ANALYTICAL SCIENCES JUNE 2018, VOL The Japan Society for Analytical Chemistry

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1 ANALYTICAL SCIENCES JUNE 2018, VOL The Japan Society for Analytical Chemistry Double-Viewing-Position Single-Particle Inductively Coupled Plasma Atomic Emission Spectrometry for the Selection of ICP Sampling Position in SP-ICP Measurements Ka-Him CHUN, Hua ZHANG, and Wing-Tat CHAN Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, PR China Double-viewing-position single-particle inductively coupled plasma atomic emission spectrometry (DVP-SP-ICP-AES) measures emission intensity at two ICP vertical positions simultaneously using a single photomultiplier tube. A particle travelling up the ICP gives two consecutive temporal emission peaks. The Yb II nm emission intensity of the two peaks for single Yb 2O 3 particles of diameter of nm are plotted against each other in a correlation plot. The correlation is poor when the gas temperature at the lower observation position is approximately the boiling point of the particles. Poor particle vaporization at the center of the central channel occurs because the gas temperature is 400 K lower than the temperature at the rim. The correlation is improved by shifting the observation positions up or using helium argon mixed carrier gas to increase the gas temperature. Gas temperature is an important parameter for precise single particle-icp measurements. DVP-SP-ICP-AES can be used to identify poor particle vaporization without the need of temperature measurement. Keywords ICP-AES, single-particle measurement, viewing position, sampling depth, boiling point, gas temperature (Received February 3, 2018; Accepted April 12, 2018; Published June 10, 2018) Introduction Single-particle inductively coupled plasma atomic emission spectrometry (SP-ICP-AES) is a powerful technique for fundamental study of droplet desolvation and particle vaporization in the ICP, 1,2 as well as determination of the elemental contents of single cells 3 and colloids. 4,5 The related technique of single-particle inductively coupled plasma mass spectrometry (SP-ICP-MS) has also been applied for the determination of the elemental and size distributions of single cells, 6,7 nanoparticles 6,8 20 and colloids SP-ICP-MS is sensitive; gold and silver nanoparticles of diameter as small as 10 nm are readily detected Quantitative analysis is best achieved by calibration using standard particles, 6,28,30 although standard-solution calibration is commonly used due to limited availability of standard particles. 1,31 The basic assumption of the standard-solution calibration method is that ICP-MS intensity depends only on the mass flux of the analyte, irrespective of the original states of the analyte. It is generally accepted that the aerosols of standard solution are completely vaporized in the ICP at typical ICP-MS sampling depth of 10 mm. 32 However, single particles of diameter as small as 250 nm may not be completely vaporized at the same sampling depth because the residence time of the particles at the sampling position is insufficient for complete vaporization. Calibration by standard solution, therefore, becomes inaccurate. It has been shown that the degree of vaporization, as well as the linear dynamic range of the single-particle calibration curve, can be increased by To whom correspondence should be addressed. wtchan@hku.hk increasing the sampling depth so as to provide more time and higher gas temperature for particle vaporization. 33 The degree of vaporization of a 250-nm Au nanoparticle increases from 70 to 92% by increasing the sampling depth of a 1400-W ICP from 5 to 10 mm. 33 To avoid underestimation of the particle size by standard solution calibration, SP-ICP-MS measurement at a position where the sample particles are completely vaporized is vital. 13 In this article, we propose an additional criterion for the selection of the sampling position for SP-ICP-MS and SP-ICP- AES measurements in that the ICP gas temperature at the sampling position must be significantly higher than the boiling point of the sample particles. Since the temperature of the ICP central channel generally increases with distance from the load coil, the optimal sampling position increases with the boiling point of the sample particles. We have previously reported that the shape of the ICP-MS sampling depth profile for standard solutions is strongly dependent on the boiling point of the dry aerosols of the solutions. 32 Analytes that give high boiling oxides as dry aerosols generally have peak positions at higher sampling depths because the aerosols start to vaporize only after the gas temperature reaches the boiling point and the vaporization process takes time to finish. We will show that the boiling-point requirement for aerosol vaporization applies to single particles in the ICP. The precision of SP-ICP measurement is poor at observation positions where the ICP gas temperature and the boiling point of the sample particles are of similar magnitude. The precision improves as gas temperature at the observation position increases. This study focuses on particle vaporization in the ICP. Ideally, the measurement technique for the study is highly sensitive and allows continuous tracking of the ion plumes of individual particles as the plumes travel through the ICP. ICP-MS has

