Studies of Analyte Particle Transport in a Particle Beam–Hollow Cathode Atomic Emission Spectrometry System JIANZHANG YOU, MELISSA A. DEMPSTER AND R. KENNETH MARCUS* Department of Chemistry, Howard L . Hunter L aboratory, Clemson University, Clemson, SC 29634-1905, USA The particle beam–hollow cathode glow discharge atomic on the GD plasma operation and energetics. The most direct emission spectrometry system appears to be a viable analytical way to accomplish this task is the application of an aliquot method for volume-limited liquid samples. Systematic (1–100 ml ) of the sample onto a target cathode ex situ, followed enhancements and reductions in instrumental response were by a solvent evaporation step prior to introduction into the related to the analyte transport eciency through the particle plasma source/cell.3,4,6,7 beam interface.The eects of liquid flow rate and analyte Because of the increasing need for chemical speciation input concentration on analyte transport eciency were information in both environmental and biological analyses, evaluated in an attempt to explain these eects on the the use of chromatographic separation techniques has seen analytical signal.Two specifically designed collectors were phenomenal growth over the last decade. While direct coupling mounted within the glow discharge source for sampling analyte of liquid chromatography (LC) with flame atomic absorption particles passed through the particle beam interface.Atomic and inductively coupled plasma (ICP) sources is relatively absorption spectrometry and scanning electron microscopy straightforward, LC solvents (and buers) can strongly aect were applied to determine analyte transport eciencies and the nebulization and spectral characteristics of these particle size distributions, respectively. In general, transport devices.13–15 In addition, many of these sorts of separations eciencies of 4–18% were achieved.Improvements in analyte present the need to detect elements that are not readily response, noted in previous studies, on addition to the sample determined in atmospheric pressure sources, including nitroof concentrated HCl (1+5, v/v) can now be attributed to gen, oxygen and halides. The low pressure GD environment enhanced analyte transport through the particle beam interface presents the possibilities to look at these ‘atmospheric’ by virtue of an increase in the size of the desolvated analyte elements.Clearly, the use of a deposition-type approach to particles. The size distribution of analyte particles appears to GD detection of LC eluents is not practical. To be most change with the distance from the center to the edge area of pragmatic, one would look to choose an interface that possesses the sample collector as most particles follow a straight the capability of operating with a variety of solvent polarities, pathway to the hollow cathode, with very little evidence of over a wide range of liquid flow rates and reasonable analyte dispersion.Typical particle sizes lie in the range 2–8 mm. transport eciency without the deleterious eects of solvent carry-over to the plasma region. In fact, some approaches have Keywords: Particle beam; hollow cathode atomic emission; looked to capitalize on methods developed for liquid chromato- transport eciency; particle size graphy–mass spectrometry (LC–MS) to introduce liquid samples into the GD.Among these have been the use of The glow discharge (GD) has a long history of application in moving belt5,9 and particle beam (PB) interfaces.10–12 the area of metal and alloy analyses.1 The application of GD The use of a PB interface for LC–MS, generally known as techniques in atomic and mass spectrometries has been the MAGIC LC–MS interface, was first developed by expanded to cover the range of direct solids elemental analysis Willoughby and Browner.16 The development and function of of both conducting and non-conducting materials by use of the PB interface for LC–MS meets many important criteria radiofrequency (rf ) powered devices.2 In all of these applifor low pressure sample presentation to an ionization volume, cations, the combination of cathodic sputtering (atomization) including: (1) highly ecient removal of solvent, (2) mechanical and electron and metastable species collisions in the gas phase simplicity and ease of operation, (3) analysis capabilities for a (excitation and ionization) present a very ecient means of wide range of sample/solvent volatility, thermal lability and direct solids analysis. The possibility of developing a GD polarity, (4) choice of detection (ionization) techniques and device as an analytical tool for both liquid and solid sample (5) preservation of sample and chromatographic integrity.The analyses has been a challenge for many chemists in this area.PB interface involves three distinct processes: (1) nebulization Researchers have looked to utilize the easily controlled negative and aerosol formation, (2) desolvation and (3) momentum glow of the plasma to eect excitation and ionization of analyte separation. The production of the primary aerosol from the species originating in the liquid