JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1986, VOL. 1 59 A Comparison of Cloud Chambers for Use in Inductively Coupled Plasma Nebulisation Systems Leslie S. Dale and Stephen J. Buchanan CSIRO, Division of Energy Chemistry, Lucas Heights Research Laboratories, New lllawarra Road, Lucas Heights, NSW, 2234, Australia A comparison of the analytical performance of five cloud chambers of different geometry used in conjunction with a commercial concentric glass nebuliser is reported. The comparison was based on measurements of transport efficiency, background equivalent concentration, analyte emission stability, detection limit, equilibration time and memory effect. Significant variations in performance indicated the importance of cloud chamber geometry on the nebulisation system.Best results were obtained with a new design based on a cylindrical chamber with a central tangential inlet. Droplet size distribution measurements revealed that although the primary distribution of the nebuliser aerosol contained a substantial proportion of small droplets of similar size to those in the ultimate distributions of the nebulisation systems, a large number of droplets are removed from the aerosol by separation processes occurring in the chambers. Keywords: Cloud chambers; nebulisation system; inductively coupled plasma; atomic emission spec- trometry The major advantages of inductively coupled plasma atomic emission spectrometry (ICP-AES) are the high analytical precision achieved as a result of the emission stability of the source and the high detection capability for a wide range of elements as a result of its high temperature.However, these properties are influenced by the performance of the nebulisa- tion system. Analytical precision is determined by the stability of analyte emission, and hence by the stability of aerosol transport rates, and sensitivity is determined by the transport efficiency. Therefore, the desired characteristics of the nebulisation system, consisting of a nebuliser and cloud chamber, are the production of a stable aerosol stream of small droplets with a narrow size distribution and a high transport rate. Commercially available nebulisers, including the concentric glass, cross-flow and those based on the Babington principle, produce aerosols with broad droplet size distributions; it is necessary to remove the larger droplets using a cloud chamber as a separator or filtering device.Consequently, the cloud chamber must be treated as an important component when assessing the performance of the nebulisation system. Although there have been a number of comparisons of the Performance of nebulisers used in ICP-AES,1-4 the effect of cloud chamber geometry in nebulisation systems has received less attention. Ebdon and Cave5 compared a Scott double- pass cylinder6 and a cyclone chamber using a commercial concentric glass nebuliser and a cross-flow nebuliser of their own design. The signal to noise ratio for aluminium was up to 30% higher with the cyclone chamber although condensation occurred in the injector tube of the torch. Olsen et al.7 carried out an extensive investigation of nebulisers and showed how the droplet size distributions were altered by different cloud chambers (cyclone, Scott single and double-pass cylinders).They concluded that chamber geometry was an important factor in achieving high signal to noise ratios. Novak and Browner8 compared a chamber used in atomic absorption spectrometry and a concentric tube chamber with a variety of nebulisers. Their measurements of droplet size distributions of a number of nebuliser - cloud chamber combinations demonstrated that the cloud chamber effectively reduced the mass distribution of the aerosol, and that chamber geometry was a critical factor in achieving high signal to background and signal to noise ratios. Because cloud chamber geometry has such a significant effect on the performance of the nebulisation system, a more comprehensive comparison of different cloud chambers was undertaken to examine, in detail, its effect on analytical performance.This was achieved by studying the characteris- tics of a number of chambers when used with the same nebuliser operating under identical conditions. Five cloud chambers were selected: the Scott double-pass cylinder, a conical chamber with an impact bead, two cyclones of 250- and 500-ml capacity and a cylinder of 180-ml capacity with a central tangential inlet. Selection was made on the basis of published performance data, use in commercial instruments or personal laboratory experience. The chambers had basic differences in their geometries with respect to shape, volume, impact surface and mode of aerosol separation.Although the effect of cloud chamber geometry on analyte emission stability and sensitivity are the most important aspects in comparing analytical performance , equilibration time and memory effect are also worthy of investigation. Rapid equilibration times and short washout times maximise the sample throughput rate and the former leads to reduced sample consumption, which may be of significance in sequen- tial instruments. The assessment of analytical performance was therefore based on measurements of transport efficiency, background equivalent concentration, analyte emission stab- ility, detection limit, equilibration time and memory effect. Droplet size distributions were also measured to assess the performance of each nebulisation system in terms of their ultimate mass distributions.Experimental All measurements were made on an ICP spectrometer based on a Labtest Plasma 2000 generator and