2 712 ANALYTICAL SCIENCES JUNE 2018, VOL. 34 excellent sensitivity (limit of detection (LOD) of Au nanoparticle = 10 nm) 30 but it can only measure the particles at a single point in the ICP because the particles are consumed during the measurement. The evolution of the average characteristics of the ion plumes in the ICP can be inferred from the changes in the peak maximum, width and shape of the SP-ICP-MS intensity distributions at different sampling depths. 37 However, the method assumes one-one correspondence of the percentiles of the intensity distributions and the particle size distribution, which is generally true for a homogeneous plasma but can be false in inhomogeneous plasma such as the ICP. The plasma excitation conditions vary significantly across the radial direction of the ICP central channel. Therefore, the measured intensity of particles of the same particle size may differ significantly, depending on the radial position of the particles in the inhomogeneous plasma. On the other hand, despite the lower sensitivity (LOD of Au nanoparticle = 200 nm), 4 ICP- AES facilitates continuous non-destructive tracking of the development of the ion plume of individual particles as it moves up the central channel. Moreover, simultaneous ICP-AES measurement of multiple emission lines is a powerful diagnostic tool to study the excitation and ionization conditions of the plasma. Axial-viewing SP-ICP-AES has been used to track the particles continuously to study the linear dynamic range and local cooling effects of Au and SiO 2 particles. 1,4 The method, however, does not provide information of the size and position of the ion plume in the ICP central channel and is, therefore, lacking in the ability for the study of particle vaporization and analyte diffusion. Radial-viewing ICP-AES with array detector can provide time-resolved spatial information of the ICP. 38,39 Radial-viewing ICP-AES using an imaging CCD spectrometer directly measures the time-varying spatial distribution of an ion plume. The time resolution, however, is limited by the frame rate of the detector. Direct imaging of the entire plume also requires a large slit width (a few mm). 40 In contrast, ICP-AES with photomultiplier tube (PMT) detectors measures ICP emission at a nominal observation position over the slit height of the spectrometer. For slit height that is small relative to the dimension of the ion plume, the method can be used to determine the dimension of an ion plume by measuring the time-resolved emission of the plume as it moves up the plasma. The dimension of the plume is simply the product of plume velocity and the duration of the plume. The PMT detector has additional advantages of sensitive detection and excellent time resolution down to the nanosecond. The method, however, can only measure the ion plume at a single observation position of the ICP. We propose a novel setup of double-viewing-position single-particle ICP-AES (DVP-SP-ICP-AES) for simultaneous measurement of the emission intensity at two vertical positions of the ICP. As the ion plume of a single particle travels up the ICP, two consecutive temporal emission peaks are observed. The emission intensity of the two peaks for a population of single particles are plotted against each other in a correlation plot. We find that the degree of scattering of the data points in the correlation plot is strongly dependent on the degree of particle vaporization at the lower observation position. In the first version of the setup, a pair of horizontal slits was placed in front of the vertical entrance slit of a monochromator with a single PMT detector. 