state.3–11 In each of these LC flow can in principle be accomplished by any number of applications, the sample is presented to the GD as a dried nebulizers common in atomic spectrometry such as concentric solution residue brought into the gas phase through cathodic and cross-flow pneumatic, ultrasonic and thermally-assisted sputtering.In the ideal case though, sputtering would not be devices. The desolvation chamber is a very important region needed, and the plasma geometry and conditions could be in which the aerosols produced by the nebulizer evaporate at optimized solely for optical emission and ionization. An or near atmospheric pressure.Obviously, heat transfer has a example of this approach has recently been presented by Olson great impact on the aerosol desolvation process, as pressure et al.,12 who used an rf-GD as an ionization source for volatile and temperature changes occur in this region. Helium is usually analytes by gas chromatography–mass spectrometry chosen as the nebulizer or sheath gas because of its high (GC–MS). The major hindrance to the direct analysis of thermal conductivity, enhancing heat transfer to the aerosol solution-based samples is the inability of the low pressure from the heated chamber components.The rate of solvent (single Torr), low temperature (slightly above room temperavaporization will vary depending on the solvent properties, an ture) plasma to eect analyte desolvation and then subsequently to eliminate the deleterious eects of solvent vapors especially important consideration for solvents such as water Journal of Analytical Atomic Spectrometry, August 1997, Vol. 12 (807–815) 807which demand high temperatures. The momentum separator input concentration, the use of chelated analytes and the addition of HCl as a carrier were evaluated to explore the serves to enrich the PB by largely removing the solvent vapors. A typical momentum separator is composed of skimmers relationship between the analyte transport eciency and response. Direct observation of the introduced particles using separating two dierential pumping stages.The mixture of sample particles and solvent vapors is forced through the scanning electron microscopy (SEM) allows assessment of the role of particle size in the transport process. By establishing a skimmers and attains momentum along the orifice axes directly related to their mass. Subsequently, the enrichment of sample relationship between sample transport eciency and analyte response, attention can be focused on improving the cumulative particles is accomplished based on the fact that light species (vapors) gain o-axis momenta and are skimmed from the nebulization–desolvation–transport processes towards the goal of higher transport eciencies, better calibration quality and expanding beam in preference to the solute (analyte) particles.The application of PB-LC–MS interfaces relies most heavily minimized matrix eects. As such, the practical use of the PB-HC-AES approach to element-specific chromatographic on the ability to operate with almost any LC solvent at flow rates of up to 2 ml min-1, with the resulting pressure in the detection can be further developed. MS ion volume easily being less than 1 mTorr.Because of this ecient solvent removal, the PB interface is attractive for EXPERIMENTAL sample introduction in LC–MS applications because of its compatibility with various ionization techniques including Glow Discharge Source electron impact, chemical ionization and fast atom bombard- The basic design of the PB-HC-AES interface and source has ment (FAB).16–22 However, studies of commercial PB interfaces been described previously,10,11 so only a cursory description is have indicated some problems for quantitative analysis at low presented here.The 3.25 mm diameter graphite hollow cathode concentrations. Bellar et al.17 first reported the existence of (HC) is mounted in the center of a heated stainless-steel cube, non-linear behavior in the region of the detection limit in the termed the thermoblock. The entire thermoblock is heated to form of depressed analyte responses. This phenomenon could #220 °C by a pair of commercially available cartridge heaters be remedied by what was described as a carrier eect that (Scientific Instrument Service, Model SC 2515, Ringoes, NY, resulted in increased target analyte ion abundance in the USA), with the block temperature measured by a W–Re presence of co-eluting compounds.The PB carrier process is thermocouple.Analyte particles enter the plasma area through postulated to be initiated by neutral molecules capable of a 1.53 mm aperture in the wall of the HC perpendicular to the forming molecular clusters in solution and/or in the latter cathode axis. Particles impacting on the opposite wall are stages of the transport processes. Apel and Perry18 have vaporized and swept into the HC–GD region by the perpen- explained the observed non-linear behavior using a high-pass dicular flow of the He discharge gas.Discharge gas inlets, filter model which addresses a hypothetical cut-o level where electrical feedthroughs, PB interface fittings and vacuum pump- the