Signal fluctuations due to individual droplets in inductively coupled plasma atomic emission spectrometry

Large emission intensity fluctuations are observed from analyte species in inductively coupled plasmas. Relative standard deviations are as large as 71% when emission is viewed with time resolution of 10 microseconds. Low in the plasma, peaks in atom emission intensity are accompanied by depressions in ion emission. This behavior appears to be due to local cooling by aerosol droplets. High in the plasma, peaks in atom emission are followed by peaks in ion emission. These emission spikes result from atomization and ionization of analyte from vaporizing particles. Laser light scattering experiments show that droplets or particles exist in a conventional 1.0-kW plasma up to 20 mm above the load coil. Emission signals detected high in the plasma correlate with laser light scattering signals below.     

Adaptive observation and estimation of trace elements in an inductively coupled plasma

Rules are proposed for observing and estimating element traces that are invariant with respect to background in an atomic emission spectrometer. The concentration dependence of the analytical parameter is used to determine samarium in aqueous solution with an atomic emission spectrometer fitted with an inductively coupled plasma source. The determination error is estimated. Translated from Izmeritel'naya Tekhnika, No. 7, pp. 65–68, July, 1998.     

Comparison of laser ablation and dried solution aerosol as sampling systems in inductively coupled plasma mass spectrometry

This paper describes a study designed to determine the possibility of using a dried aerosol solution for calibration in laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The relative sensitivities of tested materials mobilized by laser ablation and by aqueous nebulization were established, and the experimentally determined relative sensitivity factors (RSFs) were used in conjunction with aqueous calibration for the analysis of solid steel samples. To such a purpose a set of CRM carbon steel samples (SS-451/1 to SS-460/1) were sampled into an ICP-MS instrument by solution nebulization using a microconcentric nebulizer with membrane desolvating (D-MCN) and by laser ablation (LA). Both systems were applied with the same ICP-MS operating parameters and the analyte signals were compared. The RSF (desolvated aerosol response/ablated solid response) values were close to 1 for the analytes Cr, Ni, Co, V, and W, about 1.3 for Mo, and 1.7 for As, P, and Mn. Complementary tests were carried out using CRM SS-455/1 as a solid standard for one-point calibration, applying LAMTRACE software for data reduction and quantification. The analytical results are in good agreement with the certified values in all cases, showing that the applicability of dried aerosol solutions is a good alternative calibration system for laser ablation sampling.     

Numerical investigation for nano-particle synthesis in an RF inductively coupled plasma

Since plasma is regarded as one of the multifunctional fluids which has high energy density, chemical reactivity, controllability by an external electromagnetic field and variable transport properties such as electrical conductivity, it is considerably effective for the synthesis of nano-particles. Since a radio frequency inductively coupled plasma (RF-ICP) has several advantages, the synthesis of ultrafine powders of metals and ceramics with high purity can be easily achieved by the steep temperature gradients at the tail. In the present study, it is clarified how the number density, diameter and specific surface of the produced nano metal-particles of Al, Ti, Au and Pt are influenced by the operating conditions such as the quenching gas flow rate and the powder feed rate of the RF-ICP reactor by numerical investigation. For all the metals, the increase in the quenching gas flow rate results in the increase in the particle number density, the decrease in the mean diameter and the increase in the specific surface. The increase in the powder feed rate causes the increase in the mean diameter but the decrease in the specific surface. The results of four metals are markedly different from each other due to their own material properties of saturation pressure and surface tension.     

Inductively coupled plasma mass spectrometry signal fluctuations due to individual aerosol droplets and vaporizing particles.

ICPMS signal fluctuations, in some cases larger than 1 order of magnitude, are observed on a tens of microsecond time scale. The analyte ICPMS signal fluctuations are larger than those characteristic of analyte ion emission intensities observed from the side of the plasma. The analyte, solvent and plasma gas ICPMS signal fluctuations correlate with atom and ion emission intensities monitored 1.3 mm in front of the sampling cone. Analyte and O+ ICPMS signals are depressed near incompletely desolvated droplets and enhanced near vaporizing analyte particles. ICPMS molecular ion signals including ArH+, O2+, H2O+, and H3O+ are enhanced near both incompletely desolvated droplets and vaporizing particles. In contrast to other species, Ar+ ICPMS signals are depressed near a vaporizing particle. The nature of the ICPMS signal fluctuations (spikes or dips) observed at a fixed sampling position (sampling depth) is critically dependent on the center (nebulizer) gas flow rate.     

