 0.136 and 0.176 g after 12 d corre-sponding to inocula sizes of 2 4 and 6 cm3 respectively. The increases in the amount of sulphide produced with sample size of the inoculum may be related to (i) an increase in the number of micro-organisms and hence of enzyme mass trans-ferred to the basal medium; (ii) an increase in the sulphide carried over with the starter culture that will lower the reducing potential of the medium; and (iii) although unlikely unreduced sulphate in the starter culture that may increase sulphide concentration as the transferred inoculum was increased in amount.Efect of reducing the amozlnt of redox poising agent The growth of anaerobic bacteria is often stimulated by reducing agents. Thus many bacterial media contain one or more redox poising agents to lower the reducing potential of the media to more negative levels. The role of reducing agents in activating D. vulgaris growth has been studied here in relation to the roles of sodium thioglycollate and sodium ascorbate as well as sulphide brought in with the starter culture.Sulphate was used as the sulphur source and lactate as the carbon and hydrogen or electron donor (see Table I1 for parameters). In experiments 1 and 2 (Table I), sodium glycollate was substituted for sodium thioglycollate but sodium ascorbate redox poising agent was still included. The D. vulgaris grew well with stoicheiometric yields of sulphide and even the pause periodll was absent in experiment 1. The activation and initiation of the organisms in this run were probably due to the presence of ascorbate in addition to sulphide brought in with the very fresh starter culture (age 14 d). Thus sodium thioglycollate can be avoided if the inocula are fresh but it is recommended for inclusion as redox poising agent in sub-culturing if the inocula are from old starter cultures 1216 AL-HITTI et al.SULPHIDE ISE STUDIES CONCERNING TABLE I1 GROWTH OF D. vulgaris UNDER VARIOUS CIRCUMSTANCES OF ADDED POISING Analyst VoZ. 108 AGENTS P H AND LACTATE NUTRIENT Inorganic sulphur Experiment source 1 0.564 g of Na,SO, 2 0.664 g of Na,SO, 3 0.664 g of Na,SO, 0.564 g of Na,SO, 6 NoNa,SO, 6 NoNa,SO, 7 NoNa,SO, 8 0.564 g of Na,SO, 9 0.564 g of Na,SO, 10 0.564 g of Na,SO, Size of pH a t Theoretical Sulphide starter Monitoring the start sulphurlg obtained/g culture/cma method 7.35 7.30 7.35 7.35 7.30 7.20 7.30 9.00 7.50 7.35 0.127 0.127 0.127 0.127 ---0.127 0.127 0.127 0.128 0.123 0.001 0.120 0.006 0.001 0.001 0.006 0.003 0.018 2 2 2 2 2 2 2 2 -2 Indirect Direct Indirect Direct Indirect Direct Direct Direct Direct Indirect Remarks Sub-culture contains 0.5 g of sodium glycollate as alternative to thioglycollate (growth) Sub-culture contains 0.5 g of sodium glycollate as alternative to thioglycollate (growth) Sub-culture lacks thioglycollate and ascorbate (no growth) Sub-culture lacks thioglycollate and ascorbate (growth by sulphide poising) Sub-culture contains normal poising agents (no growth) Sub-culture contains normal poising agents (no growth) Sub-culture lacks thioglycollate (no growth) Sub-culture contains normal poising agents but pH set to 9.0 (no growth) No inoculation of starter culture to the normal sub-culture (no growth) Culture lacks lactate (no growth) For experiments 3 and 4 (Table 11) both sodium thioglycollate and sodium ascorbate were omitted.No meaningful growth could be detected in experiment 3 (sulphide collected was 0.001 g) but a repeat of run 2 showed very slow growth with a long lag phase and more sulphide (0.032 g). With the direct monitoring method (experiment 4) growth was readily observed (Fig. 3) despite the absence of poising agents and the sulphide yield was near stoicheiometric (Table 11). The differences between the indirect (experiment 3 Table 11) and direct (experiment 4, Table 11) monitoring methods can be attributed to the sulphide brought in by the inoculum from the starter culture on the subsequent experimental procedure Thus in the indirect method (experiment 3) this sulphide is swept off by the nitrogen stream while it remains in the culture flask for the direct method (experiment 4).For experiment 4 however the reducing potential capability brought in by the sulphide of the starter culture seems to be sufficient for Time/h Fig. 3. Direct monitoring of growth of D. vulgaris in the absence of poising agents with 0.564 g of Na,SO (0.127 g of S) (sulphide produced = 0.120 g) October 1983 DES ULFOVIBRIO SPECIES OF SULPHATE-REDUCING BACTERIA 1217 the viability of these bacterial populations in the direct monitoring procedure. This is com-patible with the observations of other worker^.^^-^' The need for reducing agents is significant as also shown28 for generation of the enzyme pyrophosphatase involved in the reduction of sulphate to sulphite.The control experiments (5-7 Table 11) confirm that sulphate (or sulphur source) is a prerequisite for growth of the bacteria with the optimum pH adjustment (experiment 8) as well as lactate (or organic carbon) as a major organic source (experiment 10). Efect of fiH Three experiments illustrate the influence of pH on the activity of D. vuzgaris in the indirect monitoring method. Each culture medium was identical except that the initial pH values were set at 6.0 7.0 and 8.0 respectively for the three cultures of D. vulgaris. The respective rates of sulphide production over 3 d were ca. 3 x ca. 7 x and mol dm-3 h-l (Fig. 4). The sulphide yield (0.002 g) was small at pH 6.0 and ceased after 3 d while sulphide continued to be produced at pH 7 and 8 for at least a further 50 h with yields of 0.128 g of sulphide com-pared with the expected 0.127 g (Fig.4). Obviously a pH of 6 is unsuitable for growth while the organisms grow successfully at pH 7 or 8 but growth is faster at a pH of 8 as observed by other w0rkers.