analyte particles suer a dramatic loss of their transport ing ports are also fixed to the thermoblock. The He discharge eciencies if their particle size is below the cut-o level. Simply, gas pressure within the block is monitored by a thermocouple the size/mass of the particles at low concentration is not gauge (Teledyne Hastings-Raydist, Model DV-4D, Hampton, sucient to ensure their direct transport through the momen- VA, USA).The GD is powered by a Kepco (Flushing, NY, tum separator. Further investigation by Ho et al.19 suggested USA) Model BHK 2000 supply operating in a constant-current that many compounds exhibit linear calibration graphs with mode. Typical source operating conditions are 3.5 Torr He particular co-eluting compounds. The trend of structural simisource pressure and a discharge current of 30 mA (#450 V).larity between a carrier and the analytes of interest is generally accepted and employed in choosing co-eluting carriers. The complex mechanism of the carrier eect, which involves physi- Particle Beam Interface cal and chemical processes, clearly indicates that no single universal additive exists and therefore there is much room for The PB-LC–MS interface employed here consists of a thermal concentric nebulizer used to generate a finely dispersed aerosol, further studies.Based on the above-cited studies, it would not be surprising a stainless-steel spray chamber for desolvation, and a twostage momentum separator which serves to remove residual to see non-ideal transport responses in all PB-based interfacing schemes. In previous studies of the feasibility of the PB sample solvent vapor and reduce the backing pressure. The liquid sample passes through a fused-silica capillary (110 mm id) introduction for flow injection-type liquid analysis by hollow cathode atomic emission spectrometry (HC-AES),10,11 there housed within a stainless-steel capillary (0.5 mm id).A fine aerosol is generated as He is introduced into the gap between was a definitive signal enhancement observed by the addition of concentrated HCl (1+5, v/v). This carrier eect yielded as the fused-silica and steel capillary (i.e., concentric, pneumatic nebulization).A dc potential applied across the steel capillary much as a 1000-fold signal enhancement in aqueous solutions, overcoming severe depressive eects seen for samples of high causes resistive heating and adds a thermal component to the nebulization process. The generated aerosol is directed into salt content (5 M NaNO3–0.1 M KOH).10 As with nebulizationbased atomic spectrometric methods employing flames and the stainless-steel desolvation chamber which is heated by electrical tape and the temperature (typically 220 °C) is moni- atmospheric pressure plasmas, there is a need to delineate the possible eects of sample transport and source operation tored by a W–Re thermocouple. In the system under study here, the commercial desolvation chamber has been modified conditions in the observed enhancement.In the absence of any changes in the HC discharge electrical characteristics (voltage with an additional gas inlet to introduce a supplementary He gas flow into the desolvation chamber.11 A vacuum gauge and current), it was suggested that addition of the acid enhanced analyte transport in particle beam–hollow cathode (TFS Technologies, Model PDR-D-1, Albuquerque, NM, USA) is mounted to the desolvation chamber to monitor the glow discharge atomic emission spectrometry (PB-HC-AES) in a manner similar to that observed in ‘organic’ LC–MS.pressure (typically#400 Torr). The flow rate of the supplemental He is measured by a mass flow meter (AALBORG Therefore, the understanding of analyte transport through the PB interface is vital to understanding the mechanism(s) of Instruments & Control, GFM-1700, Monsey, NY, USA).A thermocouple (Omega Engineering, Type J, Stanford, CT, signal enhancement (or suppression in the example above) for the future application of PB-HC-AES. USA) is mounted through the chamber walls to allow direct measurement of the gas-phase temperature (#150 °C) within In this paper, we present analyte transport studies in the PB-HC-AES system.The influence of liquid flow rate, analyte the desolvation chamber. 808 Journal of Analytical Atomic Spectrometry, August 1997, Vol. 12Sample Particle Collectors and Sampling 200, Buck Scientific, East Norwalk, CT, USA) was used with a 1+1 air–acetylene flame mixture. The alignment of the flame Two types of sample particle collectors were designed for these between the hollow cathode lamp and photomultiplier tube transport studies, shown in Fig. 1. The first was designed to was optimized for maximum absorbance during the aspiration act as an integrating collector of the analyte introduced in a of the sample solution. The amount of sample collected, and continuous-flow mode for a specified time period. This cylindrithus the eciency of the interface, was calculated based on the cally-shaped sample collector was machined from Teflon to absorbance of the sample solution as determined from calithe same diameter (3.25 mm) as the analytical HC.Analyte bration graphs prepared from a