Oxygen implantation and diffusion in pure titanium by an rf inductively coupled plasma

The superficial oxidation of pure titanium, 9 mm diameter, 5 mm thick disc samples by implantation and diffusion from inductively coupled plasmas is reported. Such rf plasmas were generated in a 15 l cylindrical Pyrex-like glass chamber containing pure circulating oxygen. A quarter wavelength solenoidal antenna capable of transmitting 500 W at 13.54 MHz was externally wound around the chamber and connected to an rf generator capable of up to 1200 W through an automatic matching network. The oxidation process was carried out for 6 h periods while varying the gas pressure between 1 × 10² and 5 × 10^?1 Pa and the sample bias up to ?3000 V DC. It was found that the sample temperature was a function both of the plasma density and the bias voltage. Without bias, the plasma heated the sample up to ～200 °C, and with maximal bias voltage, the substrate was heated to 680 °C. At the latter temperature, the presence of the rutile phase was particularly evident in X-ray diffraction patterns. According to EDX data, the average oxygen to titanium ratio rose, from ～0.06 for an untreated reference sample, to a ～1.7 value for samples treated up to 680 °C.     

Long path atomic/ionic absorption spectrometry in an inductively coupled plasma

A novel approach was taken to increase the atomic/ionic absorption path length in an inductively coupled plasma (ICP) by using a water-cooled quartz "T-shaped" bonnet. Atomic and ionic absorption spectrometry was performed utilizing a continuum source and line sources. Absorption spectra of synthetic multielement solutions were collected with a photodiode array. Sample introduction into the ICP was accomplished with an ultrasonic nebulizer. To prevent the bonnet from cracking, low radio frequency powers were utilized (i.e., 400-600 W). Plasma diagnostics were performed to study the plasma temperature and electron number density within the "T-shaped" bonnet. Analytical figures of merit were found to be better than those obtained from previous work attempted with inductively coupled plasma atomic absorption spectroscopy and approaching that of flame atomic absorption spectroscopy.     

Analysis of the parameters of plasma of an inductively coupled discharge in neon

An experimental investigation is performed of the electrical and spectral characteristics of plasma of an electrodeless neon light source operating on the principle of inductively coupled discharge. The investigations are performed under the optimal conditions of discharge burning (neon pressure of 1 torr, current frequency of 250 kHz, and discharge current of 3 to 10 A) corresponding to the high efficiency of the light source (20%). The spatial distribution of the radiation intensity of individual lines of neon is studied. Measurements are performed of discharge spectra at different points of the gas-discharge bulb and of the electrical characteristics of the discharge. The resultant experimental data are used to calculate and analyze the parameters of discharge plasma (the electron density and temperature and the concentration of excited atoms).__________Translated from Teplofizika Vysokikh Temperatur, Vol. 43, No. 2, 2005, pp. 181–187.Original Russian Text Copyright © 2005 by M. V. Isupov, and I. M. Ulanov.     

Effect of swirl flow on an atmospheric inductively coupled plasma supersonic jet

Recently supersonic jets extracted from atmospheric inductively coupled plasmas (ICP) have attracted a lot of attention. Usually atmospheric ICP generators have employed swirl flow injection in order to stabilize the plasma. However the effect of the swirl flow injection on the performance of atmospheric ICP generators has not been understood enough. This work found that the injection of the swirl flow deteriorates the performance. First we conducted the total enthalpy measurement based on the sonic flow method and evaluated the averaged specific enthalpy. Then laser absorption spectroscopy was employed in order to measure the temperature and the velocity of the extracted supersonic jet. The both measurements observed that the specific enthalpy of the jets decreased with the increase in the swirl flow fraction. This would be because the swirl flow affected the shape and the position of the plasma significantly.     

Characterization of a second-generation focal-plane camera coupled to an inductively coupled plasma Mattauch-Herzog geometry mass spectrograph

A second-generation Faraday-strip array detector has been coupled to an inductively coupled plasma Mattauch-Herzog geometry mass spectrograph, thereby offering simultaneous acquisition of a range of mass-to-charge ratios. The second-generation device incorporates narrower, more closely spaced collectors than the earlier system. Furthermore, the new camera can acquire signal on all collectors at a frequency greater than 2 kHz and has the ability to independently adjust the gain level of each collector. Each collector can also be reset independently. With these improvements, limits of detection in the hundreds of picograms per liter for metals in solution have been obtained. Some additional features, such as a broader linear dynamic range (over 7 orders of magnitude), greater resolving power (up to 600), and improved isotope ratio accuracy were attained. In addition, isotope ratio precision as low as 0.018% RSD was achieved.     

