Considerations of particle vaporization and analyte diffusion in single-particle inductively coupled plasma-mass spectrometry

The intensity of individual gold nanoparticles with nominal diameters of 80, 100, 150, and 200 nm was measured using single-particle inductively coupled plasma-mass spectrometry (ICP-MS). Since the particles are not perfectly monodisperse, a distribution of ICP-MS intensity was obtained for each nominal diameter. The distribution of particle mass was determined from the transmission electron microscopy (TEM) image of the particles. The distribution of ICP-MS intensity and the distribution of particle mass for each nominal diameter were correlated to give a calibration curve. The calibration curves are linear, but the slope decreases as the nominal diameter increases. The reduced slope is probably due to a smaller degree of vaporization of the large particles.In addition to the degree of particle vaporization, the rate of analyte diffusion in the ICP is an important factor that determines the measured ICP-MS intensity. Simulated ICP-MS intensity versus particle size was calculated using a simple computer program that accounts for the vaporization rate of the gold nanoparticles and the diffusion rate and degree of ionization of the gold atoms. The curvature of the simulated calibration curves changes with sampling depth because the effects of particle vaporization and analyte diffusion on the ICP-MS intensity are dependent on the residence time of the particle in the ICP. Calibration curves of four hypothetical particles representing the four combinations of high and low boiling points (2000 and 4000 K) and high and low analyte diffusion rates (atomic masses of 10 and 200 Da) were calculated to further illustrate the relative effects of particle vaporization and analyte diffusion. The simulated calibration curves show that the sensitivity of single-particle ICP-MS is smaller than that of the ICP-MS measurement of continuous flow of standard solutions by a factor of 2 or more. Calibration using continuous flow of standard solution is semi-quantitative at best.An empirical equation is formulated for the estimation of the position of complete vaporization of a particle in the ICP. The equation takes into account the particle properties (diameter, density, boiling point, and molecular weight of the constituents of the particle) and the ICP operating parameters (ICP forward power and central channel gas flow rate). The proportional constant and exponents of the variables in the equation were solved using literature values of ICP operating conditions for single-particle inductively coupled plasma-mass spectrometry (ICP-MS) and inductively coupled plasma-atomic emission spectrometry (ICP-AES) measurements of 6 kinds of particles in 12 studies. The calculated position is a useful guide for the selection of sampling depth or observation height for ICP-MS and ICP-AES measurements of single particles as well as discrete particles in a flow, such as laser-ablated materials and airborne particulates.     

Effect of sampler and skimmer orifice size on analyte and analyte oxide signals in inductively coupled plasma-mass spectrometry

The size of the orifice in the sampling cone of the interface in an inductively coupled plasma-mass spectrometer (ICP-MS) has a major impact on signal characteristics. In particular, for oxide forming elements, the MO⁺/M⁺ ratio is very dependent on orifice size. This is illustrated for a range of elements (La, Ho and Yb) as a function of nebulizer gas flow and for sampling cone orifice sizes ranging from 0.51 to 0.94 mm. In addition the size of the sampling orifice changes the shape of the signal vs nebulizer flow rate plot and this is illustrated. A curious and important observation is that the signal dependence on nebulizer flow rate is essentially identical for all analytes at any one orifice size if the analyte and analyte oxide (when observed) responses are summed. This indicates that the sampling orifice is the primary location of oxide formation in ICP-MS. Finally the effect of the skimmer orifice diameter on analyte signals is also illustrated for orifice diameters of 0.76, 0.89 and 1.0 mm in conjunction with sampling orifice sizes ranging from 0.51 to 0.94 mm.     

The effect of plasma operating parameters on analyte signals in inductively coupled plasma-mass spectrometry

Utilizing the SCIEX ICP-MS an extensive study of the effects that plasma operating parameters have on analyte ion signals in ICP-MS has been carried out. Parameters studied included aerosol flow rate (nebulizer pressure), auxiliary flow rate, power and sampling depth (sampling position from the load coil). The two key parameters are aerosol flow rate (nebulizer pressure) and power. Elements can be grouped into characteristic behaviour patterns based on the overall dependence of their ion count signal on these two parameters. The nebulizer pressure-power behavior patterns allow a sensible selection of compromise operating conditions and significantly clarify single parameter observations which often indicate confusing trends in behavior. In addition to characterizing analyte ion signals the parameter behavior plots have also been used to study oxide species and plus two ions in ICP-MS. While aerosol flow rate and power appear to be the key ICP parameters in ICP-MS, ion signals are dependent on sampling depth and auxiliary flow rate and some data are also presented illustrating the signal dependence on these two parameters.     