series of standard elemental particles from sample solutions were collected by positioning solutions in the range 0.1–10.0 ppm. this sample collector in the place of the HC. The analyte residues deposited in the cylindrical sample collector were then Scanning Electron Microscopy dissolved by placing the collector in 10 ml of a 1% HNO3 solution for 1 h, followed by sonication for 30 min.This Knowledge of analyte particle sizes and structure is very procedure ensures that all analyte sample deposited on the important in understanding the mechanism of analyte particle sample collector will be dissolved and transferred into solution. transport in this PB-HC-AES system. SEM oers sucient The solution was diluted to 25 ml in a calibrated flask for resolving capabilities, as well as a wide viewing angle and high measurement by atomic absorption spectrometry. The second magnification. A JEOL 848 (JEOL USA, Peabody, MA, USA) particle collector was designed for the purpose of subsequently scanning electron microscope was employed to observe the observing the particle residues with the scanning electron images of analyte particles collected in the GD area.Sample microscope. In this experiment, the flat sample collector, which preparation for SEM requires that the sample residues must is made of aluminium, is inserted in the middle of the HC be: (1) devoid of water and other solvents that could vaporize mount, perpendicular to the PB entrance direction for optimum in the vacuum system, (2) firmly mounted and (3) electrically analyte particle collection eciency.Because of the deleterious conductive. The flat aluminium sample collectors were loaded eects of residual moisture on the performance of the SEM within a sputter-deposition apparatus for coating with a thin instrument, the flat sample collectors were stored in a desic- Au film to reduce the eects of space charging by the noncator after sampling to reduce contamination.conductive particles. The SEM samples were then mounted firmly on the sample stage (stub) with metallic tape. The accelerating voltage and magnification were selected to be Solution Preparation and Delivery 15 kV and ×1000, respectively, for stable imaging. Type 55 The aqueous stock solutions of NaNO3, CuNO3, Fe(NO3)3 Kodak films and exposure times of 20–25 s were chosen for and HCl were prepared with de-ionized, distilled water from photographic recording.analytical-reagent grade inorganic salts and acid. The solvent delivery system was a Waters (Division of Millipore, Milford, RESULTS AND DISCUSSION MA, USA) Model 510 high-performance liquid chromatogra- Eects of Solution Flow Rate and Concentration phy pump. The solutions were passed through 1.53 mm stainless- steel tubing between the pump and the nebulizer, with the The understanding of flow rate as an important parameter in solution flow rate fixed at 1.5 ml min-1.For all of the experithis PB-HC-AES system not only includes its role in the ments performed here, the delivery system was operated in the nebulization process but also its eect on the particle transport continuous-flow mode. In this way, the amount of sample that through the interface. An increase in solution flow rate should ideally enters this PB-HC-AES source can be calculated with produce a corresponding, proportional increase in analytical consideration of the solution concentration, liquid flow rate, signal response.Previous studies11,12 demonstrated the existand the time of solution delivery. ence of an optimum sample liquid flow rate of 1.5–2.0 ml min-1 in this PB-HC-AES system for a range of analyte species. The general explanation for this phenomenon has been two-fold: Atomic Absorption Spectrometry (1) analyte mass transport to the GD source increases with In order to evaluate the transport characteristics of the PB increasing flow rate in the lower flow rate range interface, atomic absorption spectrometry was used to measure (<1.5 ml min-1) and (2) a continuous increase of the flow rate the actual amount of analyte reaching the GD region.The beyond 2.0 ml min-1 decreases the analyte signal because of analytical wavelengths employed were 324.7, 589.0 and degraded desolvation eciency caused by solvent overloading. 385.9 nm for Cu, Na and Fe measurements, respectively.The The eect of liquid flow rate on analyte transport, in terms of standard slotted burner supplied with the instrument (Buck the amount of Cu reaching the sample collector, is illustrated in Fig. 2. The appearance of the optimum flow rate at approximately 2 ml min-1 is consistent with the previous optical emission results,11,12 suggesting that the amount of analytical signal is an accurate reflection of the aerosol formation and eciency of the transport process.Certainly, the liquid flow rate has an eect on the nebulization processes, leading to droplets that have dierent desolvation eciencies. Harris20 characterized PB-LC–MS nebulizers with respect to the mean Sauter diameter (volume5surface area ratio) and the droplet size distributions. Those studies pointed to the need to generate the smallest diameter droplets while preventing aerosol/particle