~9,30 The highest pH recorded for cultures in an earlier studyl1J2 was ca. 8.6 after 15 d of sulphide monitoring by the indirect method. Precautions are necessary with media at a pH above 8 because calcium magnesium and phosphate ions cause precipitation. For cultures at pH 9 a considerable amount of white precipitate was produced for runs using the direct method of sulphide monitoring and no growth was identified in this medium, thus confirming the pH range of 7.0-8.0 as compatible with the activity of sulphate-reducing bacteria.This range is compatible with the viability of most enzymes that are active in sulphate to sulphide conversion at pH <8 and >6. For example Akagi and CampbelPl discovered that ATP-sulfurylase extracted from D. desulfiricans functioned over a pH range of 6.0-9.0 but was denatured beyond this range. lo-’’ 10-9 ti 1 I I I 50 100 150 Time/h 200 250 Fig. 4. Effect of pH on the growth of D. vulgaris (indirect method). A pH 8; B pH 7; and C, pH 6. Role of yeast extract For this parameter different amounts of yeast extract were added to D. vulgaris cultures and growth was observed by the indirect monitoring method. The inocula (2 cm3) of D. vulgaris were of the same age in each example.Growth of D. vulgaris was extremely poor over 15 d in the absence of yeast extract as shown by the small yield of sulphide (0.002 g) (Fig. 5) but was enhanced by yeast extract and after 6 d the expected 0.128 g of sulphide was produced €or each of the yeast runs. The relevance of yeast extract can be related to its rich content of vitamin B and organic nitrogen and carbon. The incorporation of 0.1% m/V of yeast extract has previously bee 1218 AL-HITTI et al. SULPHIDE ISE STUDIES CONCERNING Analyst Vol. 108 Fig. 5. Influence of yeast extract on the growth of D. vulgaris (indirect method). A 1.0 g of yeast extract; B 0.5 g of yeast extract; and C without yeast extract. shown32 to enhance the growth of bacteria and it is claimed33 that yeast extract is required to complete substrates for example by introducing oxamate and 2-methylpropan-1-01 and where the organic matter of yeast extract is a~similated.~' Acetate as a substrate for D.vulgaris Pure cultures of most sulphate-reducing bacteria dissimilate carbon sources with three or more carbon atoms with acetate and carbon dioxide as the end p r o d u ~ t s ~ ~ ~ ~ ~ and further oxida-tion of acetate coupled to sulphate reduction does not seem to occur. This was checked in this study for D. vuZgaris by using acetate as a carbon source. The indirect monitoring method was used but with calcium hydroxide solution in the monitoring flask instead of sodium hydroxide solution in order to monitor visually the production of carbon dioxide as well as for collecting hydrogen sulphide.The solution became turbid and finally a white precipitate was produced; this was attributed to calcium carbonate according to solubility considerations. The basal medium was also slightly cloudy. The solution in the monitoring flask at the end of the experiment was neutralised with perchloric acid and lead nitrate solution was added. The black lead sulphide precipitate collected corresponded to 0.013 g of sulphide (experiment 1 Table 111). This was consistent with the small amount of sodium sulphate (0.025g) actually reduced (Table 111). TABLE I11 GROWTH OF D. vulgaris IN MEDIA CONTAINING SODIUM ACETATE AS ORGANIC SUBSTRATE INSTEAD OF SODIUM LACTATE Sulphide Reduced expected Sulphide Age of Unreduced Na,SO calculated by sulphide Gravimetric starter Monitoring Experiments Na,SO,/g Na,SO,/g (col.2- col. 3)/g from col. 3/g electrodelg sulphide/g culture/d method Remarks 1 0.564 0.538* 0.026 0.006 - 0.013 127 Indirect No growth 2 0.564 0.538* 0.026 0.006 0.0005 0.004 133 Indirect No growth 3 0.564 0.562* 0.002 0.0004 0.0004 - 183 Direct No growth * Determined gravimetrically by the addition of barium chloride solution to the acidified medium after cessation of the experiment. The experiment was repeated and monitored potentiometrically with a sulphide ion-selective electrode by having 1 M sodium hydroxide solution and 2% ascorbic acid in the monitoring flask (experiment 2 Table 111). An even smaller indication of growth was observed by using the direct monitoring method and where of course nitrogen gas was excluded (experiment 3 Table 111).The purity of the A small yield of sulphide was obtained October 1983 DES ULFOVIBRIO SPECIES OF SULPHATE-REDUCING BACTERIA 1219 white-spot nitrogen was checked by bubbling through calcium hydroxide solution for 2-3 h and only trace amounts of carbon dioxide were found. In conclusion D. vuZgaris does not metabolise acetate thus confirming the view of Postgate.36 Consumption of acetate has however been reported with crude enrichment cultures37 and attributed to the existence of Desulfotomaculum acetoxidans isolated by Widdle and Pfennig.37 These do not utilise the substrates common to Desulfovibrio species such as lactate and pyru-vate but utilise acetate instead in the presence of sulphate as an electron acceptor with the following stoicheiometric reduction of sulphate to sulphide : CH,COONa + Na,SO -+ 2NaHC0 + NaHS .. * - (5) Salt (sodium chloride) tolerance in bacterial growth Sulphate-reducing bacteria are found in natural waters of all salinities from zero to satura-tion point.20 Therefore the sodium chloride concentration of growth media should match that of the habitat from which the organism was obtained for example Desulfovibrio salexigens have an absolute requirement of more than 1% sodium chloride.,O Nevertheless most bacterial samples from very saline environments exhibit more rapid growth in media of some-what less salinity.2* Several runs based on the indirect monitoring method tested sodium chloride tolerance in the growth of Desztlfovibrio species studied in this work.No traces of