电感耦合等离子体质谱仪(ICP-MS)在环境监测领域日常维护及故障排除

以珀金埃尔默公司(PE)生产的NexION300D为例,基于环境监测系统使用经验,总结了包括进样系统、等离子体源(ICP)、接口、透镜、四级杆分析器、检测器和内置于质谱仪中的真空系统,外连接冷却循环水及空气过滤网等部件日常维护要点,并对容易出现的点火问题、灵敏度低、氧化物高、精密度差、质量数8和220的背景值高、校准曲线异常等故障原因进行分析,并提出解决办法。认为未来电感耦合等离子体质谱仪(ICP-MS)与其它仪器联用技术的相关维护是ICP-MS从业者的挑战和机遇。     

Signal enhancement effect of halogen matrix in electrothermal vaporization-inductively coupled plasma-mass spectrometry

In tungsten furnace electrothermal vaporization(ETV)-inductively coupled plasma mass spectrometry(ICP-MS), the presence of halogen matrices caused a signal enhancement for volatile elements such as Zn, Cd and Pb, whose halides melting and boiling points were relatively low. In order to clarify the mechanism of signal enhancement in ETV-ICP-MS, the effects of chemical interaction between analytes and halogen matrices on the surface of ETV furnace, the transport efficiency of vaporized analytes from the furnace into the ICP and the physical properties of the ICP itself and of the micro plasma (interface plasma) in the interface region between the sampling and the skimmer cones were investigated by atomic absorption and atomic emission spectrometry. Among the effects mentioned above, neither the chemical interaction on the surface of the ETV furnace nor the transport efficiency of vaporized analytes could be related to the analyte signal enhancements. The degree of enhancement was found to depend on the ionization potential of the coexisting halogen and was not caused by a variation in the physical properties of the ICP but rather by a variation of those of the interface plasma. These results suggest that the halogen matrices may affect the physical properties of the interface plasma, contributing to the promotion of the ionization of analytes.     

The effect of the sampling interface on spatial distributions of barium ions and atoms in an inductively coupled plasma ion source

Planar laser-induced fluorescence was used to examine the effect of the sampling cone on analyte atom and ion distributions in the inductively coupled plasma used as an ion source for elemental mass spectrometry. Comparisons of planar laser-induced fluorescence images in the presence and absence of the sampling interface reveal that the insertion of the sampling cone into the plasma dramatically lowers singly-charged ion densities in the 1–2 mm region immediately upstream from the sampling cone, but increases densities in the region between 2 mm and 10 mm upstream from the sampling cone. Some of the drops in densities near the sampling cone can be attributed to acceleration of the plasma through the pumped sampling orifice. A shift in equilibrium between doubly and singly charged barium ions caused by cooling of the plasma is proposed to account for the increases in densities of Ba⁺ in the upstream region.     

Characterization of the ion beam inside the skimmer cone of an inductively coupled plasma mass spectrometer by laser excited atomic and ionic fluorescence

Laser-induced atomic and ionic fluorescence have been used to characterize the material extracted from an inductively coupled plasma through a differentially pumped interface of the type used in inductively coupled plasma mass spectrometry. Measurements were made in the 8 mm downstream from the tip of the skimmer cone. Reference measurements were also made outside the interface at the tip of the sampling cone. Sc and Ba ions and Pb atoms were used as test analytes. Sc⁺ densities drop more rapidly than either Ba⁺ or Pb densities. The transmission efficiencies of both Sc⁺ and Ba⁺ are suppressed by the addition of either Mg or Pb to the analyte solution. The effects of the two matrix additions are approximately the same. Based on the magnitude of the fluorescence signal for Ba⁺, a lower limit of 0.3% for the transmission of Ba⁺ from the plasma into the second vacuum stage has been calculated.     