losses in the desolvation chamber and momentum separator due to turbulence, impaction and gravitational forces.The trends described here probably reflect these characteristics also, as will be discussed in a subsequent section. As noted previously, observations of non-linear behavior of Fig. 1 Sample particle collectors designed for insertion into the PB interfaces have been attributed to low desolvation hollow cathode volume for use in subsequent, 1, atomic absorption and 2, SEM analyses. eciencies, as well as turbulence, impaction and gravitational Journal of Analytical Atomic Spectrometry, August 1997, Vol. 12 809size and shape of the particles. The particles delivered at the lower two concentrations [Fig. 4(a) and (b)] are fairly spherical in nature, with the largest particles seen in each increasing from #5 to 10 mm in diameter as the analyte concentration is increased from 10 to 50 ppm. The particles collected for the higher concentration solutions are dierent. First, the particles in Fig. 4(c) (100 ppm CuNO3) are less monodisperse than seen for the lower concentrations. Clearly, large, irregular-shaped particles are now being delivered in the presence of smaller spherical particles. The general shape seems to suggest the agglomeration of particles which could take place in either the gas phase or on the collector surface. In contrast to the analytical application of the PB-HC-AES source, the collector is not heated, so ‘wet’ particles could be envisioned to coalesce on the target surface.The departure from the ideal case of discrete particle introduction is even more dramatic in the micrograph of the ‘particles’ resulting from the 200 ppm solution [Fig. 4(d)]. In this case, the residue has the appearance Fig. 2 Eect of liquid flow rate on Cu transport expressed as the of a dried slurry. This definitively suggests a case where relative Cu concentration retrieved from the sample collector and complete desolvation has not taken place and ‘wet’ particles analyzed by atomic absorption spectrometry. are being delivered to the collector surface.The trends observed in these micrographs are consistent with conditions that would losses within the dierential pumping stages of the momentum lead to reduction in transport eciencies, most probably due separator. More specifically, these phenomena reflect that to impaction and gravitational losses. aerosol properties change with analyte concentration, basically shifting the aerosol size distribution.22 Even for 100% desolvation eciency, the dierences in initial droplet sizes will HCl Carrier Eect manifest themselves in shifting the analyte particle size distri- The basic concept of the PB is to accomplish analyte particle butions.Thus, at low analyte concentrations (small particle transport and enrichment in the detection stage via simple sizes) losses due to turbulence may play a factor, while at high physical and mechanical techniques.The comprehensive eects concentrations (large particles) impaction and gravitational of the processes of nebulization, desolvation and momentum losses would be important. These eects are analogous to separation are directly related to fine aerosol formation, solvent those seen in tertiary aerosol/particle size measurements in vaporization and analyte separation from the solvent. The nebulizer/desolvation systems used in atomic spectrometry.23,24 term ‘carrier eect’ is applied to phenomena that appear to In an attempt to evaluate the particle size/concentration eects, increase the analytical signal with the addition of certain a study was performed using the first type of sample collector physical and chemical modifiers. Bellar et al.17 reported the by continuously introducing CuNO3 solutions of dierent observation of a carrier eect associated with the LC mobile concentration. The atomic emission responses shown in Fig. 3 phase composition in the PB-LC–MS experiments.Their do not suggest suppression eects (non-linearity) at the low research explored the function of the carrier eect in the analyte concentrations, but do indeed reflect a declining transanalyte transport process and how it influenced the analytical port eciency at the higher Cu concentrations. Based on an performance of the mass spectrometer source. The addition of extrapolation of the response curve between 5 and 50 ppm chemical modifiers such as ammonium acetate and malic acid CuNO3, the relative transport eciency of the particles for to mobile phases has been shown to be an excellent approach the 100 ppm solution is depressed by approximately 30%.for improving sensitivity and linearity.24 Evidence supporting a particle size eect leading to the In our previous work,11 analyte emission signal enhancement observed non-linearity exhibited in Fig. 3 is provided through was observed by addition of HCl in the determination of Cs, scanning electron micrographs of the particles reaching the where the signal suppression caused by a high salt concen- HC volume.Fig. 4 presents the SEM images obtained for the tration matrix is in fact eliminated. In that work, it was the particles collected (30 min, 1.5 ml min-1 