JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL. 6 60 1 High Speed Photographic Study of Plasma Fluctuations and Intact Aerosol Particles or Droplets in Inductively Coupled Plasma Mass Spectrometry Royce K. Winge J. S. Crain* and R. S. Houkt Ames Laboratory US Department of Energy Department of Chemistry Iowa State University Ames /A 50011 USA Cine-films of the inductively coupled plasma were taken at 3000 frames s-I while the plasma was sampled for mass spectometry. The axial channel expanded and contracted periodically at frequencies of 260-300 Hz depending on the operating conditions. The frequency of the observed fluctuations decreased as the separation between the torch and sampling cone increased. With a concentric nebulizer emission from vapour clouds surrounding the aerosol droplets or particles was observed flowing along the axial channel and into the sampling orifice.Keywords High-speed photography; plasma fluctuation; aerosol particles and droplets; inductively coupled plasma mass spectrometry Although inductively coupled plasma mass spectrometry (ICP-MS) is very sensitive and selective the precision and stability of the instruments used are not as good as those obtained with their optical predecessors. Improved preci- sion would facilitate many analytical applications of ICP- MS particularly isotope ratio measurements. Characteriza- tion of the noise sources in ICP-MS could provide the basic information necessary for such improvements. Douglas' showed that the factors affecting precision varied with signal magnitude i.e.counting statistics limited the preci- sion at low count rates whereas flicker noise and drift were more important at high count rates. Previous studies of noise power spectra in ICP-MS revealed peaks at discrete frequencies of several hundred H z . ~ ~ ~ Similar peaks are seen in noise power spectra in ICP atomic emission spectro- metry (ICP-AES).4-8 Photographic studies in this laboratory have shown that these noise peaks are caused by physical fluctuations in the plasma i.e. by vortices originating at and progressing along the boundary between the flowing plasma gases and the static ambient atm~sphere.~ This work shows that these plasma fluctuations also occur outside the sampling orifice in ICP-MS at frequencies consistent with those measured previously in the noise spectra of the ion signal.Such fluctuations have also been described in black and white photographs published recently by FurutalO in a paper which dealt primarily with precision in isotope ratio measurements. Recent studies of time-resolved emission signals from ICPs by Olesik and c o - w o r k e r ~ ~ ~ - ~ ~ and Cicerone and F a r n ~ w o r t h ~ ~ indicated that undissociated wet droplets or solid particles may be more important and persist longer in the ICP than previously thought. The present paper shows photographs of emissions from vapour clouds surrounding these droplets or particles for one nebulizer type. These photographs illustrated that the particles are readily drawn into the sampling interface. Experimental The ICP-MS system was the same as that used for previous studies of noise spectra.2 The operating conditions and facilities are identified in Table 1.For photographic clarity the plasma was retracted to sampling positions correspond- ing to 20 and 25 mm betweeen the sampling cone and the nearest portion of the load coil; normal sampling positions for analysis with this device lie in the 10-15 mm range. The framing rate of the camera was nominally 4000 frames s-l. Approximately one half of each 30.5 m roll of film was consumed however by the initial acceleration of the film to this nominal rate. The framing rate for the sequences shown in this manuscript correspond to approximately 3000 frames s-l as determined from red timing marks at 1 ms intervals along the edge of the film.Some of the sequences shown in this manuscript were edited by the authors e.g. successive prints represent every second or third frame from the original film in some instances. Thus the time intervals between the prints varied for the different figures and is indicated in each caption. A photograph of the sampling orifice is provided in Fig. 1. The copper sampling cone employed for the bulk of the work reported here was rounded in contour near the orifice in contrast to the more sharply pointed orifices in use for example on Perkin-Elmer SCIEX ICP-MS instruments. Cine-films showing the use of both tip styles revealed no differences attributable to the geometry of the orifice tip. Also the intense induction region of the plasma was masked by the shielding box thus simplifying the