Trace anion determination in concentrated hydrofluoric acid solutions by two-dimensional ion chromatography I. Matrix elimination by ion-exclusion chromatography

Since years, ion exclusion chromatography (ICE) has been the standard method to separate strong acid analyte anions from concentrated weak acid matrices such as hydrofluoric acid (HF). In this work, the commercially available IonPac ICE-AS 1 column was used to separate trace levels of chloride, nitrate, sulfate and phosphate from HF solutions at 20% (w/w). The efficiency of the separation was studied in more detail using techniques such as ion chromatography (IC), inductively coupled plasma optical emission spectrometry (ICP-OES) and ICP-mass spectrometry (ICP-MS). For 20% (w/w) HF solutions and at a water carrier flow-rate of 0.50 ml/min, the cut window was set from 8.5 to 14.5 min. Under these conditions, analyte recoveries of better than 90% were obtained for chloride, nitrate and sulfate, but only about 75% for phosphate. The HF rejection efficiency was better than 99.9%. It was found that the ICP techniques, measuring total element levels and not species, yielded significantly higher recoveries for phosphorus and sulfur compared to IC. Evidence will be given that part of the added phosphorus (approximately 15% for an addition of 10 mg PO4/kg) is present as mono-fluorophosphoric acid (H2FPO3). In the case of sulfate, the difference between IC and ICP-MS could be attributed to an important matrix effect from the residual HF concentration.     

Local effects of atomizing analyte droplets on the plasma parameters of the inductively coupled plasma

Monodisperse droplets from aqueous analyte solutions with selected diameters in the range 35–67 µm are introduced into an inductively coupled plasma with frequencies between 1 to 10 droplets per second. The effect of desolvation and atomization in the ICP is studied end-on by optical emission spectroscopy employing simultaneously up to three calibrated monochromators with fast photomultipliers. The onsets of desolvation and analyte atomization and the extremely fast diffusion of hydrogen in the ICP and its effect on the plasma are studied by simultaneous measurements of hydrogen, analyte and Ar lines. The local cooling by analyte atomization as well as the recovering of the plasma excitation temperature after completed atomization is measured quantitatively in dependence on time applying the Boltzmann plot method to simultaneously recorded line intensities of atomized analyte atoms which act as plasma probes. Furthermore, it is shown that relatively small differences in analyte mass cause significant temperature changes during atomization and, as consequence, strong variations of the emission intensity of analyte lines during atomization if measured by end-on observation.     

Method optimization and quality assurance in speciation analysis using high performance liquid chromatography with detection by inductively coupled plasma mass spectrometry

Achievement of optimum selectivity, sensitivity and robustness in speciation analysis using high performance liquid chromatography (HPLC) with inductively coupled mass spectrometry (ICP-MS) detection requires that each instrumental component is selected and optimized with a view to the ideal operating characteristics of the entire hyphenated system. An isocratic HPLC system, which employs an aqueous mobile phase with organic buffer constituents, is well suited for introduction into the ICP-MS because of the stability of the detector response and high degree of analyte sensitivity attained. Anion and cation exchange HPLC systems, which meet these requirements, were used for the seperation of selenium and arsenic species in crude extracts of biological samples. Furthermore, the signal-to-noise ratios obtained for these incompletely ionized elements in the argon ICP were further enhanced by a factor of four by continously introducing carbon as methanol via the mobile phase into the ICP. Sources of error in the HPLC system (column overload), in the sample introduction system (memory by organic solvents) and in the ICP-MS (spectroscopic interferences) and their prevention are also discussed. The optimized anion and cation exchange HPLC-ICP-MS systems were used for arsenic speciation in contaminated ground water and in an in-house shrimp reference sample. For the purpose of verification, HPLC coupled with tandem mass spectrometry with electrospray ionization was additionally used for arsenic speciation in the shrimp sample. With this analytical technique the HPLC retention time in combination with mass analysis of the molecular ions and their collision-induced fragments provide almost conclusive evidence of the identity of the analyte species. The speciation methods are validated by establishing a mass balance of the analytes in each fraction of the extraction procedure, by recovery of spikes and by employing and comparing independent techniques. The urgent need for reference materials certified for elemental species is stressed.     