matchbox. Light from the plasma (2 mm aperture, 16 mm above the load coil) was focused on to the entrance slit of a 0.5-m Ebert monochroma- tor fitted with a 2160 grooves mm-1 grating blazed at 260 nm. The reciprocal linear dispersion was 0.8 nm mm-1. Emission signals from the photomultiplier (EMI6256S) were measured using a locally built STD bus-based microcomputer with a Motorola 6809 microprocessor. The operating power level was 1 kW. The TR-30-A3 concentric glass nebuliser was obtained from J . E. Meinhard Associates, Santa Am, CA, USA. It was operated at a gas flow-rate of 1 1 min-' and 207 kPa (30 lb in-2) pressure.The solution uptake rate was 2.7 ml min-1. The nebulisation system was located outside the60 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1986, VOL. 1 torch chamber and connected to the base of the torch by a 60-cm length of polythene tube. Cloud Chambers All cloud chambers were built in the laboratory, with the exception of the conical chamber, which was taken from an ARL instrument. The cyclone chambers were constructed from Quickfit Erlenmyer flasks; they were scaled-down versions of the 1-1 capacity chamber originally used in Unicam SP900 flame spectrophotometers. The smaller volumes were chosen on the basis of previous experience with them in flame spectrometry. The design concept for the 180-ml cylinder emerged during the course of this work; details are shown in Fig.1. Measurement of Transport Emciency A direct aerosol collection procedure similar, in principle, to that described by Smith and Browner9 was used. A fibre filter plug was used to collect the aerosol at the end of the delivery tube. Triplicate measurements of each nebulisation system were made, the nebuliser uptake rate being checked before each measurement. Other Measurements Background equivalent concentrations, emission stabilities and detection limits were measured for chromium (202.5 nm), iron (238.2 nm) , manganese (257.6 nm) , aluminium (308.2 nm), zirconium (343.8 nm) and strontium (407.8 nm). The background equivalent concentrations, expressed as the concentration of the element giving rise to a signal equal to the background level, were determined using signals from solu- tions with concentrations three to five times higher than the background level.Emission stabilities were obtained by n f Drain Fig. 1. Schematic diagram of cylindrical cloud chamber. All dimensions in millimetres calculating the relative standard deviation of ten 5-s integra- tions of signals from solutions with analyte concentrations three to five times higher than the background level. Detection limits were based on twice the standard deviation of the background (ten 5-s integrations) and solution concen- trations that were approximately 50 times higher than the detection limit. Equilibration times were determined by measuring, on a chart recorder, the time taken for a signal from a 0.05 pg ml-1 Sr solution to reach 95% of its steady value.From these chart records the delay times, that is, the time elapsed before any response was detected, were calcu- lated. The memory effect was determined by measuring the time taken for the signal from a 5 pg ml-1 Sr solution, run for 5 min, to decay to a signal equivalent to 0.005 pg ml-1 Sr ( i . e . , 0.1% of the original). Equilibration and memory effect measurements included the nebuliser solution uptake time. The above quantities were measured randomly in triplicate under identical operating conditions and then averaged. Droplet Size Distributions These were carried out on a Malvern Type 3300 particle sizer, which is based on the laser diffraction principle. The aerosol was fed through polythene tubing to a windowless cell situated in the optical path of the instrument.The mass distribution of the nebuliser alone was measured by directing the aerosol across the optical path at the cell position. The distribution was measured at a number of locations in the aerosol cloud at distances of 3-10 cm from the nebuliser nozzle. Results Data for transport efficiencies, background equivalent con- centrations, analyte emission stabilities and detection limits are shown in Table 1. The transport efficiencies could be reproduced to about 5%. The efficiency of the filter as an aerosol particle collector was checked by monitoring the plasma emission, with the filter in the delivery line, while aspirating the test solution. Some of the aerosol passed through the filter but the amount lost was insignificant (ca.1%). The procedure was therefore satisfactory for determin- ing transport efficiencies and allowed useful comparisons to be made. To facilitate comparison with the other data listed, average values for the six elements relative to the 180-ml cylinder are shown. These provided a good estimate of the over-all performance of each nebulisation system. Table 2 shows the equilibration and delay times obtained. Washout times are shown in Table 3, together with the equivalent number of volume changes that took place while the signal decayed to the specified level. The results of the droplet size distribution measurements are shown in Table 4. The mass median diameter is the droplet size below which 50% of the aerosol mass occurs.Data for the mass fractions of droplets below 1.2, 5.0 and 10.5 km give an indication of the shape of the distributions. The droplet size distribution of the nebuliser alone had an average mass Table 1. Transport efficiencies and average relative background equivalent concentrations, analyte emission stabilities and detection limits Transport Background Analyte efficiency, equivalent emission Detection Cloud chamber % concentration* stability* limit* Cylinder (180 ml) . . . . . . 