Effects of Bias Voltage and Easily-ionized Elements on the Spatial Distribution of Analytes in Furnace Atomization Plasma Emission Spectrometry VICTOR PAVSKI†a , RALPH E. STURGEON* b AND CHUNI L. CHAKRABARTIa aOttawa-Carleton Chemistry Institute, Centre for Analytical and Environmental Chemistry, Department of Chemistry, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario, Canada K1S 5B6 bInstitute for NationalMeasurement Standards, National Research Council of Canada, Ottawa, Ontario, Canada K1A 0R9 The eect of bias voltage and the presence of easily-ionized ical sources such as FAPES and its low-pressure dc glow elements (EIEs) on the spatial distribution of excited-state discharge counterparts, hollow cathode (HC)21 and hollow atoms and ions of Cu, Ag, Cs and Ca in furnace atomization anode (HA)22,23 furnace atomization non-thermal excitation plasma emission spectrometry is presented.The dc bias of the spectrometry (FANES), is the potential to decouple atomizcentre electrode significantly aects the spatial distribution of ation and excitation phenomena, thereby potentially minimiz- He I, Cu I, Ag I, Cs I, and Ca II emission in the absence of ing matrix eects.Additionally, the collision-rich environment EIEs. A reasonably uniform distribution of excited-state provided by the plasma was hoped to be suciently robust to analyte atoms over the central cross-section of the tube occurs dissociate or be insensitive to any matrix components that when the centre electrode is self-biased during the course of an were co-volatilized with the analyte.Unfortunately, complete atomization transient. A depleted area of Cs I emission around decoupling of excitation and atomization processes in FAPES the centre electrode coupled with enhanced Ca II emission in cannot be realized as the high temperature of the graphite the same region reveals that ionization of analytes is most tube influences the production of thermionic electrons which pronounced in this region.With positive dc bias, concentric can alter plasma processes.8,16 Additionally, the rf power rings of enhanced emission occur between the centre electrode influences the temperature of the centre electrode which, in and the tube wall for analyte atoms and the He I plasma gas, turn, determines the extent to which it acts as a site for primary although the overall breadth of analyte emission distribution is condensation and re-desorption of analytes.5,9,17,20 Furtherdecreased.With NaCl, NaNO3 and CsCl serving as EIEs, more, the fact that quantitative analysis of ‘real’ samples conanalyte emission from Ag I, Cu I and Ca II in the region ducted with combined ‘non-thermal’ excitation sources have between the centre electrode and the tube wall is strongly thus far employed the method of standard additions12 suggests suppressed with self-bias. The degree of the suppression that matrix eects play a significant role in altering the depends on the extent of vapour cloud overlap between analyte excitation characteristics of these plasmas. and EIE.In general, equimolar amounts of NaNO3 and CsCl A frequent interference from the sample matrix reported in suppress analyte emission similarly and both produce a greater plasma emission is that resulting from the presence of Group suppression than NaCl. Equal amounts of Fe, added as an I and II elements, commonly referred to as easily ionized interfering matrix, produces a suppression of analyte emission elements (EIEs).Electrons derived from the ionization of EIEs similar to that of EIEs, suggesting that the primary cause of will alter the discharge potential in capacitively coupled suppression is the loss of energy from the plasma (as photons) plasmas, possibly producing substantial voltage drops which due to excitation and ionization of matrix vapour. Control of attenuate the energetics of the discharge, thus decreasing the dc bias enhances the radial distribution of excited analyte analyte excitation.Less common, but also possible, are atoms in the presence of EIEs and Fe, but only at low enhancements in analyte emission as the result of the EIE (2 mg) interferent loadings. acting as a ‘buer’ for analyte ionization. Falk24 has developed general equations which permit esti- Keywords: Furnace atomization plasma emission spectrometry; mation of the concentration of sample matrix that would imaging; charge-coupled device camera; easily-ionized dissipate a given fraction of the input power of the plasma via elements; dc bias excitation losses.The maximum tolerable Na concentration which would bring about a 10% loss of discharge power is Furnace atomization plasma emission spectrometry (FAPES), 0.01% for FAPES, 0.005% for low-pressure FANES sources is a combined source for spectrochemical analysis which has and 0.05% for inductively-coupled plasma (ICP) emission been the subject of growing attention.1–20 In FAPES, a 1 mm sources.24 These calculations clearly show that matrix eects diameter (typically graphite) electrode is co-axially centred can be severe for combined ‘non-thermal’ excitation sources within a graphite furnace atomizer and coupled capacitively even at relatively low matrix concentrations, with the severity to a radiofrequency (rf ) power supply.An atmospheric pressure of the interference dependent upon the degree of temporal (glow discharge) plasma is sustained within the graphite tube overlap between the analyte and interferent.As an example, and serves as an excitation medium for analytes thermally emission from Cu was suppressed 20% in the presence of 2 mg desorbed from the graphite tube surface during the conven- of NaCl in HC FANES.24 For HA-FANES, which is geometri- tional high-temperature volatilization step. One of the primary cally identical to FAPES, Riby and Harnly25 reported that motivating factors behind the development of combined analyt- significant perturbations in discharge voltage occurred in He at 400 Torr (1 Torr=133.322 Pa) and 130 mA with only 0.25 mg NaCl. Operation at elevated pressure (600 Torr) and †Present address: Ames Laboratory USDOE and Department of Chemistry, Iowa State University, Ames, Iowa 50011-3020, USA.higher current (200 mA) was found to significantly reduce Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 (709–723) 709voltage perturbations in the presence of up to 2.5 mg NaCl. plasma and to determine whether control of the dc bias changes excitation conditions suciently to ameliorate eects Smith et al.3 found that emission from Ag in FAPES was initially enhanced in the presence of NaCl, but that it decreased of EIEs. Sodium chloride, NaNO3, CsCl and Fe were selected as matrix species with Ag, Cu, Ca and Cs as test analytes as beyond 1.2 mg of the salt.The initial enhancement was ascribed to suppression of Ag ionization by NaCl (‘buering’ eect), they exhibit mutual temporal overlaps in the vapor phase. whereas the subsequent intensity decrease at higher salt masses was believed to result from a decrease in plasma excitation EXPERIMENTAL characteristics. Hettipathirana and Blades7 subsequently found Reagents that even small masses (162 ng) of NaCl and NaNO3 were sucient to alter plasma excitation characteristics and the Liquid chromatographic grade He (Air Products Canada, extent of molecular dissociation for Pb and Ag in a FAPES Nepean, ON, Canada) was used as the plasma gas.It was first source operated at 14–40W and a low heating rate of passed through a molecular sieve before entering the graphite 360 K s-1. Significant suppression was observed for Pb emis- furnace via the internal purge gas port at a flow rate of sion signals in the presence of 1–3 mg NaCl, although high 200 ml min-1.High purity Ar (99.995%; Canadian Oxygen, input powers could decrease its severity. Using a commercial Mississauga, ON, Canada) served as the external purge gas