Some considerations regarding temperature,electron density,and ionization in the argon inductively coupled plasma

The measured density of electrons in the ICP cannot be explained on the basis of a pure LTE calculation. A mechanism which involves radiation trapping and the transfer of excitation energy from the annular regions of the ICP to the aerosol channel is offered. This mechanism called “assisted ionization” leads to a more accurate prediction of electron density at a particular temperature. Assisted ionization is the result of the coupling of high energy resonance radiation from Ar(I) in the annular regions of the ICP into the analyte channel. The response of analyte atoms and ions to temperature and electron density in the channel can be estimated by inclusion of the analyte ionization equilibrium in an overall equilibrium which includes argon atoms and excited state argon species.     

ICP-MS for elemental speciation studies

The use of inductively coupled plasma mass spectrometry (ICP-MS) coupled with separation techniques for the purpose of elemental speciation has recently gained a lot of attention. Much of this is due to ever improving separation capabilities of Chromatographic techniques, the high sensitivity of ICP-MS, and the continuing development of better interface techniques. Additionally, there is a growing awareness of the need to monitor various species of an analyte, rather than just total analyte concentrations, due to their often varying natures. For the sake of learning from different elemental speciation approaches, this review brings together some selected types of elemental speciation which have been recently seen in literature. These include separations using various forms of liquid chromatography, such as reversed phase, reversed phase ion pairing, micelle, ion exchange, and size exclusion. Elemental speciation employing gas Chromatographie separations and supercritical fluid separations are discussed as well as elemental speciation using capillary electrophoresis.     

Comparison of dielectric barrier discharge, atmospheric pressure radiofrequency-driven glow discharge and direct analysis in real time sources for ambient mass spectrometry of acetaminophen

Three plasma-based ambient pressure ion sources were investigated; laboratory constructed dielectric barrier and rf glow discharges, as well as a commercial corona discharge (DART source). All were used to desorb and ionize a model analyte, providing sampling techniques for ambient mass spectrometry (MS). Experimental parameters were optimized to achive highest signal for acetaminophen as the analyte. Insight into the mechanisms of analyte desorption and ionization was obtained by means of emission spectrometry and ion current measurements. Desorption and ionization mechanisms for this analyte appear to be identical for all three plasma sources. Emission spectra differ only in the intensities of various lines and bands. Desorption of solid analyte requires transfer of thermal energy from the plasma source to sample surface, in the absence of which complete loss of MS response occurs. For acetaminophen, helium was the best plasma gas, providing 100- to 1000-fold higher analyte response than with argon or nitrogen. The same trend was also evident with background ions (protonated water clusters). MS analyte signal intensity correlates with the ion density (expressed as ion current) in the plasma plume and with emission intensity from excited state species in the plasma. These observations support an ionization process which occurs via proton transfer from protonated water clusters to analyte molecules.     

Advances in in-situ characterizations of electrode materials for better supercapacitors

In past decades,the performance of supercapacitors has been greatly improved by rationalizing the electrode materials at the nanoscale.However,there is still a lack of understanding on how the charges are efficiently stored in the electrodes or transported across the electrolyte/electrode interface.As it is very challenging to investigate the ion-involved physical and chemical processes with single experiment or computation,combining advanced analytic techniques with electrochemical measurements,i.e.,developing in-situ characterizations,have shown considerable prospect for the better understanding of behaviors of ions in electrodes for supercapacitors.Herein,we briefly review several typical in-situ techniques and the mechanisms these techniques reveal in charge storage mechanisms specifically in supercapacitors.Possible strategies for designing better electrode materials are also discussed.     