photo- graphy of the plasma zones of interest.In one experiment the plasma was photographed from two orthogonal directions on the same film. A mirror positioned just below the gap between the torch and the sampler (Fig. 2) permitted simultaneous photography of the two views. *Present address Los Alamos National Laboratory Group ?To whom correspondence should be addressed. CLS- 1 MS G740 Los Alamos NM 87545 USA. Fig. 1 Photograph of sampling cone602 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 1991 VOL. 6 Table 1 Experimental facilities and operating conditions Ultrasonic nebulizer (Figs. 3 4 and 6) Concentric pneumatic nebulizer (Figs. 7 and 8) ICP torch Ar flow rates Sampling cone Camera (ref. 9) Film CETAC Model U-5000 uptake rate 2 ml min-l; desolvation heater 110 "C; desolvation condensor 0 "C; yttrium concentration 900 mg 1-I; gas flow rate 1.2 1 min-l uptake rate 0.5 ml min-l; spray chamber double pass Scott-type (ref. 15) uncooled; gas flow rate 0.9 1 rnin-'; yttrium concentration 5000 mg 1-1 Standard Fassel-type (ref.15) three turn load coil grounded near torch exit Outer gas 16 1 min-l; auxiliary gas used only on ignition Ames Laboratory construction (see Fig. 1) sampling orifice 0.9 mm diameter; usual sampling position 25 mm from load coil on centre Meinhard Type C1 Fastax Model WF3; 16 mm Ektachrome No. 7239 and 7251 \ Fig. 2 Mirror arrangement for photographing the ICP from two orthogonal directions. The sampler is not shown. The camera lens is shown on the right Unless specified otherwise a continuous flow ultrasonic nebulizer with desolvation was employed.Initially a concentric pneumatic nebulizer with an uncooled Scott- type double-pass spray chamber was used. This sample introduction system yielded large droplets or particles. As indicated in Table 1 a higher concentration of sample was required to yield clear photographs with the pneumatic nebulizer. Results and Discussion Photographs of ICP As is usual in ICP-MS the plasma was operated horizon- tally. In one film the system was photographed without a sampling cone to indicate whether the vortex or eddy phenomenon observed in vertical plasmas9 also occurred in horizontal plasmas. The fluctuations in this film were similar to those observed previously except that the red eddy from the yttrium oxide (YO) emission was strongest in the upper region of the plasma as illustrated by the edited sequence of three frames in Fig.3. The onset of the red eddy is shown in the top frame as the 'C-shaped' red boundary on the upper side of the ICP. A weaker red eddy can be seen on the lower side of the plasma. The eddy is more highly developed (greater curvature) and has progressed further downstream in the middle frame. The eddy is nearly dissipated in the final frame. The vortices develop symmet- rically about the axis of a vertical p l a ~ m a ~ while convection probably contributes to the asymmetry of the vortices in a horizontal plasma. Fig. 3 also illustrates the initial radiation zone (IRZ the faint red 'tongue' at the right of the axial channel) and the normal analytical zone (NAZ the blue region just downstream from the IRZ).16 Photographs of ICP and Sampling Interface The sequence of prints in Fig.4 illustrates the time fluctuation of the plasma in the vicinity of the sampling orifice. The blue central zone corresponds to emission from the excited Y+ ion. Each frame is separated by 0.6 ms. The red eddy in Fig. 3 is not as evident as in Fig. 4 because the sampling cone blocks the downstream region where the eddy is prominent. The central region of the blue NAZ enters the sampling orifice. The diameter of the NAZ at the sampler fluctuates periodically on a millisecond time scale changing in diameter (or vertical dimension in this figure) by a factor of approximately two between the views shown in the upper and middle frames of Fig.4. For example in Fig. 4 the NAZ is narrowest in the top and bottom frames and is widest in the middle frames. A similar swelling and contraction of the NAZ was observed in a previous photographic study.9 The cine-films from which this se- quence was taken clearly show that the fluctuations start at 500 * N I 2- !! 400 - 3 P LL 300 - 2oo -+- Sampling position/mm Fig. 5 Frequency of plasma fluctuation as a function of sampling position 0 noise power spectra measured by MS (see ref. 2); A measured by counting frames with plasma adjacent to sampler; and A measured by counting frames with plasma retracted fully from the sampler. The ultrasonic nebulizer was usedFig. 