Modified helium pulse-operated microwave induced plasma for spectrochemical analysis of liquid samples

To minimize problems caused by sample introduction into helium pulse operated microwave-induced plasma (He-pulsed-MIP), a simple plasma torch was developed. This torch is constructed from commonly available components with an absolute minimum of machining. In this torch, plasma is kept operating by partially isolating it from the rest of the plasma (within the plasma chamber). This auxiliary plasma is by-passed during sample or solvent injection and is therefore not affected. The design of this discharge chamber was thoroughly examined and each parameter affecting its analytical performance was evaluated. Measurements reported include effect of helium flow rate, discharge tube position and microwave power on analytical performance. Analytical calibration curves and detection limits data are shown for Ca, Cd, Cr, Cu, Fe, Mg, Ni and Zn. Plasma excitation temperature was determined using iron and copper as thermometric species. Finally, the present technique was applied to the analysis of real biological samples (liver, brain, heart, bone, kidney, tests, serum, spleen and muscles of white albino rats). The results were compared with those obtained using flame atomic absorption spectroscopy.     

An improved microwave plasma torch for atomic spectrometry

The analytical performance of a microwave plasma torch was improved through mechanical alterations. Several problems reported in earlier designs were addressed: the ignition and stabilization of a helium plasma in the MPT was difficult; high powers were required to both ignite and operate the plasma; otherwise, the plasma would erratically change from an annular to a filament type discharge. In the new torch, the helium discharge was stabilized by replacing the copper central tube with one made of quartz. In addition, air entrainment was alleviated through use of a sheathing gas. This modification simplified the background mass spectrum and raised the effective ionization temperature of the discharge. A detailed schematic diagram of the new microwave plasma torch is presented.     

Determination of arsenic by continuous hydride generation with direct introduction into an O2-argon microwave plasma torch atomic emission spectrometer

The determination of arsenic was studied with a simple and economic method. A continuous hydride generation system is interfaced to a microwave plasma torch atomic emission spectrometer (MPT-AES). Arsenic hydride is transferred directly and continuously by the carrier gas into the plasma torch without separation of hydrogen. When oxygen is introduced into the outer tube of the plasma torch, the plasma is more stable and has a higher tolerance to hydrogen. The detection limit (3σ) is 5.2 μg/L when the forward power is 100 W with argon as support gas. Application to the standard sample coal fly ash showed a comparable result to the certified quantity. Received: 3 April 2000 / Revised: 21 May 2000 / /Accepted: 25 May 2000     

Three-phase double-arc plasma for spectrochemical analysis of environmental samples

A new instrument, which uses a three-phase current to support a double-arc argon plasma torch for evaporation, atomization and excitation of solid or powder samples, is described. The sampling arc is ignited between the first and second electrode while the excitation arc is ignited between the second and third electrode. Aerosol generated from the sample (first electrode) is swept by argon gas, through a hole in the second electrode (carbon tubing electrode), into the excitation plasma. A tangential stream of argon gas is introduced through an inlet orifice as a coolant gas for the second electrode. This gas stream forces the excitation arc discharge to rotate reproducibly around the electrode surface. Discharge rotation increases the stability of the excitation plasma. Spectroscopic measurements are made directly in the current-carrying region of the excitation arc. An evaluation of each parameter influencing the device performance was performed. Analytical calibration curves were obtained for Fe, Al, K, and Pb. Finally, the present technique was applied for the analysis of environmental samples. The present method appears to have significant, low cost analytical utility for environmental measurements. Received: 7 July 2000 / Revised: 6 September 2000 / Accepted: 10 September 2000     

Preliminary spectroscopic experiments with helium microwave induced plasma produced in air by use of a new structure: the axial injection torch

In this experimental study we have used spectroscopic methods to characterize helium plasma obtained by means of a novel waveguide-fed microwave plasma torch at atmospheric pressure, the axial injection torch. This device produces a “plasma flame” by coupling high frequency (HF) power at 2.45 GHz to the discharge. Various flame parameters (namely the electron density number and the electron and gas temperatures) have been determined by using spectroscopic diagnostic techniques that provided an estimate in terms of the helium flow rate, absorbed HF power and axial position in the experiments. These preliminary results suggest some departure from local thermodynamic equilibrium (LTE) and seem to indicate the utility of the discharge as an excitation source for emission spectroscopy. Comparison with other microwave torches already described in the literature is made in terms of the electron density and the electron and gas temperature.     

