ISSN:0306-1353    

DOI:10.1039/AA97707FX001

出版商:RSC    

年代:1977

数据来源: RSC

摘要:

The Chemical Society\Analytical Sciences MonographsA series of monographs on topics of interest to analytical chemists.High Precision Titrimetryby C. Woodward and H. N. RedmanThis monograph describes how, using simple, inexpensive equipment, titrimetric procedurescan obtain results of superior precision to those achieved routinely in the laboratory. The bookdeals with visual titrations, including apparatus and preparation of substances; and in-strumented titrations, including photometric and electrometric techniques.Paperbound8i" x 6 71ppf2.50/$5.cKIThe Chemical Analysis of Water- Generat Principles and Techniquesby A. L. WilsonChemical analysis of water for human consumption has been the subject of countless books,and this volume concentrates on a critical appraisal on the analytical principles involved.Allstages of the analytical process are covered, including: sampling of time, place and techniques;the analysis proper; the reporting of results, and data handling.Clothbound 196pp 83" x 6&" f 7.5O/$15.mPyrolysis- Gas Chromatographyby R. W. May, E. F. Pearson and D. ScothernThis volume presents the available knowledge on the subject of pyrolysis-gas chromatography ina form useful to the analyst. Areas covered include: merits and demerits of particular ap-plications; major analytical uses of the technique; identification of the pyrolysis products whichare eluted from the chromatography column, and the necessity for increased standardization.Clothbound 117pp 83" x ti f 7.20/$14.10Electrothermal Atomization for Atomic Absorption Spectrometryby C.W. FullerOne of the successful alternative atomization sources to the flame is a t present electrothermalatomization, and this volume deals with all aspects of this technique, including: history;theoretical aspects; practical considerations; analytical parameters of the elements; and specificareas of application.Clothbound 135pp 4'' x %'' f6.75/$13.50Dithizoneby H. M. N. H. IrvingAs a result of his long association with analytical techniques using this reagent, the author isable to present in this volume a body of historical and technical data on the subject. Specifictopics include: the properties of dithizone; metal-dithizone complexes and their formulae; thephotochemistry of dithizonates; and organornetallic dithizones.Clothbound 112pp 83" x 3'' f7.25/$14.50/Further information about any of these publications can be obtained from: The MarketingDepartment, The Chemical Society, Burlington House, London W1V OB

ISSN:0306-1353    

DOI:10.1039/AA97707BX003

出版商:RSC    

年代:1977

数据来源: RSC

摘要:

Part I: Fundamentals and Instrumentation 9 1.2 PLASMAS Optical emission spectrochemical analysis using plasma sources has attracted considerable attention during 1977 and there appears to have been considerable advance in the under- standing of the nature of these sources and in their applications. Again it i s the r.f. induc- tively coupled plasma (ICP) for which the greatest volume of data has appeared in the literature, although microwave-induced plasmas (MIP) and d.c.arc plasmas continue to remain attractive as alternative sources.10 Analytical Atomic Spectroscopy 1.2.1 R.f. Inductively Coupled Plasmas The ‘state-of-the-art’ with respect to the excitation processes prevailing in ICP sources and their applicability for single-channel and simultaneous multi-element analysis, has been reviewed by several well known workers (583, 589. 574, 1021, 1041, 1485, 1476, 1640). Fassel (108) has reviewed the application of ICP OES, with the emphasis on small liquid samples, and compared detection limits with those attainable by AAS using electrothermal sample vaporization and atomization; on a relative basis (i.e. ng ml-I) ICP OES was shown to be capable of providing similar or superior results.Boumans and co-workers (588) have described some experimental comparisons between AAS and ICP OES in the daily practice of an analytical service group in a research laboratory. These authors point out that although such comparisons can be made on the basis of performance characteristics such as detection limits, precision, accuracy and dynamic range, assessment of the relative merits of the techniques for particular needs must also include less easily quantifiable factors, such as the time and skill needed for sample preparation, the work involved in checking spectral interferences, the convenience of operation and the over-all reliability of the analytical results in relation to the experience of the operator.Danielsson and Soderman (124) have commented on the importance of net count rate and line-to-background ratios in determining detection capability in ICP sources; they plead for publication of the corresponding measurement times and line-to-background ratios when detection-limit data are presented.In a brief article entitled ‘Thc Power Game’ Bogdain (125) has again reminded the unwary that specification of r.f.power output of generators by manufacturers and many users in publications is usually that of the plate d.c. supply (for a free-running oscillator) or that of the d.c. supply for the power amplifiers. In order to take into account losses in transformers, transmission line, matching network, etc., he recommends that, in the absence of a satisfactory method of measuring the r.f. power in the plasma itself, the r.f.power output be defined as that obtained at the induction coil connectors. This seems a reasonable compromise which should be adopted. Eckert (201, 575, 1201, 1304) has investigated the use of sealed r.f. inductively heated Iamps containing small samples as an alternative to emission spectrometry with open plasma sources.Hg, Ar and Xe were tested as plasma fillers in spherical quartz bulbs of 35 and 46 mm i.d. at r.f. power inputs of 0.3 to 1.8 k W ; the best results were obtained with 46 mm bulbs filled with Xe at 200Torr. Absolute detection limits obtained for Pb, Hg and Sb were lower by at least an order of magnitude than limits reported for open ICP sources. For the less volatile elements detection limits depend on bulb-wall temperature and the bulbs require operation near the quartz softening point.Signal intensities decreased with time at rates depending on element and concentration and imposed a limit on the observation time. 1.2.1. I Fundamental Studies. Several reports of determinations of electron numbcr densities and excitation temperatures in the ICP discharge made in order to test hypotheses con- cerned with excitation mechanisms have been published.Jarosz, Mermet and Robin (1401), in a study of a 40 MHz ICP source, measured the electronic excitation temperature using Ar, Ti, Fe and V as thermometric species, the electron number density from line profiles of Ar and H and the ionization temperatures of Mg and V. Using the Saha equation and assuming complete local thermal equilibrium (LTE), the ionization temperature of Ar (7400 K) was found to be substantially different from the excitation temperature (4800 K) and indicated LTE does not exist; ionization temperatures for Mg and V, however, were calculated as 7300 and 6700K using an excitation temperature and the experimentally determined value of the electron number density.The same workers (70) have also reported a spectroscopic study of an Ar/H,S ICP source for which electron density and excitation temperatures were recorded; some new lines for S were reported. Mermet and co-workersPart I : Fundamentals and Instrumentation 11 (578) have again postulated that metastable states of ionised Ar, whose energy levels are between 32 and 34 eV above the ground state of the Ar atom, may be responsible for excitation of lines of Al(II), Br(II), Cl(I1) and Ti(II1) up to 30 eV via the mechanism: Ar+meta 4- X 4 Ar + X+* Ar+meta 4- X -+ Ar 4- X2+* 4- e- Kornblum and de Galan (799) have proposed a novel method for the measurement of excitation temperatures that does not require a knowledge of transition probabilities; in this method both the absorption and emission intensities of the line resulting from a single transition are measured.The spatial distributions of temperature and the number densities of electrons, atoms and ions were measured in a low-power (500W) ICP at low and high carrier-gas flow-rates; it was concluded that LTE did not prevail. Departures from LTE were summarized as: Tgas < T,,, < Tion; T,,, = F(Eexc); Tion = F(Ei,,,,), where T is the absolute temperature and E,,, and Efon are the excitation and ionization energies for the transitions concerned.Boumans and de Boer (576) have suggested that the now well substantiated departures from LTE in the ICP source and the Penning ionization mechanism by metastable argon ions postulated by Mermet et al.(578) can be interpreted in terms of an hypothesis that would explain both the high sensitivity of ionic lines and the smallness of ionization interferences. It is suggested that metastable Ar acts both as an ionizer (Penning ionization) and as an ionizant, i.e., an easily ionizable constituent. This hypothesis was suggested by rcsults of experiments with a special type of d.c.plasma jet (Cabannes, F., Pure Appl. Chem., 1974, 39, 381), which shows similar characteristics to a toroidal ICP in that the plasma in the observation zone results from the mixing of cold and hot Ar. Winefordner et al. (1002) have studied a 144 MHz discharge in sealed tubes operated on Ar or Ne with a 50 W amplitude-modulated supply. Electron temperatures, positive-ion concentrations and excitation and ionization temperatures were determined.Excitation temperatures increased with decreasing pressure for Ne filled, Ar filled and Ar/TII filled sources down to 0.5 Torr. Maximum values were Ne-9000 K, Ar-6700 K, Ar/TIT-5900 K; ionization temperatures were similar for Ne and Ar and lower than the excitation tempera- tures. Addition of T1 to Ar increased the electron tcmpcrature by a factor of two and the positive-ion concentration by a factor of 5.A theoretical model for the calculation of the trajectories and temperature histories of particles injected into the fire-ball of an ICP has been proposed (1486). Calculations were made for alumina particles of diameters ranging from 10 to 250 pm; the particles were injected through a water-cooled probe upstrcam of the plasma core.The results show that the internal plasma recirculation in the coil region is responsible for bouncing the particles on the core. Particles of < 10pm are re-entrained in the core due to electromagnetic pumping and are subsequently completely evaporated. Larger particles, depending on initial position and velocity of injection, may by-pass the plasma core and be deposited on the wall of the plasma-confining tube.Particles of diameter > 100pm were found to pass through the core when injected close to the centre line of the torch. Yashida and Akashi (968) have also developed a mathematical model to predict the trajectory and temperature changes of an r.f. plasma; the model was applied to evaluate the extent of vaporization of iron particles of diameters between 20 and 80 pm.Human and Scott (504) have measured the profiles of the lines of some metal species emitted by an ICP and compared these with theoretical line profiles. Self-reversal of resonance lines was observed for very high analyte concentrations in the nebulized solutions at large distanccs (20-25 mm) above the coil.At lower observation heights (10-15 mm) the annular structure of the plasma prevented self-reversal, but line broadening by self-absorp- tion did occur, again at high analyte concentrations.12 Analytical Atomic Spectroscopy Jarosz and Robin (577) have employed an r.f. ICP source to determine the oscillator strength for Ta(1) and Ta(I1) lines in the region 290-310 nm using a pre-determined excita- tion temperature of 4800 2 300 K and an electron density of 5.5X 1014 cm-3.Other references of interest - Estimate of error of optically diagnostic methods for axially symmetric plasmas: 1317. Standards and measurement of high temperatures: 1484. 1.2.1.2 Znsfrumentation. The importance of plasma torch geometry on the analytical per- formance of the ICP source is becoming well appreciated.Allemand and Barnes (985) have studied various torch configurations in order to establish the optimum system; ease of ignition, high efficiency and freedom from clogging with organic samples were the criteria selected. Torch shape and gas flow-ratcs were found to be closely related to ignition characteristics. A computer model was used to help select an optimum configuration for a 13 mm diameter torch operable at 250-400 W and for an 18 mm torch that could be ignited at <400 W and which gave Cr(I1) 268 nm/Cr(I) 358 nm line intensities at 900 W similar to those from a conventional torch at 1200 W.The same workers have also reported a simple modification of the gas flow injection region of conventional ICP torches to promote the formation of closed loops of ionized gas to ease ignition and reduce gas consumption (951).Genna and Barnes (580, 1202) have continued their work on velocity and temperature measurements in an ICP (ARAAS, 1976, 6, 10) and reported experiments in which conven- tional photographic and Schlieren observations of the discharge have been used to give improved insight into the design of torches and choice of operating conditions to ease plasma ignition, reduce Ar consumption and improve analytical performance.In studies in which volume flow and linear introduction velocity of the injector gas were separated as independent optimizing parameters by varying the inner diameter of the injector tube, Kornblum and co-workers (581) found substantial differences in the emission intensities of different elements when the flow-rate was varied in a low-power (500 W) ICP.Quartz plasma torch designs suitable for both OES and AAS measurements have been described (991); in the torch for AAS the light beam passes axially through the plasma and the usual larger diameter injector tube is replaced by a 1.5 mm diameter quartz capillary tube. Considerable interest at present is focussed 011 the problems associated with sample introduction into ICPs.Browner (1483) has considered general problems associated with sample introduction and described rapid means of producing suspensions from micro-solid samples and a flexible scheme for the production of volatile compounds for direct intro- duction to plasmas. Kirkbright (590) reported the application of a continuous volatile hydride system to the simultaneous determination of ppb levels of As, Se, Te, Bi, Sn and Ge in an ICP and a graphite rod atomization system for the direct introduction of pl liquid samples into the plasma.Scott (1391) has described a spark discharge between two graphite electrodes situated above the sample for the elutriation of fine particles from the sample into a plasma, This technique for the introduction of powder samples was applied to the determination of Cu in geochemical samples in the range 2 to 45 ppm.Several workers have presented detailed reports concerned with the use of conventional pneumatic nebuEizers with ICP sources. Greenfield and co-workers (1475) have described a comparison of the performance of numerous commercially available nebulizers with their ICP system.A simple procedure was used to establish the optimum operating condi- tions of power, observation height, injector, plasma and coolant-gas flow rates. Wohlers (566) has compared metal concentric, glass concentric and cross-flow (right angle) nebulizers as used with a direct-reading ICP spectrometer; detection limits, long-term stabilities andPart Z: Fundamentals and Znstrumentation 13 memory effects were similar for each, but the cross-flow nebulizer was superior for solutions with a high dissolved solid content. Ohls et al.(203, 1641, 1645) have evaluated the per- formance of Eppendorf and Meinhard-type concentric nebulizers for work with an ICP system and obtained 1-2 orders of magnitude increase in intensities with the Meinhard nebulizer. Other workers (33 1) have made similar comparison studies between a concentric- flow glass-bead impinger-type nebulizer and a cross-flow nebulizer for ICP emission spectrometry and discussed the general applicability of each for the determination of trace mctals in natural waters.Olson, Haas and Fassel (570) have described the construction and performanc? characteristics of a new ultrasonic nebulizer system.This compact nebulizcr consisted of a transducer fitted with a half-wave borosilicatc-glass coupling plate mounted in the vertical plane; solutions were fed onto this plate using a peristaltic pump and Teflon tubing. The ultrasonic nebulizer gave 10-fold improvement in detection limits compared to a conven- tional pneumatic nebulizer; this was shown to result from the greater sample delivery rate (1 1 % efficiency compared to 1 % for the pneumatic nebulizer).Desolvation is required, however, to allow the system to be used due to the larger amount of water otherwise delivered to the ICP; wash-out time for the nebulizer/desolvation system was 60 s. Demers and Priede (591) have drawn attention to the importance of maintaining constant r.f.current flux density in the plasma load coil; inadequate regulation of the forward and the reflected power can lead to sudden and random shifts in the TCP back- ground intensity and, if unsuspected, cause relatively large errors in analysis. These workers recommend continuous observations of an Ar line from the source as a means of monitor- ing nebulizer and source operation.Other workers have also considered instrumental factors influencing precision and accuracy in OES with ICP sources applied to the analysis of standard reference materials (1203, 1480). The use of photodiode array detection systems in ICP spectrometers has attracted further attention during the past year (see also ARAAS, 1976, 6, 11).Thus Horlick and co-workers (1 1 17, 1434, 1436) have described a computer-coupled photodiode array spectro- meter based on a 1024-element array. This system is capable of simultaneous measurement over a 50 nm spectral range in the region between 200 and 1000 nm. The system is claimed to have several advantages for simultaneous multi-element analysis including ease of spectral background measurement and stray light correction.Mounted vertically at the exit slit of the monochromator, the photodiode array may also be employed to provide spatial profiles (intensity with respect to height in the plasma) from the ICP (1436). The same group of workers (1117) have developed a hardware read-out system for the photodiode array spectrometer that allows cross-correlation in real-time of the signal with the sought-for spectral pattern.Koirtyohann et aZ. (1433) have continued their earlier studics (ARAAS, 1976, 6, 11) with a photodiode array spectrometer controlled by a microcomputer and presented profiles for molecular, atomic and ionic species in an ICP. A photodiode array detector system has also been described by Gold (1306) to mcasure light intensity profiles across a section of a plasma; velocities of particles injected into the plasma, or of parts of the plasma itself, were then deduced from the time variations of these profilcs.Lamonthe and Seeley (329) have described a dynamic wavclength modulation system for background correction and parameter optimisation with ICP sources and Ward (567, 1116) has pointed to the advantages accruing from the use of a variable wavelength channel in addition to the fixed element programme from a polychromator system.Dalager and co-workers (1103) have discussed the optical dynamic range and spectral purity possible with modern OES systems and related these to the requirements necessary to achieve optimum analytical performance with the ICP source.Kniseley et al. (1109) have reported a versatile data-acquisition system developed for simultaneous multichannel14 Analytical Atomic Spectroscopy measurements; the system has a dynamic range greater than 105 and is based on a digital, time-sliccd averaging technique in which the analog signals are digitized 60-600 times s-1. The digitized photocurrents for each measurement channel are accumulated, averaged, and stored for future use or transferred to the selected output device.Considerations affecting the design of ICP OES spectrometers and evaluation of performance characteristics of laboratory-fabricated and commercial systems have been reported by a number of workers as conference papers and publications (587, 592, 600, 606, 1104, 1208, 1422, 1427, 1640).Complete instrumental systems are discussed in I, 2.5, and commercially available plasma spectrometers are listed in Table B. 1.2.1.3 Interference Effects. Relatively few reports of specific interference studies in ICP sources have been published recently. Several authors have studied ‘interferences’ resulting from nebulizer effects (e.g., 331, 1475) particularly with solutions of high dissolved-solid content.Abdallah, Mermet and Trassy (501, 584) have reported a study of different types of interference encountered in ICP OES; chemical interferences were claimed to be negligible and nebulization effects easily avoided, whereas spzctral interferences could be resolved using correction techniques. Residual interferences due to atomization effects were low.Kornblum and de Galan (1397) have reviewed the literature on ICP interference effects and conducted experimental work by emission and absorption measurements of atomic and ionic lines in a study of the interfcrences of phosphate and Cs on Ca and Mg; a low-power (500 W) plasma was employed at high (4.5 1 min-1) and low (1.4 1 min-1) injector-gas flow rates.As might be expected, at the high flow rate pronounced interference effects were observed attributed by these workers to a combination of volatilization interference, a change in excitation temperature and a shift in the ionization equilibrium. Interference effects under low flow rate conditions were substantially smaller, Larson and Fassel (1205) have discussed line broadening and radiative recombination background interferences in ICP OES.Changes in the concentration of concomitants can produce subtle background changes; the wings of spectral lines represent one of these background changes. Experimental evidence was reported to show that the wings of very strong emission lines can produce a significant continuum as far as 10 nm or more from the line; the origins of this broadening were discussed and examples of Stark broadened lines of metallic species of full width at half maximum of -0.05nm or greater were given.Other reports stressing the need to recognise the possibility of spectral interferences and careful spectrometer and analytical line selection have also appeared (662, 1206). 1.2.1.4 Applications. The literature concerned with the application of ICP OES in many fields is now expanding rapidly as sales of commercial sources and spectrometers increase.At present the most widespread applications appear to be to the determination of trace elements in samples of environmental importance. Thus ICP OES has been applied to trace-element determinations in natural waters (289, 586, 657, 1204, 1209, 1287, 1691), sea-water (132), soil extracts (526, 590, 1022, 1110, 1412, 1481), plant tissues (1110, 1287, 1412, 1481) and foodstuffs (127, 130, 1080, 1168, 1287, 1477).Other materials of biological origin to which trace-element analysis with ICP systems has been applied include animal tissues (1080, 1287) and urine (1290). The applications of ICP OES to materials of geo- chemical interest also appear significant.The multi-element analysis of rocks and soils (339, 1478) and routine use for rocks, soils, sediments, brines, waters and snow (198) has been described. Laqua et al. have described the determination of the rare-earth elements by ICP OES (585) and Burman (565) has reported the determination of P in geochemical samples. Golightly has used ICP OES for trace-element determinations in coal and coal-ash (1478).Scott and co-workers (1435) have discussed problems associated with the analysis ofPart I : Fundamentals and Znstrumentation 15 geological materials and industrial plant samples using ICP excitation. The commissioning of a high-power ICP system and its application to the analysis of copper, lead and zinc concentrates has also been reported (116).Trace-metal analysis in Si and A1 metals by ICP OES has been described (1482) and the use of ICP sources for analysis of high-purity substances has been discussed by Zil’berstein (620). Radiofrequency oxygen plasmas have also been used for stripping of negative photoresists from semi-conductors; Griffiths and co-workers (1285, 1286) have studied the mechanism of this process and identified the active specics producing stripping as electronically excited oxygen atoms.Further references to the above and other applications can be found in Part I1 of this volume. 1.2.2 Microwave-excited Plasmas Many publications have appeared during 1977 conccriied with the application of both atmospheric pressur: and low-pressure microwave-induced plasmas (MIP) to the detection of metallic and non-metallic elements and as element specific detectors for gas-liquid chromatography.Only a few reports concerned with capacitatively-coupled plasmas (CMP) have appeared. 2.2.2.1 Fuiidameiztal. Brassem and co-workers (51 1, 1396, 1406) have made double-probe mzasurements and optical measurements to determine the variation of electron temperature, elxtron number density and emission intensities in a low-pressure (0.1-0.5 Torr) 2450 MHz MIP with the composition of the binary gas mixture used to support the discharge.The study was an extension of earlier work with pure gases (ARAAS, 1975, 5, Ref. 206; 1976, 6, Ref. 141). Binary mixtures of Ar/He, Ar/N,, Ar/H,, He/N, and He/H, were con- sidered. It was found that at low concentrations of an added component, the excitation parameters had the values typical for the major constituent, whereas above 1% addcd componcnt the excitation conditions changed and log(intensity) was no longer linear with respect to log(concentration) of added component.Changcs in electron tcmpcrature had the greatest effect on spectral-line intensities.The same workers (1 396) have considzred the applicability of the radiative-ionization rccombjnation model to the description of the e.rcitation conditions in an MIP at 0.2 Torr in Hc and Ar. Tests compared theoretically predicted and experimental values of absolute intensities and the intensity ratio of the Ha and Hp lines; the absolute intensities exceeded those pxdicted from thermal considcra- tions by eight orders of magnitude.The predictions from the radiative-ionization recombina- tion model, however, were two orders of magnitude too low. Kawaguchi and co-workers (446, 1 167), studying the enhancemeizt of line iizteiisities for analytes in the MIP produced by the addition of KC1 at 2-10 m moll-1 to sample solutions, concluded that the effect of KC1 i s largely to alter thz species entering the plasma by producing chlorides instead of oxides.They therefore proposed that if the radiative recom- bination model proposed by Busch and Vickers (ARAAS, 1974, 4, Ref. 14) i s to be accepted. it must be preceded by steps such as - MCl+e- + M+ +C1 +e-+e- and Hemeta+MCl+ He +M++Cl+e- Evidence was given to support the hypothesis of direct formation to the ion from the chloride rather than by a stepwise process.Lampe, Risby and Serravallo (919) have pointed out that the populations of He states in a 1 Torr, 2450 MHz, MIP cannot be Calculated from thermal equilibrium parameters as the states are only populated by electron impact events. Rate cquations were developed to represent the dynamic balance equilibrium state populations under steady-state conditions. Using published values of electron impact cross-sections these workers calculated the16 Analytical Atomic Spectroscopy population of each He state; the values obtained were compared with measurements of absolute radiant powers from each state and showed quite good agreement.Such calcula- tions may have limited application, however, as the excitation mechanism in the He plasma may be considerably modified in the presence of easily ionized elements when, for example, Penning ionization may contribute significantly. 1.2.2.2 Instrumentation. Beenakker (506, 1405) has described the construction and perform- ance characteristics of a novel cylindricd TM,,, cavity by means of which MIP discharges can be supported in He and Ar at atmospheric pressure.The operating characteristics were discussed for (a) He and Ar plasmas coupled to a gas chromatograph for element-selective detection, and (b) He and Ar plasmas coupled to a continuous feed, cross-flow nebulizer for emission spectrometric analysis of solutions. For the latter application, data concerned with the adaptation of the cavity, the use of systems with and without desolvation, detection limits and interferences from alkalis and phosphate (for Ca) have been presented, Beenakker and Boumans (1402) have described the low microwave stray field of this cavity and also given two alternative modifications to achieve impedance matching when Ar rather than He is used; the use of a double-stub tuner in thc coaxial line i s recommended as it permits the discharge tube to be positioned where the electric field strength is maximum and thus makes possible the use of the Ar MIP with a nebulizer without the need for desolvation.Others workers have also reported use of this cavity (1225). Alger, Johnson and Kirkbright (595, 11 13) have described an atmospheric pressure Ar 2450 MHz MZP operated at 30-200 W within a 2mm i.d.silica tube in a <-wave resonant cavity as a source for simultaneous multi-element analysis. Sample introduction by ETA from a Ta ribbon was employed. A 6- or 8-channel 4-m concave-grating direct-reading polychromator with silica-plate wavelength-modulation facilities, for background correction. was described for the simultaneous determination of volatile elements such as Cd, Pb, Hg, Zn, As and Se at trace levels in water samples.Mitchell et al. (1560) have described the determination of organic C in water samples in a technique in which differential volatiliza- tion of organic and inorganic C from samples heated at 850 O C in a furnace into a 2450 MHz MIP for atomization and measurement at the C 193.0 nm line was employed. Fricke, Robbins and Caruso (349) have developed a procedure that permits application of the hydride-generation technique with MIP discharge sources.The hydrides were generated and collected in a liquid-N, trap; H, liberated from the NaBH, was vented. The hydndes were then swept with He into the Ar/He MIP; the procedure was applied to As, Se, Sb, Sn, Ge, Bi and Te. The instrumentation and applications of Zow-pressure MIPS have been studied recently by several workers.A detailed discussion has been given (596, 1278) of the optimization of conditions for a low-pressure He MIP operated at 2450 MHz as an element-selective detector for C, H, N, 0, S, P, F, C1, Br and I. The plasma was operated at 90 Torr and 75 W within a silica tube 150 mm in length and 2 mm id.; limits of detection ranged from 0.01 ng s-1 for H, 0.05 ng s-1 for halogens, S and C to 0.4 ng s-1 for 0 and N.Sulik and Skogerboe (330) have compared AAS and AES characteristics of a low-pressure Ar MIP and Alder and da Cunha (1019) have described a novel sample introduction system for a low pressure MIP source in which samples were dispensed on to a graphite rod at atmospheric pressure and desolvated and ashed before being introduced into the low-pressure side of the device; with this device the plasma was not extinguished during sample introduction and pre-treatment. Atsuya et ul.(982) have utilized ETA from Ta, Mo and W loops into a He low-pressure MIP for the determination of As in the enzyme preparation monoarsanilazo Tyr 248; the detection limit was 10-10g As and a RSD of 0.054 at the 7 ng (1.4 p g ml-1) level was obtained.Part I : Fundamentals and Instrumentation 17 The interest in the application of MIP sources as element-specific detectors for GC continues to expand.The use of this type of system for detection of volatile metal com- pounds has been reported (321) and modifications to the system to allow better long-term stability and greater sensitivity have been described.Fricke, Robbins and Caruso (1298) have described the determination of As, Se, Sb, Sn, Ge, Bi and Te via generation of their volatile hydrides and chromatographic separation of these before their atomization and excitation in an Ar/He MIP. Beenakker (599, 802, 1095) has discussed the use of the cylindrical resonant cavity described above (TM,,, cavity) with a He plasma for element- specific detection in GC.The detector was evaluated for C, H, C1, Br, I and S and the detection limits reported were considerably better than those in the literature for He MIPS at reduced pressure and Ar plasmas at atmospheric pressure. Kirsten (1008) has recom- mended the use of a pre-column-capillary column system with a microwave plasma detection to determine small traces of compounds in large volumes of crude extracts and to identify the compounds simultaneously by a semi-quantitative spectral analysis.Kawaguchi et al. (40) have determined Cu and A1 in zinc in a procedure in which Cu and A1 were extracted as their trifluoroacetylacetonates into CHCl,, chromatographed and determined at 324.8 (Cu) and 396.2 nm (Al) using a 50 W 2450 MHz MIP detector.The use of a 2.5 kW 2450 MHz microwave plasma source for atomization for AAS has been investigated by Korovin and co-workers (1364); Ar or N, mixed with 3% CH, was employed as the carrier gas. Detection limits were presented for Zr, Nb, B, Ta, Fe, A], Cu, Cay Sr, Mg and Ge and the system was applied to the determination of B in steel over the concentration range 0.5-2.5% with a detection limit of 0.02%.Agterdenbos and colleagues (594, 1399) have continued their studies of the direct use of emission from sealed microwave-excited electrodeless discharge sources for quantitative analysis at the pg level (see also ARAAS, 1976, 6, 14). The work reported has been con- cerned with the choice of excitation buffer (BiI,+CsI), the influence of external heating or cooling, the pressure of filler gas and some practical applications.Capacitatively coupted microwave plasmas (CMP) have been employed for the deter- mination of A1 in metals (1023) and As, Sb, Bi, Ge, Sn, Se and Te via hydride generation before delivery to the plasma (279). Nakashima and Sasaki (233) have developed a CMP system in which Ag and Cu were determined after mixing with graphite powder; vaporiza- tion was then effected in a h.f.induction furnace before plasma excitation in a high-power CMP. 1.2.3 D.c. Arc Plasmas Keirs and Vickers (990) have reviewed the application of d.c. plasma arcs for elemental analysis and Kubota (360) has described a modified version of a wall-stabilized capillary arc source for emission spectrochemical analysis of solutions. The effects of KCl on the spectra, background intensities, calibration graphs, precision and sensitivity obtained with this type of source for analyte elements introduced as solution samples via a pneumatic glass nebulizer have been investigated (319); the use of 0.1M KC1 gave improved detection limits for all elements tested except for W and Zr. There have recently been numerous reports of the application to analysis of d.c. arc plasmas, These result principally from the availability of a commercial d.c. arc plasma spectrometer system. This system, in which the d.c. arc plasma is most frequently coupled to an echelle spectrometer to attain high resolution, has been employed in the determination of trace and minor components in steel (1106), B in fertilisers (457), Ba in diatom ash (918), Fe, Ni and V in petroleum (1264), trace elements in natural, sea- and waste-waters (365, 327), Hg in marine sediments and organisms (1153), and Cu in phytoplankton media (479). Other reports of the capabilities of the system for metal analysis have also been published (837,18 Analytical Atomic Spectroscopy 1220, 1221, 1222, 1426). Ellebracht and Fairless (1 112) have described the determination of the elements S, Hg, P, Se, As and I using this system in the V.U.V. region. Uden and Bigley (1279) have described a system in which a d.c. arc plasma-echelle spectrometer was interfaced to a HPLC to produce an element specific detector; the system has been applied to the determination of metals after chromatographic separation of their chelate complexes (195, 1295). Uden and co-workers (1079) have also reported a system in which the d.c. arc plasma spectrometer was interfaced to a GC system for the detection of metals in novel volatile organometallic compounds and metal chelates.