41 The positions of the two horizontal slits correspond to two vertical observation positions of the ICP. The emission of an ion plume at the two selected observation positions of the ICP was measured as two consecutive temporal emission peaks with microsecond time resolution. Fast data acquisition rate is essential for precise measurement of the shape of the 100-μs transient peaks. The experimental set up is relatively simple and low cost. The increase in background intensity and background noise due to the additional horizontal slit is also relatively small, which is important for the measurement of the emission of small particles. Data processing of the single-column data file is also straightforward. The setup reported in this article makes use of a bifurcated fiber optic bundle in place of the double horizontal slits. The setup allows flexible selection of ICP observation position by simply varying the position and relative distance between the optical fibers. A fundamental study of particle vaporization in the ICP using the optical fiber setup is reported in this article. Ytterbium oxide is selected as the model particle in this study. The emission line of Yb II nm (LOD = μg/ml) is selected instead of Yb I nm (LOD = μg/ml) because of its higher sensitivity. For the study of particle vaporization, it is immaterial whether an ionic line or an atomic line is used because the vaporization process is relatively slow (complete vaporization takes 10 2 to 10 3 μs) 33,37 compared to the ionization and excitation processes (time constant of nanoseconds). 42 In other words, particle vaporization is the rate determining step and the dominating factor of the signal production process of single particles in the ICP. Yb is selected over typical SP-ICP-AES test elements of gold and silicon 1,4,5 because the LOD of Yb is approximately 10 times lower than that of gold (0.017 μg/ml at Au I nm) and silicon (0.012 μg/ml at Si I nm). 43 The strong emission of Yb is beneficial for the detection of small particles over the strong ICP background. In addition, the atomic weight of ytterbium (173.04) is high compared to other strong-emitting elements such as magnesium (24.305) and calcium (40.078). The diffusion coefficient of the high atomic-weight Yb atoms is relatively small. The ion plumes are, therefore, relatively compact. Overlapping of the two consecutive ICP emission peaks is relatively minor in this study and the analysis of the peak shape and peak intensity is relatively simple. The typical sample introduction method of single particles into the ICP for SP-ICP measurement involves preparation of suspension of the particles in aqueous medium and nebulization of the suspension into fine aerosols. 10,44 The aqueous aerosols can change the electron number density, thermal conductivity, and temperature of the plasma. 45 In this study, dry Yb 2O 3 particles were introduced into the ICP directly to eliminate the effects of water on the ICP excitation conditions. In addition, particle vaporization is delayed in the presence of a water droplet because particle vaporization can only start after complete evaporation of the water droplet that encloses the particle. Variation in the size of the polydisperse water droplets leads to uncertainty in the vertical position of the ICP where particle vaporization starts. The use of dry sample particles avoids the convoluted effects of droplet evaporation and particle vaporization. Several studies have used a monodisperse microdroplets generator 2,4,5,46 49 and a monodisperse dried microparticulate injector 40,50,51 to produce monodisperse droplets reproducibly to study the local effects of the droplet evaporation and particle vaporization on plasma cooling, excitation conditions, and emission intensity. The introduction of monodisperse and polydisperse water droplets invariably changes the plasma conditions. 45 Introduction of dry particles provides a direct way to study the particle vaporization. Experimental Figure 1 shows the schematic diagram of the experimental setup of DVP-SP-ICP-AES. The operating parameters are given in

3 ANALYTICAL SCIENCES JUNE 2018, VOL Fig. 1 Schematic diagram of the experimental setup of DVP-SP-ICP-AES. Table 1. The ICP is based on a solid-state MHz RF generator and matching network (CV-2000 and CPMX-2500, Comdel, USA). The 1/4-m monochromator (Cornerstone 260, Newport Corporation, USA) is equipped with a ruled grating with line density of 1200/mm (Model 74063, Newport Corporation) and a photomultiplier tube (Model 77348, Newport Corporation). The entrance slit width was 200 μm. The PMT anodic current was amplified using a current amplifier (SR570, Stanford Research Systems, USA) and the transient signal was digitized at a sampling rate of 100 khz using an analog-todigital conversion card (PCI-6251, National instruments, USA) and LabVIEW 7.0. The plasma was imaged onto the two branched bundles of a high grade fused silica bifurcated fiber optic bundle (transmittance range nm) (Model 77565, Newport Corporation, USA) using a fused silica lens (focal length = 10 cm, diameter = 50.8 mm) with 2 magnification. The bundles were separated by 22 mm vertically, equivalent to displacement in vertical observation position of 11 mm in the ICP. As the diameter of a typical ion plume is generally less than 5 mm at full-width at half-maximum (FWHM), the 11-mm vertical displacement of the observation positions is sufficient to give minimal peak overlap. The diameter of each bifurcated bundle is 2.2 mm. Each bundle monitors a region of the ICP central channel of diameter of 1.1 mm. The common end of the fiber bundle (diameter = 3.2 mm) was placed at the center of the entrance slit of the monochromator. It is noted that the sensitivity of the PMT is dependent on the position of the photocathode. 