FAPES source based on an integrated contact cuvette, Gilchrist and was maintained at a flow rate of 1 l min-1. Stock solutions and Liang10 found that the emission intensity of Tl was (1000 or 10000 mg ml-1) were prepared from dissolution of suppressed by the presence of only 0.08 mg NaCl. the high-purity metals (Cu, Ag and Fe) or their salts (NaCl, Easily-ionized elements may aect analyte excitation in NaNO3 and CsCl).Working standards were prepared fresh FAPES by various means. Analyte emission intensity may be daily by dilution of the stocks with high-purity (18.3MV cm decreased by physical expulsion of analyte from the excitation/ resistivity) distilled, de-ionized water and acidified to 1% v/v observation volume as the result of: co-vaporization with the with HNO3 (Ultrex grade; J.T. Baker Canada, Toronto, EIE matrix; chemical interference due to analyte molecule ON, Canada). formation; decreased plasma power available for analyte excitation as the result of photon emission by excited EIE matrix Instrumentation species; de-tuning, or loss of plasma power coupling eciency because of changes in the load impedance of the plasma A detailed description of the imaging spectrometer, atomizer workhead, rf generator and dc bias control unit used has brought about by the EIE; and alterations in the self-bias voltage resulting from changes in electron density in the already been provided in a previous publication.20 For the present study, a Perkin-Elmer (Norwalk, CT, USA) model presence of the EIE.Enhancements in analyte emission intensity in the presence of EIEs may occur as a result of an HGA-500 graphite furnace power supply/controller was used in place of the HGA-76B power supply employed previously. increased gas density of the diusion medium as a significant amount of He is replaced by matrix vapour and shifts in The HGA-500 supply has a maximum power heating rate of ~2000°C s-1, compared with the ~1250°C s-1 achievable analyte ionization equilibrium due to ionization of matrix elements.Finally, analyte excitation may either be enhanced with the HGA-76B controller. This is advantageous in terms of analytical sensitivity. Furthermore, the HGA-500 is a micro- or decreased by an alteration of the electron energy distribution function (EEDF) due to the release of electrons from the EIE processor-driven unit, permitting acquisition of emission transients in increments of 1 s before or after the actual through ionization, or by attenuation of plasma electron energy from excitation or molecular dissociation of matrix species, commencement of the high-temperature volatilization step.Because time-resolved imaging of analyte emission transients depending on the analyte excitation energy relative to the original and altered EEDF.Imai and Sturgeon18 have shown was desired, the charge-coupled device (CCD) camera (Photometrics, Tucson, AZ, USA) used to image the central that suppression of analyte excitation in the presence of EIEs is most likely due to radiative power losses as a result of cross-section of the graphite furnace interior was operated in frame-transfer mode. The CCD chip was binned 4×4 to obtain excitation of EIE matrix species and alterations in the EEDF, rather than by analyte molecule formation, gas-phase expulsion a nominal image array size of 62×62.A 5 ms exposure time was used resulting in an acquisition rate of 50 ms per image. and de-tuning of the plasma. All prior investigations into the eect of EIEs on FANES To obtain the greater spatial resolution required for imaging He I emission from the plasma, the full resolution of the CCD and FAPES response have sampled one region of the plasma exclusively (typically immediate to the centre electrode).chip was used (i.e., 1×1 binning), whereas all other image acquisition steps remained unchanged. As a consequence of Previous investigations4,8,20 have revealed that FAPES is a highly non-homogenous discharge with regions of emission the larger image array size that is produced when the chip is not binned, the time required for the CCD system to read out that vary widely in intensity and that the spatial structure of the plasma is strongly aected by the dc bias of the centre the contents of the image array increased.Thus, for imaging of emission from excited state He I, the acquisition rate was electrode.20 Sturgeon et al.16 found that control of this dc bias decreased fluctuations in excitation temperatures during the 400 ms per image, with an exposure time of 100 ms. Optical resolution in the imaging system is achieved through course of an atomization transient. Since EIEs alter excitation temperatures in non-thermal excitation sources,25 bias control the use of interference filters.Although this results in a bandpass of 10 nm or more, the CCD allows the background may aord a more robust plasma to minimize their eects. Several studies of ICP discharges26,27,28 have revealed that to be stored and subtracted from the analytical run for accurate correction of analytical transients. The spectral transitions EIE eects are highly spatially-dependent. Since spatial eects are observed for EIEs in such a comparatively symmetric monitored and interference filters used are listed in Table 1.The wide spectral bandpass of the system means that, in the plasma, it is not unreasonable to assume that the spatial response of analytes to the introduction of EIEs would be case of Cu and Ca II, the images obtained were composites of emission from at least two spectral transitions. Imaging of Ag even more pronounced for FAPES. This study was undertaken in an eort to determine whether changes in excitation con- in the presence and absence of NaCl and NaNO3 was accomplished using a 330 nm central maximum (10 nm ditions of the plasma (induced by the dc bias of the centre electrode) lead to significant changes in the spatial distribution FWHM) interference filter to isolate the Ag I 328.07 nm resonance line.Although Na I transitions at 330.23 nm and of analyte emission, to ascertain whether the introduction of EIEs significantly alters the excitation characteristics of the 330.30 nm are passed by this filter, emission from Na I was 710 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12Table 1 Spectral features monitored and interference filters used for changes in the spatial response of the plasma and analytes detection of Ag I, Cu I, He I, Ca II and Cs I occur in the absence of matrices during atomization. In particular, the spatial distribution of excited state He I is important, Spectral Excitation Central wavelength FWHM of as this will serve as a ‘template’ of plasma excitation against transition energy/eV of interference interference which all analyte emission is convoluted.There is no practical filter/nm filter/nm way of masking the intense blackbody radiation from the Ag I 328.33 nm 3.45 330 10 heated tube walls using the imaging system. At long wave- Cu I 324.75 nm 3.82 326 10 lengths and high temperatures, blackbody emission over- Cu I 327.40 nm 3.78 326 10 whelms the He I signal, giving the illusion that the plasma has He I 388.86 nm 23.00 390 10 ‘collapsed’.Thus, imaging of He I under high temperature Ca II 393.37 nm 9.26 395 10 conditions must be performed in the ultraviolet and is restricted Ca II 396.85 nm 9.24 395 10 Cs I 852.11 nm 1.45 850 25 to the 388.86 nm line. This is unfortunate as it is spectrally isolated with the imaging system using a 10 nm FWHM, 390 nm central maximum interference filter. Studies by not detected by the CCD as a consequence of its insensitivity.Sturgeon et al.11 and Hettipathirana and Blades4 have revealed Before recording emission images from analytes in the presence that strong second positive and N2+ first negative system of interfering species, images from the interfering species alone bands are excited in