Atomization and Excitation 25 1.4 ELECTROTHERMAL ATOMIZERS With the widespread use and acceptance of ETA in routine analysis, developments in applications would be expected to be more prevalent than fundamental studies of the atomizer and atomization processes, and new methodologies. It is refreshing to find, how- ever, that this is not the case, and that this year there have been a number of papers directed at improving the performance of ETA.There have, for example, been significant develop- ments in the use of tube coatings, temperature optimization and background reduction. It would appear that there is still much scope for improvement and that any complacency over problems at this stage would be dangerous. 1.4.1 Atomizer Design and Modifications Several groups of workers have modified the design of Massmann-type furnaces to allow atomization of samples under isothermal conditions.A graphite platform, similar to that proposed by L’vov (see also ARAAS, 1976, 6, Ref. 1141), has been shown to reduce matrix interferences substantially, especially for the more volatile elements (557, 1016, 1344). An analogous approach involved the use of a wire or rod onto which the sample was deposited and dried prior to its introduction into a pre-heated graphite furnace (463, 1124, 1340, 201 1).Chakrabarti and co-workers (886, 1339, 1341, 1994) have reported enhanced sensitivity for many elements by using a tube atomizer of anisotropic pyrolytic graphite, heated by capacitive discharge and maintained at constant temperature by an auxiliary power supply.Temperatures of up to 3300K, and heating rates of up to 100 Kms-1 were achieved independently of each other, The use of anisotropic graphite resulted in a more even temperature distribution along the length of the tube. To avoid memory effects in the atomization of U, Tarui and Tokairin (636) atomized samples from Ta boats, which were inserted into the graphite tube.Additional reference on the preceding topic - 1190. A device by which direct temperature control of a furnace was achieved by monitoring the thermal expansion of the graphite within a feedback circuit was proposed by Czobik et al. (1200). Wassall (1020), however, stated that only optical feedback systems could successfully cope with the rates of temperature rise achieved by modern furnaces.Research continues on the use of metal carbide coatings, both to prolong tube life and to reduce memory effects. Norval et al. (887, 1233, 1648) employed a cathodic sputtering technique for coating tubes with metals (usually W) prior to the application of a pyrolytic graphite coating. They claimed that over 3000 firings at 3000 K could be achieved with test solutions of 2% V/V perchloric acid and 10% V/V nitric acid.Additional reference on the preceding topic - 888. There have been some interesting papers on the use of metals as atomizers. Sychra et al. (263, 898, 1563, 1564) described a simple W tube furnace, fabricated from two U-shaped metal strips, pressed together to form a tube. The furnace was claimed to give comparable performance to that of commercial graphite atomizers for most elements, but gave superior detection limits and tube lifetimes for the more refractory elements.Ohta and Suzuki (622, 1582, 2018) used a Mo microtube to determine volatile elements, using a sheath gas mixture of Ar and H, to prevent oxidation of the tube and also possibly to provide a reducing atmosphere. Additional references on the preceding topic - 409, 539, 2010.Theoretical calculations by Frigieri and Trucco (1 39) predicted improvements in scnsi- tivity when the internal shape of the tube was tailored to fit the geometry of the hollow- cathode light beam. Practical results have substantiated these predictions (see also ARAAS, 1977, 7, 26 and Refs. 719, 778).26 Analytical A tomic Spectroscopy Other references of interest - Sample introduction: I1 1, 523, 524, 889, 939, 1977. 1.4.2 Atomization Processes A new approach to the investigation of atomization processes has been proposed by L’vov and Pelieva (715, 749). Using a W probe to introduce samples into a preheated commercial graphite atomizer, they concluded that gaseous monocyanides were formed as intermediate species during the atomization of 32 out of 42 elements studied (in particular Al, Ba, Cs, Sr).Dissociation energies of Al, Ga and rare-earth monocyanides (1 81 5) were calculated by applying the second and third laws of thermodynamics, and found to be in agreement with mass spectrometric data. The feasibility of these calculations when related to conventional pulse-heated atomizers has yet to be investigated.Tessari and Torso (1771, 1916) have further extended their theoretical model, which describes atom release from graphite rod atomizers. This model assumes conditions of pseudo-equilibrium at the solid/gas interface. When the model was examined with respect to different gas atmospheres (Ar, He and H2), some inconsistency was found as to whether the major factor was diffusive or convective transport.In a H, atmosphere (8971, local equilibrium at the solid/gas interface was no longer established and atomization was found to be kinetically controlled. Falk and Thann (900) similarly described the processes occurring in the vapour above open atomizers. They concluded that vapour transport was principally diffusive and explained how such information could be used to optimize atomic signals both spatially and temporally. Chakrabarti er al.