52 The bifurcated optic fiber bundle with a common end ensures that the image from both branched fiber bundles are projected onto the same region of the photocathode of the PMT and there is no need to account for any difference in detector sensitivity for the two observation positions. The branched fiber bundles and the focusing lens were mounted on an optical platform (part number , Optosigma, USA). The assembly is in turn mounted on a laboratory jack (part number , Optosigma, USA) for adjustment of the observation positions of the ICP. In this study, the emission line of Yb II nm was measured with two sets of ICP observation positions at 8.5 and 19.5 mm and 11.5 and 22.5 mm above the top of the load coil (ALC). Yb 2O 3 particles (Nanopowder, <100 nm (BET), >99.7% trace metal basis, Aldrich, MO, USA) were used in this study. The boiling point of Yb 2O 3 is 4343 K. The particles were dried in an oven at 130 C for 6 h before use. The particles formed Table 1 Operating parameters of DVP-SP-ICP-AES ICP forward power 1000 W Reflected power 10 W RF frequency MHz Outer gas flow rate 15 L/min Intermediate gas flow rate 1 L/min Carrier gas flow rate 1 L/min (argon with 0, 13 or 50% helium) Injector diameter of ICP torch 1.5 mm Observation position 8.5 and 19.5 mm ALC; and 11.5 and 22.5 mm ALC Grating line density 1200 lines/mm Grating blaze wavelength 350 nm Monochromator slit width 200 μm Wavelength Yb II nm Data sampling rate 100 khz Measurement time 400 s aggregates despite drying and gentle grinding of the particles. Dynamic light scattering measurement (3000HS Zetasizer, Malvern Instruments, Worcestershire, UK) shows that the particle aggregates are up to 3000 nm in diameter with polydispersity index of 1. The size of the Yb 2O 3 particles introduced into the ICP for SP-ICP-AES measurement was estimated by solution calibration using standard solution of 125 μg/ml of Yb. The estimated particle diameter is nm. Dried particles of Yb 2O 3 were introduced into the ICP using a custom-made sample introduction device (Fig. 2). For each experiment, 0.3 mg (approximately 10 5 particles) of the dried Yb 2O 3 particles was placed on the sample holder (a plastic disc, 2 cm in diameter, 0.4 cm in height) which was supported on a spring. The solenoid underneath the sample holder was energized every 3 s to drive the small metal rod on the solenoid, which in turn struck the disc to generate a suspension of Yb 2O 3 particles above the inner plastic cylinder of the sample introduction device. The suspended particles were carried to the ICP by a stream of carrier gas of flow rate of 1 L/min as a quasi-continuous flow of Yb 2O 3 particles. The efficiency of particles introduction is approximately 1% (10 3 temporal peaks were obtained per sample). The Yb 2O 3 particles enter the ICP randomly. The probability of particle overlap in the ICP can be estimated using Poisson statistics. 5 Within the first 15 s of the sample introduction cycle, the frequency of the occurrence of the peak pairs was 160 Hz or

4 714 ANALYTICAL SCIENCES JUNE 2018, VOL. 34 Fig. 3 Typical background-corrected temporal profile of DVP-SPICP-AES measurement of Yb2O3. λ = Yb II nm. Observation position = 8.5 and 19.5 mm. Fig. 2 Schematic diagram of the sample introduction device. higher. The probability of peak overlap is more than 9%. The data were rejected because of high probability of particle overlap. The frequency of the occurrence of the peak pairs decreased to 50 Hz or lower after 15 s of sample introduction. The probability of peak overlap is reduced to 3% or less. Temporal data of Yb II emission over 400 s after the first 15 s were acquired for each experiment. The temporal profiles were inspected manually to remove any overlapped double-peaks of which the individual peaks in the double-peak are asymmetric, the base width is in excess of 1.2 ms, and/or the number of local maxima within a duration of 500 μs is larger than 2. The rejection rate is approximately 5%. MgO particles (Nanopowder, <50 nm (BET), Aldrich, MO, USA) were also used as test particles. The size of the particles is estimated to be nm and the boiling point is 3873 K.53 Results and Discussion Figure 3 shows a typical temporal profile of the transient emission of single