3 Three prints from high-speed film of horizontal ICP retracted fully from sampling cone.The plasma is flowing from right to left. Most of the torch and induction region are hidden behind a shielding box; only the end of the torch is visible. Note blue emission from Y+ in the axial channel and red emission from YO and/or neutral Y at the outer edge of the plasma. The three frames shown are 1.2 ms apart; intervening frames have been removed. The ultrasonic nebulizer was used. Time progression is from top to bottom in all the sequences shown Fig. 4 A typical sequence of prints of the ICP during sampling. Note the fluctuation in the diameter of the axial channel. The ultrasonic nebulizer was used. Each frame is separated by 0.6 ms [to face page 6021Fig. 6 Orthogonal views of the ICP during sampling. Note that the eddy develops in the same frame when viewed from either direction.The direct view (i.e. from the side) is on the left; and the reflected view (Le. from the bottom) is on the right. Each frame is separated by 0.6 ms [to face page 6031 Fig. 8 Individual prints (not a time sequence) showing red vapour clouds from (a) a single very large particle and (b) two large particles. The concentric nebulizer was usedJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL. 6 603 the periphery of the plasma and then couple into the axial channel although this coupling is not easily seen in the still photographs reproduced here. Also the plasma fluctuations are shown much more clearly in Figs. 3 and 4 than in the recent black and white photographs of Furuta.’O The frequency of the plasma fluctuations was estimated either from noise-power spectra2 or in the present work by counting the frames and timing marks on the developed film.These frequencies are plotted in Fig. 5 for five sampling positions and also when the sampling cone is completely removed from the plasma. The frequencies decrease as the plasma is retracted from the sampler. The same trend was seen in the noise power spectra (circled data points in Fig. 9 although higher frequencies were seen in that work because the plasma was positioned closer to the sampler in order to yield useful ion signals. Fig. 5 shows that the frequencies measured from noise power spectra and the frequencies measured by counting frames are congruent on a single smooth curve. Thus the noise peaks observed in the 400-550 Hz range at the ‘normal’ sampling positions ( 10- 1 5 mm) arise from the same basic phenome- non as the plasma fluctuations shown photographically in Fig.4. With some intruments these plasma fluctuations are clearly audible with the unaided ear. Both the pitch and amplitude of the resulting whine decrease as the plasma is retracted from the sampling cone in agreement with the trends seen in this study. Simultaneous photographs of the ICP-MS system from two orthogonal directions (with the imaging system as in Fig. 1) are shown in Fig. 6. Note that the camera orientation was changed in order to optimize the framing of the orthogonal views. Also the two images on each frame are not exactly parallel because the mirror was misaligned slightly.In Fig. 6 as in Fig. 4 the NAZ swells and contracts around the tip of the sampling cone and these fluctuations are in phase in the two orthogonal directions. Several frames e.g. the bottom one show that the NAZ is symmetrical with respect to the sampler in one direction but is displaced to one side when viewed from the other direction. Considering the NAZ in the fourth and fifth frames of Fig. 6 i.e. those in which the blue NAZ is narrowest nearly all the blue Y+ ion emission from the NAZ is apparently flowing into the orifice. In other frames where the NAZ is at its maximum width or is oriented symmetrically with respect to the orifice small amounts of Y+ ion emission from the periphery of the NAZ miss the orifice and travel down the outer surface of the cone.This observation illustrates that a large fraction of the NAZ passes into the orifice as indicated by the gas dynamic calculations of Douglas and French.” Because the NAZ broadens as it progresses from the torch the fraction of the NAZ that passes into the sampling orifice decreases as the distance between the torch and the sampling orifice increases. Thus the ion signal decreases as the sampling position is moved downstream from the tip of the IRZ. As mentioned before much of the red eddy (Fig. 3) is blocked by the sampler in Figs. 4 and 6. In some frames a faint red plume from the eddy can be seen to pass downstream along the surface of the cone. All the time- dependent phenomena such as the red plume and the fluctuation in the width of the NAZ are seen much more clearly when the films are either projected or televised from videotape than from the still photographs reproduced in this paper.Emission from Intact Particles Discrete clouds of red emission travelling along the axial channel of the ICP were occasionally observed in the high- speed films with the concentric nebulizer and a 5000 mg 1-’ Y solution. The red clouds are presumably from the Y neutral atom or oxide emission in the vicinity of large undissociated aerosol droplets or solid particles as de- scribed by Olesik and c o - ~ o r k e r s ~ ~ - ’ ~ and Cicerone and Farnsworth.14 The clouds are also reminiscent of the expanding atomic vapour clouds from discrete droplets introduced into flames in early studies by Hieftje and Malmstadt.’