A close-up of three microwave plasma sources in view of improved element-specific detection in liquid chromatography

Progress in the features of three types of microwave plasmas is discussed, in view of the development of successful methods for atomic spectrometric element-specific detection in liquid chromatography. For the low-power microwave induced plasmas the development of the toroidal plasma in a TM010 cavity according to Beenakker is mentioned as the break-through for the introduction of wet aerosols. Capacitively coupled microwave plasmas (CMP), which can be operated with helium, argon and even air as working gases, are robust and allow obtaining of detection limits for Fe, Cr, Ni and Co in aqueous solutions in the 0.02 to 0.06 μg/ml range and in light oils, as an example of organic liquids, between 0.08 and 0.13 μg/ml. Special attention should be given to the microwave plasma torch (MPT) in which aerosols from aqueous as well as from organic solutions produced by a Légère nebulizer can be introduced without desolvation. Here, detection limits for Cd, Cr, Li and Pb range from 0.02 to 0.5 μg/ml. For the case of chromium dissolved as dithiocarbamate complex in an acetonitrile/H₂O mixture (2:1), its detection limit is 0.12 μg/ml, being already below that obtained with UV spectrophotometry. The limits of detection achieved with the sources discussed in the case of atomic emission spectrometry show the prospective for further development.     

Helium microwave-induced plasma mass spectrometric detection for reversed-phase high-performance liquid chromatography

Reversed-phase high-performance liquid chromatography (HPLC) is directly coupled to helium microwave-induced plasma mass spectrometry (He MIP-MS) for the element-selective detection of halogenated organic compounds. Absolute detection limits are approximately 50 pg Br for brominated compounds, 1 pg I for iodinated compounds, and 10 ng Cl for chlorinated compounds. The linear dynamic range for Br- and I-containing compounds is 3-4 orders of magnitude. However, the linear range for chlorinated species is severely limited by high background at m/z = 35. The relative standard deviation for repetitive injections is less than 10%. The helium microwave-induced plasma is operated at moderate powers (300-350 W) and with a total helium consumption of 6-8 L/min. The effect of organic solvents on the background mass spectrum is investigated.     

微波等离子体光谱技术的发展(二)

微波等离子体光源是一类重要的有较强激发能力的原子发射光谱光源,主要包括微波感生等离子体光源,电容耦合微波等离子体光源及微波等离子体炬光源。本文是微波等离子体光谱技术发展的第二部分,主要介绍了电容耦合微波等离子体光源及微波等离子体炬光源的结构原理和性能。并对它们的技术特点和进展进行评述。     

A capacitively coupled microwave plasma atomic emission spectrometer for the determination of trace metals in microsamples

A capacitively coupled microwave plasma operating at 450 to 850 W is used for atomic emission spectroscopy. The laboratory-constructed system contains a tungsten cup electrode capable of holding a volume of up to 30 μl. Microwaves are used to dry the sample, while at higher powers the plasma is ignited for sample vaporization and excitation. The entire analysis can be carried out in less than 5 min. A mixture of helium and hydrogen is used as the plasma gas. A spherical or cylindrical shaped plasma can be formed depending upon the gas flow rate and the microwave power selected. The effects of experimental parameters, such as gas flow rate, atomization power, electrode position and plasma shape are examined. Detection limits for Cd, Mg and Zn are in the low picogram range for a 10 μl sample; the relative standard deviation is less than 10%.     

Investigations on laser ablation–microwave induced plasma–atomic emission spectrometry using polymer samples

The potential of laser ablation–microwave induced plasma–atomic emission spectrometry (LA–MIP–AES) for the analysis of plastic materials has been investigated. A Nd/YAG laser, operated in its fundamental mode at 1064 nm, was used to ablate small amounts of various plastics. The sample atoms were transported and excited in a closely neighbored continuously running microwave induced plasma (MIP) operated in argon or helium at reduced pressure. A 0.5-m échelle spectrometer, equipped with an intensified charge coupled device (ICCD) as a detector was used for recording the spectra. The amount of ablated material was found to be strongly dependent on the matrix (10–190 ng/shot). Signals for some metals often used as additives in polymers (Al, Ca, Cu, Sb, Ti) and for the elements F, Cl, Br, J, and P in various polymers were recorded in the spectral range 250–840 nm. The estimated detection limits were found to be in the range 0.001–0.08% for metals and 0.05–0.7% for non-metals. Spectral lines of fluorine and iodine could only be measured in the helium MIP. For high concentrations of chlorine and carbon in the samples (polyvinylchloride), a memory effect was observed.     