particles of Yb2O3 measured by DVP-SPICP-AES. The temporal profile takes the form of two consecutive peaks, corresponding to observation positions of 8.5 and 19.5 mm for the first and second peaks, respectively. The pair of transient peaks are referred to as double-peak in the following discussion. The basewidth of a double-peak is approximately 1.2 ms. The two temporal peaks are generally well resolved. The time difference between the two peaks is 600 ± 60 μs. The average particle velocity is, therefore, 18 m/s, which is simply the quotient of the distance between the two observation positions and the time difference. The FWHM of the first and second peaks are 150 ± 17 and 250 ± 35 μs, respectively, equivalent to an ion plume of dimension of 2.7 and 4.5 mm FWHM at 8.5 and 19.5 mm ALC, respectively. The height of the temporal peaks is taken as the intensity of the peaks, which are referred to as I1 and I2 for the first and second peaks, respectively, in the following paragraphs. Figure 4 shows the correlation plot of I2 versus I1. The large Fig. 4 Correlation plot of I2 versus I and 19.5 mm. Observation position = degree of scattering of the data points in the plot is unexpected. Ideally, particles of the same particle diameter would follow the same signal production processes in the ICP (heating to melting and boiling, followed by particle vaporization, atomization, ionization, and excitation) and give constant intensities of I1 and I2 at the first and second observation positions, respectively. A correlation plot of I2 versus I1 of any selected particle diameter would give a single point in the plot. It follows that a population of particles with a range of particle diameter would ideally give a line in the correlation plot. The scattering in the correlation plot is not a result of changes in the sensitivity or random errors of ICP-AES measurement. Introduction of particles into the ICP does not cause significant change in the plasma temperature and electron number density.54 The sensitivities at the two observation positions should, therefore, remain constant throughout the experiment. The scattering of data points in the correlation plot is also unlikely due to random variation of the measured intensity because the signal-to-noise ratio (SNR) of the temporal peaks is relatively large, ranging from 10 to 350. In addition, similar patterns of scattering of data points in the correlation plots are also observed in separate experiments using MgO as sample particles. We propose that the large degree of scattering of data points in

5 ANALYTICAL SCIENCES JUNE 2018, VOL the correlation plot in Fig. 4 is related to heat transfer from the ICP to the Yb 2O 3 particles and the degree of vaporization of the particles. The boiling point of the Yb 2O 3 particles is 4343 K. Before the start of particle vaporization, the particle must be heated up to the boiling point. The measured excitation temperature of the ICP at 8.5 mm ALC is 4300 and 4700 K at radial position of 0 and 1.5 mm from the center of the ICP central channel, respectively. The temperature increases to 4700 and to 4800 K, respectively, at 19.5 mm ALC. The excitation temperature was determined using a Boltzmann plot of Fe I , , and nm. 55 The measured excitation temperature is assumed to have the same magnitude of the gas-kinetic temperature. 42,43 It is interesting to note that the measured temperature is comparable to gas temperature reported in the literature for ICP of similar operating parameters. 34 The temperature at the first observation position of 8.5 mm ALC was estimated to be 4400 to 4800 K by interpolation of the ICP gas temperature at 5 and 15 mm ALC in Ref. 34. It is noted that the gas temperature at 8.5 mm ALC (the first observation position) is approximately the boiling point of the Yb 2O 3 particles at the center of the central channel and increases to 400 K above the boiling point at the rim of the central channel. Since the particles enter the ICP central channel at random locations within the cross sectional area of the injector tube of the ICP torch, 56 large variation in the degree of particle vaporization at the first observation position is expected. Particles travelling along the center of the ICP central channel have barely reached the boiling point at the first observation position. The degree of vaporization is expected to be very small. On the other hand, particles travelling along the rim of the central channel experience a gas temperature of a few hundred degrees above the boiling point at the same observation position. The degree of vaporization is expected to be substantially higher. In other words, particles of the same diameter will give a wide range of intensity at the first observation position (I 1), depending