Performance of a microwave induced plasma (MIP) operated in a liquid-cooled discharge tube for atomic emission spectrometry

Different types of microwave induced plasma (MIP) discharge operated in liquid-cooled tubes, namely a glass tube of Duran^®, a quartz tube of Herasil^®, and a very simple demountable discharge tube made of glass and quartz have been investigated. The last tube leads to the best analytical properties and the longest lifetime. The intensities of silicon lines and of the continuum spectral background, together with the signal-to-background ratios for B, Ca, Cd, Co and Zn in the case of the pneumatic nebulization of solutions have been measured and used as an indicator for the cooling efficiency. The MIP torch was cooled with a thermostated silicon oil. The decrease of the temperature of the cooling medium causes a measurable decrease of the spectral background intensity. Diagnostic measurements of the plasma include radial profiles of spectral line intensities and excitation temperatures with the lines of Fe I; values of 5000–6000 K are found. The influence of different plasma parameters, e.g. microwave power and helium flow rate, is investigated. The preliminary analytical characterization of a helium MIP maintained with the liquid-cooled demountable discharge tube is presented. Limits of detection for Al, B, Ca, Co, Fe, P, Sb and Zn (between 0.002 and 1.2 μg ml⁻¹) are comparable with or better than those reported for low power helium MIPs with sample introduction in the form of a wet aerosol.     

A status report on helium inductively coupled plasma mass spectrometry

The current status of helium inductively coupled plasma - mass spectrometry (He ICPMS) is examined, its potentials and limitations are reviewed, and a summary of fundamental properties of atmospheric pressure He ICP discharges is presented. Also included are results of He ICPMS studies with a new helium plasma torch (18 mm i.d.) operated at four sets of operating conditions. Under the "cold plasma" condition (600 W forward power), no secondary discharge is observed and ion kinetic energies ranging from 2.0 eV to 9.5 eV for 6 elements (mass range: 39-208) are measured. At higher power levels, the secondary discharge still is strong. In general, detection limits for certain elements are improved by 1-3 orders of magnitude compared to previous data acquired in 1993 with a 13-mm He ICP torch. Elements such as K, Fe, Cr, Mn, Ni, and Co that suffer from spectral interferences in Ar ICPMS can be detected at pg/mL-levels with an analogue detector and a prototype ICPMS instrument having no photon stops or obstacles present in the ion trajectory path.     

Analytical Potentials and Features of Atomic Emission Analysis Using Electron-Impact Excitation of Atoms in Helium under Atmospheric Pressure

A device is described for the atomic emission analysis of vaporous samples using electron-impact excitation in helium under atmospheric pressure. The device consists of a cathode atomizer with a test sample applied onto it and the anode located at 1–3 mm from the cathode. The electrons emitted by the cathode upon heating are accelerated by applying a constant voltage to the electrodes. The mechanism for the formation of a non-self-sustained gas discharge between the cathode and anode is considered and the properties of the discharge are compared to those of the known discharges used in atomic emission spectrometry. The influence of atomization temperature and helium pressure on the analytical and background signals was studied. It is shown that, under certain conditions, the analytical signal increases with helium pressure. The relative detection limits attained for a number of elements are from tenths to dozens of nanogram per liter; this is two or three orders of magnitude lower than those in inductively coupled plasma atomic emission spectrometry and of the same order of magnitude as detection limits in inductively coupled plasma mass spectrometry.     