A possibility of element specific detection in HPLC by means of MIP-AES coupled with hydraulic high pressure nebulization

An interface for coupling hydraulic high pressure nebulization (HHPN) with microwave induced plasma (MIP) atomic emission spectrometry (AES) is described. An appropriate spray chamber and aerosol desolvation system has been constructed for matching the HHPN generated aerosol flow with the loading capacity of toroidal argon and cylindrical helium MIP sources. The system has been optimized for aqueous solutions. Nanogram amounts of metals and nonmetals could be detected by the HHPN-MIP-AES technique developed. The HHPN devices are directly compatible with HPLC solvent flow, therefore they can be directly coupled with HPLC separations in aqueous media.     

A possibility of element specific detection in HPLC by means of MIP-AES coupled with hydraulic high pressure nebulization

An interface for coupling hydraulic high pressure nebulization (HHPN) with microwave induced plasma (MIP) atomic emission spectrometry (AES) is described. An appropriate spray chamber and aerosol desolvation system has been constructed for matching the HHPN generated aerosol flow with the loading capacity of toroidal argon and cylindrical helium MIP sources. The system has been optimized for aqueous solutions. Nanogram amounts of metals and nonmetals could be detected by the HHPN-MIP-AES technique developed. The HHPN devices are directly compatible with HPLC solvent flow, therefore they can be directly coupled with HPLC separations in aqueous media.     

Implementation of acoustic, radiofrequency and microwave rotating fields in analytical plasma sources

Four helium plasma sources operating at atmospheric pressure have been developed for analytical emission spectrometry by applying a synchronically rotating field with three or more phases operating at 1 kHz, 27 MHz or 2.45 GHz. The plasma takes the form of a disk and has minimum field strength at the axis. Thus, a channel is formed at the center through which the sample in the form of wet aerosol or a chemically generated vapor of halogen may be introduced. A dual-flow concentric ceramic injector was used to supply helium plasma gas and the sample to the plasma. The helium plasma operated at low power levels (40-300 W) and low gas flow rates of below 3 L min^− 1 and was self-igniting. The acoustic, radio-frequency (rf) and microwave-driven plasmas can withstand wet aerosol loadings of 5, 30 and 100 mg min^− 1, respectively, generated by an ultrasonic nebulizer without a desolvation unit. The plasma physical characteristics were compared at these three frequencies under otherwise similar operating conditions. The helium excitation temperature, OH rotational temperature and electron number density increased with increasing frequency in ranges of 2800-4000 K, 1100-3200 K and 0.1-7 × 10¹⁴ cm^− 3, respectively. To demonstrate the effect of frequency on the plasma excitation efficiency the emission intensity from halogen ions was evaluated using chemical vapor generation with continuous sampling without desiccation. Using 3-phase microwave, 6-phase microwave, 4-phase rf and 1 kHz helium plasma sources the detection limits (3σ) for chlorine at 479.40 nm were 26, 60, 230 and 1200 ng mL^− 1, respectively. The microwave-driven plasma was the densest and had the highest excitation potential toward chlorine and bromine ions.     

Non-thermal features of atmospheric-pressure argon and helium microwave-induced plasmas observed by laser-light Thomson scattering and Rayleigh scattering

Laser-light Thomson scattering and Rayleigh scattering have been measured from a microwave-induced plasma sustained at atmospheric pressure, using both argon and helium as a support gas. The measurements were performed at several spatial positions in each plasma, and at forward microwave power levels of 350 W for argon, and at 350 W and 100 W for helium. It was found from these measurements that both argon and helium plasmas deviate substantially from local thermodynamic equilibrium (LTE), Measured electron temperatures range from 13 000–21 500 K, whereas gas temperatures are generally lower by a factor of 2 to 10, depending on the support gas and the spatial position in the discharge. At the same forward microwave power, the electron temperature of the helium plasma is about 3500–7000 K higher than that of the argon plasma. Yet, the argon plasma has a higher electron number density than the helium plasma. Electron number densities in both argon and helium plasmas are roughly two to three orders of magnitude lower than what LTE would predict, based on the measured electron temperatures and the Saha Equation. Even more interestingly, signals in the far-wing portion of the Thomson-scattering spectrum were found to be significantly higher than are predicted by a fitted Maxwellian curve, indicating that there exists an over-population of high-energy electrons. It is concluded that, compared to the inductively coupled plasma, the microwave-induced plasma is highly non-thermal and remains in an ionizing mode in the analytical zone.     