on the radial location of the particles. The correlation plot for any particle is, therefore, not a single point but rather a collection of data points scattered along the I 1 axis (Fig. 4). It is noted that the range of I 1 is broader for large particles (of large I 2) because the mass of vaporized materials is proportional to the power 3 of the particle diameter. In addition, the effect of gas temperature on particle vaporization is more prominent for large particles because the particles take more energy and time to heat to the boiling point, leading to an even lower degree of vaporization in the center of the ICP central channel. Therefore, I 1 varies significantly for large particles, depending on the pathway of the particles in the ICP central channel. It should be noted that the lower limit of I 1 in Fig. 4 is not zero. The lower end of I 1 is approximately 0.01 V for all particle size in Fig. 4, which is the measurement limit of the current DVP-SP-ICP-AES system. To summarize, large scattering of I 1 for the large particles in the correlation plot in Fig. 4 is due to relatively low temperatures (approximately the boiling point of the sample particles) in the center of the central channel and a large temperature difference across the central channel at the first observation position. The degree of particle vaporization in the center of the ICP central channel is very small or negligible. It follows that increasing the plasma temperature and the heating rate will increase the degree of vaporization of the large particles and reduce the degree of scattering of I 1 in the correlation plot. The hypothesis is supported by reduction in the degree of scattering of the data points in the correlation plot by moving the observation positions upward by 3 mm to 11.5 and 22.5 mm (Fig. 5). Displacement of lower observation position upward by 3 mm increases the Fig. 5 Correlation plot of I 2 versus I 1. Observation position = 11.5 and 22.5 mm. plasma gas temperature to 4500 and to 5000 K at the center and the rim of the central channel, respectively. The 200-K increase in gas temperature and additional time for particle vaporization substantially increase the degree of particle vaporization at the new observation position. The relative difference in the degree of vaporization for particles located at the center and the rim of the ICP central channel is expected to be reduced. Indeed, a relatively large degree of scattering of data points in the correlation plot is observed only for the large particles (I V, the top 10 percentile of the particle mass), probably because a relative long duration of time is required to heat up the large particles to boiling and the degree of vaporization of the particles is still relatively small in the center of the ICP central channel. The hypothesis in the last paragraph that low plasma temperature results in a small degree of particle vaporization and large degree of scattering of data points in the correlation plot is further supported by introduction of helium in the argon carrier gas flow to increase the heat conduction rate and temperature of the plasma gas. In Figs. 6(a) and 6(b), 13 and 50% of helium, respectively, was added to the argon carrier gas. The degree of scattering in the correlation plots indeed decreases as He content in the carrier gas increases. The gas-kinetic temperature, 57 excitation temperature and ionization temperature 58 were reported to increase by 10 3 K with the addition of 16.7% of He in the carrier gas flow. For the 13% helium mixed gas plasma, the measured excitation temperature at 8.5 mm ALC is 5000 and 5100 K at the center and the rim of the central channel, respectively, which is approximately 700 K higher than that of pure argon. The heat conduction rate in the mixed gas ICP also increases substantially. At 5000 K, the thermal conductivities of helium and argon are 0.9 and 0.1 W/mK, respectively. 59 In Fig. 6(a), with 13% of helium in the carrier gas flow, the degree of scattering of I 1 for I 2 smaller than 0.9 V is significantly reduced. Further increase of helium content to 50% in the carrier gas flow results in a relatively narrow band over the entire intensity range in the correlation plot (Fig. 6(b)). All particles, no matter the particle size, probably have reached the boiling point well below the first observation position and have sufficient time to vaporize. The difference in gas temperature across the central channel also decreases with the introduction of He in the argon carrier gas flow, 57 which further reduces the range of I 1 for all particle sizes.