1.26 1 .o 1 .o 1 .0 Cyclone (250 ml) . . . . . . 1.21 1.1 1.4 1.9 Cyclone (500 ml) . . . . . . 0.91 1.5 2.4 2.3 Cone (50 ml) . . . . . . . . 0.52 2.1 2.3 3.6 Double-pass cylinder (250 ml) . . 0.39 2.9 2.9 4.1 * Values are relative to those for the 180-ml cylindrical chamber.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1986, VOL.1 61 median diameter of 18.4 pm with 11.7% m/m of the droplets being below 10.5 pm. Discussion Transport Efficiency The observation that cloud chamber geometry changed the transport efficiency by as much as a factor of three is indicative of the importance of this factor to the nebulisation system. To explain these variations in transport efficiency, reference is made to the aerosol transport model of Browner et al. , l o who described the aerosol in terms of its primary, secondary and tertiary distributions. The primary droplet distribution of the nebuliser is modified by impingement of the aerosol on an impact surface where larger droplets are removed, small droplets follow the gas stream and intermediate droplets of sufficiently high velocity may shatter and generate smaller droplets.The aerosol then assumes its secondary droplet size distribution. Beyond the impact surface, further separation occurs through processes of impaction, turbulence, centrifugal loss, gravitational settling and evaporation. This is the tertiary distribution that is transported to the plasma. Based on this model, differences in transport efficiencies may be explained. In the cylinder and cyclone chambers, for which the highest values were obtained, the droplets which remain after initial impaction with the chamber walls follow the circular path of the gas stream when larger droplets are removed by centrifugal loss. There is little visual turbulence in these chambers.In contrast, the conical chamber and double- pass cylinder, which yielded lower efficiencies, have different impact surfaces from the other chambers. In the conical chamber impaction of the aerosol on the bead, located in a direct line with the nebuliser nozzle, produces considerable condensation on its surface. Some aerosol turbulence was visible in the region between the nozzle and the bead, which suggests further separation by this process. In the double-pass cylinder, the impact surface is the relatively large area of the inner cylinder, and the end of the outer cylinder. Considerable Table 2. Equilibration times for various cloud chambers Equilibration Cloud chamber timels Delayls Cylinder . . . . . . 11 4 Cyclone (250 ml) . . . . 19 3 Cyclone (500 ml) .. . . 25 4 Cone . . . . . . . . 9 2 Double-pass cylinder . . 28 9 Table 3. Washout times for various cloud chambers No. of volume Cloud chamber Volume/ml Timels changes Cylinder . . . . . . 180 33 3.1 Cyclone . . . . . . 500 110 3.6 Cone . . . . . . 50 38 12.7 Double-pass cylinder . . 250 63 4.2 Cyclone . . . . . . 250 53 3.5 condensation was visible up to half way along the inner cylinder. Turbulence was also obvious in this region. The probability of further separation by secondary impaction on the chamber walls is high, owing to the large surface to volume ratio. Variations in transport efficiencies may therefore be attri- buted mainly to the influence of the cloud chamber geometry on secondary and tertiary mechanisms induced by the shape and position of the impact surface.The major mechanisms that limit the transport efficiency in the conical chamber and double-pass cylinder appear to be condensation and, to a lesser extent, turbulence. In the cylinder and cyclone cham- bers, the higher efficiencies are due to the lack of turbulence, and the predominant tertiary mechanism appears to be centrifugal loss. Condensation on the impact surface must be lower as there is considerable drainage of waste solution on the chamber walls as a result of this centrifugal loss. Background Equivalent Concentration, Analyte Emission Stability and Detection Limit The 180-ml cylinder gave the best over-all performance. The background equivalent concentrations for this chamber and the 250-ml cyclone are only marginally different.This would be expected from their similar transport efficiencies, as this quantity depends on the amount of aerosol reaching the plasma. However, the significant differences in analyte emission stability and detection limit indicates a more stable aerosol mass transport rate with the 180-ml cylinder. For the other chambers, the background equivalent concentrations are in line with their lower transport efficiencies and their analyte emission stabilities result from less stable aerosol mass transport rates. This combined with lower background equi- valent concentrations, is responsible for the decline in detection limits. Equilibration Time Although the conical chamber had the fastest equilibration time, it was only marginally better than that of the 180-ml cylinder.However, allowing for the delay period, both chambers had about the same response time. The cyclone chambers had a relatively short delay time but their equilibra- tion times were long, A possible explanation for this is that because of the close proximity of the impact surface to the top outlet, some of the aerosol has a short residence time in the chamber. This portion of the aerosol therefore circumvents the system by entering the gas stream near this outlet. This explanation is supported by the fact that the 180-ml cylinder produced a longer delay time than the 250-ml cyclone. With the cylinder, the impact surface is the centre of the chamber wall and the aerosol paths to the outlets are longer, thus minimising this effect. The double-pass cylinder has a long delay time and a long equilibration time.This is probably due to the long aerosol path length and substantial mixing within the chamber. This effect is exemplified by the similarity of the times for this chamber and the 500-ml cyclone, although the volume of the cylinder was only half that of the cyclone. Table 4. Droplet size distribution data for various cloud chambers Mass median % mlm below diameter/ Cloud chamber Pm 1.2 pm 5.0 pm 10.5 pm Cylinder . . . . . . . . 