the FAPES discharge in the 385–395 nm were recorded. In all cases, no emission from the interfering wavelength region. Additionally, CN violet system bands species was detected.arising from reaction of entrained air with volatilized carbon The temperature–time characteristics of the furnace were cannot be ignored. Because the Massmann furnace used in recorded following the output of a calibrated Ircon series 1100 this study operates in an ‘open’ environment, it is not possible automatic optical pyrometer (Ircon, Niles, IL, USA) focused to completely remove the contribution of the nitrogenous onto the graphite tube wall through the dosing hole.background species, making unambiguous determination of He I emission structure at high temperature impossible. Procedures Nevertheless, the spatial distribution of excited N2 species in the FAPES source has been shown to be very similar to that Images of FAPES transients were obtained as follows. of He I.4,8,20 Furthermore, although mechanisms of excitation Solutions (5–40 ml) of the analyte and/or interferent were of other gas species may be dierent, it is reasonable to assume manually pipetted into the graphite furnace using fixed-volume Eppendorf pipettes.The samples were then dried at 120°C for that all excitation mechanisms are ultimately powered by 60 s and ‘charred’ at 400°C for 60 s. Temperature readings electrons24,31 so that the spatial distribution of such species refer to those read directly from the HGA-500 power supply. should still be provided by the excited state He excitation The lengthy drying and ‘charring’ times were used to minimize ‘template’.the eect of entrained water vapour which has been shown to False-coloured images (where white represents the highest have deleterious eects on similar ‘non-thermal’ discharges.29,30 intensity and black the lowest) obtained for the He I 388.86 nm The graphite tube was then allowed to cool to room tempera- line in a dry, unloaded furnace are presented in Fig. 1. A ture, after which the plasma was ignited. The tube temperature forward rf power of 50W was applied and the centre electrode was then ramped to 400°C for 5 s (to function as a plasma was allowed to self-bias while the tube wall temperature was ‘pre-stabilization period’). Using maximum power heating con- ramped to 2600°C using maximum power heating. Under such ditions, atomization at the desired temperature was then conditions the dc self-bias exhibits its greatest change from initiated for 5 s.For Cu and Ca II, this was set to 2600°C, -10 to -125 V and it is in this ‘free-running’ mode that whereas for Ag this was set to 1700°C.Cesium and sodium FAPES sources are most commonly operated. Before pro- were atomized at both 2600°C and 1700°C to reflect the gressing to a discussion of the image analysis it must be relative temporal overlap of these elements with Cu, Ca and mentioned that each image was scaled for optimal viewing Ag analytes. In the case of an analyte in a clean matrix, the and, hence, no absolute intensity intercomparisons can be background contribution to the emission intensity was assessed drawn from the series of false-coloured images.The imaging by atomizing a blank consisting of 1% HNO3 (under the same software associated with the CCD permits no direct detailed conditions as the analyte). In the case of analyte/interferent image analysis (e.g., no spatial integration of emission intensity) combinations, the blank consisted of the interferent alone in and only the highest pixel intensity in the array is readily 1% HNO3, which was again atomized under conditions ident- displayed.As an aid to interpretation of the false-coloured ical to that of the analyte. Emission from He I and plasma images presented in Fig. 1, selected three-dimensional response background species were imaged under transient conditions surfaces of emission intensity are plotted in Fig. 2 and a plot by heating an empty tube from 400°C to 2600°C. The blank of maximum pixel intensity values versus time is given in Fig. 3. for such a system consisted of heating the empty tube in the The intensity values plotted in Fig. 3 do not necessarily absence of a plasma. In all cases, the blank series of images correspond to the same pixel in each image frame of Fig. 1, was subtracted from those of the analyte to yield background- but simply represent the maximum intensity recorded, which corrected images of analyte distributions. In most cases, rf in all cases corresponds to a position proximate to the centre forward power was 50W (as read on the port of the rf tuner), electrode.In general, the overall shape of the curve in Fig. 3 although for other experiments the rf power was varied from is similar to that obtained by the PMT-based system used by a minimum of 20W to a maximum of 100 W. Reflected powers Sturgeon et al.16 were tuned to 1W in all cases (‘room temperature’ value). The initial image in the series is typical of the room The dc bias on the centre electrode was varied from -50 to temperature He I images previously shown for the He I +40 V using a series of 12 V motorcycle batteries, as described 667.82 nm line (which is devoid of spectral interference) and previously.16,20 features the centre electrode and weak outer ring plasmas already noted.20 Thus, it may be tentatively concluded that RESULTS AND DISCUSSION this series of high-temperature images should provide a good Eect of Dc Bias on Excited State of Helium Distribution assessment of the actual excited state He I spatial distribution.During an Atomization Transient From 0.8 s to 1.6 s, over which the furnace reaches a steadystate temperature of 2600°C, He I emission intensity gradually Before obtaining images of the spatial response of analytes in the presence of EIEs, it was first necessary to determine if any increases. This arises despite a 3–4-fold lower neutral He Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 711Fig. 1 False-coloured images for plasma gas species, including He I, N2, N2+ and CN obtained using a 390 nm peak maximum, 10 nm FWHM interference filter for an empty tube ramped to 2600°C under maximum-power heating conditions in a 50 W plasma with a self-biased centre electrode. number density in the graphite tube. Furthermore, the centre electrode plasma increases in thickness, commensurate with the increased mean free path of electrons and increased sheath thickness at the lower He gas density.An additional factor accounting for this enhanced centre electrode plasma intensity is the high negative dc self-bias developed on the centre electrode. From an initial value of ~-10 V at room temperature, this reaches a maximum negative value of -125 V at 2.0 s, causing the centre electrode to behave increasingly as the cathode in the system during a given half-cycle and enhancing the intensity of the plasma around the centre electrode.20 Comparison of the image at 1.6 s relative to that at 0.0 s reveals a decreased thickness of the outer ring plasma, which eventually collapses the 90 mm ‘dark space’.This may arise as a result of liberation of thermionic electrons from the tube wall at this temperature (2600°C), since dark space thickness decreases as secondary electron production increases.32 By 2.0 s into the transient, maximum emission intensity is attained and a dramatic decrease in the thickness of the plasma around the centre electrode occurs.This is the time when the dc selfbias reverses polarity, eventually resuming its ~-10 V ‘room Fig. 2 Three-dimensional response surfaces for plasma gas species, temperature’ value near 3.2 s. The flux of thermionic electrons including He I, N2, N2+ and CN obtained using a 390 nm, 10 FWHM from the centre electrode surface is responsible for the interference filter for an empty tube ramped to 2600°C under maxi- decreased centre