growth were observed with D. gigas and D. desulfuricans even with just 1% sodium chloride but D. vulgaris thrived well even at 3y0 sodium chloride (15 g per 500 ~ m - ~ ) in the culture. Full yields of sulphide were realised with D. vulgaris (0.124,0.128 and 0.127 g at 1,2 and 3% sodium chloride respectively) compared with very low yields (G0.003 g ) from D. desuljkicans and D. gigas. Thus D. vulgaris is more active and more adaptable environmentally than the other two species, although growth of the species in the presence of 3% sodium chloride was rather slower than at lower levels of the salt. Sodium chloride might be used for inhibiting growth of both D. desulfuricans and D. gigas. Inhibition of Bacterial Growth Minimum inhibitory conditions are influenced by the nature of the test media and by inoculum size.Desulfovibrio species and their resistance to inhibitors can cause conversion problem^.^^^^^ Also the existence of mixed populations shows different patterns of sensitivity to inhibitors to those shown by individual species. Experiments were carried out in this study with D. vulgaris using the indirect monitoring method for testing sodium tetraborate(II1) (2% m/V) and 2,4-dinitrophenol (2% m/V) as inhibitors. For sodium tetraborate(III) the pH of the culture initially around 9.0 was adjusted to 7.4 with sterilised hydrochloric acid after sterilisation. No growth was observed according to the low yield of sulphide (0.007 g) sensed by the sulphide ion-selective electrode. The inhibition of D.vulgaris was attributed to tetraborate(II1) ions while in an earlier study with D. desulfuricans38 similar inhibitors might have been brought about by pH as well as the tetraborate(II1) ions because the pH of this earlier study was not adjusted to the appropriate value for bacterial growth. The pH of one was adjusted to 7.40 with sterilised sodium hydroxide solution after sterilisation of the culture and the other was used at the prepared pH of ca. 4. No growth of D. vzdgaris was identified in either of the cultures as indicated by the low yield of sulphide (0.006 g), thus confirming the pronounced inhibiting effect of DNP. DNP has previously been shown41 to inhibit completely the reduction of sulphate in whole cells but had no effect on the reduction of thiosulphate indicating that DNP prevented the cells from producing the high energy phosphate required for reducing sulphate.Two other cultures for D. vulgaris contained 2% m/V of 2,4-dinitrophenol (DNP). Conclusion The sulphide ion-selective electrode is confirmed as a very useful and convenient means of monitoring the growth patterns of sulphate-reducing bacteria and especially for determinin 1220 AL-HITTI MOODY AND THOMAS the role of various culture media parameters. The study also illustrates a wider role for ion sensors and similar potentiometric devices in the monitoring of fermentation type processes. The authors thank the University of Al-Deen Iraq for paid leave of absence to I. K. A1-H. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. References Hussein W. R. and Guilbault G. G. Anal. Chim. Acta 1974 72 38. Hussein W. R. and Guilbault G. G. Anal. Chim. Acta 1975 76 187. Ladenson J. H. Huebner M. and Marr J. J. Anal. Biochem. 1975 63 56. Mor J. R. Zimmerli A. and Fiechter A. Anal. Biochem. 1973 152 614. Alico R. K. and Liegey F. W. J. Bacteriol. 1966 91 1112. Crombie D. J. Moody G. J. and Thomas J. D. R. Lab. Pract. 1980 29 259. Nedwell D. B. and Barat I. M. Microbiol. Ecol. 1981 7 305. Burker H. J. J . SOC. Chem. Ind. London 1939 58 93. Dostalek M. and Spurny M. Folia Biol. (Prague) 1956 2 338. Spurny M. Dostalek M.and Vlehla S. Folia Biol. (Prague) 1957 3 202. Al-Hitti I. K. Moody G. J. and Thomas J. D. R. Analyst 1983 108 43. Al-Hitti I. K. Moody G. J. and Thomas J. D. R. Anal. Proc. 1983 20 119. Machenzie A. R. “National Collection of Industrial Bacteria,” Aberdeen personal communication, Postgate J. R. in Schlegel H. H. and Kroger E. Editors “Anreichungskulfur and Mutanteri-Pankhurst E. S. in Slapton D. A. and Board R. G. Editors “Isolation Anaerobes,” Academic Postgate J. R. J. Gen. Microbiol. 1951 5 725. Nakatsukasa W. and Akagi J. M. J. Bacteriol. 1969 98 429. Ephraim F. C. “A Textbook of Inorganic Chemistry,” Second Edition Gurney and Jackson, London 1934. Senez J. C. and Leroux-Gilleron J. Bull. SOG. Chim. Biol. 1954 36 553. Postgate J. R. “The Sulphate-reducing Bacteria,” Cambridge University Press Cambridge 1979.Postgate J. R. Annu. Rev. Microbiol. 1959 13 505. Tobolosky A. V. and Eisenberb A. J. J . Am. Chem. Soc. 1959 81 780. Thomas P. J. Microsc. Paris 1972 13 349. Moura J. J . G. Xauier A. V. Bruschi M. LeGall J. Hall D. D. and Cammack R. Biochem. Stephenson M. and Stickland L. H. Biochem. J. 1933 27 1517. Grossman J. P. and Postgate J. R. J. Gen. Microbiol. 1955 12 429. Postgate J. R. J. Bacteriol. 1963 85 1450. Ware D. A. and Postgate J. R. J Gen. Microbiol. 1971 67 145. Postgate J. R. J. Gen. Microbiol. 1951 5 714. Starkey R. L. and Wight K. M. Am. Gas. Assoc. Proc. 1945 25 307. Akagi J. M. and Campbell L. L. J. Bacteriol. 1963 85 1450. Butlin K. R. Adams M. E. and Thomas M. J. Gen. Microbiol. 1949 3 46. Mechalas B. J. and Rittenberg S. C. J. Bacteriol. 1960 80 501. Senez J. C. Bull. SOG. Chim. Biol. 1954 36 541. Grossman J. P. and Postgate J. R. Proc. SOC. Appl. Bacteriol. 1953 16 1. Postgate J. R. personal communication 1980. Widdle F. and Pfennig N. Arch. Microbiol. 1977 112 119. Crombie D. J. PhD Thesis University of Wales 1979. Roberts G. A. H. BY. Corros. J. 1969 4 318. Crombie D. J. Moody G. J. and Thomas J. D. R. Chem. Ind. (London) 1980 500. Peck H. D. J. Biol. Chem. 1960 235 2434. 1976. auslese,” Fischer Stuttgart 1965 p. 190. Press London 1971 p. 223. Biophys. Res. Commun. 1976 72 782. Received April 20th 1983 Accepted May 19th 198