气动雾化进样时微波等离子体炬作为激发光源的性能研究

以气动雾化进样研究了微波等离子体炬（ＭＰＴ）放电作原子发射光谱法激发光源的性能，包括ＭＰＴ的获得、操作参数的影响、样品引入及其分析性能，并与微波诱导等离子体进行了比较，证明ＭＰＴ放电作激发光源有良好的分析性能．     

The electromagnetic performance of a surfatron-based coaxial microwave plasma torch

Low power microwave plasma torches are of particular interest to analytical chemists. The torch design investigated herein, called TPS, is based on the known surfatron structure to which a coaxial section is added consisting of an impedance transformer followed by the metallic nozzle at the tip of which the discharge occurs. A series of experiments illustrate the main electromagnetic features and performance of this novel coaxial microwave plasma torch operating at 2.45 GHz and with input power in the range 10–180 W. A specially devised slotted coaxial line with a movable probe arrangement can be inserted into the torch in place of the transformer section to provide in situ measurements of the plasma impedance. Analyzing these results, we show that the shape of the torch tuning characteristics can be controlled to improve the power transfer to the plasma and stability of operation with respect to changes in discharge conditions; under these conditions, the design of the device can be simplified. The procedures presented have a general character and can be applied to various torch configurations.     

Correlation Between the Electrical and Optical Properties of an Atmospheric Pressure Plasma During Siloxane Coating Deposition

The effect of varying process parameters on atmospheric plasma characteristics and properties of nanometre thick siloxane coatings is investigated in a reel-to-reel deposition process. Varying plasma operation modes were observed with increasing applied power for helium and helium/oxygen plasmas. The electrical and optical behaviour of the dielectric barrier discharge were determined from current/voltage, emission spectroscopy and time resolved light emission measurements. As applied power increased, multiple discharge events occurred, producing a uniform multi-peak pseudoglow discharge, resulting in an increase in the discharge gas temperature. The effects of different operating modes on coating oxidation and growth rates were examined by injecting hexamethyldisiloxane liquid precursor into the chamber under varying operating conditions. A quenching effect on the plasma was observed, causing a decrease in plasma input power and emission intensity. Siloxane coatings deposited in helium plasmas had a higher organic component and higher growth rates than those deposited in helium/oxygen plasmas.     

Solution nebulization of aqueous samples into the tubular-electrode torch capacitatively-coupled microwave plasma

This work shows the feasibility of using nebulization for introduction of aqueous samples into the tubular-torch capacitatively-coupled microwave plasma (CMP). Previously, solid electrodes were used with this type of plasma, in which analyte carrier and plasma support gases are premixed and swept around the electrode tip. With the new design, the analyte carrier gas passes through the centre of the hollow tubular electrode and mixes with the plasma support gas at the tip of the electrode where the plasma is formed. Sample solutions are nebulized with a Meinhard nebulizer and a laboratory-constructed spray chamber and desolvation system. The tubular torch is made of tantalum. Plasma gases investigated include argon, helium and nitrogen. Typical operating powers are 300-350 W. Elements studied include Ag, Al, Ba, Ca, Cd, Cr, Cs, Cu, K, Li, Na, Pb, Pd, Sr and Zn.     

Sampling modulation technique in radio-frequency helium glow discharge emission source by use of pulsed laser ablation

An emission excitation source comprising a high-frequency diode-pumped Q-switched Nd:YAG laser and a radio-frequency powered glow discharge lamp is proposed. In this system sample atoms ablated by the laser irradiation are introduced into the lamp chamber and subsequently excited by the helium glow discharge plasma. The pulsed operation of the laser can produce a cyclic variation in the emission intensities of the sample atoms whereas the plasma gas species emit the radiation continuously. The salient feature of the proposed technique is the selective detection of the laser modulation signal from the rest of the continuous background emissions, which can be achieved with the phase sensitive detection of the lock-in amplifier. The arrangement may be used to estimate the emission intensity of the laser ablated atom, free from the interference of other species present in the plasma. The experiments were conducted with a 13.56 MHz radio-frequency (rf) generator operated at 80 W power to produce plasma and the laser at a wavelength of 1064 nm (pulse duration:34 ns, repetition rate:7 kHz and average pulse energy of about 0.36 mJ) was employed for sample ablation. The measurements resulted in almost complete removal of nitrogen molecular bands (N₂⁺ 391.44 nm). Considerable reduction (about 75%) in the emission intensity of a carbon atomic line (C I 193.03 nm) was also observed.     