flow rate) for 10, 50, presence of the chloride ion which was deemed the important 100 and 200 ppm CuNO3 solutions [Fig. 4(a)–(d), respectfactor, as use of other acids (HNO3, H2SO4) did not yield ively].The micrographs reflect distinct changes in both the such enhancements. By the same token, the addition of other chloride salts did not prove to be eective at enhancing transport. The eect of carrier addition was studied by comparing the transmission eciencies of metal nitrate salts with and without HCl addition. In each case, known concentrations (10 ppm) of the metal ions were introduced under continuous- flow conditions (1.5 ml min-1) for a period of 30 min.The sample particles were collected on the Teflon sample collector (sample collector 1, Fig. 1) and analyzed by atomic absorption spectrometry. Table 1 presents the results of these transport studies in terms of the percentage of the total analyte input for each solution. The data point to a number of interesting trends. First, and probably most important in terms of analytical applications of the PB-HC-AES approach, is the fact that in every case the addition of HCl increases the analyte transport Fig. 3 Eect of Cu input concentration on Cu transport expressed eciency. The extent of the improvements ranges from #43% as the relative Cu concentration retrieved from the sample collector for the Fe3+ solution to 73% for Cu2+. On the other hand, and analyzed by atomic absorption spectrometry (1.5 ml min-1 liquid flow rate). there are also appreciable dierences in the transport 810 Journal of Analytical Atomic Spectrometry, August 1997, Vol. 12(a) (c) (b) (d) Fig. 4 Scanning electron micrographs of Cu particles resulting from dierent CuNO3 input concentrations: (a) 10, (b) 50, (c) 100 and (d) 200 ppm (all micrographs taken at ×1000 magnification). Table 1 Multielement transport studies of the PB–HC-AES interface is often used as a drying agent in humid atmospheres. Based on the same argument of relative solubility, the improvements Without addition of HCl — seen on addition of the chloride are easily explained.In each Analyte Cu Fe Na case, the metal chlorides are less soluble than the corresponding Transport eciency (%)* 6.61 3.83 11.67 nitrates. Therefore, as the droplet containing both the NO3- With addition of HCl (155) — and Cl- counter ions begins to desolvate, the vastly lower Analyte Cu Fe Na solubility of the metal chlorides initiates their formation in Transport eciency (%)* 11.47 5.47 17.80 lieu of the nitrates. As with the metal nitrates, the observed transport eciencies are inversely related to the solubilities of * Transport eciency(%)=(Caa×F1)/(F2×Co×LFR×T )×100%, the metal chlorides.While the knowledge of using solution where Caa is AA measured concentration, mg ml-1; Co input analyte concentration, 10 mg ml-1; F1 volume dilution factor, 25; F2 volume chemistry to improve analyte transport can be used to analytdilution factor with 551 HCl, 5/6; LFR liquid flow rate, 1.5 ml min-1; ical advantage, the comparison between the dierent metals T time of sample input, 30 min. points to sources of discrepancies in the analytical performance (sensitivity) of dierent metal ions in solution.It is interesting to compare the obtained transport eciencies eciencies between each of the metal nitrates and the HClwith nebulization systems commonly employed in atomic added solutions. Let us first consider the dierences between spectrometry. While the eciencies depicted in Table 1 (nom- the respective metal nitrate solutions.As will be seen in the inally 4–18%) may not, at first glance, seem impressive, they next section, the most eective analyte transport will be are substantially higher than those typically quoted for pneu- achieved for those cases where completely dry particles of matic nebulizers operating in the #1 ml min-1 solution flow finite size are passed through the PB interface. Thus, desolvrate regime, #1%.26,27 On the other hand, the use of nebulizer ation eciency will be a key determining factor in transport systems which operate at very low solution uptake rates processes.To a first approximation, desolvation eciency will (1–100 ml min-1) has shown eciencies in the 50–100% be related to the solubility characteristics of the salt in question. range.28,29 The values reported for this thermoconcentric For the Na, Cu and Fe nitrates, there is a large variation in nebulizer-PB introduction system reflect optimization of nebul- the solubilities, ranging from 39 g ml-1 (at 100 °C) for NaNO3 ization and desolvation parameters and may be improved to infinite solubility for the Fe3+ salt.25 The relative solubilities further through optimization of the PB interface itself.For are inversely related to the observed transport eciencies. This example, no eort has been made to optimize the spacing of makes sense as the higher the solubility, the greater the propensity for the solute to ‘hold’ solvent. In fact, Fe(NO3)3 the skimmer cones, nor their aperture sizes.Journal of Analytical Atomic Spectrometry, August 1997, Vol. 12 811SEM Images reflect some degree of hydration on the surface, which