ISSN:0003-2654    

DOI:10.1039/AN9830801209

出版商:RSC    

年代:1983

数据来源: RSC

摘要:

Analyst October 1983 Vol. 108 pp. 1221-1226 1221 Electrochemical Studies on Minoxidil and its Determination in Tablets by Differential-pulse Pola rog rap hy Lawrence Amankwa Leslie G. Chatten and Stanley Pons* Faculty of Pharmacy and Pharmaceutical Sciences University of Alberta Edmonton Alberta T6G 2N8, Canada A simple differential-pulse polarographic method has been developed for the determination of minoxidil in pharmaceutical dosage forms. The extracting solvent was methanol and the supporting electrolyte was 1.0 N sulphuric acid. An excellent linear relationship was obtained between the concentration and current peak height with a correlation coefficient of 0.999 9. Good agreement was obtained between results with the differential-pulse polarography method and those by the manufacturer’s method of assay.There was no interference by the tablet excipients. In acid solution a mechanism for reduction a t - 1.2 V is proposed which involves the transfer of four electrons as well as dehydration and deamination steps. Keywords Minoxidil determination ; diflerential-pulse polarography ; con-trolled-potential coulometry ; cyclic voltammetry Minoxidil (2,4-diamino-6-piperidinopyrimidine-3-oxide) (I) is a recently marketed orally ad-ministered peripheral vasodialator which is useful for the treatment of patients with refractory hypertension.1 The detection and determination of this substance continues to be of interest, particularly because of its associated numerous side effects.2 As yet no official method for the determination of minoxidil has been listed in the US Pharmacopeia3 and the few reported methods of assay involve radioimmunoassay,4 thin-layer chromatography5p6 or radiochromato-gram scanning.’ Investigation with a gas - liquid chromatographic procedure revealed that substantial amounts of some derivatives were lost on the c01umn.~ H2NTyNH2 In addition to its basic properties minoxidil has two structural features that could be used for analytical purposes (earlier reports8-1° have shown that both the N-0 bond and the carbon-nitrogen double bond are electroreducible) .Accordingly in this paper we present a diff erential-pulse polarographic procedure for the determination of minoxidil in tablets which involves only a single extraction prior to the electroreduction. The method is sensitive, accurate and easy to perform for routine analysis.Experimental Apparatus and Conditions for Polarographic Analysis to measure the pH values of the solutions. A Fisher Model 320 pH meter fitted with a glass - calomel electrode system was employed * Department of Chemistry 1222 Analyst VoZ. 108 A PAR Model 174 polarographic analyser equipped with a drop timer Model 172A and a Houston Ominigraphic Model 2000 recorder were used in the investigations. A three-electrode combination was employed which consisted of a saturated calomel electrode a dropping-mercury electrode and a platinum wire as the auxiliary electrode. A conventional H-type cell was maintained at 25 & 1 "C and all sweeps utilised a scan rate of 2 mV s-l and a drop time of 2 s. In 1.0 N sulphuric acid (pH rn 0.5) the instrumental parameters were applied potential range -0.6 to -2.1 V; current 100 pA full scale; height of mercury column 75 cm; flow-rate of mercury 1.176 mg s-l; modulation amplitude set at 50 mV; and low pass filter set at a time constant of 1 s.Controlled-potential Coulometry A PAR Model 173 potentiostat - galvanostat equipped with a PAR Model 377A three-component coulometric cell system was connected to a Hi-Tek digital integrator and digital voltmeter. A 19-ml volume of 1.0 N sulphuric acid was placed in the coulometric cell on top of a 5-ml layer of triply distilled mercury and 1 ml of M solution of minoxidil in methanol was added. The system was purged for 10 min with purified nitrogen. The applied potential was set at -1.2 V with a current range of 10 pA full scale and the solution was electrolysed until the digital readout indicated a constant but small count.One hour was required to complete the electrolysis. The process was repeated with a blank consisting of 19 ml of 1 .O N sulphuric acid and 1 ml of methanol. Cyclic Voltammetry Cyclic voltammetric experiments at a hanging mercury drop electrode were performed with a four-component system consisting of a PAR EG and G Model 175 Universal Programmer a PAR Model 173 potentiostat - galvanostat a Houston Model 2000 Omnigraphic recorder and a PAR Model 9323 hanging mercury drop electrode fitted with a polarographic cell. Two supporting electrolyte systems were employed. In 1 .O N sulphuric acid the instrumental parameters were as follows potential range -0.8 to -1.3 V; current range 10 PA; and scan rate varied from 10 to 200 mV s-1.In a dimethylformamide - tetraethylammonium bromide system the following settings were used potential range -1.2 to -2.2 V; current range, 10 pA; and scan rate as in the previous system. Reagents The following reagents were used all of analytical-reagent grade barbitone boric acid, citric acid potassium dihydrogen orthophosphate dimethylformamide (DMF) anhydrous methanol tetraethylammonium bromide (TEAB) 0.2 N sodium hydroxide solution 1 .O N sulphuric acid and 1% tetraethylammonium bromide in DMF. Britton - Robinson buffers were prepared with distilled and de-ionised water at intervals of 0.5 pH unit over the pH range AMANKWA et al. ELECTROCHEMICAL STUDIES ON The instrument was operated in the differential-pulse mode.2.6-7 .O. Reference Standard further purification. Dependence Studies Preparation of Calibration Graph Five test solutions of varying concentrations from 1 to 5 x 1 0 - 5 ~ were prepared by appropriately diluting the stock solution with 1.0 N sulphuric acid while in the total sample volume of exactly 20 ml the amount of methanol was always maintained at 1 ml. All samples were purged with oxygen-free nitrogen for 10 min prior to each run and a stream of nitrogen was allowed to flow gently over the surface of the solution during the electro-reduction. Samples of each of five concentrations were run five times and resulted in a corre-lation coefficient for the graph of 0.9999. Minoxidil (100.4y0) was obtained from Upjohn Company of Canada Ltd.and used without These studies were carried out in Britton - Robinson buffer over the pH range 2.6-7.0. A stock solution of minoxidil (10-3 M) was prepared in anhydrous methanol October 1983 Diffusion Dependence Studies These studies were carried out in the aforementioned sulphuric acid - methanol system on a 5 x lo-* M solution of minoxidil. The applied potential was from -0.6 to -2.1 V and the height of the mercury column ranged from 60 to 80 cm. The mercury flow-rate was measured at each of five heights over that range. Analysis of Pharmaceutical Dosage Forms MINOXIDIL AND ITS DETERMINATION IN TABLETS BY DPP 1223 Two dosage forms 2.5 and 10.0 mg tablets were available from the manufacturer. Twenty tablets were weighed and finely powdered and an amount of powder was taken that, according to the label would result in an approximately M solution of minoxidil in 50 ml of solvent.The accurately weighed sample was stirred magnetically for 20 min in 20 ml of methanol. The mixture was quantitatively transferred into a 50-ml calibrated flask diluted to volume with methanol and then filtered through a Whatman No. 1 filter-paper discarding the first 5 ml of the filtrate. A 0.6-ml aliquot of the filtrate was transferred into the polaro-graphic cell and 19 ml of 1.0 N sulphuric acid and 0.4 ml of methanol were added. As previ-ously described the solution was purged for 10 min with purified nitrogen prior to recording the polarogram. The amount of minoxidil was determined from a calibration graph.Content Uniformity Test Ten tablets were randomly selected from the sample. Each tablet was placed in an individ-ual 150-ml beaker 20 ml of methanol were added and the system was allowed to stand for 5 min in order to promote disintegration of the tablets. The remaining larger lumps of tablet mass were crushed with a glass rod and the mixture was stirred magnetically for 20 min. After transferring the mixture quantitatively into a 50-ml calibrated flask the determination was continued as described in the previous section except that 1 ml of the filtrate and 19 ml of 1.0 N sulphuric acid were used. The amount of minoxidil in each tablet was calculated by the direct comparison method using reference standard solutions of 0.238 6 x and 0.9546 x