Validated method for the determination of the novel organo-ruthenium anticancer drug NAMI-A in human biological fluids by Zeeman atomic absorption spectrometry

NAMI-A is a novel ruthenium-containing experimental anticancer agent. We have developed and validated a rapid and sensitive analytical method to determine NAMI-A in human plasma, plasma ultrafiltrate and urine using atomic absorption spectrometry with Zeeman correction. The sample pretreatment procedure is straightforward, involving only dilution with an appropriate hydrochloric acid buffer-solution. Because the response signal of the spectrometer depended on the composition of the sample matrix, in particular on the amount of human plasma in the sample, all unknown samples were diluted to match the matrix composition in which the standard line was prepared (plasma-buffer 1 : 10 v/v). This procedure enabled the measurement of samples of different biological matrices in a single run. The validated range of determination was 1.1–220 μM NAMI-A for plasma and urine, and 0.22–44 μM for plasma ultrafiltrate. The lower limit of detection was 0.85 μM in plasma and urine and 0.17 μM in plasma ultrafiltrate. The lower limit of quantitation was 1.1 and 0.22 μM, respectively. The performance of the method, in terms of precision and accuracy, was according to the generally accepted criteria for validation of analytical methodologies. The applicability of the method was demonstrated in a patient who was treated in a pharmacokinetic phase I trial with intravenous NAMI-A. Received: 1 September 2000 / Revised: 1 November 2000 / Accepted: 12 November 2000     

Mini, micro, and high-efficiency torches for the ICP - toys or tools?

Because the inductively-coupled plasma (ICP) is expensive both to purchase and to continue in operation, many workers have sought to reduce both the radiofrequency power and coolant argon ordinarily required to sustain the discharge. In one approach, the ICP has been reduced in size, in the hope that its analytical strengths could be retained but the power and argon required for it reduced. In the second approach, the torch used to support the discharge has been modified extensively. The final and most recent approach utilises alternative torch cooling schemes using, for example, water or high air flows as a coolant. In this paper, these alternative approaches will be reviewed and assessed in terms of their practicability and analytical capability. Prospects for future torch modification and low-flow, low-power ICP instrumentation will be considered and discussed.     

Development of ambient sampling chemi/chemical ion source with dielectric barrier discharge

The development of a new configuration of chemical ionization (CI)‐based ion source is presented. The ambient air containing the gaseous sample is sniffed into an enclosed ionization chamber which is of sub‐ambient pressure, and is subsequently mixed with metastable species in front of the ion inlet of the mass spectrometer. Metastable helium atoms (He*) are used in this study as the primary ionizing agents and are generated from a dielectric barrier discharge (DBD) source. The DBD is powered by an AC high‐voltage supply and the configuration of the electrodes is in such a way that the generated plasma is confined within the discharge tube and is not extended into the ionization chamber. The construction of the ion source is simple, and volatile compounds released from the bulky sample can also be analyzed directly by approaching the sample to the sampling nozzle. When combined with heated nitrogen or other desorption methods, its application can also be extended to non‐volatile compounds, and the consumption for helium can be kept minimum solely for maintaining the stable discharge and gas phase ionization. Applications to non‐proximate sample analysis, direct determination of active ingredients in drug tablets and the detection of trace explosive such as hexamethylene triperoxide diamine are demonstrated. Copyright © 2010 John Wiley & Sons, Ltd.     

A status report on helium inductively coupled plasma mass spectrometry

The current status of helium inductively coupled plasma – mass spectrometry (He ICPMS) is examined, its potentials and limitations are reviewed, and a summary of fundamental properties of atmospheric pressure He ICP discharges is presented. Also included are results of He ICPMS studies with a new helium plasma torch (18 mm i.d.) operated at four sets of operating conditions. Under the cold plasma condition (600 W forward power), no secondary discharge is observed and ion kinetic energies ranging from 2.0 eV to 9.5 eV for 6 elements (mass range: 39–208) are measured. At higher power levels, the secondary discharge still is strong. In general, detection limits for certain elements are improved by 1–3 orders of magnitude compared to previous data acquired in 1993 with a 13-mm He ICP torch. Elements such as K, Fe, Cr, Mn, Ni, and Co that suffer from spectral interferences in Ar ICPMS can be detected at pg/mL-levels with an analogue detector and a prototype ICPMS instrument having no photon stops or obstacles present in the ion trajectory path.     