6 716 ANALYTICAL SCIENCES JUNE 2018, VOL. 34 Fig. 7 Temporal profile of DVP-SP-ICP-AES measurement of Yb 2O 3 particles with large I 2 and small I 1. λ = Yb II nm. Observation position = 8.5 and 19.5 mm. Conclusions Fig. 6 Correlation plots of I 2 versus I 1 with (a) 13 and (b) 50% helium in the argon carrier gas flow. Observation position = 8.5 and 19.5 mm. Similar reduction in the degree of scattering of I 1 (Mg I nm) in the correlation plots of MgO particles (boiling point 3873 K) has been observed by moving the observation position upward or introduction of helium to the carrier gas flow. The results of MgO and Yb 2O 3 particles show that SP-ICP-MS or SP-ICP-AES measurement at the sampling position of a gas temperature around the boiling point of the sample particles is erratic because of small degree of vaporization of the particles. Apart from the large range of I 1 for large Yb 2O 3 particles in the correlation plot (Fig. 4), it is also observed that, for doublepeaks of large particles (I 2 > 0.7 V), approximately 66% of the first transient peak is asymmetric with long tail (N = 35) (Fig. 7). The asymmetric peaks are generally low in intensity. In contrast, for those double-peaks with relatively large I 1, the first peak is relatively symmetric (Fig. 3). Furthermore, with 50% of helium in the carrier gas flow, all first peaks become symmetric with high intensity. The asymmetric peaks may be related to a very small degree of particle vaporization at the first observation position. The mechanisms for the generation of the asymmetric peaks require further investigation. A computer model is being developed to study the evolution of the ion plumes of single particles in the ICP. The model accounts for the rate of heating and vaporization of the single particles and ionization, diffusion, and excitation of the analyte atoms. The investigation will be reported in a separate article. Careful selection of observation position is essential for accurate and precise SP-ICP measurement. The observation position should be at least a few millimeter higher than the vertical position of the central channel, the gas temperature of which equals the boiling point of the sample particles. At an observation position of gas temperature approximately equalling the boiling point, the significant variation in the measured intensity will lead to incorrect estimation of the distribution of particle size. Incomplete particle vaporization also gives a measured intensity that is not directly proportional to the particle mass. In addition, some of the larger particles may not have reached the boiling point in time due to large heat capacity and are not detected, leading to an underestimation of particle number density. We propose that the rule of selecting sampling position with ICP gas temperature well above the boiling point of the sample particles is applicable to all particle sizes, although only particles of relatively large particle diameters were used in this study. High sampling position for SP-ICP-MS and SP-ICP- AES is beneficial in that the degree of particle vaporization is high and variation in the measured intensity and negative errors in the determination of particle size of the large particles are reduced. The position of complete particle vaporization estimated using an empirical equation is a convenient starting point for optimization of the sampling depth. 37 The double-viewing-position method will enhance the analytical performance of SP-ICP-AES in that the double-peaks can be used for unambiguous identification of emission of single particles against the random background noise. Correlation of the peak pairs may also improve the SNR of SP-ICP-AES measurements. The DVP-SP-ICP-AES setup is also a useful tool for SP-ICP-MS measurement. The degree of scattering in the correlation plot can be used as a criterion for optimization of the sampling position of SP-ICP-MS. The large degree of scattering in the correlation plot hint to a small degree of particle vaporization at the first observation position and sampling at this position should be avoided. Acknowledgements This work was supported by a grant from the Research Grant Council of the Hong Kong Special Administrative Region,