was followed by further dehydration. Second, there appears to be Because analyte transport in the PB interface is a mechanical much more material for the Cu salts. The relative inhomogenprocess, it is obvious that the size of the particles will have a eity of the distribution in Fig. 6(a) could be due to a process bearing on the overall eciency.Studies by Harris20 suggest whereby the primary particles may be scattered from the that the ideal particle size distribution is in the range 2–5 mm, central portion of the target and are re-deposited to form the so that particles can possess sucient momentum to cross observed pattern. As with the Na particles, a comparison over into the source region while at the same time promoting between the center and edge of the deposition [Fig. 6(b) and vaporization. Scanning electron micrographs are powerful (d), respectively] reveals that the central portion has a contools in studies involving detailed observation of such particles. densed appearance while the image taken from the edge of the For the sake of comparison with the previously presented deposit is composed of dispersed, discrete particles. Also constudies, Na and Cu were chosen as the analyte species in the firming the observations in Fig. 5, the micrographs for HCl SEM studies.Inspection of dierent areas of the sample addition [Fig. 6(c), central portion, and 6(e), edge] indicate collector provides information on both the particle sizes and the presence of larger particles, with some evidence that the their spatial distribution, giving insights into carrier eects. It larger particles are the result of agglomeration of smaller should be emphasized that the particle collection and imaging particles. The presence of HCl seems to induce aggregation of process employed here is not ideal as the samples are brought small sample particles to yield larger particles.Interestingly, into contact with ambient atmosphere on transfer between the the space charging observed for the sodium salts [Fig. 5(c) and GD source, the deposition chamber and the electron micro- (e)] is not observed in Fig. 6(c) and (e), possibly because the scope. Obviously, in situ observations of the sample surface copper-containing particles have better conductive character.would be preferable. Fig. 5(a) is a low magnification (×20) image of NaNO3 CONCLUSIONS particles on the sample collector. The image indicates that the sample particles basically concentrate in the center of the In the development of the PB-HC-AES system, it is important target area, with some dispersed particles surrounding the to understand the eect of the analyte transport phenomenon central region. The fact that the majority of the deposition on analytical performance.The high eciency solvent removal occurs in an approximately 1 mm diameter region reflects the of the PB interface eliminates interferences with regard to well collimated nature of the PB, as the spot size is smaller plasma processes and the observed optical emission spectra. than the entrance aperture through the side of the HC. Fig. 5(b) The absence of solution residues in the desolvation chamber (magnification=×1000) depicts a fairly homogeneous sample suggests that the sensitivity of the technique is controlled by particle distribution in the center of the collector; however, the the dynamics of the analyte transport across the PB interface.size of the sample particles is still not clear because of particle Studies of the overall transport eciencies reflect the cumulatcondensation. The cause of the condensation appearance could ive aspects of solvent loading, particle sizes, and the eects of either be due to hydration of the surface as it was brought scattering, impaction and gravitational forces on the resulting into ambient atmosphere, or by continuous impaction of the particles. Many of the sample introduction parameters that high momentum particles.Alternatively, the image may be aect the observed analyte emission intensities are shown to blurred due to space charging eects. The inability to discern be the direct result of analyte transport characteristics.accurately the particle sizes is overcome by examining the edge Inspection of collected analyte particles gives insights into the area around the concentrated particle accumulation where roles of analyte concentration and carrier eects on particles there is a sparse particle distribution as shown in Fig. 5(c) sizes and transport eciencies. (magnification=×1000). This image reveals discrete particles An optimum liquid flow rate in the range 1.5–2.0 ml min-1 in this area that are in the size range from 2 to 5 mm in can be seen in the curve of transport eciency aected by diameter.Deposits of the Na sample with HCl carrier addition liquid flow rate. Heavy solvent loading at high liquid flow were also examined through SEM images in both the center rates causes analyte transport eciency to decrease as the and edge areas and are presented in Fig. 5(d) and (e), respect- result of low desolvation eciency. The deviation from linearity ively. By comparing Fig. 5(b) with (d) and Fig. 5(c) with (e), of