ISSN:0306-1353    

DOI:10.1039/AA9770700009

出版商:RSC    

年代:1977

数据来源: RSC

摘要:

18 Analytical Atomic Spectroscopy 1.3 LOW PRESSURE DISCHARGES 1.3.1 Glow Discharge Lamp The stable and reproducible emission characteristics of the GDL offer attractive advantages over many other less-easily controllable forms of excitation (see ARAAS, 1976, 6 , IS). Harrison et al. (344) have illustrated the use of the discharge as an excitation source for OES, a sputter atom-source for AAS and an ionization source for atomic MS.The dischargz allows the control of parameters such as the choice of gas and its pressure in order to optimize excitation conditions, Surface effects in the sample are of special importance and studies using scanning electron microscopy show the formation of spires and cones, which can influence analytical results. The growing versatility of the technique has been demon- strated by several application studies.An automatically stabilized GDL has been used by Czakow (603) for the analysis of powders, metals and solutions. Kenawy et al. (637) and Fijalkowski (638) have determined Br, C1, F, Se and S in non-metallic materials. Butterworth (43) has compared the pa-form- ance of a GDL with that of a spark source for low-alloy steel analysis using a constant-time exposure, i.e., by exposure until a preset integrated-current value was reached, and by the method of Jaeger (ARAAS, 1972, 2, Ref. 18) in which sputtering rate was used as zn internal standard. Calibration curves were rectilinear and the precision obtained compared favour- ably with that obtainable by using the spark source. A similar comparison has been made for a range of matrices including low- and high-alloy steel, aluminium, copper and lead alloys (606).For rare-earths analysis, solutions (10~1) have been evaporated on to a polished copper plate (548); for Sn in tin ores a technique in which the powdered sample was mixed with high-purity Cu in the ratio of 1 : 20, and then compressed and degassed, proved successful in the hands of unskilled staff (631).A glow-discharge atomizer described earlier (Anal. Chem., 1975, 47, 194) has been further improved and characterized as an atom source for multi-element analysis (1224). Excitation temperatures of the order of 5000 K in the Ar discharge were reported; the addition of 1% H, assisted in the production of a reproducible, stable glow. Butler (878) has described a new AFS apparatus incorporating two GDLs.The analyte was atomized cathodically in a first GDL to emit primary radiation; this radiation was focussed through a quartz lens into a second GDL operating in a pulsed mode to produce resonant fluorescent radiation. The fluorescence was passed through an interference filter to a photomultiplier tube and the average fluorescence was recorded.Using this equipment for the determination of Mg and Cu in steel, and Cu in aluminium, detection limits of 20, 10 and 25 ppm respec- tively were obtained; for Cu and Ag in gold, the limits were 2 and 3 ppm, respectively (682). In the latter application the analytical curve tended to bend towards the concentra- tion axis above 0.5% concentration, but this would be reduced by operating the GDL at a power lower than the 0.01 kVA used in these experiments.TV camera used with a GDL: 1574. Other reference of interest -Part Z: Fundamentals and Znstrumentation 19 1.3.2 Cathodic Sputtering and Hollow Cathode Discharges Work of the type first reported by Walsh (Spectrochim. Acta, 1955, 7 , 108) on the atomic absorption measurement of vapours produced by cathodic sputtering has continued at C.S.I.R.O.(684). Internal standards were used for the determination of Fe, Cr, Ni and Cu in metallic and non-metallic samples; the ratio of the absorbance of the analyte to that of the internal standard was compared with the corresponding ratio in the reference material (452). This allowed accurate determinations to be made even when the sputtering rate was quite different from that of the reference sample.The assumption that concentration ratios in the vapour phase were identical to those in the solid phase was supported by experi- mental measurements of Ni/Cu ratios in different alloys. Theoretical studies of mechanisms in the hollow cathode discharge have included work by Semyonova et al. (635), who considered the influence of resonance and recombination processes upon the radiation of Cu, Ag and Au vapours.Torok et al. (915) examined the radial distribution of electron temperature in a liquid-air cooled discharge; Zhiglinskii et al. (1316) proposed a method for determining the temperature of the plasma and the concentration of metal atoms from changes in the intensity distribution of the hyperfine structure of the spectral line.Atomization caused by a sputtering effect as in the HCL discharge should lead to reduced matrix effects; results of matrix effect studies have been reported by McCamey et al. (332), and by Szilvassy (744) who studied the determination of rare earths in oxides. Caroli et al. (869) obtained emission spectra of Cu and steel by using HCL and spark excitation sources and of Cu using a GDL.The HCL radiation source was the most accurate for OES analysis. Pichugin et al. (166) stabilized the discharge in a HCL by packing the analyzed material into a cavity drilled into the bottom of a demountable graphite electrode. Improved detec- tion limits at the ppb level are reported for impurities in graphite powder. Sabatovskaya et al.(1 139) cxamined the influence both of the inside diameter of the cathode and the gas pressure on the detection limits obtained using a HCL. Line intensities increased with decreasing internal diameters and gas pressures, but discharge stability was poor when using a cathode of reduced internal diamcter. A cathode in which the cavity had a ‘stepped’ profile (4 mm at inlet, 1.5 mm at base) ensured good stability and gave a 10-fold improve- ment in detection limits at a gas pressure of 20 Torr.Hollow cathode excitation with vidicon detection offers a promising alternative to convc.Ttiona1 OES with photomultiplier detection, Good results were reported for the simultanecus dctermination of rare earths in minerals when a standard-addition procedure was used (272), and also for Pb and Cd determination in airborne dusts using graphite porous-cup electrodes as the filter material in the sampling device (659, 1646). Other references of interest - Determination of Eu in rare earths by isotope dilution using a HCL discharge: 633. Electron impacr 3xcitation sources: 650. Hanle effect in atomic vapours produced by cathodic sputtering: 380. Surface analysis by ion-beam sputtering: 930.

ISSN:0306-1353    

DOI:10.1039/AA9770700018

出版商:RSC    

年代:1977

数据来源: RSC

摘要:

Part Z: Fundamentals and Znstrumentation 19 1.4 FLAMES 1.4.1 Fundamental Studies Theoretical studies of flame systems continue to be reported and in recent times the emphasis has shifted from the flame itself to the processes involved in atomization. New techniques and instrumentation often enable workers to consider new approaches to old problems and in some cases the flame provides a convenient test cell for examining the20 Analytical Atomic Spectroscopy validity of theoretical predictions.L’Vov er al. (171) have continued their serics of discus- sions on the theory of AAS with a paper concerned with the effect of incomplete sample vaporization. It is shown that curvature of the log A versus log C curve can be charac- terised by the ratio of the particle radius to the mean free path.When the particle radius is less than the mean free path the slope may decrease to 0.67 and when the radius is greater than the mean free path the slope may decrease further to 0.33. Experimentally found slopes for Ni and Co sulphates were 1 in an oxidising flame, and 0.57 in a reducing flame where reduction to involatile carbides was assumed. The difference between 0.57 and the predicted 0.67 was explained by poly-dispersity of the aerosol. A stochastic approach to the theoret- ical prediction of the processes of desolvation and atom production has been given by Li (922, 11 85).The expressions developed describe the atomic concentrations for a certain position in the flame as a function of solution uptake rate, droplet distribution, flame gas velocity, atom diffusion coefficient, atomization efficiency, mass of the analyte atoms, desolvation, solute vaporization and size of burner, The atomic concentrations calculated can be used to determine absorbances for AAS or radiances for AFS and AES.The dynamics of desolvating aerosol particles in high-temperature sources has been the sub ject of work by Hieftje and Boss (790, 11 87, 1242).Halls has shown (796, 1686) that the dissociation equilibrium theory of atom production fails in a number of cases to predict atomization efficiencies that agree with experience. In such cases atomization is achieved by fast binary reactions involving reduction of analytc oxides by flame species. The free energy available from the oxidation of C, or H radicals is just sufficient to permit the reduction of the oxides of elements that are known to be atomized well.Calculations of atomization efficiency for Na and Mg assuming reduction by H radicals gave good agreement with experimentally determined values. Alternatively, Hayashi et at. (849) have performed modelling of flame species in the N,O/C,H, flame based on equilibrium calculations and generally found satisfactory agreement between theory and experiment.Alkemade et al (812) have studied the emission and absorption line profiles of Sr and Ba in an air/C,H, flame using both a pressure scanned interferometer and Zeemaii scanning and use the results to show that the free atoms are in thermal equilibrium with the flame gases. In further work on Sr (68, 69) in O,/H,/Ar and N,O/CO flames they have deter- mined adiabatic collision cross-sections for Ar and H,O and quenching cross-sections for 0,, CO,, CO and N,.A further paper by the same authors (803) discusses the effect of line profile on the curve-of-growth, and gives numerically computed curves for various shapes, including the Voigt profile. The study of flame species provides continuing interest since they form the chemical environment in which atomization must occur and also are responsible for the background radiation against which the analytical signal is measured, Fowler and Winefordner (91 7) have studied scatter and fluorescence spectra of the air/C,H, flame irradiated under pulsed and d.c.conditions by a high pressure Xe lamp. The principal band spectra observed were those due to PO (220-275nm; 323-330nm), OH (280-295nm; 305-320nm) and CN (380-390 nm).A commercially available PH, gas filter when placed in the C,H, line was found to be effective in removing fluorescence contributions from PO. A report from Steiner (1381) suggested that impurities in C,H, can affect the accuracy of analyses if the variable composition of the cylinder outflow i s not taken into account.The use of molecular absorption for flame photometric determination has been limited because of the complex background spectra that occur. A twin magnetic-tape analogue signal-processing system capable of analyzing flame absorption spectra has been described by Haraguchi et al. (925). By recording signal and background spectra separately on different tracks and then playing back simultanewsly through a differential amplifier, background correction was readilyPart I: Fundamentals and Instrumentation 21 achieved and with the addition of time displacement, operations such as differentiation could be accomplished.The interferences caused by flame background absorption spectra have been studied by Daidoji (1 501).A CO, laser has been used to irradiate Ca and Mg containing flames (338) and has been found to produce unattributed emission peaks. It is suggested that the emission may be from metal oxide particles undergoing candoluminescence, which is in some way stimulated by the i.r. radiation. Baronavski and McDonald (877) used a flash lamp pumped dye laser to excite fluorescence of the C , Swan band system in an O,/C,H, flame, From these measurements they determined the total lower electronic state population of C, to be 4.5X 1015 molecules cm-3.The concentrations of NO and free 0 atoms in the air/C,H, flame have been studied using gas probe extraction methods (240). The maximum concentration of NO was 1600 ppm, which occurred 60mm from the burner, while maximum 0 concen- tration was 3000 ppm and occurred 30mm from the burner; peak temperature was attained 40mm above the burner.Halls (1400, 1693) has measured the atomizaton efficiencies of Na, K and Li in the air/C,H, flame and obtained values of 0.98, 0.85 and 0.26, respec- tively; the value for K is somewhat higher than that normally expected. Atomization efficiency was found to be controlled by the equilibrium of the frec metal with the hydrox- ide and no evidence was found supporting the formation of oxides or carbides.Temperature-measurement techniques are of considerable importance and new methods are continually being sought. Winefordner et al. have published further papers (987, 993, 997, 1421) describing use of the “two-line fluorescence method” (see ARAAS, 1972, 2, Ref. 616 and ARAAS, 1973, 3, Ref. 711) for the measurement of localised temperatures in air/C,H,, N,O/C,H, and Ar/H, flames. The fluorescence technique has the advantage over other methods that it does not integrate the temperature and allows measurement of the temperature in localised volumes without the need for Abel transformation. Fassel and co-workers (1 537) have compared flame-temperature measurements using the reversal method and Fe two-line or slope technique and show that serious errors may arise in the latter case.These were attributed t o the uncertainties in the transition probabilities. 1.4.2 Interference Studies A large proportion of flame literature is devoted to the study of interferences. Much of the information is, however, of limited general use since in relatively few cases do authors give the mechanism of the process behind the interference.Often success amounts to circumven- tion of the problem, entirely satisfactory when answers are required, but this pragmatic approach leads to much duplication of effort and the recent literature suggests that progress towards the systematic treatment of interferences is relatively slow.This view is supported by Cresser (534) who emphasized the inadequacy of many interference studies. Fortunately the above comments do not apply universally and valuable work on inter- ferences has been described by several authors. Varju (715) has studied the effect of HCI and HNO, in the range 04.5 M on solutions containing Zn, Cu or Mn. In each case, plots of the percentage interference against acid molarity yielded straight lines that were mutually parallel.This was taken to indicate that the apparent interference was non-specific and in this case a correlation was indicated with the density and viscosity of the solution. Rubeska (688, 1142; see also ARAAS, 1976, 6, Refs. 861, 868, 924) has continued his excellent work on interference mechanisms associated with refractory species in the N,O /C,H, flame.By considering the parallel processes of volatilization and reduction, it has been found that interference behaviour can be classified according to the relative volatility of the metal, oxide and carbide species. Elements for which volatility increases from oxide to carbide, e.g., Al, Be, La and Y, are least susceptible to interference, elements for which volatility decreascs from oxide to carbide, e.g., Mo, B, Si, Ti, V, W, are more22 Analytical A tomic Spectroscopy susceptible, and the interference pattern for elements that are involatile in all three forms, e.g., Hf, Nb, Ta, Th and Zr, depends on the nature of their crystalline structure.Chakrabarti and Naranjit (1443) have studied the interference of phosphoric acid on Ca in the N,O/C,H, flame.It was suggested that the high boiling-point H,PO, produced larger droplets from the nebulization chamber, which therefore limited the occurrence of lateral diffusion in the flame. Hence, absorbances were enhanced at the flame centre as compared with CaCI, and reduced at the flame edge. The interference was overcome by determination of the Ca by ETA.The addition of carbon black (1516) to the air/C,H, flame has been found to produce enhanced atomization for several elements, particularly those having a strong affinity for 0, e.g., A1 and Ti. Further, the interferences of phosphate and fluoride on Ca and Fe determinations were eliminated. Pszonicki and Krupinski (690) have observed that inter- ference effects in the N,O/C,H, flame are consistent and that single species interferences can be predicted by exponential equations employing two coefficients, one describing the maximum degree of interference and its sign and one defining the shape of the interference function.It is stressed that the effects of individual interferents are not additive and there- fore the method cannot be applied when several interferents are present.A potential source of error in the determination of Si in natural waters has been identified by DeVine and Suhr (136). They showed that the absorption of Si in the N,O/C,H, flame could be enhanced by as much as 50% by the presence of Na. Although some background absorption was observed in the presence of Na, background correction had only a minimal effect in correcting the interference.The authors pointed out that as most Si standards were made from commercially available sodium silicates with unspecified Na content, existing analytical results had to be treated with some caution. The detennina- tion of Ba in silicates has been shown (293) to be affected by interferences from Ca and K and by synergistic interference effects, e.g., A1 in the presence of Ca and K, and Na and K in the presence of Ca.Dithiocarbamate has been found to be an effective releasing agent (846) in controlling the mutual interferences of the noble metals when determined directly in organic extraction systems, Kaszerman and Theurer (75) have reported increased sensitivity for the As(II1) state compared with As(V) in an Ar/H, diffusion flame; similar but less pronounced variations were found in the N,O/C,H, flame.Further, the presence of acids produced marked interferences on As(V) but had less effect on As(1II); KT was therefore used to reduce all As to As(I1I) before analysis. Oguro (92, 820, 544, 827, 340, 226) has reported an enhancement effect of NH,C10, in the determination of Eu and Yb by both AAS and AES in air/C,H, and air/H, flames.A twin-nebulizer technique has been employed by Kato et al. (318) to study the inter- ference of Ti and Nb peroxo-complexes on the determination of Cu, Mn and Co in Ni-based alloys in the air/C,H, flame. The interference was attributed to the formation of refractory compounds that occlude the Cu, Mn and Co leading to incomplete dissociation. Other rcference of interest - Removal of A1 interferences on Be by HF: 140. 1.4.3 Devices for Sample Introduction The sampling and atomization of solid materials presents particular difficulties for flame- spectrometric methods, which have been developed almost exclusively for solution analysis. Some solid materials are difficult to dissolve, contamination by the reagents used can prcsent problems and the dilution factor can render the analyte species unmeasurable. A common approach to overcoming these problems is to sample the solid directly using a higher temperature source, such as an arc, to vaporize the matrix material.Thus, Pungor et al. (235, 704) have described a d.c. arc nebulizer for use on small powder, metal or liquidPart I : Fundamentals and Instrumentation 23 samples and have used the device for the determination of Na and K in industrial Al,O, and electrocarborundum.Using an air/C,H, flame, detection limits were also established for Ca, Cd, Cr, Cu, Fey K, Li, Mg, Na, Pb and Zn in synthetic materials. Similarly, Posta and Papp (906) have designed an arc cell with an electrode specially configured for vapor- ization purposes and claimed increases of sensitivity in AAS of 2 orders of magnitude, compared with the equivalent solution technique.Pungor et al. (235) have proposed an apparatus for the nebulization of samples with a d.c. arc and subsequent analysis of the fine particles by AES. Operational parameters have been studied and methods presented for the determination of Na and K in aluminium and Pb in copper. A somewhat unusual device has been described by Razumov (466), which consists of an arc cell with the sample elec- trode at one end, and the whole is mounted in the flame produced by a standard long-path absorption burner.Other arc atomizaton devices have been reported by Chupakhin et af. (1362) and Rasumov (1512), and a review of the determination of powdered samples by AAS has been given by L'Vov (1667).The simplest approach to direct solid sampling is the dispersion method, in which the sample is finely ground, dispersed in a solvent and then sprayed in the usual manner. Labrecque (728) has reported results by this technique for the determination of Mo and Co in desulphurization catalysts and for Al, Si and Ti in laterites using both air/C,H, and N,0/C2H, flames. Fuller (294) has used the dispersion method for the determination of tracc elements in TiO, pigments.Chemical transforrnatiort has become more common in recent years as a mcans of improving the volatility of materials or of avoiding interferences. An example is flame gas-solid interaction, which has been employed by Fike and Frank (1094) for the determina- tion of I and Br at the ppb level.A standard O,/H, Beckmann burner was fitted with a stainless-steel chimney drilled with 6 holes to allow the passage of combustion air. A silver tube was placed on top of the chimney. When aqueous iodide or bromide was sprayed, AgX was formed on the tube surface, spraying was maintained for 1-5 min to allow accumu- lation of AgX.The solvent was then changed from H,O to 95% ethanolic solution, the rise in flame temperature vaporized the AgX and the halide was determined by measurement of the emission intensity of the Ag line at 338.2nm. Mallett and Royal (818) have reported the determination of 0 s in the N,0/C2H, flame by the prior formation of volatile ammo- nium chloro-osmate in a heated quartz furnace.Several metals, e.g., Au, Ru, Ni, Fe, Se, La and Al, interfered, but this was overcome by distillation of the 0 s and collection in HC1 solution before analysis. Similarly, Chapman and Dale (524) have reported improved sensitivities for the determination of Si and B by introduction of the volatile fluoride into the flame. The fluoride is generated by reaction with CuOHF at 345 "C.The use of the Delves cup technique for the determination of Cd in blood has been hindered by the large background absorption /scatter signals produced by combustion of the sample. It has been found (305, 710) that addition of (NH,),HPO, to the sample retards the atomization of Cd allowing resolution of its absorption peak from that of thc sample matrix.A detection limit of 16X 10-12 g was reported with an RSD of 0.056 at the 10 ppb level. MetaE speciation studies have become increasingly important particularly in the environmental ficld where chemical form rather than total content is often the determining factor in toxicity. Flame emission and absorption spectroscopy have been used for some time as detectors for GC and now their use is being extended for LC and HPLC.Van Loon et al. (1083) have demonstrated the use of AAS for the determination of Cr(V1) and Cr(II1) in the eluent from ion-exchange columns and have reported the analysis of alkyl and aryl Zn compounds separated on HPLC columns. Van Loon and co-workers (1423, 833) have also employed non-dispersive AFS as a means of chromatographic detection.GC coupled24 Analytical A tomic Spectroscopy with a flame emission detector has been used by Jungers et al. (932) to determine total S content in petrol. Analysis time was less than 5min with a detection limit of 0.002% and RSD of 0.1. Similar equipment has been used by Pearson and Hines (931) for the analysis of H,S, COS, CS, and SO, in inert gases and flowing hydrocarbon streams.Koop et al. (1062) have demonstrated that metal speciation in solid samples can be investigated by ramp heating of an ETA furnace, followed by flame or ICP analysis of the evolved vapcrur. Improvements in nebulizer design and use continue to be reported and indeed there is still much scope for development. The Babington nebulizer (see ARAAS, 1975, 5, Ref. 164) has been found to be an effective device for nebulizing high solids content solutions or those having high viscosity. Fry and Denton (484) have described the design of such a device and reported its use for the determination of Cu and Zn in whole blood, urine, sea-water, evaporated milk and tomato sauce. Hieftje and Savage (1238) have continued development of the electric-field pneumatic nebulizer (see AKAAS, 1976, 6, Ref. 11 37) and have described operating conditions and parameters for a practical device that can be coupled to flame or plasma atomizer instruments. The discrete-droplet generator of Malmstadt et al. (Anal. Chem., 1968, 40, 1860) has been employed to produce a “source modulator flame” for AAS with a continuum source (147). The atomic clouds produced by the atomization of indivi- dual droplets pass through the source beam causing it to be modulated.Synchronous detection becomes possible and the authors demonstrate that such a system is capable of simultaneous multi-element analysis, with relative freedom from narrow spectral line inter- ference. Tominaga et al. (1152) have found that the addition of the surfactant Na dodecyl sulphate (SDS) can increase Cr absorption by up to 120% in the air/C,H, flame, provided that it is present above the critical micelle concentration. Several interferences, i.e., those due to Fe, Ni, Co, C1-, NO,- and SO,2- were eliminated by the presence of SDS and it is suggested that the surfactant both improves nebulizer performance and enhances the atomization of the Cr. Other references of interest - Arc atomizer and source for AAS: 170.Arc discharge atomizer for AAS: 1502. Automation of sample introduction in FAAS: 1066. Controlling sample flow in flame AAS: 1122. GC-AAS for detection of volatile metal chelates: 420. Gas dynamics studies of chamber-electrode atomizer: 1509. Review of direct atomization of geological samples: 487. Ta cup for determination of Pb in sea-water: 76. 1.4.4 Other Studies Flame methods are widely used for routine analysis in laboratories throughout the world and an inevitable consequence of this is the appearance of papers describing devices that improve the performance of the flame method. Most of the devices reported do have some advantages to offer, usually increasing the residence time of the analyte in the flame, but none so far has given universal improvement and the benefits are usually related to a specific group of elements in a certain matrix type. An example of such a device is that dcscribed by Watling (1281).A slotted quartz tube was placed in the flamc such that a portion of the flame gases entered the slot and burned inside the tube, while the remainder formed a flame sheath round the outside.The absorption was measured along the tube axis and 4-10 fold improvements in sensitivity for the volatile elements As, Sb, Se and Hg were obtained. The improved sensitivity was gained at the expense of enhanced interference effects, which might preclude use of the device for all but simple sample matrices.Part I: Fundamentals and Instrumentation 25 Solvent extraction provides the analytical chemist with a tool for overcoming some of the difficulties posed by complex sample matrices.These may be of the interference type, or those related to sensitivity limits often imposed by the dilution factor necessary to take solid samples into solution form. The high-background levels and flame instability caused by spraying organic solvents is well known and often necessitates inclusion of a back- extraction step in the analytical procedure to obtain the more favourable aqueous form.Kono (31 1) has described an 0, sheathed flame system which apparently overcomes some of the problems and allows satisfactory analyses to be carried out directly on the organic phase. Similar claims for the N,0/H2 flame have been made by Lukasiewicz and Buell (1212) where the presence of the organic solvent had a beneficial effect in removing strong NO absorption bands from the background spectrum.Hieftje and Saturday (1262, 1179) have continued their studies on inert gas diluted flames (see ARAAS, 1976, 6, Ref. 1179) and have determined free-atom fractions under various flame conditions. Improvements in technique continue to be reported, particularly for samples that have specific composition or present difficulties when analyzed by conventional methods.Henrion et al. (267) have determined Sb in Bi-Sb semiconductors by chemiluminescent excitation in an air/H, flame. Propanol was the active species and was added to the sample solution; Sb or added Sn was used as internal standard.Two methods have been reported for the indirect determination of fluoride, the first involved precipitation of the fluoride as CeF, followed by filtration and determination of Ce by flame emission (222), and the second, the direct measurement of the intensity of the InF band at 328.12 nm in a N,/H, diffusion flame (429). The emission determination of Ag at 328.1 nm at low resolution is hindered by OH back- ground emission. Improved precision has been obtained using the Cr line at 357.9 nm as an internal standard (103).The excitation energy of the two lines differs by less than 1 eV. Urbain and Cattenot (689) have found that the atomization efficiency and therefore sensitivity of determination of the noble metals Pt, Rh and Ru is greatly enhanced by the presencc of SO,,- ions, The authors claim that SO,,- acts as an oxidising agent thereby preventing the formation of stable inter-metallic aggregates.Preconcentration techniques present several problems in use; a loss of precision is almost inevitable and the ever present danger of contamination becomes critical. Neverthe- less such techniques are often essential when adequate sample is available but the concentra- tion is too low for direct analysis. Jackwerthe and Messerschmidt (502) have described the preconcentration of trace metals dissolved in HCIIHNO,, on to high purity Ga. Those elements which are nobler electrochemically than Ga, e.g., Ag, Au, Bi, Co, Cd, Fe, Hg, Ni, Pb, Pd and Sn, are spontaneously deposited and enrichment factors of 103 can be obtained. Certain elements, e.g., Cd, Co, Fe, T1 and Zn, can be directly extracted from the Ga without the need for dissolution using HI and I,. A similar procedure whereby trace elements are electrolytically deposited onto a platinum spiral has been described by Lund et al. (1625). The spiral is then placed into an air/C,H, flame to volatilize the deposited material and the resulting vapour is localised by placing a quartz tube just above the spiral position. The technique was applied to the determination of the volatile elements Ag, Bi, Cd, Hg, Pb, Se, Te, Th and Zn in sea-water, urine and biological samples. Other references of interest - Absorption-line modulation by Zeeman scanning: 864. Characterisation of signals for micro-sample AAS: 894. Comparison of AES and AAS for steel analyses: 1452. Computer assisted construction of AAS response surfaces for Ru and Rh: 1231. Determination of halides by GaX and InX band spectra: 1656. Gas control system for flame photometer: 1505. MECA: determination of C,H,Br,: 190; determination of P compounds: 1018.