* In the subsequent discussion the species responsible for these red emission clouds are simply referred to as ‘particles’.The present study cannot discrimi- nate between wet droplets (desolvation was not employed) and dry solid particles. A short sequence of prints in Fig. 7 illustrates these particles. The top frame does not show a particle but in the next frame a relatively sharp red ‘plume’ has appeared at the tip of the IRZ. This plume is attributed to emission from the vapour cloud surrounding a particle that is just emerging from the IRZ. Such particles are likely to be responsible for the ‘flicker’ often seen in the spatial position of the tip of the IRZ. In the third frame from the top the tip of the IRZ has retreated to its usual position and the particle has moved along the NAZ and is about to flow into the orifice.In the bottom frame the particle is gone. Between the second and third frames the particle has travelled approximately 8 mm estimated relative to the width of the torch. The flow velocity is then about 8 mm per 0.3 ms or about 27 m s-l which is in reasonable agreement with other estimates of flow velocity in the axial channel of the ICP.19920 Fig. 8(a) illustrates a large emission cloud presumably from a particularly large particle. The simultaneous pres- ence of at least two particles is shown in Fig. 8(6). These vapour clouds are elongated because of the distance travelled during the exposure time of the frame. Faint emissions from large particles in the axial channel were seen previously9 with an Ames Laboratory cross-flow nebulizer although such emissions were much less evident than in the present work.No such particles were observed in Figs. 3-5 of the present work which employed an ultrasonic nebu- lizer with desolvation. The sequences in Figs. 7 and 8 also illustrate clearly that most of the axial channel flows into the sampling orifice as described above. In a recent paper Montaser et aL2’ reported time- resolved measurements of the size and spatial position of wet aerosol droplets. This study indicated that at least in some instances individual droplets in a flowing gas stream were not evenly distributed in space. Instead some of the droplets tended to associate closely with one another in small groups of 3-5 droplets per group. Such groups of droplets passing through the plasma could also produce the red emission clouds seen in Figs. 7 and 8 and many of the time-dependent emission characteristics described by Ole- sik and co-workers’ ‘-I3 and Cicerone and F a r n s ~ o r t h .~ ~ Intact particles such as those shown in Figs. 7 and 8 are probably not desirable in ICP-MS instruments. They probably promote deposition in and plugging of the sampling and skimming orifices.22 Those that pass through the sampler might deposit on the skimmer photon stop or perhaps elsewhere in the ion lens. Such insulating deposits would accumulate charge change the effective potentials inside the ion lens and destabilize the ion beam. This effect could be one source of long-term drift in ICP-MS and might explain why the first small stop (sometimes called the ‘shadow stop’) at the base of the skimmer in Perkin-Elmer SCIEX instruments greatly reduces such drift.23 Transit of these large particles through the ICP could also contribute to noise in the signal observed.The deleterious effects of large droplets or particles such as these could be another factor that limits the maximum concentration of matrix elements that is tolerable in ICP-MS. As an extension of these studies the effect of other sample introduction systems e.g. laser ablation on particle behaviour in the ICP should be evaluated photographically.604 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY DECEMBER 199 1 VOL. 6 Ames Laboratory is operated by Iowa State University for the US Department of Energy under Contract W-7405-Eng- 82. This work was supported by the Office of Basic Energy Sciences.References 1 Douglas D. J. Can. J. 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