Visualizing Events Under Solvent-Gradient and Insights Thereof

This paper describes graphical methods to visualize variations in mobile phase composition inside a chromatographic column under solvent-gradient conditions, and how these variations affect movement of analyte bands. Based on the visualization techniques, the report explains how variations in composition “experienced” by an analyte, which is moving, are different from variations experienced by a static point inside the column. For example, if a linear gradient is created by the system, every point inside the column will experience the same linear variation of composition, although at different times, assuming that the solvent-gradient created at the mixer propagates through the system intact and undisturbed. For an analyte, on the other hand, the variation could be strongly non-linear because normally it travels with varying speed along the column during a gradient run. And because different analytes travel with different speeds, solvent-gradient experienced by the analytes will not be the same, even under the same method condition. Although these events are captured in the fundamental equation of solvent-gradient chromatography and generally understood by researchers working with gradient theories, the visualization methods discussed here may provide a clearer imagery of events for wider appreciation.     

Heavy metals in lake water: a review on occurrence and analytical determination

Many lakes especially in Asia are source of livelihood for the surrounding communities. With increased urbanisation and industrialisation, however, these lakes are threatened with emerging environmental contaminants, including heavy metals. Some heavy metals are harmful to human health and the environment. This review aims to describe the different sampling, sample preparation and pretreatment, and instrumental methods of analysis for heavy metals in lake water. Filtration and acid digestion are common sample treatment methods used prior to analytical determination. Atomic absorption spectroscopy and inductively-coupled plasma – mass spectrometry (ICP-MS) are typical analytical techniques but nowadays ICP-MS is frequently used. This review also describes the sources and extent of heavy metals contamination in different lakes. Although some lakes still have natural levels of heavy metals in the water, many have elevated concentrations due to anthropogenic sources, such as vehicular, household, agricultural, industrial and mining activities.     

Time-resolved measurements of individual ion cloud signals to investigate space-charge effects in plasma mass spectrometry

A new approach to directly monitor space charge induced effects due to high concentrations of efficiently ionized elements in inductively coupled plasma mass spectrometry (ICP-MS) is described. The broadening of ion clouds produced from individual, monodisperse drops of sample is measured by using time-resolved ICP-MS. The extent of broadening due to high concentrations of Pb in the sample is related inversely to the analyte mass. For the lightest analyte investigated, Li⁺, the relative width of the time-resolved analyte peak increases and then shows a dip in the center as the Pb concentration is increased to 500 and then 1500 µg/mL. The initial results of experiments that investigated chemical matrix effects as a function of concomitant species concentration, analyte mass, and sampling location in ICP-MS are consistent with space-charge effects.     

电感耦合等离子体质谱中的基质效应

电感耦合等离子体质谱(Inductively coupled plasma mass spectrometry,ICP-MS)是痕量元素分析中最常用的检测技术,尽管ICP-MS在元素分析中表现出诸多优势,但其在检测复杂基质样品时,仍会遇到许多问题。复杂基质所引起的基质效应通常会导致分析物信号的抑制或者增强。基质效应影响程度取决于基质成分的绝对浓度,还与基质的种类、分析物的物理和化学性质以及仪器条件有关。该文介绍了ICP-MS中几种常见的基质效应及其影响因素,包括元素基质、含碳基质、酸基质和仪器条件等,探究了基质效应产生的可能原因,对国内外去除基质效应的方法,如样品前处理方法、样品引入系统、优化仪器参数和校准法等进行了系统的归纳和总结,并对基质效应的研究进行了展望。     

High resolution imaging of barium ions and atoms near the sampling cone of an inductively coupled plasma mass spectrometer

Planar laser-induced fluorescence was used to map density distributions of ground state barium atoms, ground state barium ions, and excited-state barium ions in the region between the load coil and the sampling cone of an inductively coupled plasma mass spectrometer. The effects of power, nebulizer gas flow rate, and the addition of lithium to the sample on the distributions were studied. The maps reveal that the radial distributions of atomic species across the diameter of the plasma are compressed as the plasma is drawn into the sampling orifice, and that as a result of that compression, the distribution of ions across the 1-mm diameter of the sampling orifice is non-uniform. The distribution changes as conditions in the plasma change. The skimmer cone that separates the first and second stages of the differentially-pumped vacuum interface transmits ions exclusively from the center of the distribution that exists at the sampling cone. As a result, the overall efficiency with which ions are transmitted through the vacuum interface varies as conditions in the plasma change.