electrode sheath, analogous to the situation mum-power heating conditions in a 50 W plasma with a self-biased described above for the tube wall.Near the end of the transient, centre electrode. from 3.6 to 4.0 s, there is a general ‘levelling out’ of the spatial emission structure such that it begins to resemble that obtained prior to heating the tube. These factors are probably a consequence of the equalization of temperatures of the centre electrode and the tube wall, resulting in the resumption of a small and constant dc self-bias.Emission transients obtained under negative bias conditions were not significantly dierent from those arising from self-bias runs; consequently, no specific data is presented for these conditions. Figs. 4 and 5 show the false coloured images and selected three-dimensional response surfaces, respectively, for He I 388.86 nm emission intensity during an atomization transient in which the dc bias of the centre electrode is held at +40 V.A plot of maximum pixel intensity versus time is presented in Fig. 6. This graph does not correlate well with the He I line Fig. 3 Emission intensity at 390±10 nm versus time for a 50 W forward power plasma with a self-biased centre electrode. intensities at +38 V bias obtained by Sturgeon et al.16 In the 712 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12Fig. 4 False-coloured images for plasma gas species, including He I, N2, N2+ and CN obtained using a 390 nm peak maximum, 10 nm FWHM interference filter for an empty tube ramped to 2600°C under maximum-power heating conditions in a 50 W plasma with a +40 V dc bias imposed on the centre electrode.latter study, intensities for a number of He lines first increased to 2 s and then exhibited a precipitous drop, possibly as a result of the formation of a mini-arc which resulted in a loss of the plasma image on the entrance slit of the monochromator. 16 In the present study, He I intensity progressively increases to 3.2 s, after which a smooth decline occurs.The overall He I intensity is lower than that for the self-bias case, consistent with the intensity of the centre electrode plasma decreasing with increasing positive bias.20 In terms of spatial features, the most striking characteristic is that of expanded radial emission occurring at 0.4 s, in agreement with earlier observations.20 ‘Ring-like’ structures arise which persist, in general form, throughout the heating transient. The origin of these structures is, presently, unknown and they are detected only when a 0 V or positive bias is imposed on the centre electrode.Their intensity increases with increasing positive bias whereas their diameter correspondingly decreases. They may simply be the result of the positive dc bias repelling He+ into a region intermediate between the electrode and the wall where they are ‘held’ as a consequence of the reversal of the rf field.The Fig. 5 Three-dimensional response surfaces for plasma gas species, formation of a ring of alternating positive and negative space including He I, N2, N2+ and CN obtained using a 390 nm, 10 FWHM charge in the annular space of concentric cylindrical corona interference filter for an empty tube ramped to 2600°C under maxi- discharges under an ac field has also been noted.33 Additionally, mum-power heating conditions in a 50 W plasma with a +40 V dc it may be that the positive bias of the centre electrode induces bias imposed on the centre electrode. a resonance interaction between the electrons in the plasma and the magnetic field of the rf antenna (centre electrode), which propagates circularly around the circumference of the graphite tube,34 thus creating an environment for the formation of ring-like regions of emission.Other salient features of the images merit discussion. At a dc bias of +40 V, the outer ring plasma merges towards the wall, producing an intense wall glow, as noted previously.20 From 0.8–1.6 s the tube wall is ramped to its steady-state temperature.Beyond 2 s the center electrode has radiationally heated suciently to commence significant thermionic emission from its surface. The plasma near the center electrode increases in intensity (Fig. 6) and breadth (Figs. 4 and 5) in response to the enhanced electron density. Beyond approximately 3 s a decrease in plasma intensity occurs, likely as a consequence of Fig. 6 Emission intensity obtained at 390±10 nm versus time for a enhanced reflected power losses.16 50 W forward power plasma with a +40 V bias imposed on the centre electrode. Several general conclusions can be drawn from Figs. 1 to 6. Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 713At no time does the plasma collapse as a consequence of Fig. 9 shows the atomization sequence for 10 ng of CsCl at 2600°C with a forward rf power of 50 W and a self-biased injected thermoelectrons, even for prolonged heating at 2600°C.This is also supported by monochromator-PMT- centre electrode. Emission from Cs I commences from the tube wall at approximately 0.05 s and gradually fills the internal based measurements of He I line intensities and excitation temperatures during atomization transients.16 Recently, tube volume relatively uniformly, with the exception of the area proximate to the centre electrode.A dark gap remains in LeBlanc and Blades8 reported that the plasma in their FAPES source extinguished at tube wall temperatures in excess of this region throughout the duration of the atomization transient. This gap is probably the consequence of ionization of Cs 1330°C and suggested that this arose as a result of thermionic electron emission changing the impedance of the source beyond within the intense centre electrode plasma. The apparent ‘intensity’ at the tip of the centre electrode is an artifact of the range of, or faster than the response time of, their matching network.Since the rf generator and tuner used by these authors thermal and plasma-induced heating of the centre electrode, which becomes so intense at ~850 nm that complete back- is identical to that used in this study, it is evident that the plasma quenching described by LeBlanc and Blades8 is unique ground correction is problematic. The ‘rings’ of excitation observed for He I in Fig. 3 and Cu I in Fig. 8 are absent in to their system, rather than being emblematic of a general limitation of FAPES. Possibly, the commercial graphite furnace this self-biased system. As the forward rf power to the plasma is increased to 100 W the depletion region around the centre recently employed by these authors,8 with its very high maximum power heating rate10 of 3500 K s-1, is part of the electrode grows, consistent with plasma-induced ionization of Cs in the (expanded) centre electrode plasma.This plasma- problem. The data additionally reveal that no emission ‘plume’ from induced ionization process was verified in a subsequent experiment in which 100 mg CsCl was atomized and excited in the the center electrode to the sample injection hole is present, despite the increased flux of He through it during the 0–1.6 s absence of a plasma at 2600°C (the higher analyte mass being required to obtain an emission signal in the absence of a period when rapid heating of the graphite tube causes expansion of the internal gas out of the injection hole.The absence plasma). Fig. 10 clearly shows that excited-state Cs fills the tube volume reasonably uniformly before dissipating. of a ‘plume’-like structure also suggests that the contribution of N2 and CN background species to the image is small, as an Furthermore, the entire event is delayed more than 1 s compared to that in the presence of a plasma, indicating the key emission plume has previously been observed from one or both of these sources when the tube wall was heated.20 Perhaps role the plasma plays in excitation and ionization of Cs.Only at very