Macro-scale Electrochemical Synthesis of 2-Amino-6-piperidinopyrimidine (X) from Minoxidil The procedure was similar to that used for the controlled-potential coulometry with the exception that the cell contained 150 mg of minoxidil in 25 ml of 20% V/V methanal in 1.0 N sulphuric acid.The applied potential was held at -1.2 V and the reduction time was 6 h. On completion of the reduction the product together with the supporting electrolyte was M for the 2.5- and 10-mg tablets respectively. I I I I -0.8 -1.0 -1.2 -1.4 -1.6 Applied potentialN Fig. 1. Effect of pH on the differential-pulse polaro-graphic waves of minoxidil (5 x M) in A 1.0 N H,S04; B C and D Britton - Robinson buffer a t pH 3.0 5.0 and 7.0 respectively 1224 AMANKWA et aZ. ELECTROCHEMICAL STUDIES ON AnaEyst VOZ.108 separated from the mercury the pH adjusted to 7.0 with ammonia solution and the resulting solution extracted with chloroform. The organic layer was separated dried over magnesium sulphate and concentrated to 1 ml. The concentrated solution was applied to a 1-mm thin-layer silica gel plate and then developed with a methanol - ammonia solution (100 + 1.5) solvent system. Two spots were observed. The RF value of one corresponded to that of minoxidil while the other component with an RF value of 0.60 was scraped off the plate leached out with methanol and the methanolic solution was evaporated to dryness. This isolation yielded X a yellow crystalline powder that decomposed between 80 and 100 "C. Other experimental data were as follows NMR (200 MHz CDCl,) 6 1.6 (m 6H), 3.5 (m 4H) 5.9 (m 3H) and 7.9 (d 1H) (see Discussion); M+ m/e 178; IR (KBr disc) 930, 1060 1090 1400 1680 and 3000 cm-l.I I -1.6 -1.8 -2.0 -2.2 Applied potentialN Fig. 2. Cyclic voltammogram of minoxidil (5 x ~ O - * M ) in a solution of TEAB in DMF. -0.8 -1.0 -1.2 Applied potentialN Fig. 3. Effect of scan rate on the cyclic voltammogram of minoxidil (5 x 10-4 M) in 1.0 N H,SO,. Scan rates A 10 mV s-1; B 100 mV s-l; and C 200 mV s-l. Results and Discussion Minoxidil exhibits two d.c. and d.p. polarographic waves in 1.0 N sulphuric acid and in Britton - Robinson buffer (pH 3.0-7.0). In sulphuric acid the first wave is intense and well resolved from the second wave which is partially overlapped by the supporting electrolyte discharge current (Fig.1). At that pH the first wave had an Et (half-wave potential) value of 0.95 V. In the Britton - Robinson system however the E moves cathodically while the i d decreases with increasing pH. The second wave which has an E value at approximately -1.20 V has a diffusion current that is very much higher than that of the first. This unexpected height increase is the result of interference by reduction of the supporting electrolyte. Both the i d (diffusion current) and E of this wave vary with pH in the same manner as those of the first. The first wave is attributed to the reduction of the fully protonated N-oxide while the second probably results from the reduction of the 3,4 carbon-nitrogen double bond. The latter functional group is more difficult to reduce owing to the presence of the amino group at position 4 of the pyrimidine moiety.l October 1983 MINOXIDIL AND ITS DETERMINATION IN TABLETS BY DPP 1225 The first step in the process would then involve protonaton of the N-oxide to an N-hydroxy intermediate which undergoes a two-electron reduction to 2,4-diamino-6-piperidinopyrimidine intermediate.At higher potentials the 3,4 carbon-nitrogen double bond can be reduced by a two-electron process to an unstable intermediate 3,4-dihydro-2,4-diamino-6-piperidinopyrimi-dine. This intermediate undergoes deamination to give the final product 2-amino-6-piperi-dinopyrimidine which was isolated in this work. The coulometric analysis of minoxidil in 1.0 N sulphuric acid indicates that four electrons per molecule were involved in the electro-reduction process.The graph of limiting current versus the square root of corrected height of the mercury col-umn is a straight line that does not pass through the origin. It is possible under certain conditions to delineate the processes in the first wave and con-sequently voltammetry of 5 x M minoxidil in the aprotic solvent DMF is shown in Fig. 2. Stabilisation of the initially formed anion radical of minoxidil is clearly indicated by the quasi-reversible one-electron voltammogram. A slow following chemical reaction of the anion radical is indicated by the non-linearity of a plot of ip (cathodic peak current) with scan rate. Strong multiple adsorption peaks as illustrated in Fig. 3 were observed in the cyclic voltam-mogram of minoxidil in 1 .O N' sulphuric acid.The peak centre occurs at about - 1.0 V and the multiplicity of the peaks decreases with increasing scan rate. The major product extracted from the macro-scale experiment emits a pinkish fluorescence under short-wave ultraviolet light. It also produces a negative result with the iron(II1) chloride test indicating the absence of an N-oxide group. The strong IR N-oxide absorption peak between 1250 and 1300 nm is absent in the IR spectrum of the product. The NMR (200 MHz) spectrum of the product exhibits four distinguishable peaks in CDC1,. In the CDC1 - D,O system however the peak at 6 5.9 is reduced to a doublet with an integration value corresponding to one proton. No other change in the spectrum was observed. From all of the foregoing observations the following pathway is proposed as minoxidil pro-ceeds by electrochemical reduction in aqueous media to the final product 2-amino-6-piperi-dinopyrimidine (Scheme I).0 H t I 2e 2H+ ___) - - H20 X Scheme 1 Resolution provided by the differential-pulse wave at an E value of -0.95 V was better than that of the d.c. wave; consequently the former mode was utilised for the analysis of the dosage forms. The peak height varied linearly with the concentration of the drug over the range 1 x 10-5-5 x M. Table I provides the results of the assay for each of the minoxi-dil dosage forms. Values obtained by the manufacturer's quality control laboratory are TABLE I ASSAY OF MINOXIDIL TABLETS BY DIFFERENTIAL-PULSE POLAROGRAPHY IN' A 1.0 N SULPHURIC ACID - METHANOL SYSTEM Recovery by Recovery by Recovery by Recovery by Tablet Label manufacturer/ manufacturer differential-pulse differential-pulse lot No.claimlmg mg % polarography /mg polarography % H-651 2.6 2.49 99.6 2.46f0.09 98.2 H-710 10 9.69 96.9 9.66f0.12 96.6 * Each value is the average of five determinations 1226 AMANKWA CHATTEN AND PONS presented for comparative purposes and excellent agreement is observed between the two results. Table I1 lists the average of the results obtained for each dosage form when ten single tablets were analysed. Common excipients do not interfere with the electrochemical method. TABLE I1 AVERAGE VALUE FOR THE ANALYSIS OF TEN INDIVIDUAL MINOXIDIL TABLETS IN A 1.0 N SULPHURIC ACID - METHANOL SYSTEM Recovery by Recovery by Tablet Labelled diff erential-pulse diff erential-pulse lot No.claimlmg polarographylmg polarography % H-651 2.5 2.49fO.l 99.9 H-710 10 10.08f0.7 100.8 The proposed method has the advantages of simplicity high sensitivity and rapidity. It can distinguish between minoxidil and those degradation products that do not contain the N-oxide group and consequently the method can be applied in purity and stability studies on minoxidil. The authors gratefully acknowledge the samples of pure minoxidil and tablets as well as the useful information that they received from Upjohn Company of Canada Ltd. In addition, they thank B. Speiser Department of Chemistry of the University of Alberta for his helpful suggestions and assistance with the reduction products. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. References Miller D. D. and Love D. W. Am. J . Hosp. Pharm. 1980 37 808. Gilmore E. Weil J. and Chidsey C. N . En@. J . Med. 1970 282 521. “United States Pharmacopeia,” Twentieth Revision Mack Easton PA 1980. Royer M. E. KO H. Gilbertson T. J. McCall M. J. Johnston K. T. and Stryd R. J . Pharm. The Upjohn Company of Canada Ltd. personal communication. Thomas R. C. Hsi R. S. P. Harpootlian H. and Judy R. W. J . Pharm. Sci. 1975 64 1360. Thomas R. C. and Harpootlian H. J . Pharm. Sci. 1975 64 1367. O’Reilly J. E. and Elving P. J. J . EZectroanaZ. Chem. 1969 21 169. Tachibana M. Sawaki S. and Kawazoe Y. Chem. Pharm. BUZZ. 1967 15 1112. Brooks M. A. DeSilva J. A. F. and D’Arconte L. J . Pharm. Sci. 1973 62 1395. Smith D. L. and Elving P. J. J . Am. Chem. SOC. 1962 84 2741. Sci. 1977 66 1266. Received March 22nd 1983 Accepted May 4th 198