Characterization of an inductively coupled plasma with xylene solutions introduced as monodisperse aerosols

The optimum operating parameters of the inductively coupled plasma (ICP) with organic solvents are different from those with aqueous solutions. With organic solvents, the ICP is operated at higher power to overcome plasma cooling due to the organic solvent, and aerosol desolvation is necessary in order to reduce solvent load into the plasma. The monodisperse dried microparticulate injector (MDMI) offers the possibility of controlling solvent load by controlling the frequency with which droplets are introduced into the plasma. A test solution that contained 0.5 mg/L Ba in xylene was used to study the influence of MDMI operating parameters on the behavior of the ICP with an organic solvent. The spatial and temporal distribution of the solvent in the ICP was determined for different droplet production frequencies and MDMI oven temperatures, and the optimum operating conditions were established. The best detection limit for Ba in xylene was 1.5 ng/mL, or 0.16 pg (in 200 droplets).     

Quantitative characterization of gold nanoparticles by field-flow fractionation coupled online with light scattering detection and inductively coupled plasma mass spectrometry

An analytical platform coupling asymmetric flow field-flow fractionation (AF(4)) with multiangle light scattering (MALS), dynamic light scattering (DLS), and inductively coupled plasma mass spectrometry (ICPMS) was established and used for separation and quantitative determination of size and mass concentration of nanoparticles (NPs) in aqueous suspension. Mixtures of three polystyrene (PS) NPs between 20 and 100 nm in diameter and mixtures of three gold (Au) NPs between 10 and 60 nm in diameter were separated by AF(4). The geometric diameters of the separated PS NPs and the hydrodynamic diameters of the Au and PS NPs were determined online by MALS and DLS, respectively. The three separated Au NPs were quantified by ICPMS and recovered at 50-95% of the injected masses, which ranged between approximately 8-80 ng of each nanoparticle size. Au NPs adhering to the membrane in the separation channel was found to be a major cause for incomplete recoveries. The lower limit of detection (LOD) ranged between 0.02 ng Au and 0.4 ng Au, with increasing LOD by increasing nanoparticle diameter. The analytical platform was applied to characterization of Au NPs in livers of rats, which were dosed with 10 nm, 60 nm, or a mixture of 10 and 60 nm nanoparticles by intravenous injection. The homogenized livers were solubilized in tetramethylammonium hydroxide (TMAH), and the recovery of Au NPs from the livers amounted to 86-123% of their total Au content. In spite of successful stabilization with bovine serum albumin even in alkaline medium, separation of the Au NPs by AF(4) was not possible due to association with undissolved remains of the alkali-treated liver tissues as demonstrated by electron microscopy images.     

Water analysis by inductively coupled plasma mass spectrometry

The major problems in the analysis of various natural and potable waters by the method of inductively coupled plasma mass spectrometry (sampling, matrix effects, and spectral interferences) are studied; recommendations for addressing them are given. New data on the use of robust conditions for spectrometer settings to increase its tolerance to matrix effects are considered. The advantages of combination of mass spectrometry with the simpler atomic emission method, which allows expanding the range of determined elements and increasing the reliability of the analysis, are discussed.     

Investigation of the plasma uniformity in an internal linear antenna-type inductively coupled plasma source by applying dual frequency

A large area inductively coupled plasma source with an internal linear-type antenna was operated in dual frequency mode (2 MHz/13.56 MHz), and the electrical/plasma characteristics of the ICP source were examined as a function of the relative rf power ratio. When the source was operated in single frequency mode (13.56 MHz only), approximately 8.5% plasma uniformity was observed at 5 kW of 13.56 MHz rf power for the substrate size of 880 mm × 660 mm. The plasma uniformity improved with increasing rf power. However, a further improvement in plasma uniformity to approximately 6.3% could be obtained using the dual frequency mode by applying 0.9 kW of 2 MHz rf power in addition to 5 kW of 13.56 MHz. For 15 mTorr Ar, the plasma density at a dual frequency rf power of 0.9 kW 2 MHz/5 kW 13.56 MHz was 1.6 × 10¹¹/cm³ and the electron temperature was approximately 3 eV. The addition and increase in 2 MHz rf power to the 13.56 MHz power increased the plasma density without increasing the electron temperature.     

Applicability of self-consistent global model for characterization of inductively coupled Cl2 plasma

Investigations of the influence of gas pressure and input power on the Cl₂ plasma parameters in the inductively coupled plasma system were carried out. The investigations combined plasma diagnostics by Langmuir probe with plasma modeling represented by the self-consistent global model with Maxwellian approximation for the electron energy distribution function. From the experiments, it was found that the increase of gas pressure in the range of 0.27-3.33 Pa for 400-700 W of input power results in a decrease in both electron temperature (3.3-2.0 eV) and density (6.5×10¹⁶−3.0×10¹⁶ m⁻³ for 400 W and 1.2×10¹⁷-6.7×10¹⁶ m⁻³ for 700 W). For the given range of experimental conditions, the model showed an outstanding agreement with the experiments and provided the data on kinetics of plasma active species, their densities and fluxes.     