weak plasma power (20W) could a condition be most importantly, the images presented in Figs. 1 and 4 suggest that a relatively small change in the positive dc bias can realised in which the ‘depletion zone’ due to ionization around the centre electrode was eliminated, owing to the decreased produce very significant changes in the He emission ‘template’, with the formation of an annulus or ‘rings’ of enhanced production of Cs+ in this region.Similar distributions, in terms of ionization depletion and power dependencies, were excitation around the centre electrode plasma. Recent experiments have revealed that a helium FAPES plasma operated at also observed for Na (whether injected as the chloride or nitrate salt). The ‘depletion zone’ detected for Na was smaller 40.08 MHz is sustained with a peak-to-peak rf voltage of only ~75 V.35 It is not unreasonable to assume that an rf voltage in diameter than that for Cs, as a consequence of its higher ionization potential. of similar magnitude exists in the 13.56 MHz system used in this work. This is particularly significant since the measured rf Experiments were also undertaken to determine whether bias control had any eect on the ionization ‘depletion zone’ voltage is more than an order of magnitude less than that previously assumed to exist in FAPES.4 These small rf voltages around the centre electrode.Fig. 11 shows the results obtained for 10 ng of CsCl atomized as before into a 50 W rf power suggest that the relatively small dc bias potentials applied to the centre electrode may influence the voltage gradients present plasma with a +25 V bias on the centre electrode. Fig. 12 displays the results obtained for a -47 V bias. At +25 V bias in the source and the resulting emission patterns.The implications of changes in the centre electrode dc bias on the the emission rings again form (this is particularly evident when images obtained at 0.15 and 0.20 s in Fig. 11 are compared atomization of analyte transients will now be discussed. with those at the same time intervals as in Fig. 9) and the size of the ‘depletion zone’ is enhanced relative to the self-bias run Eect of Dc Bias on the Spatial Distribution of Excited-state shown in Fig. 9.For the -47 V bias situation the results are Atoms during an Atomization Transient reversed and Cs I emission encroaches much more closely around the centre electrode than for a self-bias run at the same Figs. 7 and 8 show the data obtained for atomization of 200 ng of Cu at 2600°C with a forward rf power of 50 W. The centre rf input power (although not as closely as the results obtained for 20W or in the absence of a plasma). Initially, this would electrode was allowed to self-bias (Fig. 7) or was maintained at +25 V (Fig. 8). In the self-bias transient, significant Cu I appear to be counter-intuitive, since the results presented earlier20 show that the intensity of the centre electrode plasma emission begins to appear at 0.45 s and generally fills the tube volume from 0.75 to 1.25 s before dissipating beyond 1.35 s. decreases with positive bias, implying that positive bias should be more favourable for decreasing the extent of Cs ionization. The ‘crescent’ at the left side of images from 0.75–1.15 s is a (recent) artifact of the CCD chip which becomes most evident However, positive bias also has the eect of enhancing both the radial emission intensity of the plasma and creating rela- when the emission intensity recorded by the chip is high.Positive bias produces enhanced Cu emission directly around tively intense annular structures surrounding the centre electrode. Since the ‘emission event’ for Cs ends at ~0.4 s, the bias the centre electrode, which is particularly evident at 0.85 s and beyond.Bias control (primarily 0 V bias) increases the eective potential at the centre electrode never exceeds ~-20 V for the images obtained in a self-biased run.16 Thus, a more likely length of the atomization event, possibly by providing a more consistent excitation environment. This was also observed for explanation for the dependence of the Cs I emission distribution on the dc bias can be obtained by realizing that a Fe and Pt by Sturgeon et al.16 Silver exhibits similar overall excitation characteristics to that for Cu, with the exception positive bias extends the radial excitation capacity of the plasma such that significant ionization of Cs occurs further that its distribution more uniformly extends to the walls, consistent with the higher anity of Cu for graphite surfaces.36 away from the centre electrode (i.e., proximate to the ‘excitation annulus’ developed at positive bias). A negative bias of -47 V, For comparison with the atomization characteristics of transition elements, the atomization of an easily ionized on the other hand, is more negative than the ‘indigenous’ value obtained in a self-bias run and has the eect of decreasing the element, Cs, was examined as bias and power were varied. 714 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12Fig. 7 False-coloured images for 200 ng Cu atomized from the tube wall for a forward rf power 50 W, with the centre electrode allowed to self-bias. Fig. 8 False-coloured images for 200 ng Cu atomized from the tube wall for a forward rf power 50 W, with +25 V dc bias imposed on the centre electrode. intensity of the radial emission region near the centre electrode conductive sample deposit is observed at 1.00 s and Ca II emission begins at 1.30 s, progressing radially around the plasma. Thus, despite an increase in the centre plasma intensity, a decrease in the radial excitation capability of the plasma centre electrode from 1.55 s onward.Intense Ca II emission occurs around the centre electrode between 1.6 and 2.0 s. Thus, permits a closer encroachment of Cs I to the centre electrode. This eect may have more practical uses in future applications Ca II emission is concentrated primarily around the centre electrode, providing a ‘mirror image’ to the depletion in of FAPES (e.g., as an ion source for mass spectrometry). Complementary information regarding ionization in FAPES emission noted for Cs in this region.This relatively localized Ca II emission pattern reflects the high excitation energy of was obtained by directly imaging an excited-state ion population. Calcium was selected for study and emission from the the Ca II emission lines and is consistent with that obtained for other high-lying transitions (e.g., CO+ and N2+).20 Biasing Ca II 393.37 nm and Ca II 396.85 nm lines was isolated using a 10 nm FWHM, 395 nm peak maximum interference filter the centre electrode to 0 V or positive voltage results in more enhanced wall emission, in accordance with an expansion of (see Table 1).Fig. 13 displays the results obtained for atomization of 100 ng Ca at 2600°C into a 50 W plasma with a self- the plasma (0 V bias) and the formation of a wall plasma at positive biases. biased centre electrode. Initial arcing of the plasma to the Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 715Fig. 9 False-coloured images for 10 ng CsCl atomized from the tube wall for a forward rf power 50 W, with the centre electrode allowed to self-bias. Fig. 10 False-coloured images for 100 mg CsCl atomized from the tube wall at 2600°C in the absence of a plasma. Eect of Easily-ionized Elements on the Spatial Distribution of the same pattern of excitation suppression as Cu, although the duration of the eect is longer. This pattern of decreased Excited-state Analyte Atoms During an Atomization Transient excitation suggests that it is a consequence of reduced plasma electron energy, or a lowered electron energy distribution The eect of 10 mg NaCl on the emission from Cu atomized at 2600°C into a 50W plasma with a self-biased electrode is function arising from excitation and/or ionization of Na.Beyond 