(1997) have utilized Fuller’s kinetic model (see ARAAS, 1974, 4, Ref. 1131) to describe atomization in their capacitively heated atomizer. Since this atomizer operates virtually at constant temperature, the absorbance is propor- tional to the efficiency with which atoms are initially produced. The authors intend more closely to define this kinetic model in order to design “a perfect atomizer”.Explanations of signal enhancements and depressions based on gas-phase reactions have been forwarded by Eklund and Holcombe (542, 904, 1672, 1709). They have suggested that competition for 0, traces in the gas atmosphere above open atomizers could result in an increase (or decrease) in free-atom populations.It was shown that metal-oxide bond strengths can be used to predict such changes. The controlled electrothermal heating of a graphite crucible in combination with an air/C,H, flame, by Kantor er al. (903, 1478), has been used to study condensed-phase reactions taking place within the furnace. Experimental results on various Cu, Sn and Zn compounds showed close agreement between this technique and differential thermogravimetry as far as initial vaporization temperatures were concerned.In a further study (1009), the same authors added CC1, and CF,C12 to the sheath gas of the furnace and noted that Al, Ca, Co, Cu, Fe, Ga, Mn and Ni were now vaporized at 560-600 “C. This system was seen as being useful for selective volatilization of analytes from refractory matrices.In the first part of a comprehensive study on the atomization of noble metals in a graphite furnace, Rowston and Ottaway (1 180) critically compared thermogravimetric, X-ray diffraction, vacuum deposition and thermodynamic data to support their theory that atomization occurs via evaporation of the metal only. The mechanism of atom excitation in graphite furnace AES has been clarified by Littlejohn and Ottaway (508).Electronic, vibrational and ionization temperature measure- ments wcre used to prove the existence of local thermal equilibrium in commercial tube atomizers operated under static gas conditions. It was concluded that molecular collision was the major process contributing to atom excitation, though radiative excitation was con- sidered significant when a monatomic purge gas, with molecular impurities of less than 0.01 %.was used.Atomization and Excitation 27 Other references of interest - High temperature equilibrium calculations: 101 2. Thermogravimetric studies: 899, 1862. 1.4.3 Interferences The concept of achieving isothermal conditions within pulse-heated atomizers in order to reduce matrix interferences has gained much popularity (see also Sction 1.4.1).The applica- tion of the L’vov platform to commercial graphite atomizers by Manning and Slavin (1642) for example, substantially reduced anion interferences on Pb. This type of work has also caused a resurgence of interest in the constant temperature or Woodriff-type furnace.In some comparative studies this furnace was found to give substantially reduced interferences for Co (891), Mn (1731), Pb (555, 1547, 1716) and Se (106) in a variety of matrices, compared to pulse-heated atomizers of the mini-Massmann-type, In no case was the inter- ference effect greater than 10% in the Woodriff furnace. Interferences caused by molecular volatilization during ashing steps were also reported to be less in Woodriff-type furnaces (165).Additional references on the preceding topic - 467, 556, 648, 1470, 1630, 1838, 1877. The use of matrix modification to reduce interferences has gained in popularity. Koirtyohann et al. (104) described studies of various matrix modifiers used in the determina- tion of Cd, Cu, Fe, Ni and Pb in blood, urine, waters and tissue.They concluded that although no universal reagent has been found, considerable progress towards a general reduction in interferences has been made. Some interesting spectral interferences have recently been recorded. Frigieri and Trucco (919) reported that Co and Fe showed a very strong spectral interference on Cs. It would appear that AE is still superior to absorption for Cs determinations.Iron has also caused some spectral overlap problems when Pb was determined in iron-based alloys (120). The effect was however minimized by selective volatilization of analyte and matrix. An apparent spectral interference of Fe on Se at 204.0 nm was observed by Langmyhr et al. (1494) when determining Se in whole blood. The authors commented that the interference could be reduced by measuring at the 196.0nm Se line.Additional references on the preceding topic - 645, 888, 1444, 1455, 1480, 1583. Other references of interest - Accuracy: 13 1 3. Background correction: 127, 457, 522, 1297, 1935. Contamination: 322. Solvents: 925. Tube lifetime: 1056. 1.4.4 Emission There have been several interesting developments on the use of the graphite furnace as an emission source (885).The mechanism of excitation in commercial tube atomizers has been discussed by Littlejohn and Ottaway (508) (see Section 1.4.2). The same authors showed that for instrumcnts without dynamic background correction, e.g., wavelength modulation, optimization of temperature may be necessary to achieve the best SBR (1461). A graphite furnace with additive excitation has been produced by Falk et al.(895). In this device, the vaporized sample from the graphite tube was excited in a hollow-cathode discharge. Detection limits similar to AAS values and superior to AES values were reported (see also ARAAS, 1977, 7, Ref. 1001). The authors have proposed a non-thcrmal mechanism to explain the processes involved (1688).28 Amlyrical Atomic Spectroscopy Some new equipment for graphite furnace AES has been evaluated by Papp and Bodnar (892, 1008).Different graphites and alternative furnace geometries were examined in an attempt to optimize the atomizer. Other investigations showed that modifications to the graphite tube may improve detection limits (2074). However, no tube design giving the best sensitivity for all elements has yet been proposed.Detailed studies of ionization phenomena in graphite furnaces were reported by Ottaway and co-workers (902). Calculated electron concentrations ranged from 5.2 X 10-16 m-3 at 2558 K, to 1.2X 10-17 m-3 at 2761 K. An attempt was made to produce a simple expression, formulated from the Saha and thermionic emission equations, to predict the degree of ionization under practical conditions, Several reviews of the characteristics of the graphite furnace as an emission source, which demonstrate the potential of the technique.have been published (761, 1362, 1468). Other references of interest - Applications of graphite furnace emission: 1545. Chemiluminescence measurements in a furnace: 1 10. 