analyte transport eciency at high analyte input concenthe dierences caused by addition of HCl are readily observed. trations is observed and attributed to a large-particle cut-o. First, in the micrographs of the center area, Fig. 5(b) and (d), As expected, the eects of liquid flow rate and analyte input space charging eects of the incident electron beam appear as concentration on analyte transport eciency are the same as lines across Fig. 5(d), although both images show the same their eects on emission signal. This coincidence demonstrates condensed and homogeneous surface. The space charging the direct relationship between analyte transport eciency and suggests the presence of either more non-conductive material analytical signal. The function of HCl as an analyte carrier (i.e., more analyte) or a particle deposition that is more non- was evaluated by studying its eects on both analyte transport conductive than in the simple nitrate salts. It can be seen that and particle size changes.Without exception, experimental the addition of HCl in the sampling process seems to have a data show analyte transport enhancements for elements tendency to cause particles to be more closely packed on the with the addition of HCl, although this transport eciency sample surface. A comparison of the edge region micrographs, improvement is analyte-dependent. Fig. 5(c) and (e), reveals more details of larger particle sizes The analyte particle size and particle size distribution were produced by HCl addition.While there is still some evidence examined by SEM analysis at both the center and edge areas of charging eects, the presence of particles in the 10 mm size of the sample collectors. The micrographs of Na and Cu at range is clear. the edge area show clear, discrete particles, with sizes on the In order to avoid false assumptions based on scanning single micrometer level. However, condensed and clustered electron micrographs of Na samples, the same experiments particle aggregation is observed in the center areas of the were performed for Cu samples, with the results presented in collected Na and Cu samples. The dierence between the Fig. 6(a)–(e). Fig. 6(a) shows the Cu sample particles in the center and edge area micrographs indicates that the analyte center portion of the collector under low magnification (×20). particles travel in a very straight path with high momentum.Comparison of the low magnification images for the sodium Based on this observation, the HC, which is perpendicular to and copper nitrate salts [Fig. 5(a) and 6(a)] is interesting. First, the PB entrance, is being redesigned with a longer hollow cylinder for more ecient acceptance of the analyte particles the cracking of the residue of the Cu deposition appears to 812 Journal of Analytical Atomic Spectrometry, August 1997, Vol. 12(a) (c) (e) (b) (d) Fig. 5 Scanning electron micrographs of Na particles: (a) low magnification (×20), (b) central portion of target area at high magnification (×1000), (c) edge area at high magnification (×1000), (d) central portion with HCl addition at high magnification (×1000) and (e) edge area with HCl addition at high magnification (×1000). and more ecient atomization/excitation. Scanning electron The observed transport eciencies are in the 4–18% range, which is higher than values typically quoted for other atomic micrographs also depict the particle size increasing with HCl addition for both Na and Cu samples.At this point, it is easy spectrometry systems employing pneumatic nebulization and desolvation chambers operating at similar solution uptake to see that the improvement of analyte transport eciency by HCl addition is due to an increase in analyte particle size rates. It is believed that this number may be improved as no eort has been made to date to optimize the spacing of the which overcomes the PB interface cut-o limitation.The formation of larger particles would seem to be related to the skimmers in the interface or in the sizes of the apertures. Future studies of this PB-HC-AES system will focus on its relative insolubility of the analyte metal chlorides versus nitrates and the like. Measurements of the primary aerosol application to organic compound analyses. For example, research using this system for amino acid analyses is currently droplets sizes would lend insights into whether the improvements are related more to nebulization characteristics or underway in an eort to determine molecular composition with atomic emission information.desolvation. Journal of Analytical Atomic Spectrometry, August 1997, Vol. 12 813(a) (c) (e) (b) (d) Fig. 6 Scanning electron micrographs of Cu particles: (a) low magnification (×20), (b) central portion of target area at high magnification (×1000), (c) edge area at high magnification (×1000), (d) central portion with HCl addition at high magnification (×1000) and (e) edge area with HCl addition at high magnification (×1000). 2 Marcus, R. K., Harville, T. R., Mei, Y., and Shick, C. R., Anal. The authors acknowledge the financial support from the Chem., 1994, 66, 902A. 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