ISSN:0306-1353    

DOI:10.1039/AA9770700019

出版商:RSC    

年代:1977

数据来源: RSC

摘要:

26 Analytical A tomic Spectroscopy 1.5 ELECTROTHERMAL ATOMIZERS Electrothermal atomizers have established a clearly defined role within the analytical laboratory and are widely applied to small samples and for analyses where flames offer insufficient sensitivity. Some of the more extravagant claims of early workers in ETA have not come to pass and fundamental studies are largely directed to eliciting information about atomization and interference mechanisms and to improving practical performance. 1.5.1 Atomizer Designs and Modifications Grushko et al. have described a graphite tube furnace (12mm long by 2.5mm diameter), enclosed in a silica chzmber, in which the electric current flows perpendicularly through the tube (463). The authors studied the effects of current intensity, graphite-tube size and porosity of the graphite material used for the furnace.The results were as expected, Le., peak signals increased with increasing tube length, decreasing tube diameter and decreasing porosity of the graphite. Detection limits for the 8 elements studied were inferior to those obtained with commercial atomizers, The effects of pressure on atomization and absorbance in a graphite furnace have been studied by Sturgeon et a!.(1688). Pressures up to 13 atm were used and shown to reduce both the integrated and peak sensitivities, improve the linear range and raise the appearance temperature. The major factor contributing to reduced sensitivity was the Lorentz shift in the absorption line profile causcd by applied pressure. Other work rcported on atomizer design features concentrated on modifications to commercial atomizers. A graphite tube designed to fit the light-beam geometry of a commer- cial AA spectrometer has been described (71 9).Experimental results using this design for the determinations of Cd and P showed significant enhancement of sensitivities compared to standard cylindrical graphite tubes (778).Ure and Hernandez-Artiga have modified their own graphite rod atomizer design by drilling two vertical holes either side of the sample cavity (1284). This produced a more laminar flow of the sheath-gas, which the authors claimed produced better precision in the determination of Cd by AFS. Gregoire and Chakrabarti (1 180, 1431) introduced a pyrolytic graphite platform into a graphite furnace.The intention of this was to slow down the rate of atomization and delay appearance of AAS signals to a later time when the graphite tube itself had reached thermal equilibrium (see ARAAS, 1976, 6, Ref. 1141). It has been shown that Mo and V peak height signals increased exponentially with increasing heating rate (143 1). Instrument manufacturers of graphite furnaces have also extolled the virtues of rapid heating rates for the atomization stage (197, 779, 780, 788, 1025, 1073, 1088).The advantages of rapid heating have been known, however, for some time (see ARAAS, 1974, 4, Refs. 576, 1325). A novel method of sample introduction has been described by Matousek (489). The sample was introduced to the graphite furnace, at 70 "C, for 5-20 s using a conventional pneumatic nebulizer and spray chamber connected with a large inverted-cone secondary droplet separator. Precision approaching that obtainable by FAAS was achieved.Two automatic sampling systems have been described which are claimed to give improved precision (277, 1076). Using manual pipetting, Kirkbright and Snook (1 194) have obtained improved precision by adding water-soluble organic solvents (acetone, butan-2-one) to samples.They used the non-specific absorption peak height caused by these compounds during the drying stage to correct for pipetting errors. In one test with deliberately careless pipetting the RSD for Pb determinations was improved from levels of 0.13-0.16 by a factor of 3. Garnys and Smythe (1385) described a novel sampling system, which consisted of a tungsten or graphite wire passing through a conventional graphite furnace.Several samples could be placed in series along the wire and dried and ashed externally by passing a lowPart I : Fundamentals and Znstrumentation 27 heating current. The wire was then passed through the furnace to atomize each sample in turn. A rapid rate of sample throughput was claimed by the authors,, Detailed results of the work first reported last year (see ARAAS, 1976, 6, 21) using Zeeman splitting, of the atomic species produced in an ETA, as a background correction system have now been published by Grassam et al.(1388). For further details of this and other work on background correction see I, 2.2. Other references of interest - Atomizer control unit: 955, 1322.Atomizer design: 389, 1197, 1323. Autosampler: 398, 956. Furnace temperature measurement: 426, 427, 1547, 1687. Pyrolytic graphite coating: 148, 1464. 1.5.2 Atomization Mechanisms A major session on atomization mechanisms was arranged at the XXth C.S.I. / 7th I.C.A.S. meeting in Prague. Although no outstanding new ideas were presented it can be hoped that the meeting stimulated considerable interest and that the fruits of the conference will be secn in the coming year.Maessen et al. (770, 1404) reported the effects of concomitant species [Al(NO,),, BaCI,, BaF,, CsC1, CsF, CuCl,, CuF,, LiF, NaF, and Ga(NO,),] on the atomization parameters of Be, Mn and Zn. They used radioactive isotopes to establish the relationship between the amount of analyte in the furnace at any time and the character- istics of the absorption signal.The results indicated that the atomization interferences observed were closely related to the thermochemical reactions between analyte and con- comitants. De Galan (766) and, elsewhere, Van den Broek (1403) showed by atomization studies that the poor atomization efficiences obtained with ETA could be improved by an order of magnitude through the correct dimensional construction of furnaces and choice of atomization heating rates.Rowston and Ottaway, using X-ray diffraction and thermogravimetric data, deduced that the rate-determining step for the atomization of the noble metals involved vaporization of the free metal (772). Rubeska and Koreckova (771) explained the cause for the increase in sensitivity of Te in the presence of Cu.The AAS signal for Te is controlled by the dissociation of Tc, in the gas phase therefore, as the addition of Cu raises the appearance temperature of Te, a higher degree of dissociation of Te, is obtained at this elevated temperature. Other authors presented data at the conference covering atomization kinetics (765, 767) and atomization processes (768, 769, 781, 782, 783, 796, 943, 1178).Stone and Woodriff have used a modified version of the appearance temperature technique (see ARAAS, 1974, 4, 13, 14) to elucidate reaction mechanisms (1051). A temperature, T,, corresponding to the lowest temperature at which the maximum AAS signal was reached was defined. They found a good correlation between T, and the free energy of formation of the metallic oxides for the 15 elements studied and concluded that carbon reduction of the metal oxides was the most probable mechanism, Howevcr, although they calculatcd thcrmodynamically that in all cases the gaseous metal monoxides would be at least 99% dissociated, they were dubious as to whether the rate of dissociation of gaseous diatomic molecules would be fast enough to achieve this in the furnace.Hasegawa et al. studied the atomization mechanisms for Al, Ca and Mn on a molybdenum filament and concluded that reduction of the metal oxides by Mo was the likely mechanism (357). X-ray diffraction and differential thermal analysis with on-line mass spectrometry have been used to study the atomization mechanism of Co from the Co-APDC complcx (1236). Other references of interest - Atomization mechanisms: 699, 764, 786,933, 1172, 1419, 1544, 1607,28 Analytical Atomic Spectroscopy 1.5.3 Interferences Frech et al.have continued their work on the study of interferences using thermodynamic data. They have reported on the determination of A1 (530, 531) and of Pb in the presence of NaCl (358) and sulphate (1630) media.A theoretical treatment of the A1 system based on equilibrium calculations showed that H, 0, N, C1 and S might give rise to severe inter- ferences. Practical studies showed that the interferences could be minimized by using an ashing temperature as high as possible. The use of statistical studies of interference effects has been described for the determination of Cd (1366, 1469), Pb (366, 1470), and Zn (1471).Some reports have appeared describing the use of releasing agents to minimize inter- ference defects. A favourite from flame AAS, La, has been used for the determination of Be (110, 409), Pb and Cd (514), and P (81). In the determination of Pb the use of La was found to be superior to the use of organic or NH,NO, additions for the determination of Pb in non-saline waters. Molybdenum has been used for the determination of Pb (109, 298) and Cd and Se (109) and A1 for the determination of V (253).Hocquaux ef al. (1701) have advocated the use of in situ chemical treatment to minimize problems, both from chemical and non-specific absorption-type interferences, in the analysis of solutions with high-salts content.Interference by halide-containing compounds, a major problem in AAS with ETA, has been overcome by the addition to samples of (NH,),CO, (1045), NH,ClO, (254), H,SO, (2, 89, 525) and HNO, (525). Adler and Hickman (448, 1195), however, found that the additions of HCl, together with H,O,, stabilised Hg in a graphite furnace and produced enhanccd sensitivity (up to 20-fold).This was attributed to the effect of the high concentra- tions of HCl gas in the vapour phase, which inhibited production of free-Hg atoms at low tcmperatures by formation of HgC1,. Fuller has reported his findings on the effect of graphite tube condition on the deter- mination of Pb in the presence of MgCl, (1193). The interference was most noticeable with new tubes; oxalic acid was used as a releasing agent.Person er d. (531) have also reported an effect of graphite-tube age on the determination of Al. McLaren and Wheeler (354) have studied the causes of the production of double absorption peaks in the determina- tion of Pb. They postulated that the peaks were caused by the formation of two Pb oxide forms in the graphite furnace, litharge and massicot, before atomization.Other references of interest - Classification of interferences: 1086. Effect of anions on atomization temperature: 1565. Graphite-braid atomizer: 1047. Matrix interference: 495, 954. Specific elements, effects on: Ag: 545, 1333, 1540 Biz 966 Ca: 1443 Cd: 229, 725, 773, 1540 c o : 1532, 1540 cu: 229 Fe: 1532, 1540 Mn : 229, 773 Ni: 1532, 1540, 1700 Pb: 229 Rare earths: 1416, 1417 Si : 1419 Sn: 725, 1123Part I: Fundamentals and Instrumentation 29 V: 1700 Zn: 773. 1.5.4 Speciation Studies Coupling ETA detectors with chromatographic separation tecllniques for the identification of molecular species continues to be exploited. Perhaps the most significant development has been made by Robinson et al. (350, 1009, 1274), who have designed a graphite rod atomizer specifically for this application.The detector consisted essentially of a T-shaped cavity drilled into a vertical graphite rod atomizer. The eluent from a GC column was continuously swept into the base of the cavity where decomposition of the compounds occurred. The decomposed components were transported into the horizontal cavity where atomization of the metals occurred. This atomizer is likely to give more efficient atomiza- tion than can be obtained with commercial atomizers.Other references of interest - Speciation studies: 152, 153, 345, 1190. 1.5.5 Atomic Emission Background emission (from the atomizer, molecular species or reagents used in the analy- tical procedure) is a limiting factor on detection limits obtained by ETA emission spectro- scopy. Ottaway and co-workers (785, 902) described an optical baflle system that reduced the amount of black body radiation, from a graphite furnace, reaching the monochromator.The residual background signal was assigned to Rayleigh scattering of f urnace-wall emission by species in the vapour phase. Two groups working on furnace AES collaborated to report on the observed emission spectra of CN, C,, CaO, SrO and MgO from graphite furnaces (964).A variety of techniques was used to measure the spectra: (i) sequentially, from successive atomizations at varying wavelengths; (ii) using a vidicon detector, over wave- length ranges of 70 nm; (iii) using a vibrating quartz refractor plate, in the optical path of a monochromator, over wavelength ranges of 1.0 nm and (iv) using an oscillating mirror mounted at the exit slit of the monochromator over wavelength ranges of 10nm.By the addition of H, to the atomizer purge gas (975) it was shown that the detection limit for Ba in the presence of Ca could be improved substantially over that reported previously by the same authors (903). This improvement was ascribed to a reduction in the background signal generated by CaOH. Ottaway and Shaw have shown that the emission signals from elements, atomized in a graphite furnace, follow the predicted Boltzmann distribution as a function of temperature (1001).It was also reported, however, that the optimum experimental temperature was frequently not the maximum obtainable temperature of the atomizer, owing to thermal diffusion effects, changes in appearance temperatures and the degradation of the background signal with increasing temperature (777).Epstein has shown that the thermal gradient in many furnaces can cause self-reversal of emission lines (1077). If the bandwidth of the emission line is close to the spectral band pass of the spectrometcr (say 0.1 nm) then the intensity-concentration calibration relationship is no longer solely dependent on the source emission.In addition, the use of wavelength modulation under these conditions can result in negative emission signals and double-valued analytical calibration graphs. Alder and Snook (130) have made the first report of simultaneous multi-element analysis using ETA emission spectroscopy. Emission signals were obtained for Al, Bi, CO, Fe, Mo, Ni, Pt and Zn using a vidicon tube.However, the detection limits were, as expected, inferior to those obtainable with a photomultiplier tube. The same workers (509) have reported on temperature measurements made in the gas phase using the two-line method.30 Analytical A tomic Spectroscopy The calculated excitation temperature (2450 K) agreed closely with the furnace-wall tempera- ture (2420 K).Presently work on AES using ETA is performed on furnaces designed exclusively with AAS in mind. A logical next development would be the design of furnaces specifically for AES. 1.5.6 Analytical Methods The determination of P by ETA has been reported by several workers this year (81, 720, 899, 1071, 1075, 1084).Ediger has shown that the addition of 1% m/V La as the nitrate significantly enhances sensitivity and also minimizes differences in sensitivity for various forms of inorganic P (81, 1084). Strecker et al. showed that it was also necessary to convert organic compounds in to inorganic form, by ashing with magnesium sulphonate, before analysis (1071). Driscoll et al., however, overcame this problem for the determination of P in petroleum by adding an ethanolic solution of La(NO,), to the petroleum before direct analysis (1075).Vacher (1707) has reported the determination of B in plants, fertilizers and steel, using a pyrolytically coated graphite tube with rapid heating. Slovak has used a fine-grained ion-exchange resin to separate metal ions from solution.He then slurried the resin in water and injected a sample of the suspension into a graphite furnace for determination (775). Two authors have independently used an aqueous slurry technique for the determination of trace clements in TiO,. One used monoisopropanolamine (290) and the other used sodium hexametaphosphate (294) as the dispersing agent to pro- duce stable slurries. Fuller and Thompson (900) have further developed this sampling technique so that it should be applicable to a wide range of determinations. Finely-ground samples are suspended in water and a thixotropic agent added, together with a defoaming reagent. After thorough shaking, suspensions of ground rock samples were obtained, which were stable for several days. Samples could be injected directly into the ETA using con- ventional micropipettes. There have been two reports of the determination of F using molecular absorption spectroscopy. Dittrich (678) used absorption by GaF or InF after the addition of Ga(NO,), to the samples while Tsunoda et a!. (1182) used the AlF species after addition of Al. Chloride has been determined by molecular absorption after formation of InCl (1 590). Nickel, as might be predicted from its interference effect in the determination of Bi by the hydride technique, has been shown to stabilize Bi in a graphite furnace during pyrolysis of samples (1198). Other references of interest - Measurement of physical parameters: 703, 838, 1257. Separation proccdures: 205, 1 181, 1382, 1506.