1.4.5 Advances in Methodology The potential of ETA for the direct analysis of solids continues to be exploited and several on-going studies as well as some novel applications have appeared this year.One of the problems often associated with solid sampling is over-sensitivity. Using less sensitive atomic lines, Backman and Karlson (1682) overcame this problem and determined Ag, Bi, Pb, Sb and Zn in steels and nickel-based alloys.Some erratic results were, however, obtained; these were attributed to sample heterogeneity. Headridge and his group have continued to demonstrate the utility of their induction furnace for the direct analysis of metallurgical samples. Ag (311), Sb and other more volatile metals (1445) were determined in a variety of iron and steel samples. Some difficulties were experienced with Sn and other metals that were retained by the matrix.A similar technique has been employed for the analysis of Bi (491) in chips of nickel-based alloys. Solution analysis and direct solid analysis of mild steels for Pb and Sb were used in homogeneity studies by Frech and Lundberg (1119). Their results indicated that even for sample weights as low as 2 mg, no heterogencity could be detected.The same authors have modified a commercial auto-sampler to accept and deliver solid samples to a furnace held at constant temperature; Bi, Ag, Cd and Zn were determined in steels with improved precision (1675). The importance of isothermal conditions and of peak area measurements was stressed in further work (1112). Several novel applications have been proposed.Nakano et al. (644) analysed Cu, pre-adsorbed on to a Dowex A-1 ion-exchange resin. Nichols and Woodriff (553) atomized trace clenients from APDC precipitates using their constant-temperature furnace. The use of ETA coupled to chromatographic systems has increased. One of the original exploiters of the technique, Van Loon, has with co-workers determined Pb alkyl compounds in air (1562).They passed the eluate from a gas chromatograph directly into a preheated graphite furnace. Copper in different amino acid complexes (436) has been similarly determined after separation of the amino acids on a silica-gel ion-exchange column. Two systems for the analysis of WPLC eluate, using Zeeman AA, have been described. In the first (1774), the continuous mode of operation was compared with a temporary storage mode, which was shown to give a more complete description of the concentration profile.The second (1964) utilized a new furnace design to dstermine organic Pb compounds in automotive exhausts. By using indium chloride band emission measured at 359.9nm (see ARAAS, 1977, 7, Ref, 1590), Gutsche and Rudiger (39) used a graphite micro-furnace as a chloride-specific detector on a GC system.As little as 1.3 ng of chlorine could be determined in this manner. Speciation studies using electro-deposition of metallicA tomizatiorz and Excitatiort 29 species onto graphite tubes were described by Batley and Matousek (195, 1192). Chromium (VI) could be differentiated from Cr(II1) by altering the deposition potential.Acidified NaCl solutions were electrolysed at 3.2-5.2V versus S.C.E. to deposit Ag, Cd, Cu, Pb and Zn on to a W wire, before atomization in a graphite furnace (1193). The method eliminated background problems and could detect 0.3 ppb of Pb. Another area of activity has been biological analysis. A novel application was described by Ottaway and co-workers (1871) who evaluated the use of electrophoresis to separate Cu protein fractions before analysis of the cellulose acctate strips by ETA.A Zeeman spectro- meter was used by Pearson and Pleban (170) to determine Se in human kidney cortex. Magnesium was added to the matrix to prevent Se losses on ashing. The importance of temperature optimization in the determination of Cd in urine samples was stressed by two groups of workers (1544, 1586).Whole blood has also been analysed for Cd (741) by placing an aliquot onto a carbon or filter paper, which was then inserted into an r.f. heated atomizer. In this work aqueous and internal standards were compared. The most novel applications of ETA have bceen reported in the field of eiiviroizmentd analysis. Thompson and Wagstaff (961) used the molecular absorption of a sample as it undergoes evaporation or slow pyrolysis to detect and characterize organic pollutants in natural waters (see also Analyst, 1979, 104, 668). The addition of Ba was found to enhance the absorption signal of B in a graphite tube furnace (502). The method was used to monitor B in natural waters with a rapid sample throughput of 10 samples h-I. No mention of memory effects was given. Airborne particulates have been electrostatically precipitated onto a graphite furnace by Torsi and Desimoni (1749) prior to analysis. L’vov and Pelieva (1839) have investigated the factors affecting the determination of C e using ETA. Ce in steel was successfully determined at the 0.05-0.23% level. Other references of interest - Application of magneto-optics: 124, 200, 912. Rare earths: 2020. Trace impurities in metallic Na by ETA: 1027. Tube cleaning: 479.