5.43 4.3 28.4 83.9 Cyclone (250 ml) . . . . . . 6.78 2.7 16.1 67.2 Cone . . . . . . . . . . 3.91 5.1 49.4 90.4 Double-passcylinder . . . . 4.72 4.7 37.3 90.8 Cyclone (500 ml) . . . . . . 6.22 1.4 29.0 75.762 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1986, VOL, 1 Memory Effect The 180-ml cylinder and the cone gave similar washout times, which were substantially better than the others.With the exception of the cone, the number of volume changes required to reach the 0.1% level was between three and four. On the basis of chamber volume alone, and taking the average number of changes of 3.6 obtained for the other chambers, the washout time for the conical chamber should have been about 10 s instead of the 38 s obtained. This suggests that the washout time for this chamber is prolonged by inefficient mixing. Droplet Size Distributions The measurements of droplet size distributions show that although both the mass median diameter and the size distribution were altered by the cloud chamber geometry, there was no correlation with any of the other parameters measured.Both the cone and the double-pass cylinder had lower mass median diameters and narrower distributions, but also they had the lowest transport efficiencies and poorer emission stabilities. The higher mass median diameters and broader distributions obtained with the cyclones are pro- bably due to the larger droplets contained in that part of the aerosol that circumvents the system. It can be seen, from the droplet size distribution data for the 180-ml cylinder, that placement of the nebuliser at the mid-point prevents this occurrence. Of most significance is the comparison between the droplet size distributions obtained with the chambers and that of the nebuliser alone. The latter distribution contained 11.7% mlm of the droplets below 10.5 pm whereas the cloud chambers had distributions in which 67-90% mlm of the droplets were below this value.As the highest transport efficiency obtained was only about 1.2%, up to 90% of droplets suitable for transport to the plasma were removed by separation mechanisms in the chambers. The cloud chambers therefore not only perform their desired function of filtering the larger droplets from the primary distribution, but also considerably reduce the popula- tion of those droplets suitable for transport to the plasma. Performance of Cylindrical Chamber It has been shown that the cylindrical chamber generally provides the best performance. This may be attributed to certain features of its design. The tangential inlet results in little turbulence, and separation of the aerosol particles is achieved largely by centrifugal loss.This leads to high transport efficiency and stability. The favourable equilibration and memory characteristics are due, in part, to its small volume and the central location of the nebuliser inlet. With this arrangement the two outlets reduce the aerosol residence time, leading to rapid response and short washout times. The droplet size distribution produced by this chamber is compar- able to those of the cone and double-pass cylinder while maintaining a higher transport efficiency. Prevention of aerosol particles from circumventing the system contributes to the narrow size distribution. Conclusion Cloud chamber geometry is an important consideration when optimising the analytical performance of a nebulisation system.Of the chambers studied, those based on centrifugal loss for separation provided best performance. The cylindrical chamber with a central tangential inlet gave the best over-all performance. The consistently high performance level of this chamber has been demonstrated by its continuous and routine use in these laboratories for over 1 year. A limiting factor in achieving higher transport efficiencies is the loss of a large proportion of droplets of suitable size range produced by the nebuliser . The authors gratefully acknowledge the skill of W. T. Williams, Scientific Glassblower, who fabricated the cloud chambers. They are also indebted to D. Shirlaw for carrying out the droplet size distribution measurements. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. References Gustavsson, A., Spectrochim. Acta, Part B , 1984, 39, 743. Wohlers, C. C., ZCP Znf, Newsl., 1977, 3, 2. Greenfield, S., McGeachin, H. McD., and Chambers, F. A., ICP Znf, Newsl., 1977, 4, 117. Garbarino, J. R., and Taylor, H. E . , Appl. Spectrosc., 1980, 34, 584. Ebdon, L., and Cave, M. R., Analyst, 1982, 107, 172. Scott, R. H., ZCPZnf. Newsl., 1978, 3, 425. Olsen, S. D., Strasheim, A., and Perry, A., paper presented at the 9th International Conference on Atomic Spectroscopy and the XXIII Colloquium Spectroscopium Internationale, September 4-8, 1981, Tokyo, Abstract No. 5All. Novak, J. W., and Browner, R. F., Anal. Chem., 1980,52,792. Smith, D. D., and Browner, R. F., Anal. Chem., 1982,54,533. Browner, R. F., Boorn, A. W., and Smith, D. D., Anal. Chem., 1982,54, 1411. Paper J5l15 Received July 15th, I985 Accepted August 2 7th, 1985