ISSN:0306-1353    

DOI:10.1039/AA9770700026

出版商:RSC    

年代:1977

数据来源: RSC

摘要:

30 AnalyticaZ A tomic Spectroscopy 1.6 VAPOUR GENERATION 1.6.1 Hydride Generation A novel method for generating ASH, has been dscribed by Rigin and Verkhoturov (1608). They used electrolytic reduction in KOH medium using platinum electrodes. Improvements in sensitivities for the determination of As and Sb havc been achicved by collecting the liberated hydrides in a liquid-N, cold trap before introduction to the atomizer (942, 1258, 1291).Vijan and Wood (295) have improved the detection limit for determination of Pb to 0.1 ng ml-1 by the addition of H,O, to the samples. They suggest that the improved sensi- tivity is brought about by the oxidation of Pb to a metastable tetravalent state. The current trend is to the use of ekctrically-heated silica tube furnaces, rather than flame and flame-heated silica tube atomizers, for the decomposition of volatile metalPart I : Fundamentals and Znsttumentation 31 hydrides.An electrically-heated furnace has been described where the silica tube is sup- ported by two ceramic holders, which contain the heating elements (307). This design permits rapid replacement of the silica tube alone. The growing popularity of electrically- heated furnaces is also reflected by the fact that one is now available from an instrument manufacturer (702, 1259, 1451).Other references of interest - Interferences: 215, 949, 961, 1175, 1635. Non-dispersive AFS detection: 1069. Se recovery: 927. 1.6.2 Mercury Determination Some minor modifications to the Hg gencration system have been described: (i) a sample decomposition train that allows samples containing up to 50% organic material to be analyzed (304); (ii) the use of a sintered-glass bubbler in an Hg generation vessel-the lig is collected by absorption on the sintered glass before removal - to double the sensi- tivity of the method (353); (iii) a dual-channel Hg generation system to incrcase the sample throughput rate (59).The addition of ammonia solution to samples containing chloride was claimed to give a significant improvement in the AAS signal (859). Thorburn Burns et al. have demon- strated that an alkaline Sn(II)/Cd reduction method improves recoveries and precision in the analysis of organo-Hg compounds (1030). Other references of interest - Sample preparation: 832, 1375.