0.90 s greater radial Cu emission is evident, approxi- presented in Fig. 14. In contrast to the Cu emission transient obtained in the absence of NaCl (Fig. 7), there is significant mating the distribution found in the absence of NaCl. Dissipation begins at 1.30 s. This implies that the most signifi- suppression of Cu excitation in the annular region between the centre electrode and the graphite tube wall. This is particu- cant vapour cloud overlap between Cu and Na occurs between 0.40 and 0.80 s, which was confirmed by direct CCD imaging larly evident from 0.45–0.90 s. Indeed, at these times Cu I emission is generally confined to a region around the centre of emission from the Na I 589.00 nm and Na I 589.59 nm lines (figures not shown).In any graphite furnace source the degree electrode and the tube wall. This is not surprising, considering that these regions contain the centre electrode plasma and the of vapour cloud overlap between interferent and analyte will determine the extent of the interference eect. As a consequence, outer electrode plasma, which are the most intense regions of excitation.Silver, atomized in the presence of EIEs, produces one would expect analytes of volatility lower than that of Cu 716 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12Fig. 11 False-coloured images for 10 ng CsCl atomized from the tube wall for a forward rf power 50 W, with a +25 V dc bias imposed on the centre electrode. Fig. 12 False-coloured images for 10 ng CsCl atomized from the tube wall for a forward rf power 50 W, with a -47 V dc bias imposed on the centre electrode.to exhibit a decreased interference eect, whereas more volatile complete suppression occurring at 20 mg NaCl (compared with 100 mg NaCl in the HGA-500 system). This is the consequence analytes would be more severely aected by the presence of NaCl. For Cu, the above ‘quenching’ of analyte emission begins of the lower maximum power heating rate of the HGA-76B supply relative to the HGA-500 power supply (~1250°C s-1 in the presence of 2.5 mg NaCl. This is in accordance with the results of Falk24 who calculated that a concentration of 0.01% versus ~2000°C s-1) and is consistent with the work of Hettipathirana and Blades7 who observed significant suppres- NaCl (equivalent to 2 mg in a 20 ml aliquot) would be sucient to bring about a 10% power loss in FAPES.With 100 mg NaCl, sion of emission for Pb at NaCl masses of only 162 ng with a heating rate of 90°C s-1. In contrast, Imai and Sturgeon18 only weak Cu emission around the centre electrode is evident.Previous Cu atomization experiments conducted with an using a nominal 1600°C s-1 heating rate and an identical ICC FAPES source, observed a 50% enhancement in Pb emission HGA-76B power supply revealed that the interference eect manifested itself at much lower NaCl concentrations, with signals in the presence of 500 ng of NaCl, which they ascribed to the early volatilization and subsequent plasma-induced substantial suppression occurring at 5 mg NaCl and essentially Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 717Fig. 13 False-coloured images for 100 ng Ca atomized from the tube wall for a forward rf power 50 W, with the centre electrode allowed to self-bias. Fig. 14 False-coloured images for 200 ng Cu atomized from the tube wall in the presence of 10 mg NaCl for a forward rf power 50 W, with the centre electrode allowed to self-bias. dissociation of PbCl. Suppression of the Pb emission signal arc. Formation of the arc must therefore be related to ionization and changes in plasma conductivity.It is interesting that the only manifested itself at NaCl masses of 10 mg or more. These data serve to point out the critical importance of the heating arc occurs exclusively in the direction of the dosing hole and not to other points along the outer electrode (tube wall) rate of the furnace in aording a ‘plug’ of atomic vapour to the plasma, readily available for excitation. surface.This may be related to the convective flow of He gas inside the tube, or it may simply be an edge eect due to An additional salient feature of Fig. 14, which is common to all the EIE matrices examined, is the development of an voltage fields near the sample injection hole. No arc occurs when the dosing hole is plugged with a graphite rod during emission ‘arc’ from 0.50–0.80 s between the sample injection hole and centre electrode. This arc forms readily when analytes atomization. Note that because the graphite plug protruded into the graphite tube somewhat, it is conceivable that an are atomized in the presence of an EIE matrix or when suciently large (e.g. 0.2 mg) amounts of EIEs themselves are ‘edge eect’ may still arise due to perturbations of voltage fields around it. Similarly, when Ca is atomized and emission atomized. It is not significant for atomization of other elements or in non-EIE matrices. For example, atomization of Cu in from the ionic state is monitored, no arc forms to the dosing hole.Ionic calcium emission commences at 1.30 s (Fig. 13), the presence of only 5 mg NaCl was sucient to produce the arc, but atomization in the presence of 40 mg Fe yielded no after the furnace has reached a steady-state temperature and 718 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12rapid expulsion of He out of the dosing hole has ceased. The prior to the appearance of Cu at ~0.45 s. This is not observed lack of arc formation under these experimental conditions, in when an equimolar mass of NaNO3 is used, suggestive of early addition to when the dosing hole is plugged, suggest that it is transfer of molecular NaCl to the centre electrode, which then related to the flow of He and its rapid expulsion from the enables the arc to form more readily.As well, images obtained dosing hole as the furnace reaches a steady-state temperature for equimolar amounts of NaCl and NaNO3 at high charring (at ~1 s under maximum power atomization conditions).temperatures are more intense for the latter salt, suggesting Imaging of Na transients has shown that, although arc forma- more ecient removal of NaCl at these charring temperatures. tion accompanies it as well, it has ceased by the time of Cu On the other hand, NaNO3 is reduced to metallic sodium on arc formation and Na is simply present within the volume of the graphite surface. Hence, the extent of EIE eects for Na the graphite tube (cf.Cs, Fig. 9, at 0.15 s). Thus, Cu is not (and for other EIEs as well) may depend upon the chemical co-vaporized in an arc along with the Na matrix. Arcing to form in which the EIE is present as well as its thermochemical the sample injection hole for Cu is a consequence of the properties. If the EIE is present as a less volatile salt, it will presence of EIE vapour which forms a more conductive path be more eciently retained within the graphite tube, producing from the centre electrode to the tube wall.Imaging these a more pronounced interference eect. The beneficial eects of analyte–interferent systems in an enclosed integrated contact thermal pretreatment (charring) for removal of volatile forms cuvette (ICC) furnace without a flow of gas would prove useful of EIEs will be more restricted in FAPES than graphite furnace in further elucidating the reason(s) underlying arc formation AAS as a consequence of the cooler centre electrode acting as in EIE matrices.an ecient condensation site for matrix vapour. The eect of 7.3 mg NaNO3 (equivalent to 5 mg NaCl) on In the presence of 14.4 mg CsCl (equivalent to 5 mg NaCl), the emission from 200 ng Cu atomized at 2600°C into a 50W Ca ion emission for 100 ng of Ca atomized at 2600°C into a plasma with a self-biased electrode is shown in Fig. 15. Despite 50 W self-bias plasma is suppressed everywhere except around the lower mass of EIE, suppression of Cu emission is much the centre