ISSN:0003-2654    

DOI:10.1039/AN9830801221

出版商:RSC    

年代:1983

数据来源: RSC

摘要:

Analyst October 1983 Vol. 108 pp. 1227-1234 1227 Differential Electrolytic Potentiometry with Periodic Polarisation Part XXVII." Polarisation in Substitution Addition and Oxidation Titrimetry with Dibromine in Anhydrous Acetic Acid Direct and Mark-space Biased Periodic Adballa M. S. Abdennabit and E. Bishop University of Exeter Chemistry Department Stocker Road Exeter EX4 4QD Applications of direct current and mark-space biased square wave differential electrolytic potentiometry to bromination and oxidation reactions in anhydrous acetic acid have been examined. A few oxidation reactions e.g., of iodine and ascorbic acid are fast enough to permit direct titration. Other reactions such as nuclear bromination of aromatic hydroxy and amino com-pounds addition to unsaturated aliphatic moieties and oxidation of covalent group 5B compounds require double excess back-titration.Of the three electrolytes examined perchloric acid lithium chloride and sodium acetate, the last gives the best results for aromatic substitution being alkaline enough to allow the phenoxide ion intermediate to form. Certain oxidation indicators of the diphenylamine and benzidine classes and aminotriphenylmethanes are not oxidised but react by substitution by nuclear bromination and are cleanly determined without interference from unstable oxidation products. Addition to double bonds is slow but complete. Keywords ; Difleerential electrolytic potentiometry ; non-aqueous bromination ; oxidation titrimetry ; anhydrous acetic acid solvent Direct current differential electrolytic potentiometry (d.c.DEP) has been applied to oxidation -reduction titrations in aqueous media,l and gives two types of curve (Figs.2-6) a peak when the electrode processes are of comparable speed (type I) or a 2-shaped curve when one of the electrode processes is slow rising if the slow process originates from the titrant falling if from the titrand [types II(a) and II(b) respectively]. Symmetrical periodic current (~.c.DEP)~ and mark-space biased (rn.s.b.DEP)39* polarisation have also been applied to aqueous oxida-tion - reduction titrimetry and have been found to lead to rapid equilibration of potentials and sustained electrode activity due to the high current densities required and the continued polarity reversal that prevents accumulation of films on the electrodes.Little use has hitherto been made of polarised electrodes in non-aqueous titrimetry; although studies of DEP in non-aqueous acid - base titrimetry have been r e p ~ r t e d . ~ ~ ~ This paper presents an examination of the application of DEP to substitution addition and oxidation reactions in anhydrous acetic acid with dibromine. Experimental Apparatus and titration processes have been des~ribed.~,~ Zero-current potentiometry, d.c.DEP and m.s.b.DEP measurements were conducted simultaneously to afford compar-ability. Platinum electrodes were used. Reagents The calculated amount of acetic anhydride' was added to glacial acetic acid allowed to stand for 24 h refluxed for 8 h and finally distilled. In a fume cupboard approximately 0.1 mol of Aristar grade dibromine was transferred into a 1-1 flask containing anhydrous acetic acid mixed and Anhydrous acetic acid.Dibromine solution 0.1 moll-l. * For Part XXVI of this series see reference list p. 1234. t Present address University of Petroleum and Minerals P.O. Box 144 Dhahran International Airport, Dhahran Saudi Arabia 1228 ABDENNABI AND BISHOP DIFFERENTIAL ELECTROLYTIC Analyst Vol. 108 diluted to volume. The solution was standardised each day against the 0.2 moll-1 arsenic(II1) solution by DEP lithium bromide being added as electrolyte. AnalaR grade arsenic(II1) oxide was dried at 120 "C for 3 h and 9.981 g were dissolved in 100 ml of warm 2.5 moll-1 sodium hydroxide solution, 100 ml of 2.5 mol 1-1 hydrochloric acid were added and the solution was cooled and diluted to 1 1 with water.4-Hydroxybenzonitrile phenol quinol 4-aminopheno1 thymol 4-methoxyphenol 1-naphthyl-amine aniline 1,3-phenylenediamine and 2-nitrophenol solutions 0.1 moll-1. These solutions were prepared by dissolving 0.01 mol of the pure amine or phenol in anhydrous acetic acid and diluting to 100 ml. Dipkenylamine benxidine N-phenylanthranilic acid rosaniline and 3,3'-dimethoxybenxidine solutions 0.01 mol 1-l. The indicator solutions were prepared by dissolving 1 mmol of pure indicator in anhydrous acetic acid and diluting to 100 ml. Limonene cyclohexene oleic acid and pentane-2,4-dione solutions 0.1 moll-l. The solutions of unsaturated compounds were prepared by dissolving 0.01 mol of the pure substance in anhydrous acetic acid and diluting to 100 ml.Diiodine and L-ascorbic acid soltxtions 0.01 moll-l. A 1-mmol amount of the AnalaR grade compound was dissolved in anhydrous acetic acid and diluted to 100 ml. The limited soh-abilities precluded the use of stronger solutions. Bis(pentane-2,4-dionato)oxovanadium(IV) (vanadyl acetylacetonate) soltxtion 0.002 moll-1. Ammonium metavanadate(V) (4.6 g) was dissolved in 60 ml of 2.0 mol 1-1 sodium hydroxide solution 170 ml of 1.0 mol 1-1 sulphuric acid were added and sulphur dioxide was passed through until no further change in the blue colour occurred. The excess of sulphur dioxide was expelled by boiling and the solution was allowed to cool and filtered. To the stirred filtrate 10ml of pentane-2,4-dione were added followed by a solution containing 20g of anhydrous sodium carbonate in 120 ml of water.The precipitate was filtered off and recrystal-lised from chloroform. The low solubility in anhydrous acetic acid permitted dissolution of 2 mmol in the solvent and dilution to 100 ml. Electrolyte solutions 0.1 moll-l. The required volume of 72% m/m Aristar grade perchloric acid was diluted with acetic acid the amount of acetic anhydride required to react with the water present was added the solution was diluted to volume with acetic acid and stored until the reaction was complete. For lithium chloride and sodium acetate 0.1 mol of the anhydrous compound was dissolved in anhydrous acetic acid and diluted to 1 1. The standard 1 mg ml-l arsenic solution (BDH Chemicals Ltd.) in hydrochloric acid was standardised by the blank titration with the standardised dibromine solution, Arsenic(III) solution 0.2 moll-l.Arsenic(1II) chloride solution 0.0133 moll-1. Procedures Direct titration The method with the simultaneous use of zero-current potentiometry d.c.DEP and m.s.b.DEP by means of five platinum wire indicator electrodes and the double bridge filled with the appropriate non-aqueous electrolyte solution connecting the S.C.E. to the titration cell has been de~cribed.~ The current density was 3 x 10-6 A for d.c.DEP and the bias for m.s.b.DEP was 23% at a frequency of 60 Hz. Double excess back-titration.s To take account of the volatility of dibromine and its slow reaction with the electrolyte the method was modified by conducting the extended reaction alongside the blank in two stoppered 100-ml Quickfit flasks.To each 20 ml of the appropriate non-aqueous electrolyte solution is added. To one an aliquot of 0.1 mmol of sample is added followed by 0.5 mmol of dibromine (an excess of 40-400~o); to the other the blank 0.5 mmol of dibromine is added. After the selected time of reaction the same amount of arsenic(II1) chloride in an excess of that needed to react with the unconsumed dibromine is added to each vessel. The contents are transferred into the titration cell washed with similar amounts of electrolyte solution and the excess of arsenic(II1) is back-titrated with dibromine as in the direct method. The difference between the two back-titrations gives the amount of dibromine consumed by the sample October 1983 POTENTIOMETRY WITH PERIODIC POLARISATION.PART XXVII 1229 Results and Discussion Glacial acetic acid has proved to be a suitable solvent for oxidation - reduction titrations.799-12 Phenols have been directly titrated potentiometrically by dibromine in glacial acetic acid with sodium acetate as the ele~trolyte.