一种原子光谱分析用新激发光源——千瓦级微波等离子体炬(kW-MPT)

Based on the simulation of transmission and distribution characteristics of the electromagnetic field inmicrowave plasma torch(MPT) torches with different configurations using electromagnetic simulation software and experimental study, a new MPT torch with double resonant configuration was, for the first time, developed. The results show that the inner tube of MPT torch plays an important role in strengthening the electric field intensity at the open end of the MPT torch and redistributing the electromagnetic field in the whole torch by the formation of the double resonance. It contributes also to enhance the macroscopic stability and the self-sustaining of the plasma. The stability of the plasma was shown to be excellent when the spacer between inner and intermediate tubes is located about 20—30 mm from the top opening of the torch. Preliminary study showed that the analytical performance for 13 common elements was approaching that of traditional ICP-AES.     

Evaluation of a radio frequency plasma for oxygen-selective detection in capillary gas chromatography

A radio frequency plasma detector for capillary GC has been modified for oxygen-selective detection. Purification of the plasma gas and purging of both ends of the discharge region with helium were crucial to minimizing oxygen background emission from impurities in the plasma. With a pure helium plasma, eluting hydrocarbons released oxygen from the discharge region resulting in interfering signals on the oxygen channel. These interfering signals were efficiently reduced by using a methane-doped (0. 15%) low power RF plasma (15 W) sustained in a high make-up flow (150 mL/min). With this plasma, a 10³:1 oxygen-to-carbon selectivity and a 100 pg oxygen/s detection limit were obtained. The detector was linear over three orders of magnitude. The detection system has been used to screen for oxygenated compounds in two environmental samples.     

Direct solution introduction using conventional nebulizers with a short torch for plasma mass spectrometry

A new torch, a shortened version of a standard demountable torch, is proposed for facilitating direct injection of liquid samples into an inductively coupled plasma mass spectrometer using conventional and micro-pneumatic nebulizers. The proposed arrangement reduces the cost of the direct injector nebulizer by a factor of 5, typically from $2000 to $400, although a different torch is required. The analytical performance of the high efficiency nebulizer-short torch arrangement is compared to that obtained with the direct injection high efficiency nebulizer interfaced to the conventional torch. Optimum operating conditions for the high efficiency nebulizer-short torch arrangement are generally similar to those of the direct injection high efficiency nebulizer: high RF power (1500 W), low nebulizer gas flow rates (0.09 L/min) and low solution uptake rates (5–85 μL/min). Sensitivity with the high efficiency nebulizer-short torch system at 85 μL/min is improved by a factor of 2.4 on average compared to the direct injection high efficiency nebulizer, while precision values (%RSD) and detection limits are generally comparable or slightly degraded (on average by a factor of 1.7), respectively. Sensitivity is also better at lower solution uptake rates (5 μL/min) by factors ranging from 2 (⁸²Se) to 7 (⁵⁹Co) compared to the direct injection high efficiency nebulizer. Additionally, the %RSD values are better at 5 μL/min, ranging from 3.5% to 6.0% for the high efficiency nebulizer-short torch combination compared to 4.7 to 9.1% for the direct injection high efficiency nebulizer. The utility of the high efficiency nebulizer-short torch arrangement is demonstrated through the microscale flow injection analysis of Cr-DNA adducts and the analysis of four certified reference materials (Lyphochek urine metals control, SRM 1515: Apple Leaves, SRM 1570a: Spinach Leaves, SRM 1577b: Bovine Liver). Peak to peak precision values (N = 3) for the microscale flow injection analysis-high efficiency nebulizer-short torch system is 3.1% and 3.7% based on peak areas and heights, respectively, at a solution uptake rate of 85 μL/min. Good agreement is obtained between certified and measured concentrations for several elements across the mass range (e.g., Al, V, Mn, As, Cd, Pb, U). The proposed system is novel because it potentially offers a lower-cost and a more universal arrangement for improved direct solution introduction in plasma mass spectrometry using off-the-shelf commercial nebulizers.