7 ANALYTICAL SCIENCES JUNE 2018, VOL China (Project No ) and the Seed Funding Programme for Basic Research of The University of Hong Kong. We would like to thank Prof. Gary Horlick for his generous donation of the Plasma Therm RF 2500D generator and matching network. The initial work of the current study was based on the instrument. References 1. S. Groh, C. C. Garcia, A. Murtazin, V. Horvatic, and K. Niemax, Spectrochim. Acta, Part B, 2009, 64, A. Murtazin, S. Groh, and K. Niemax, Spectrochim. Acta, Part B, 2012, 67, T. Nomizu, S. Kaneco, T. Tanaka, D. Ito, H. Kawaguchi, and B. T. Vallee, Anal. Chem., 1994, 66, C. C. Garcia, A. Murtazin, S. Groh, V. Horvatic, and K. Niemax, J. Anal. At. Spectrom., 2010, 25, A. Murtazin, S. Groh, and K. Niemax, J. Anal. At. Spectrom., 2010, 25, K.-S. Ho and W.-T. Chan, J. Anal. At. Spectrom., 2010, 25, W.-Y. Lau, K.-H. Chun, and W.-T. Chan, J. Anal. At. Spectrom., 2017, 32, T. Nomizu, H. Hayashi, N. Hoshino, T. Tanaka, H. Kawaguchi, K. Kitagawa, and S. Kaneco, J. Anal. At. Spectrom., 2002, 17, C.-N. Tsang, K.-S. Ho, H. Sun, and W.-T. Chan, J. Am. Chem. Soc., 2011, 133, J. W. Olesik and P. J. Gray, J. Anal. At. Spectrom., 2012, 27, H. E. Pace, N. J. Rogers, C. Jarolimek, V. A. Coleman, E. P. Gray, C. P. Higgins, and J. F. Ranville, Environ. Sci. Technol., 2012, 46, J. Tuoriniemi, G. Cornelis, and M. Hassellöv, Anal. Chem., 2012, 84, K.-S. Ho, K.-O. Lui, K.-H. Lee, and W.-T. Chan, Spectrochim. Acta, Part B, 2013, 89, L.-N. Zheng, M. Wang, B. Wang, H.-Q. Chen, H. Ouyang, Y.-L. Zhao, Z.-F. Chai, and W.-Y. Feng, Talanta, 2013, 116, A. Hineman and C. Stephan, J. Anal. At. Spectrom., 2014, 29, F. Laborda, E. Bolea, and J. Jiménez-Lamana, Anal. Chem., 2014, 86, J. Liu, K. E. Murphy, R. I. MacCuspie, and M. R. Winchester, Anal. Chem., 2014, 86, S. Miyashita, A. S. Groombridge, S. Fujii, A. Minoda, A. Takatsu, A. Hioki, K. Chiba, and K. Inagaki, J Anal. At. Spectrom., 2014, 29, W.-Y. Lau, K.-H. Chun, and W.-T. Chan, J. Anal. At. Spectrom., 2017, 32, M. H. P. Yau and W.-T. Chan, J. Anal. At. Spectrom., 2005, 20, C. Degueldre and P.-Y. Favarger, Colloids Surf. Physicochem. Eng. Asp., 2003, 217, C. Degueldre, Talanta, 2004, 62, C. Degueldre, P.-Y. Favarger, and C. Bitea, Anal. Chim. Acta, 2004, 518, C. Degueldre, P.-Y. Favarger, R. Rossé, and S. Wold, Talanta, 2006, 68, C. Degueldre, P.-Y. Favarger, and S. Wold, Anal. Chim. Acta, 2006, 555, F. Laborda, J. Jiménez-Lamana, E. Bolea, and J. R. Castillo, J. Anal. At. Spectrom., 2013, 28, S. Gschwind, L. Flamigni, J. Koch, O. Borovinskaya, S. Groh, K. Niemax, and D. Günther, J. Anal. At. Spectrom., 2011, 26, F. Laborda, J. Jiménez-Lamana, E. Bolea, and J. R. Castillo, J. Anal. At. Spectrom., 2011, 26, B. Franze, I. Strenge, and C. Engelhard, J. Anal. At. Spectrom., 2012, 27, R. Peters, Z. Herrera-Rivera, A. Undas, M. van der Lee, H. Marvin, H. Bouwmeester, and S. Weigel, J. Anal. At. Spectrom., 2015, 30, S. Lee, X. Bi, R. B. Reed, J. F. Ranville, P. Herckes, and P. Westerhoff, Environ. Sci. Technol., 2014, 48, K.-S. Ho, W.-W. Lee, and W.-T. Chan, J. Anal. At. Spectrom., 2015, 30, W.-W. Lee and W.-T. Chan, J. Anal. At. Spectrom., 2015, 30, M. Huang, S. A. Lehn, E. J. Andrews, and G. M. Hieftje, Spectrochim. Acta, Part B, 1997, 52, P. Yang, J. A. Horner, N. N. Sesi, and G. M. Hieftje, Spectrochim. Acta, Part B, 2000, 55, M. Huang, D. S. Hanselman, P. Yang, and G. M. Hieftje, Spectrochim. Acta, Part B, 1992, 47, K.-S. Ho, K.-O. Lui, K.-H. Lee, and W.-T. Chan, Spectrochim. Acta, Part B, 2013, 89, M. W. Blades and G. Horlick, Appl. Spectrosc., 1980, 34, M. W. Blades and G. Horlick, Spectrochim. Acta, Part B, 1981, 36, J. W. Olesik, J. A. Kinzer, and G. J. McGowan, Appl. Spectrosc., 1997, 51, H. Zhang, MPhil Thesis, Characterization of Signal- Production Processes of Single Particles in ICP by Timeresolved ICP-AES, The University of Hong Kong, J. M. Mermet, Anal. Chim. Acta, 1991, 250, A. Montaser and D. W. Golightly, Inductively Coupled Plasmas in Analytical Atomic Spectrometry, 1992, VCH Publishers. 44. H.-A. Kim, B.-T. Lee, S.-Y. Na, K.-W. Kim, J. F. Ranville, S.-O. Kim, E. Jo, and I.-C. Eom, Chemosphere, 2017, 171, I. Novotny, J. C. Farinas, W. Jia-liang, E. Poussel, and J.-M. Mermet, Spectrochim. Acta, Part B, 1996, 51, S. Groh, C. C. Garcia, A. Murtazin, V. Horvatic, and K. Niemax, Spectrochim. Acta, Part B, 2009, 64, G. C.-Y. Chan and G. M. Hieftje, Spectrochim. Acta, Part B, 2016, 121, G. C. Y. Chan, Z. Zhu, and G. M. Hieftje, Spectrochim. Acta, Part B, 2012, 76, G. C. Y. Chan, Z. Zhu, and G. M. Hieftje, Spectrochim. Acta, Part B, 2012, 76, J. W. Olesik and S. E. Hobbs, Anal. Chem., 1994, 66, J. W. Olesik, Appl. Spectrosc., 1997, 51, 158A. 52. Newport Corporation, Photomultiplier Tube, CRC Handbook of Chemistry and Physics: a Readyreference Book of Chemical and Physical Data, ed. D. R. Lide, 1998, 79th ed., CRC, Boca Raton. 54. J. A. Horner, G. C.-Y. Chan, S. A. Lehn, and G. M. Hieftje, Spectrochim. Acta, Part B, 2008, 63, N. Furuta, Spectrochim. Acta, Part B, 1985, 40, A. Bogaerts and M. Aghaei, J. Anal. Spectrom., 2017, 32, N. N. Sesi, A. Mackenzie, K. E. Shanks, P. Yang, and G. M. Hieftje, Spectrochim. Acta, Part B, 1994, 49, C. Chan, MPhil Thesis, Investigation of Matrix Effects on Excitation Conditions of Dry Inductively Coupled Plasma Using Laser Ablation, The University of Hong Kong, J. O. Hirschfelder, C. F. Curtiss, and R. B. Bird, Molecular Theory of Gases and Liquids, 1954, John Wiley and Sons Inc., New York.