electrode (Fig. not shown).The eect of CsCl on more extensive for NaNO3 than for NaCl. This enhanced Ca II emission is similar to that obtained for NaCl, but more suppression from NaNO3 was a general trend observed for all pronounced due to its lower ionization potential. The suppress- analytes examined (Ag, Cu, Ca). Indeed, for all images through- ive eects of equimolar amounts of CsCl and NaNO3 are, out the Cu transient, emission is confined to a small annulus however, comparable, suggesting that ionization of Cs does around the centre electrode with only very weak emission near not contribute as significantly to the loss of excitation capa- the tube walls and radial emission being almost negligible.The bility as photons radiated during the excitation of Cs and Na. reason for this may be related to dierences in the thermal In addition, attenuation of plasma electron energy from exci- properties of NaCl and NaNO3.Hettipathirana and Blades7 tation and dissociation of molecular matrix species may occur. measured Na emission arising from the vaporization of NaCl In general, NaCl, NaNO3 and CsCl produce the same overall and NaNO3, and concluded that NaCl vaporized earlier from pattern of suppression of excitation for all analytes studied the tube wall, consistent with a mechanism for formation of (Cu, Ag and Ca). The extent of the suppression is dependent Na on the graphite surface by carbon reduction of Na2O.37 upon the degree of vapour cloud overlap between analyte and When NaNO3 is used: EIE.Cesium should, however, have a greater vapour cloud overlap with analytes at higher temperatures due to the greater 4 NaNO3(s)�2 Na2O(s)+4 NO2(g)+O2(g) involatility of Cs vis a ` vis Na. It should be emphasized that Na2O(s)+C(s)�2 Na(s)+CO(g) the CCD imaging software does not permit calculation of integrated emission intensities over any region of the image, When a charring temperature of 1300°C is employed for the so no conclusion concerning the eect of EIEs on spatially vaporization of Cu in the presence of NaCl, an early arc (at ~0.10 s) arises from the centre electrode to the dosing hole integrated signals can be made.However, it is reasonable to Fig. 15 False-coloured images for 200 ng Cu atomized from the tube wall in the presence of 7.3 mg NaNO3 (equivalent to 5 mg NaCl) for a forward rf power 50 W, with the centre electrode allowed to self-bias.Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 719assume that the suppressed ‘emission patterns’ observed are imposed on the centre electrode. Clear in these images (and also strongly evident for a 0 V bias on the centre electrode) is reasonable representations of results that would be obtained in monochromator/PMT-based detection systems, from the the expansion of the centre electrode plasma, as was observed for He and analytes with no matrices.Emission in this region simple consideration that the analysis volume imaged in such systems consists of a thin slice of the plasma near the centre is enhanced relative to the self-bias case and the arcing of Ag to the sample injection hole, lasting up to 1.0 s under self-bias, electrode from the top to the bottom of the graphite tube. Any suppression in radial emission in that slice would thus be is significantly reduced for the zero or positive bias condition.Applied negative biases produce very pronounced arcs to the manifest as a loss of integrated emission detected by the PMT. The fact that suppressions in integrated emission signals have dosing hole throughout the atomization transient. The lack of arc formation at 0 V or slightly positive biases (i.e., 12–25V) been observed in PMT-based detection systems7,18 for similar analytes and the same masses and types of EIEs used in this compared to self-bias or large negative bias may derive from a reduced potential dierence between the centre electrode and study supports this.the outer tube wall during a given half-cycle under these conditions. At 0 V dc bias, the centre electrode and tube wall Eect of Dc Bias on the Spatial Distribution of Excited-state serve as cathode and anode for equal periods of time during Analyte Atoms in the Presence of Easily-ionized Elements a given rf half-cycle, while for a +12 to+25 V bias, the tube wall is the anode for only a slightly greater period of time Dc bias has been shown to significantly influence the spatial than the centre electrode during a given rf half cycle.Because distribution of analyte emission in clean matrices and its an arc forms most readily when this potential dierence is control aords robustness of excitation temperatures during accentuated (i.e., in the self-bias or large negative bias case), an atomization transient.16 Thus, the influence of dc bias in its formation may be somewhat attenuated under 0 V or small controlling the eects of the ‘excitation suppression pattern’ positive bias.Copper and Ca exhibit similar eects. observed for EIEs in self-bias systems was investigated. Unfortunately, study of this radial enhancement was restricted Fig. 16 shows the images obtained during atomization of to low interferent loadings as application of a positive bias 50 ng Ag at 1700°C into a 50W self-bias plasma, whereas with interferent masses in excess of 0.5 mg NaCl for Ag or Fig. 17 shows those for the atomization of the same mass of 2.5 mg for Cu and Ca produced a suciently conductive sample Ag with 125 ng NaCl under the same operating conditions. deposit to encourage arcing of the plasma to the site of sample Clearly, extensive suppression of Ag emission throughout the deposition. course of the transient occurs at even these low interferent loadings and despite a larger eective plasma power (reflected power is 1–3 W at 1700°C compared to 33–35 W at 2600°C). Eect of Dc Bias on the Spatial Distribution of Excited-state The primary reason for this is that, unlike Cu, for which Na Analyte Atoms in the Presence of an Iron Matrix vapour overlap occurred for ~0.3 s into the 1.0 s transient, CCD imaging of Na I reveals that it persists throughout the The similarity in suppression eects observed for CsCl and NaNO3 implies that attenuation of plasma excitation power entire Ag transient at 1700°C.Early (0.10–0.20 s) emission of Ag is completely absent, arcing is prevalent and, unlike Ag is primarily a consequence of loss of photons from the excitation of the EIE. To support this, Fe was used as an interferent without a matrix, a complete cross-sectional filling of the graphite tube with emission from the Ag I 328.07 nm line is since excitation of its large number of available spectral transitions may bring about a similar suppression in analyte never realized.As with the other analytes and, for reasons already discussed, the magnitude of this eect increases when intensity as a result of photon losses.24 The use of Fe is also fortuitous in that its atomic mass is close to the molecular equimolar amounts of NaNO3 or CsCl are present. Fig. 18 shows the results obtained for the same interferent mass of NaCl, thus permitting ready comparison of these two interferents at approximately the same mass loadings. and operating conditions as above but with a +25 V bias Fig. 16 False-coloured images for 50 ng Ag atomized from the tube wall for a forward rf power 50 W, with the centre electrode allowed to self-bias. 720 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12Fig. 17 False-coloured images for 50 ng Ag atomized from the tube wall in the presence of 0.125 mg NaCl for a forward rf power 50 W, with the centre electrode allowed to self-bias. Fig. 18 False-coloured images for 50 ng Ag atomized from the tube wall in the presence of 0.125 mg NaCl for a forward rf power 50 W, with +25 V dc bias imposed on the centre electrode.Fig. 19 shows the images obtained during the atomization being expelled from the dosing hole