ISSN:0306-1353    

DOI:10.1039/AA9770700030

出版商:RSC    

年代:1977

数据来源: RSC

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2 lnstrumentatlon 2.1 LIGHT SOURCES 2.1.1 Lasers The growth in the literature dealing with the application of lasers to analytical atomic spectroscopy continues to accelerate and a wider range of techniques is now being reported. An important advance has been the use of direct electrical detection of optical processes either by counting ionization events or by measurement of the ionization current, This leads to a far more efficient detection method, with single-atom monitoring being feasible through selective excitation followed by ionization and detection of the free electrons.Direct electrical detection offers many advantages, including obviating the need for an optical detection system, i.e., lenses, monochromators, filters, photomultipliers, etc. All optical laser fluorescence measurements are ultimately limited by scatter; it is claimed that electrical detection is not affected by scatter and therefore allows the full power capability of laser sources to be employed.Single-atom detection has been demonstrated by Young et al. (341) using what is termed resonance ionization spectroscopy (RZS). In this case Cs atoms were ionized by a two-photon process involving saturation of the 72P,, state by 455.5 nm photons followed by ionization by a second photon of the same energy.The Cs atoms were located in a proportional counter tube, capable of responding to single-electron events. Travis et al. (1 1 1 , 1067, 1068, 1237) have demonstrated “uptu-galvanic detection” of Na and Ba in analytical flames using a tunable laser source.The voltage drop produced between electrodes placed in a flame, when the resonance line was irradiated, was found to be linear with concen- tration over 4 orders of magnitude and yielded detection limits similar to those obtained by emission methods. The technique has also been applied to a graphite furnace atomizer (685) and it was suggested that the mechanism was optical excitation of the resonance transition followed by collisional ionization of the excited atom.Opto-galvanic detection has also been used as a means of calibrating laser wavelength and bandwidth (1603) by monitor- ing the variation in voltage drop across a standard HCL when the atomic cloud was illumin- ated by a fraction of the laser beam. Bandwidth was determined by comparison with the known line widths produced by the HCL.An alternative or perhaps additional mechanism for laser ionization has been discussed by Measures (969), who notes that the ionization produced by pumping the resonance transition of Na is unusually efficient. This author proposes that three-photon ionization occurs, creating a pool of electrons that are then accelerated by collisions of the second kind with excited Na atoms.The accelerated elec- trons have sufficient energy to ionize the excited atoms, which because of their large number behave like a pseudo-ground state of low ionization potential. The high spectral power density and narrow line output of lasers makes them obvious sources for AFS. However, the laser is not simply a super-intense conventional source, it is coherent and therefore its properties are in many ways more similar to those of a radio- or microwave beam than a light source.The combination of high intensity and coherence produces a range of effects when lasers are used for excitation that arc quite different to those encountered from conventional sources. Saturation spectroscopy i s now well known and during the past year more papers have appeared discussing its importance in analytical spectroscopy.Piepmeier et ul. (1233) have studied saturation effects in different atomizers, i.e., air/C,H, flame, Ar d.c. arc, carbon rod and laser microprobe plume. Line-width studies showed that “power broadening”, or “saturation broadening” as it is sometimes known, became evident as saturation was approached.The phenomenon of power broadening has 32Part I: Fundamentals and Instrumentation 33 been well known in microwave spectroscopy for many years, but its observation in the optical region of the spectrum is relatively recent. Power broadening occurs because the frequency of population-depopulation of the excited state is so great at or near saturation, that there is an apparent shortening of the spontaneous lifetime leading to increased spectral line-width, Sharp and Goldwasser (1 504) have presented theoretical calculations showing the power required to produce saturation under various excitation conditions; namely for homogeneous line profiles, for an inhomogeneous Doppler profile and for a completely inhomogeneous line.The latter case was included in an attempt to show the limitations of rate equations in describing high irradiance phenomena.This point has been taken further by Daily (493), who has used density-matrix formalism to show precisely the conditions under which rate equations can be used and where it is necessary to take account of the coherance of the excitation field. Winefordner et al. (810) have observed saturation effects in Na vapour using a CW dye laser and Alkemade (664) has described experiments demonstrating sane of the unusual effects associated with laser excitation, such as power broadening, two-photon absorption and intra-cavity quenching.Li (823) has investigated the effect of phase and intersection angle on the amplitude of Doppler-free two-photon absorption.The laser-excited atomic fluorescence of Pb has been studied by Bolshov et aZ. (1395). These authors showed that 40% of the Pb atoms were transferred to the metastable level within 5 ns of excitation and this process leads to an optimal excitation pulse length for maximum analytical sensitivity. New laser systems and modifications to commercial lasers are still being described and it is to be hoped that this trend will continue, with particular emphasis on producing equipment that can yield long-term reproducible performance, as required for analytical applications.Malmstadt et al. (1174, see also ARAAS, 1976, 6, Ref. 1158) have published a paper describing the design of their microprocessor controlled N,-laser-pumped tunable dye laser. The dye laser gives output from 350-650 nm with a line width of 0.06 nm and can be scanned at speeds of up to 2nms-1 with an added 5 s required each time a dye is changed.Wallenstein (16) has given details of the construction of tunable dye lasers pumped by high-power-gas and solid-state lasers. This author emphasiscs the superior spectral perform- ance of CW systems, but points out that their wavelength range is generally limited to 460-640 nm and that pulsed systems are necessary to achieve efficient frequency doubling for output in the U.V.region. The problem of Rayleigh scattering in laser excited AFS has been investigated by Yeung and Goff (1234). By the addition of a commercially available electro-optic tuning element to a CW dye laser it was possible to achieve frcquency modula- tion of the beam; lock-in detection was used to remove scatter signals.The high sensitivity obtainable by use of laser-excited AFS is demonstrated in work by Bolshov et al. (507). A frequency-doubled dye laser was used to detect sub-pg quantities of Fe and Pb atomized in a graphite tube furnace. Similarly Hohimer and Hargis (819) have determined pg quantities of Cs using ETA and direct line fluorescence detection.Gelbwachs et al. (1293, 1668) have reported detection limits in the 1-100 atoms range for Na, Pt and Ni using what they term saturated optical non-resonance emission spectroscopy (SONRES). A CW laser was used for excitation and under saturated conditions non-resonant fluxes of about 107 photons s-1 for each atom were obtained, These authors quoted detection limits of fractions of an atom; it might be considered more meaningful to report single-atom detection at a given SNR.Winefordner et d. (1420) have presented analytical data for the determination of Ba, Cu, Li, Mo, Nd, Rh, Sc, Na, U, V and Sr in either air/C,H, or N,O /C,H, flames using CW laser excitation. The limited excitation-wavelength range was offset by using lower excited states and non-resonance fluorescence processes.Other references of interest - Fluorescence excitation profile in flames: 1186.34 Analytical Atomic Spectroscopy Fluorescence of diatomic sodium: 228. Intra-cavity atomic absorption: 184, 665. Plasma inhomogeneity studied by He/Ne laser: 1312. Pulsed versus continuous wave excitation sources; 920, 989.Temperature and OH measurement by frequency-doubled CW dye laser: 1536. Tunable laser AFS: 378. 2.1.2 Continuum Sources Continuum sources have some advantages for spectroscopic analysis. They enable a con- siderable simplification of the instrumentation in that only one source has to be used and optimized, and simultaneous multi-element analysis is possible. A compact, flexible con- tinuum source operable at wavelengths from the visible to V.U.V.has been described by Norton and Wooding (1308). It consisted of a plasma produced by the ablation of a solid dielectric surface in the presence of an intense electric discharge in a vacuum of 1.3 mPa. The device operated by applying a voltage of 5-10 kV between Mo electrodes, which were mounted on a dielectric surface with a separation of 3 mm.The discharge duration could be varied between 1 and lops. Butler and Human (671) have described a pulsed flash lamp source consisting of a He-jet guided spark which was discharged on the axis of a 0.1 pF coaxial capacitor charged to 12 kV. The light pulse had a duration of 1 ps and showed an equivalent black-body temperature of 20 000 K yielding a spectral radiance of 400 W cm-2 sr-l nm-1 at 300 nm.A pulsed microwave discharge in He at pressures of about 1.5 Torr has been shown to exhibit intense emission in the V.U.V. between 30 and 70 nm (926). The He I atomic lines between 50 and 60 nm were found to dominate the emission spectrum with the He 30.38 nm line becoming evident at pressures below 1.5 Torr. The spectral characteristics and properties of a 300 W ‘Eimac’ continuum lamp have been investigated and compared with those of a conventional 150 W Xe high-pressure lamp (1183). The ‘Eimac’ lamp was found to provide up to 37 times more radiant power than the Xe lamp.However, it was noted that the SNR varied by a factor of 5 over the window of the lamp, indicating that careful consideration of its spatial properties is necessary in analytical applications (see also 1528).Tamura et al. (1049) have described a combination tungsten (halogen) lamp with a deuterium lamp, which provides continuum output between 190 and 900 nm. The application of the device as a background corrector for AAS was demonstrated, Improvements in continuum source AAS using an echelle spectrometer have been reported by O’Haver et al.(572). The improvements resulted from modifications to the spectrometer including the use of a larger order-sorting prism thereby reducing order- overlap stray light, and a refinement to the slit mounting enabling the use of narrower spectral bandpasses. Other references of interest - AAS with a continuum source: 374, 1361. Exploding wires as a continuum source: 892. 2.1.3 Hollow-Cathode and Electrodeless Discharge Lamps The established position of hollow-cathode lamps as sources for analytical spectrometry is reflected in the relatively few research papers that are now publishcd on their design and characteristics. However, some workers are still active in this field. Schearer and Rambow (553) have described a demountable, free flowing HCL in which the cathode was machined from boron nitride, lined with Ta foil, and mounted on a stainless-steel rod insulated by borosilicate glass, The cathode was mounted in a glass jacket opposite a water-cooled brassPart I : Fundamentals and Instrumentation 35 anode.The quartz window was flushed with flowing He such that the lamp pressure was maintained between 1 and 50Torr.The lamp was used as a source of resonance radiation for Ca, Ba, Zn, Mg, Sr, Yb and Eu and was operated at the unusually high current of 3 A. Mehs and Niemczyk (333) have measured the electron temperature in HCL discharges using a floating double-probe technique. The plasma electron temperature was used to discuss the operating mechanism of the source and comparisons were drawn with similarly measured temperatures in electrodeless discharges.Falk (1 393) has carried out theoretical calculations of the intensity of line emission from HCLs and has shown that these agree well with experimental rcsults. In comparing local thermodynamic equilibrium (LTE) with non-LTE sources the theory shows that line to background ratios should be higher in non- LTE sources.The determination of B isotope ratios using HCL sources has been reported by Hannaford and Lowe (1176). Three methods were described based on measurerncnt of the ratio of the intensities of the 208.89/208.96nm doublet: an accurate method using an enriched lOB source (Ne filled) with a water-cooled sputtering-cell atom reservoir, a less accurate determination using an enriched source with a N20/C,H, flame atom reservoir and an approximate technique based on a natural-boron source with the N20/C,H, flame reservoir.The eiectrodeZess discharge lamp has perhaps had an unfortunate history as a source for analytical atomic spectroscopy. Many laboratories attempted to make and use EDLs when their initial potential was realised, but owing to the lack of truly systematic study, erratic rcsults were obtained and the source fell somewhat into disrepute.In more recent times much has been done to rectify the position and EDLs at least for some elements may yet prove the most practical source for AFS. The value of tempcraturc control has been established and there is now a move toward the use of r.f. rather than microwave frequen- cies for the excitation field.This has obvious advantages in that r.f. generators of high power are easier and cheaper to construct and inductive coupling can be used rather than expensive, precision-made resonant cavities. Browner and Novak (668, 1070) have describcd the construction and operation of r.f. (35 MHz) excited lamps in both CW and pulsed mode with peak powcrs up to 1 kW.It was found that for pulsed operation source radiance increased linearity above 200W and showed no levelling off at the maximum available power of I kW. The onset of line broadening was also less severe in pulsed as compared with CW operation. A vacuum-jacketed EDL powered by an r.f. field has been designed by Baranov et al. (1331). Sources were constructed for the elements Cd, Cs, Cu, Fe, K, Na, Pb, Rb, Sb, Se, Te and T1 and yielded equivalent performance to conventional EDLs. Similar results have been reported for r.f. excited EDLs by Winefordner et at. (1418). Stephens ( I 614) has described the construction of an r.f. capacitively-coupled vapour discharge lamp, specially designed to facilitate Zeeman modulation of the emitted spectra. Parsons and Bentley (568) have continued to refine their hydride formation method for filling EDLs and have applied the “Simplex” algorithm to optimization of the construction parameters. These lamps were doped with KBr to increase the electron density, thereby facilitating Penning ionization in the lamp. Other references of interest - Line-profilc studies in EDLs: 669. Noise studies on pulsed HCLs: 1503. Studies on HCLs: 670.

ISSN:0306-1353    

DOI:10.1039/AA9770700032

出版商:RSC    

年代:1977

数据来源: RSC

摘要:

Part I : Fundamentals and Instrumentation 35 2.2 OPTICS 2.2.1 Background Correction Studies Background correction facilities are now provided on many commercial atomic absorption36 Analytical Atomic Spectroscopy instruments and for emission studies background correction by use of a vibrating quartz plate with lock-in detection is commonly employed. Background correction is an important tool, particularly in absorption studies, for the removal of systematic error due to spurious absorption; however if used without regard for the true nature of the spectrum, errors may actually be introduced that would not have been encountered with single-channel measure- ment.Massmann et al. (63, 1407) have pointed out that conventional background correction with a continuum source can only be effective if the background is also a continuum, such as that yielded by the dissociation spectra of molecules.Erroneous results arc obtained if the background i s comprised of line-rich electronic spectra where the true background depends on whether or not the absorption line coincides with a rotational line of a molecule. This information is not available with conventional spectrometers where the line is not resolved, and it is concluded that the only way of avoiding error is to undertake a prior detailed analysis of the background spectra using a high resolution instrument.Zander (373, 557) has also discussed the errors introduced by unresolved background line spectra and has described methods for dealing with them. Zeeman scanning was used as a means of providing correction at small spectral intervals from the absorbing line and the use of a wavelength-moddated high-resolution echelle spectrometer (160) was described as a means of discriminating against background continua and for correcting for line overlap.An interesting paper by Marks and Wclcher (1241) pointed out that background absorption may result from atomic lines greatly broadened by self-absorption.These authors estimated that in ETA, where atomic densities of matrix concomitants can be very high, line widths at one-tenth maximum height may reach 3.0 nm. Many hitherto unassigned absorption backgrounds might well be attributed to this mechanism. In recognising the problems that can be introduced by use of background correction it is pointed out that error occurs when the average absorption over the bandpass of the spectrometer does not equal that under the absorbing line.Grassam et al. (1388) have described the use of the inverse Zeernari eflcct for back- ground correction in ETA. An electromagnet was placed round a specially designed graphite furnace atomizer and the Zeeman effect generated in the sample was used for correction of the absorption due to the matrix.The inherent polarization properties of the Zeeman effect were used to generate two signals, one corresponding to the atomic line plus background absorption and the other to the background absorption alone. Analytical measurements on Ag, Au, Cd, Cr, Cu, Hg, K, Mg, Mn, Na, Ni and Pb showed a reduction in sensitivity of up to 35%; however a background absorbance of 2.0 was corrected to better than 0.005.A similar approach to background correction has been adopted by Koizumi et al. (1508). A further refinement by these authors (944) employs modulation of the plane of polariza- tion (at 100 Hz) of the incident light coupled with modulation of the light intensity (1.5 kHz). This scheme facilitates background correction and the intensity modulation discriminates against radiation from the ETA.The ability of a wavelength-modulated continuum-source spectrometer to correct for background absorbances above 2.0 has been demonstrated by O’Haver et al. (1072). The system was used for the direct determination of Pb and Cu in salt water and of sub-ppb levels of Cr in blood serum in a graphite tube atomiser.A novel method of background Correction using closely spaced non-absorbing lines has been proposed by Minami ~t al. (998). It was found that in the pulsed operation of u HCL (40 ps at 200 mA), the rise times of the resonance and non-resonance lines were sufficiently different to allow gating of the detector to measure Zinc and background absorptions separately without the need for spectral resolution.Koirtyohann et al. (946) have employed a stepped refractor plate modulator for back- ground correction in emission studies. They claim advantages for the system in that thePart I : Fundamentals and Instrumentation 37 dwell time at each wavelength is greater than for sinusoidal modulation, yielding lower detection limits; also, precise wavelength selection for background correction is possible. An analysis of SNR improvement in wavelength-modulated emission systems has been given by Epstein (648).The major noise sources are delineated and a computer model described that is capable of correctly predicting enhancements in SNR. Other references of interest - Background correction in flame AES using dual-flame technique: 804.Review of background correction methods: 11 34, 1600, 1662. Zeeman background correction: 673. 2.2.2 Optical Systems The availability, at “modest prices”, of fast digital computers with large memories has provided a stimulus to the development of multiplex spectrometers, which require signal decoding to reconstruct the original spectrum. Simultaneous measurement at several wave- lengths using one detector is convenient and can yield considerable SNR improvements (the Fellgctt advantage) in detector noise-limited situations by virtue of the increased measure- ment time available for each spectral interval.In the case of interferometrically-based spectrometers high resolution with large aperture can also be realised. Horlick and Yuen (950, 1429) have described the design of a Michelson interferometer for simultaneous multi-element emission analysis, The spectrometer produced double-sided interferograms thereby obviating the need for phase corrections; a spectral resolution of 0.1 nm in the range 316.4-421.9 nm was achieved.It was pointed out that while the true Fellgett advantage may not be realised in the u.v.-visible region because of signal noise, there are important additional advantages such as high resolution, high aperture and simultaneous wavelength monitoring.Further information has been published on the selective modulated interferometric dispersive spectrometer (SEMIDS) (see ARAAS, 1976, 6, Refs. 564, 910) regarding its SNR characteristics. It has been shown (150) that although the luminosity-resolution factor may be improved by 103, there is no SNR gain, as com- pared with a dispersive spectrometer, for u.v.-visible spectroscopy wherc signal noise is the limiting factor (see also 151).Theoretical studies of Hadamard transform spectroscopy (HTS) (798) have shown that it offers a slight Fellgett advantage over single-slit scanning of a spectrum when intense spectral signals are available as in AAS, but is disadvantageous for the measurement of weak signals as encountered in AES and AFS.To test the feasibility of simultaneous multi-element AAS based on HTS, a two-channel instrument was con- structed (for Pb and Mg), which employed a novel continuously moving Hadamard mask. Sensitivities similar to those of conventional systems were obtained, although because of instrumental limitations detection limits were worse.The high resolution, moderate aperture characteristics of echelle spectrometers makes them attractive instruments for analytical spectroscopy; the two-dimensional spectrum display is, however, more difficult to process than the linear spectrum of conventional spectromctcrs. Pardue and Felkel (1246) have overcome this problem by using a silicon- target vidicon tube detector.The instrument provided 40000 spectral elements in the range 180-700 nm (half-width 0.04 nm at 300 nm, 0.09 at 700 nm), and was used for the simul- taneous determination by AAS of Co, Cr, Cu, Fe, Mn and Ni. Using a multi-element HCL source, detection limits were similar to single-channel measurement for Cu, Cr, Mn and Ni, but worsc for Fe and Co. Parsons et al.(1213) have described the design of a scanning echelle monochromator in which selection of the XY co-ordinates was accomplished by microprocessor control. O’Haver et al. (1087) have discussed the shape of the analytical curve produced by wavelength-modulation continuum-source A AS. Lock-in detection of the modulated signal38 Analytical A tomic Spectroscopy led to measurements of the difference between the incident and transmitted intensities, which were only proportional to absorbance (A) over a limited range.It was suggested that the separate measurement of incident intensity necessary for calculation of the absorbance can be obtained by monitoring the d.c. component of the photo-signal.Thus the absorbance is calculated from the expression A =log[(Ep+ K,EJ /(EP - K,E,)I - where Ep is the average d.c. level of the photo-signal, E, is the lock-in output voltage and K, and K, are constants. This method in fact led to over-estimation of absorbance at high concentration, but this was shown to be useful in correcting negative deviations due to the failure to conform to the Beer-Lambert law.Further work by the same authors (156) discussed the importance of the waveform driving the refractor plate in modulation spectro- metry. Using computer simulation techniques it was claimed that waveform shape was relatively unimportant, except that for systems limited by background photon-shot noise, square-wave modulation offered a SNR gain of 2 when measurements were made at twice the modulation frequency.The statistical characteristics of non-dispersive atomic-absorption spectrometers have been investigated by Chupakhin et al. (1 76). Multiplicative and additive sources werc defined and their dependence on instrument type (Le., single or double beam) and concentration range was discussed. In many spectroscopic measurements, particularly those involving intense sources such as lasers, it is necessary to employ pre-spectrometer rcjcction filters to avoid stray light affecting the measurement channel.Dielectric bandpass interference filters have very low losses and therefore whcn used in reflection mode they act as band-rejection filters. Mahdavi (1 367) has used this property to construct combination interference-filter polychromators employing both bandpass and band-rejection modes.Other references of interest - Aberration corrected holographic gratings: 1043, 1576. Adjustable optical mount for high-precision optical rails: 994. Determination of the instrument function of the Fabry-Perot interferometer: 649. Digitally-controlled shutter system for spectrography: 984.ETA with multichannel spectrometer: 106. 5 m Echelle spectrograph: 1604. Twin hollow-cathode interferometer spectrometer: 508. 2.2.3 Magneto-optical Effects Perhaps one of the more important advances during the past year has been the application of magneto-optic effects to atomic spectral analysis. Thus Kitagawa et al. (1681) have employed the Faraduy effect in the determination of Cd, Cu and Pb.For realization of this technique the atom reservoir is placed between crossed polarizers and a magnetic field applied along the optical axis. In the absence of the ficld no radiation from the source reaches the detector, but when a field is applied there is an effective rotation of the plane of polarization, which whcn resolved into its orthogonal components yields a component at right angles to the original plane, which therefore passes through the second polarizer to fall on the detector, The Faraday effect is directly related to the longitudinal inverse Zeeman effect.The anomalous dispersion in the region of the Zeeman components rcsults in different refractive indexes for different components of the polarization. As the light beam transverses the sample, phase differences are introduced between the components causing a net rotation of the electric vector with respect to its original plane.The direction of the rotation i s dependent on the field direction and the magnitude of the field strength. Passage of the beam back through the sample increases the degree of rotation, unlikcPart Z: Fundamentals and Znstrumentation 39 natural optically-active materials which would restore the electric vector to its original plane.An analysis by Corney et al. (Proc. Roy. SOC., 1966, A293, 70) has shown that the transmitted intensity may be expressed by Z 00 Z,(NL)2f(H) where Z, is the incident intensity, N is the atomic density, L is the length of the atomic vapour and H is the field strength.Thus although the optical arrangement i s similar to that of AAS the characteristics of the technique are similar to AFS in that the source is not directly viewed by the detector and the signal is directly proportional to the source radiance. It is also important to note that the signal is proportional to (NL)2 rather than linearly proportional as with conventional methods. Laser sources possess both high radiance and a high degree of polarization and could therefore prove useful sources for studies of the Faraday effect. In the paper by Kitagawa et al.HCLs and EDLs were used as sources and samples (5 PI) were atomized in a graphite tube furnace (3.5 mm internal diamctcr; length 12 mm). Dctcction limits were 5XlO--13g for Cd, 3x10-11 g for Cu and 5XlO-llg for Pb.Background correction capability was demonstrated by adding 5p1 of 25000 ppm NaCl or cigarette smoke to the atomizer, neither of which gave any signal. A similar technique, but this time with the field placed transverse to the optical axis, has bccn described for the detection of Hg (558) and Cd (1692) by Yamamoto et al. For transverse fields the phenomenon is known as thc Voigt effect or magnetic birefringence (double refraction).It i s evident that the Voigt effect is directly related to the transverse Zeeman effect just as Faraday rotation is related to the longitudinal Zeeman effect. The incident beam was sent through the sample with its plane of polarization orientated at 45" to the magnetic field. The field induced refractive index variation caused the different polarization components to travel at differcnt velocities and the resultant phase diffcrcnces caused the emergent beam to be elliptically polarized. As before, the analyzer polarizer passed the component orthogonal to the incident plane. The concentration dependence is the same as for the Faraday effect and in this case the degree of birefringence introduced is proportional to the square of the field strength. It was pointed out that atom-reservoir geometry is simpler and the influencc of co-existing atoms and molcculcs is less with the Voigt effect than with Faraday rotation, although the latter gives signals which are two or three times more intense for the same NL and magnetic field values.

ISSN:0306-1353    

DOI:10.1039/AA9770700035

出版商:RSC    

年代:1977

数据来源: RSC

摘要:

Part Z: Fundamentals and Znstrumentation 39 2.3 DETECTOR SYSTEMS Developments in solid-state electronics have led to the production of a range of electro- optical devices that can be used for photo-detection. Although considerable advances have been made, most of these dcvices suffer from limited wavelcngth coverage, exhibiting rather poor sensitivity at wavelengths below 300 nm. The increasing demand for simultancous or rapid sequential multi-element analysis has placed particular emphasis on imaging type detectors, which can monitor complete spectra.The range of devices is considerable and includcs the silicon vidicon (SV), the silicon-intensified-target vidicon (SIT), the intensified silicon-intensified-target vidicon (ISIT) and the secondary electron-conduction vidicon (SEC).The image dissector tube (ID) can also be used for multi-spectral caverage; it is a rapid sequential device based on a photomultiplier in which segments of the photocathodc can be selected by means of magnetic deflection coils. Winefordner, Caoney and Boutilier (921) have compared the properties of all the above devices in relation to analytical spectroscopy and drew the following conclusion: the SV is not useful for AFS or molecular fluorescence spectrometry, the SEC and ID are potentially useful for AFS particularly with echelle spectrometers and the SIT possesses considerable potential for molecular luminescence spectroscopy in the visible region (> 350nm).An experimental comparison of a U.V. sensitised vidicon and a SIT with photomultiplier tube detection has been prcscnted by Howell and Morrison (149).Studies on the flame emission detection of 23 elements showed40 Analytical Atomic Spectroscopy that the SIT gave equivalent detection power to commonly used photomultiplier tubes in the visible region only. Chiu (1366) has described the use of a slow-scan SEC for the measurement of stellar spectra at high resolution.Several problems were encountered in the use of the SEC tube, namely low target gain (-50), target gain noise, non-linear response and low storage capacity (< 2000 photoelectrons per pixel). In addition considerable varia- tions were found between different tubes, and industry figures showed that only 10% of manufactured tubes yielded good quality dztectors. In spite of these difficulties it was concluded that the SEC was the only vidicon tube capable of low-level light integration with high spatial resolution.A vidicon-based rapid-scan spectrometer capable of producing first derivative and intensity spectra has been described by Pardue et al. (162). The wavelength modulation used to produce the derivative signal was created by the electron-bzam scan pattcm.The vidicon spectrometer was linked to a mini-computer and its performance in atomic emission and molecular absorption was evaluated. Alder and Snook (130) have employed a vidicon detector for the measurement of emission signals from Al, Ni, Mo and Pt in a carbon tubc furnace. The determination of trace metals in Au using vidicon detection has been described by de Villiers et al.(64). Katzenberger, Fassel and Kniseley (1207) have evaluated the use of a silicon-intensified target detector (SIT) for OES with the ICP excitation source. Recent advances in SIT fabrication have brought their performance characteristics closer to those achieved in conventional multichannel polychromators; a comparison of powers of detection, linearity and precision for a modern SIT and for typical polychromators with an ICP source was presented.Pardue and McDowell (947) have investigated the use of a SIT tube as a multi- wavelength detector for LC peaks. Detection limits and performance were found to be similar to those obtainable with currently available commercial systems. Self-scanned linear phorodiode arrays are one of the cheapest and most convenient solid-state detectors for monitoring linear spectra.Sandell and Broadfoot (1 83) have obtained improved sensitivity with such a device by coupling it (via fibre optic) to an image-intensifier tube. In another paper they report (494) the use of a 128-element self- scanned anode array as a detector for a multichannel plate electron multiplier. O’Keefe and Chuang (1089) have studied various self-scanned diode arrays as detectors for FAAS and found that a 1024-element device mounted in the focal plane of a 0.35m Czerny-Turner monochromator provided the most useful performance. Spectral coverage of 55 nm with resolution of 0.2nm was achieved, and yielded good detection for several elements with absorbing lines in the wavelength range 214-589 nm.A similar arrangcment was reported by Horlick (1434), who used the device for studying emission spectra from an ICP. The ability to make simultaneous measurements on different parts of the spectrum was shown to be particularly useful in elucidating the effect of concomitants on line emission. Koirtyohann et al. (1525) have used a 512-element array placed parallel to the exit slit of a stigmatic monochromator to study spatial variations in spectral line emission from an ICP and a N,0/C2H, flame.A device known as the “photosil detector” (PSD) has been used (1309) in a low-noise photon counting system. The PSD is a combination of a conventional photocathode with a solid-state detector. Hieftje (1223) has described a method for measuring sub-ns response times of fast detectors. The technique uses a free running CW laser as a multi-frequency modulated source and a ref. spectrum analyzer to monitor detector response. Detector response times down to 200 ps could be measured. Other references of interest - Automatic photon-counting system: 1394. Comments on signal flicker noise and noise power spectra: 445.Part I : Fundamentals and Instrumentation New vidicon spectrometer design for laser-excited fluorescence : 320. Performance of a vidicon detector at low light levels: 182. Review of multi-channel TV type detectors: 1575. Silicon-photodiode array for AAS and A E S : 601. Vidicon detector for Stark modulation spectroscopy: 1307. 41

ISSN:0306-1353    

DOI:10.1039/AA9770700039

出版商:RSC    

年代:1977

数据来源: RSC