~ IngbermanlO has catalysed the reaction with pyridine, which permits direct tit rat ion using constant - curren t pot ien t iomet ry . l1 Glacial acetic acid has also been used as the solvent for iodosobenzene diacetate titrations.12 Initially the direct titration of phenol thymol 4-hydroxybenzonitrile and quinol in glacial acetic acid with dibromine was attempted. Although the potential of the zero-current electrode was reasonably steady the potentials of the polarised electrodes fluctuated because the water present affected the electrode processes.On substituting anhydrous acetic acid in the same titrations all potentials became steady but the bromination reactions became excessively slow ; direct titration proved to be impossible even in the presence of sodium acetate or pyridine. The difference from earlier observation^^^^^ is due to the presence in glacial acetic acid of water which acts as a base assisting the formation of the phenoxide ion which is easily attacked by the bromonium ion. Brominations in glacial acetic acid are accelerated by small amounts of added water.13 Brominations in anhydrous acetic acid had perforce to be studied by the tedious and time-consuming double excess back-titration method as the only alternative in anhydrous systems.Reaction with Solvent and/or Electrolyte To 20 ml of anhydrous solvent or a solution of the electrolyte therein 0.628 mmol of dibromine was added and after 24 h the residual dibromine was determined by back-titration with arsenic(II1). For the solvent alone the loss was 4.00%. In increasingly alkaline solution as the sodium acetate concentration was increased the loss increased more or less linearly approaching lo%, and levelling at 1.0 mol 1-1 sodium acetate as shown in Fig. 1. This is due to the base enhanced solvolysis High blanks suggested significant reaction with the solvent and/or electrolyte. 0 0 II II CH3COH + Br + CH,CO-Br+ + HBr which is retarded by the accumulation of hydrogen bromide. Acyl hypobromite has been is01ated.l~ In the slightly acidic 0.1 mol 1-1 lithium chloride the loss fell to 2.90y0 while in the strongly acidic 0.1 mol 1-1 perchloric acid solvolysis was suppressed to 0.96Y0.4 ’ 1 I I I I 0.2 0.4 0.6 0.8 1.0 Concentration of CH3COONa/M Fig. 1. Loss of dibromine in 24 h from a solution initially 0.1 mol 1-1 in anhydrous acetic acid as a function of the concentration of sodium acetate electrolyte. Substitution Reactions of Aromatic Amines and Phenols The bromination of a selection of amines and phenols in basic (0.1 mol 1-1 sodium acetate), slightly acidic (0.1 moll-1 lithium chloride) and strongly acidic (0.1 mol 1-1 perchloric acid) electrolyte solutions in anhydrous acetic acid was examined by double excess back-titration. It is immediately evident from the results in Table I that all the reactions are very slow and o 1230 ABDENNABI AND BISHOP DIFFERENTIAL ELECTROLYTIC Analyst VoZ.108 small analytical utility. They are fastest and most complete in the basic electrolyte appreci-ably incomplete in lithium chloride and grossly incomplete even for the amines in the strongly acidic electrolyte. The low dielectric constant of t.he solvent favours ion pairing hindering the dissociation of Br+-Br- and slowing down the bromination reaction. The acidity of the solvent discourages the formation of the active phenoxide ion and the protonation ion pair e.g., C6H50H,+ -OCOCH, a much less effective intermediate is stabilised. The sodium acetate electrolyte is sufficiently basic to allow the phenoxide mechanism to operate but at a slow rate.The reactions are mainly substitution phenol thymol and (surprisingly) 4-hydroxybenzo-nitrile are tribrominated ; 1-naphthylamine is 2,4-dibrominated with 50-60% of a tribromo compound ; 4-methoxyphenol is partially demethoxylated as well as 2,6-dibrominated. Other reactions are incomplete even in sodium acetate. Possibly after very prolonged reaction further brominations might reach completion but the mounting blank causes uncertainty. In addition to dibromination to the 2,4-derivative quinol and 4-aminophenol are oxidised to the anion radical consuming 2.5 mol of dibromine. A mixture of 1 mol each of these two com-pounds consumes a total of 7 mol of dibromine. This synergic effect as demonstrated by the superadditivity effect in photographic development by metol - quinol mixtures most probably leads to the 2,3,5,6-tetrabromo-l,4-benzoquinone and the 2,5-dibromo-4-aminophenol.Amino oxidation - reduction indicators dication radicals by a two-electron step.15 Colourless indicators of the diphenylamine - benzidine class are oxidised to deeply coloured Direct titrations of diphenylamine N-phenylan-TABLE I BROMINATION OF PHENOLS AND AMINES IN VARIOUS 0.1 moll-1 ELECTROLYTE SOLUTION'S IN ANHYDROUS ACETIC ACID Sample* 4-Hydroxybenzonitrile . . . . 4-Aminophenol . . . . . . . . Quinol +'4-ami,ophenbit . . . . Phenol . . . . . . . . . . 4-MethoxGhendl' . . . . . . 2-N i trophenol . . . . . . . . 1-Naphthylamine . . . . . . Aniline . . . . . . . . 1,3-Phenylenedi&ne . . . . Quinol . . . . Thymol .. . . . . Sodium acetate Time of Amount of Br, standing/h consumed/mmol 24 0.304 2 0.242 8 0.244 6 0.247 24 0.705 24 0.304 16 0.302 16 0.236 16 0.123 16 0.256 L I 7 Lithium chloride Perchloric acid Time of Amount of Br Time of Amount of Br, standing/h consumed/mmol standing/h consumed/mmol A L I r > 24 0.281 24 0.075 24 0.246 72 0.054 24 0.228 48 0.048 24 0.281 48 0.218 16 0.278 16 0.197 16 0.087 16 0.156 48 0.056 96 0.161 * A 0.1-mmol amount of sample was taken. t 0.1 mmol of each. thranilic acid benzidine 3,3'-dimethoxybenzidine and the bromine indicator rosaniline were attempted. The reactions were very slow gave a white precipitate and showed no colour changes. Double excess back-titration was adopted with the results shown in Table 11.In TABLE I1 DOUBLE EXCESS BACK-TITRATION OF 0.1 mmol OF VARIOUS SUBSTANCES IN ANHYDROUS ACETIC ACID Compound Diphenylamine N-Phenylanthranilic'&id : Benzidine 3,3'-Dimetho~~benzidine : Rosaniline . . . . . . r Bromination of indicators Time of s t anding/h 6 6 24 8 2 4 5 24 72 24 Amount of Br, consumed/mmol 0.406 0.306 0.308 0.204 0.085 0.097 0.101 0.148 0.183 0.184 Addition to double bonds Time of Amount of Br, Compound standing/h consumed/mmol Limonene 6 0.097 Oleic acid 48 0.102 Pentane-2,4-dione . . . . 48 0.204 . . . . . . Cyclohexene . . . . . . 48 0.103 . . . . . . Oxidation of covalent inorganics r 1 Time of Amount of Br , standing/h consumed/mmol Antimony(II1) chloride .. 48 0.086 Phosphorus(II1) chloride . . 24 0.102 Sulphur dioxide . . . . 