Analysis of pure scandium,yttrium, and their oxides using methods of inductively coupled plasma atomic emission spectrometry and inductively coupled plasma mass spectrometry

The analytical possibilities of direct detection of rare earth and non-rare-earth impurities in pure scandium, yttrium, and their oxides using the ICP-AES and ICP-MS methods are investigated. The analytical lines and isotopes of the sought elements that are the most free from folding are selected. The effect of the sought impurities of the matrix elements on the analytical signal is studied. The detection and determination limits for impurities in scandium and yttrium are estimated. The lower limits of determining impurities in scandium, yttrium, and their oxides at the level of n × 10⁴ mass fractions, %, are reached using the ICP-AES method; those at the level of n × 10⁻⁶ mass fractions, %, are reached using the ICP-MS method. The joint application of the ICP-AES and ICP-MS methods for the analysis of standard specimens of yttrium and scandium oxide is implemented. The control of the validity is accomplished by comparing the obtained results with the certified values of standard specimens and by the method of addition.     

In situ measurements of human cough aerosol hygroscopicity

The airborne dynamics of respiratory droplets, and the transmission routes of pathogens embedded within them, are governed primarily by the diameter of the particles. These particles are composed of the fluid which lines the respiratory tract, and is primarily mucins and salts, which will interact with the atmosphere and evaporate to reach an equilibrium diameter. Measuring organic volume fraction (OVF) of cough aerosol has proved challenging due to large variability and low material volume produced after coughing. Here, the diametric hygroscopic growth factors (GF) of the cough aerosol produced by healthy participants were measured in situ using a rotating aerosol suspension chamber and a humidification tandem differential mobility analyser. Using hygroscopicity models, it was estimated that the average OVF in the evaporated cough aerosol was 0.88 ± 0.07 and the average GF at 90% relative humidity (RH) was 1.31 ± 0.03. To reach equilibrium in dry air the droplets will reduce in diameter by a factor of approximately 2.8 with an evaporation factor of 0.36 ± 0.05. Hysteresis was observed in cough aerosol at RH = ∼35% and RH = ∼65% for efflorescence and deliquescence, respectively, and may depend on the OVF. The same behaviour and GF were observed in nebulized bovine bronchoalveolar lavage fluid.     

Studies of chemical interferences in an inductively coupled plasma using moment analysis of space-resolved emission profiles

Interelement effects induced by sodium, aluminum, and phosphate ion on calcium and magnesium emission were studied with a two-channel spectrometer that could record spatially resolved emission profiles of an atom line and an ion line simultaneously. The statistical moments of the digitized emission profiles were calculated and were employed for rate-constant estimation using our previously established theory. Interelement effects are results of interferences in the analyte atomization, ionization, and/or recombination steps. Characteristic changes will be induced in the emission profiles by each of these interferences. These changes can be characterized by the statistical moments of the corresponding profiles. Therefore, by investigating variations in the rate constants caused by the specific concomitant, one may better understand the analyte atomization-excitation mechanisms. Sodium enhances the atomic emission of both calcium and magnesium but has little effect on their ionic emission. Based on the trends of variation in the rate constants, the atomic signal enhancement seems to be attributed more to the facilitation of analyte atomization rather than to the shift of ionization equilibrium. Aluminum and phosphate ion appear not to induce significant interferences on either elements.     

Diode laser thermal vaporization inductively coupled plasma mass spectrometry

An approach of sample introduction for inductively coupled mass spectrometry (ICPMS), diode laser thermal vaporization (DLTV) is described. The method allows quantitative determination of metals in submicroliter volumes of liquid samples. Laser power is sufficient to induce pyrolysis of a suitable substrate with the deposited sample leading to aerosol generation. Unlike existing sample introduction systems based on laser ablation, it uses a NIR diode laser rather than an expensive high-energy pulsed laser. For certain elements, this sample introduction technique may serve as an alternative to solution analysis with conventional nebulizers. Using a prearranged calibration set, DLTV ICPMS provides rapid and reproducible sample analysis (RSD ~ 10%). Sample preparation is fast and simple, and the prepared samples can easily be archived and transported. The limits of detection for Co, Ni, Zn, Mo, Cd, Sn, and Pb deposited on the preprinted paper were found to be in the range of 0.4-30 pg. The method was characterized, optimized, and applied to the determination of Co in a drug preparation, Pb in whole blood, and Sn in food samples without any sample pretreatment.