during the first 1.6 s in both cases. This further implicates the increased conductivity of 50 ng Ag in the presence of 20 mg Fe at 2600°C into a 50W self-bias plasma. Unlike Ag vaporizing with no matrix, for imparted to the plasma by the sample matrix (i.e., EIE) in being responsible for the formation of the arc to the sample which Ag I emission commences at 0.10 s, the onset of Ag I is delayed until 0.35 s in the presence of 20 mg Fe, presumably injection hole.A larger mass of Fe than NaCl is required to produce an equivalent matrix eect. For example, whereas because Ag is embedded in the less volatile Fe matrix. More significantly, however, the radial distribution of Ag is altered only 0.125 mg NaCl produces substantial suppression of radial emission from Ag I, significant suppressions by Fe only begin throughout the entire transient such that the only regions of high Ag intensity are those around the centre electrode and at interferent loadings of 5 mg.Suppression eects for Cu and ionic Ca in the presence of Fe are generally similar to the the tube wall. This situation is similar to the radial ‘excitation suppression’ already noted for Ag in the presence of those observed for Ag, except that a larger mass of Fe (20 mg) is required to produce the onset of quenching of the radial 0.125 mg NaCl, with the exception that there is essentially no arc formation in the presence of Fe, despite the fact that He is distribution.Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 721Fig. 19 False-coloured images for 50 ng Ag atomized from the tube wall in the presence of 20 mg Fe for a forward rf pow0 W, with the centre electrode allowed to self-bias.Fig. 20 illustrates the eect of application of a +22 V bias their influence was not addressed in studies involving interfering matrices. Biasing eects for ionic Ca and Cu were similar on the centre electrode during the atomization of Ag in the presence of the 20 mg Fe. Greater radial emission is evident in to those observed for Ag. The general trend of suppression of excitation by Fe for all analytes studied and the similarity of all images as the development of the radial ‘excitation annulus’ occurs.The general emission pattern is analogous to that the spatial response of dierent analytes in the presence of this interfering element to the dc bias suggest that the major obtained for similar bias conditions for Ag and 0.125 mg NaCl, with the exception that formation of excited-state Ag atoms is mechanism underlying the suppressive eects of EIEs on analyte distributions is associated with the loss of photons due delayed due to the Fe matrix and the radial excitation pattern is wider in the case of Fe.As with NaCl (or NaNO3 or CsCl), to the excitation of the matrix (Fe or EIE) and the less energetic plasma which ensues. Although ionization eects negative biases were unusable as these resulted in significant arcing to the site of sample deposition. Application of a positive likely play some role in analyte suppression, it is probable that this mechanism does not cause a significant energy loss in the bias up to +38 V decreased the volume of the excitation annulus somewhat (as with the EIEs) and, as positive biases plasma, since equimolar amounts of Cs (as CsCl) and Na (as NaNO3) produce similar quenching eects.in excess of 38 to 40 V tend to cause arcing in clean systems, Fig. 20 False-coloured images for 50 ng Ag atomized from the tube wall in the presence of 20 mg Fe for a forward rf power 50 W, with +22 V dc bias imposed on the centre electrode. 722 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 1212 Sturgeon, R. E., Willie, S. N., Luong, V. T., and Berman, S. S., CONCLUSIONS J. Anal. At. Spectrom., 1990, 5, 635. The dc bias of the centre electrode plays a significant role in 13 Sturgeon, R. E., Willie, S. N., Luong, V. T., and Berman, S. S., J. Anal. At. Spectrom., 1991, 6, 19. determining the spatial distribution of analyte emission in the 14 Sturgeon, R. E., Willie, S. N., Luong, V. T., and Dunn, J. G., FAPES plasma for both clean samples and when interfering Appl.Spectrosc., 1991, 45, 1413. matrices are present. As a relatively small voltage (~75 V 15 Sturgeon, R. E., and Willie, S. N., J. Anal. At. Spectrom., 1992, peak-to-peak) sustains the FAPES plasma,35 application of a 7, 339. relatively small dc bias voltage to the centre electrode signifi- 16 Sturgeon, R. E., Luong, V. T., Willie, S. N., and Marcus, R. K., Spectrochim. Acta, Part B, 1993, 48, 893. cantly alters the voltage gradients present in the source and, 17 Imai, S., and Sturgeon, R.E., J. Anal. At. Spectrom., 1994, 9, 493. by extension, the spatial distribution of analyte emission. The 18 Imai, S., and Sturgeon, R. E., J. Anal. At. Spectrom., 1994, 9, 765. utility of bias control is limited to low interferent loadings 19 Imai, S., Sturgeon, R. E., and Willie, S. N., J. Anal. At. Spectrom., (2 mg, generally), otherwise arcs and instabilities begin to 1994, 9, 759. manifest themselves. It should be stressed, however, that the 20 Pavski, V., Chakrabarti, C. L., and Sturgeon, R. E., J. Anal. At. Spectrom., 1994, 9, 1399. current CCD imaging system did not permit emission intensit- 21 Falk, H., Homann, E., and Ludke, Ch., Prog. Anal. Spectrosc., ies to be spatially integrated over any given cross-section of 1988, 11, 417. the tube. As such further investigation will be required (e.g., 22 Ballou, N. E., Styris, D. L., and Harnly, J. M., J. Anal. At. with conventional PMT detection) to ascertain the full extent Spectrom., 1988, 3, 1141. of some of the suppressions and enhancements observed. 23 Harnly, J. M., Styris, D. L., and Ballou, N. E., J. Anal. At. Spectrom., 1990, 5, 139. 24 Falk, H., J. Anal. At. Spectrom., 1991, 6, 631. 25 Riby, P. G., and Harnly, J. M., J. Anal. At. Spectrom., 1993, 8, 945. REFERENCES 26 Tripcovic�, M. R., and Holclajtner-Antunovic�, I. D., J. Anal. At. Spectrom., 1993, 8, 349. 1 Liang, D. C., and Blades, M. W., Spectrochim. Acta, Part B, 1989, 27 Galley, P. J., Glick, M., and Hieftje, G. M., Spectrochim. Acta, 45, 1059. Part B, 1993, 48, 769. 2 Sturgeon, R. E., Willie, S. N., Luong, V. T., Berman, S. S., and 28 Kitagawa, K., and Horlick, G., J. Anal. At. Spectrom., 1992, 7, 1207. Dunn, J. G., J. Anal. At. Spectrom., 1989, 4, 669. 29 Larkins, P. L., Spectrochim. Acta, Part B, 1991, 46, 291. 3 Smith, D. L., Liang, D. C., and Blades, M. W., Spectrochim. Acta, 30 Ratli, P. H., and Harrison, W. W., Spectrochim. Acta, Part B, Part B, 1990, 45, 493. 1994, 49, 1747. 4 Hettipathirana, T. D., and Blades, M. W., Spectrochim. Acta, Part 31 Hieftje, G. M., Spectrochim. Acta, Part B, 1992, 47, 3. B, 1992, 47, 493. 32 Von Engel, A., Ionized Gases, Oxford University Press, London, 5 Hettipathirana, T. D., and Blades, M. W., J. Anal. At. Spectrom., 2nd edn., 1965. 1992, 7, 1039. 33 Cobine, J. D., Gaseous Conductors: T heory and Engineering 6 Banks, P. R., Liang, D. C., and Blades, M. W., Spectroscopy, Applications, Dover Publications, Inc., New York, 1958. 1992, 7, 36. 34 Gilmour, Jr., A. S., Microwave T ubes, Artech House, Inc., 7 Hettipathirana, T. D., and Blades, M. W., J. Anal. At. Spectrom., Dedham, MA, 1986. 1993, 8, 955. 35 Sturgeon, R. E., unpublished data. 8 LeBlanc, C. W., and Blades, M. W., Spectrochim. Acta, Part B, 36 McNally, J., and Holcombe, J. A., Anal. Chem., 1987, 59, 1105. 1995, 50, 1395. 37 Campbell, W. C., and Ottaway, J. M., T alanta, 1974, 21, 837. 9 Gilchrist, G. F. R., Celliers, P. M., Yang, H., Yu, C., and Liang, D. C., J. Anal. At. Spectrom., 1993, 8, 809. Paper 7/00843K 10 Gilchrist, G. F. R., and Liang, D. C., Am. L ab., 1993, 25, 34U. Received February 5, 1997 11 Sturgeon, R. E., Willie, S. N., Luong, V. T., and Berman, S. S., Anal. Chem., 1990, 62, 2370. Accepted April 15, 1997 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12