48 0.09 October 1983 POTENTIOMETRY WITH PERIODIC POLARISATION. PART XXVII 1231 no instance was a two-electron mole per mole ratio obtained despite the formal potentials favouring direct oxidation. Instead the ratios corresponded to or approached full nuclear bromination. The method is therefore useful for the determination of these substances and it avoids the formation of unstable products.15 Analysis of the product from diphenylamine confirmed tetrabromination. The products from the above-named compounds were di(2,4-dibromophenyl)amine N-2,4-dibromophenyl-4-bromoanthranilic acid 3,3’-dibromobenzidine, 5,5’-dibromo-3,3’-dimethoxybenzidine and 2,2’-dibromo-2’’-rnethyl-4,4’,4’’-triaminotriphenyl-methane respectively.The example of 3,3’-dimethoxybenzidine is of interest as the first bromine enters fairly quickly but the second approaches completion exponentially the plot of moles of dibromine consumed veYsus the logarithm of time being a straight line. 100 a 60 20 800 4 e E a 0 uj 700 > 600 - - - _ _ - - -\ --8.4 8.6 8.8 9.0 9.2 Volume of titrantlml Fig. 2. Titration of 10 ml of 0.1 mol 1-1 limonene with 0.109 moll-’ dibromine. Conditions as follows electrolyte lithium chloride A. anode - S.C.E. Dotential 2. 6oo t 0.6 0.8 1.0 1.2 1.4 Volume of tit rantlml zero-current electrode - S.C.&. potential Titration of 10 ml of 0.002 mol 1-1 bis-C cathode - S.C.E. potential; EA differ- (pentane-2,4-dionato)oxovanadium(IV) with 0.081 ential potential.Solid line d.c. DEP; and Conditions and symbols as in broken line m.s.b. DEP. Fig. 3. moll-’ dibromine. Fig. 2. Addition to Double Bonds The direct titration of various unsaturated compounds was attempted but the reactions were too slow although limonene gave interpretable curves (Fig. 2) in a titration lasting 6 h; extrapolation of the lines on either side of the first break of the differential curves to inter-section gives the end-point. Double excess back-titration reveals complete saturation of the side-chain methylene bond (Table 11). Clearly the charge-separation process producing the bromonium ion intermediate or the bridging species,ls is strongly inhibited by the solvent an indication of the necessity for trace amounts of water in Wijs-type reagents1’ An attempt was made to solubilise an oxidisable ion by formation of a neutral complex that might be titratable in the solvent.Bis(pentane-2,4-dionato)oxovanadium(IV) (vanadyl acetylacetonate) proved to be usable although the reaction was slow taking 2 h. The curves in Fig. 3 are like those for limonene (Fig. 2) and are similarly interpreted but oxidation of vanadium(1V) would require 2 mole of vanadium(1V) per mole of dibromine whereas th 1232 ABDENNABI AND BISHOP DIFFERENTIAL ELECTROLYTIC Analyst VoZ. 108 experimental ratio was 1 mol of vanadium(1V) to 4 mol of dibromine indicating enolisation of the ligand and addition of two dibromines to the double bonds of each ligand as shown in Table 11.Oxidation Reactions The end-point of the back titrations reported above is of course the oxidation of arsenic(II1) by dibromine in anhydrous acetic acid but the arsenic(II1) solution used is aqueous as there is no point in employing a non-aqueous solution. The medium therefore contains 10% or more of water and the titration curves shown in Fig. 4 are similar to those found in bromate titra-tions of arsenic(II1) in aqueous media,l and are of excellent quality. Diiodine in anhydrous acetic acid titrates cleanly and rapidly duration 10 min in a two-electron 1 1 reaction with dibromine to give iodine monobromide. The electrode reactions are fast and type I peaks are produced as shown in Fig. 5. The two-electron dibromine also oxidises L-ascorbic acid cleanly and directly into dehydroascorbic acid the m.s.b.(Fig. 6 ) is a sharp falling-Z of type II(b) but the solvent and the inactive product inhibit any fast response by the other electrodes. Other covalent p-block compounds were examined by double excess back-titration and showed slow reaction with consumption of one dibromine per molecule as in Table 11. 700 600 4 500 s !i E a 4 0 a > 30C 20( 3.0 3.2 3.4 3.2 3.4 Volume of titranvml Fig. 4. Titration of 20 ml of 0.013 mol 1-l arsenic(II1) chloride with 0.081 mol 1-1 dibromine. Conditions and symbols as in Fig. 2. Anhydrous Acetic Acid - Methanol Solvent In an attempt to encourage formation of bromonium and phenoxide ions by increasing the dielectric constant of the solvent the use of a 1 + 1 V/V mixture of anhydrous acetic acid an October 1983 POTENTIOMETRY WITH PERIODIC POLARISATION.PART XXVII 1233 methanol was examined with lithium chloride as the supporting electrolyte. Direct titration of phenols was unsuccessful while back-titration led to unacceptably excessive blanks arising from the reaction of bromine with methanol. 600 I I I I I 0.4 0.6 0.8 1.0 1.2 1.4 Volume of titrantlml Fig. 5. Titration of 10 ml of 0.01 moll-' diiodine with 0.090 moll-' dibromine. Conditions and symbols as in Fig. 2. 700 600 > E . Lu" 500 400 30C / I *' I -- I- ' I I I I I I I I I I I I I I I I I I 0.6 0.8 1.0 1.2 1.4 Volume of titrant/mI Fig. 6. Titration of 10 ml of 0.01 mol 1-' ascorbic acid with 0.081 moll-' dibromine.Conditions and symbols as in Fig. 2. Conclusions Although successful in solubilising organic and covalent inorganic compounds without solvolysis anhydrous as opposed to glacial acetic acid is not a good solvent for bromine reactions and oxidations. The reagent has poor stability and reaction is inhibited by the ion-pairing effect of the solvent which additionally inhibits phenoxide ion formation even with the strongly basic sodium acetate electrolyte. Although the double excess back-titration method gives reproducible and reasonable results and excellent end-points it is tedious and time-consuming and the blanks are high. Aromatic substitution and addition to double bonds are slow in this solvent and few oxidations are fast enough for direct titration.Clean bromi-nation of certain oxidation indicators may be of advantage despite the long reaction times. The reaction mechanisms and kinetics are at fault in this solvent rather than the titration procedures but where the titrations are feasible DEP offers the advantages of enhanced response speed and sharpness of the end-point. In direct titrations of limonene and acetyl-acetone DEP permits location of the end-point where the classical method fails and in direct titration of ascorbic acid the m.s.b. method alone gives a good end-point by virtue of its electode activation effect 1234 ABDENNABI AND BISHOP 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. References Bishop E. Analyst 1958 83 212. Bishop E. and Webber T. J. N. Analyst 1973 98 712. Bishop E. and Webber T. J. N. Analyst 1973 98 769. Hartshorn L. G. PhD Thesis University of Exeter 1972. Abdennabi A. M. S. and Bishop E. Analyst 1982 107 1032. Abdennabi A. M. S. and Bishop E. Analyst 1983 108 71. Kucharskf T. and Safaiik L. “Titrations in Non-aqueous Solvents,” Elsevier Amsterdam 1965. Bishop E. and Jennings V. J. Talanta 1962 8 679. TomiEek O. and Heyrovsky J. Chem. Listy 1950 49 169. Ingberman A. K. Anal. Chem. 1958 30 1003. Huber I. O. and Gilbert J. M. Anal. Chem. 1962 34 247. Sivasankarapillia V. N. and Nair C. G. R. Talanta 1975 22 57. Keefer R. M. and Andrews L. J. J. J . Am. Chem. SOC. 1956 78 3637. Hundiecker C. PhD Thesis Koln (Wintgen) ; privat labor. Koln (Braunsfeld). Bishop E. and Hartshorn L. G. Analyst 1971 96 26. Gold E. S. “Mechanism and Structure in Organic Chemistry,” Holt and Reinhart Winston New Wijs J. J. A. J . SOC. Chem. Ind. 1898 17 698. York 1959. NOTE-References 1 2 3 5 and 6 are to Parts 11 XXII XXIII XXV and XXVI of this series respec-Received March 7th 1983 Accepted May 9th. 1983 tively

ISSN:0003-2654    

DOI:10.1039/AN9830801227

出版商:RSC    

年代:1983

数据来源: RSC