Adirectly Coupled Microwave-Induced Plasma Atomic Emission Liquid Chromatograpy Detector for Nonmetals: Preliminary Characterization with Halides and Oxohalogen Salts

Abstract

A liquid chromatography system is directly coupled to a moderate power helium microwave induced plasma for the selective determination of no metals in aqueous solutions. The detector is a large volume helium microwave-induced plasma operated at 500 watts with a helium support gas flow of 21L/min. A sample set of halides and oxohalgen salts are separated by ion exchange chromatography and introduced into the plasma as mist generated with an ultrasonic nebulizer. Detection limits range from 1.5 to 6 μg. Calibration plots are presented. Selectivity is observed when monitoring the elemental signals of co-eluting peaks.     

Preliminary investigation of a medium power argon radiofrequency capacitively coupled plasma as atomization cell in atomic fluorescence spectrometry of cadmium

The single ring electrode radiofrequency capacitively coupled plasma torch (SRTr.f.CCP) operated at 275W, 27.12 MHz and Ar flow rate below 0.7 lmin(-1) was investigated for the first time as atomization cell in atomic fluorescence spectrometry (AFS) using electrodeless discharge lamps (EDL) as primary radiation source and charged coupled devices as detector. The signal to background ratio (SBR) and limit of detection for Cd determination by EDL-SRTr.f.CCP-AFS were compared to those obtained in atomic emission spectrometry using the same plasma torch. The detection limit in fluorescence was 4.3 ngml(-1) Cd compared to 65 ngml(-1) and 40 ngml(-1) reported in r.f.CCP-atomic emission (AES) equipped with single or double ring electrode. The lower detection limit in EDL-SRTr.f.CCP-AFS is due to a much better SBR in fluorescence. The limit of detection was also compared to those in atomic fluorescence with inductively coupled plasma (0.4 ngml(-1)), microwave plasma torch (0.25 ngml(-1)) and air-acetylene flame (8 ngml(-1)). The influence of light-scattering through the plasma and the secondary reflection of the primary radiation on the wall of the quartz tube on the analytical performance are discussed. The non-spectral matrix effects of Ca, Mg and easily ionized elements are much lower in EDL-SRTr.f.CCP-AFS compared to SRTr.f.CCP-AES. The new technique was applied in the determination of Cd in contaminated soils, industrial hazardous waste (0.4-370 mgkg(-1)) and water (113 microgl(-1)) with repeatability of 4-8% and reproducibility in the range of 5-12%, similar to those in ICP-AES. The results were checked by the analysis of a soil and water CRM with a recovery degree of 97+/-9% and 98+/-4%, for a confidence limit of 95%. The present EDL-SRTr.f.CCP-AFS is a promising technique for Cd determination in environmental samples.     

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.     

Microwave plasma torch analytical atomic spectrometry

The microwave plasma torch (MPT), as a relative new source, has found extensive use in atomic spectrometry. In this review, the fundamental features and characteristics of the MPT are summarized and compared with other kinds of analytical atomic sources, such as the more popularly used inductively coupled plasma (ICP), the direct current plasma (DCP), as well as other kinds of microwave plasmas (MWPs). Since the MPT offers some attractive features, it has been used as an excitation source for atomic emission spectrometry (MPT-AES), including the atomic emission detection (AED) for gas chromatography (GC), liquid chromatography (LC) and supercritical fluid chromatography (SFC). Also, it has been used either as an ionization source for atomic mass spectrometry (MPT-AMS) or an atomization source for atomic fluorescence spectrometry (MPT-AFS). The historical development and recent improvements in these MPT atomic spectrometric techniques are evaluated with emphasis on the analytical advantages and limitations. In addition, the future research directions and the application prospects of MPT atomic spectrometry (MPT-AS) are discussed.     

Recent trends in atomic spectrometry with microwave-induced plasmas

The state-of-the-art and trends of development in atomic spectrometry with microwave-induced plasmas (MIPs) since the 1998s are presented and discussed. This includes developments in devices for producing microwave plasma discharges, with reference also to miniaturized systems as well as to progress in sample introduction for microwave-induced plasmas, such as pneumatic and ultrasonic nebulization using membrane desolvation, to the further development of gaseous analyte species generation systems and to both spark and laser ablation (LA). The features of microwave-induced plasma mass spectrometry (MIP-MS) as an alternative to inductively coupled plasma (ICP)-MS are discussed. Recent work on the use of microwave-induced plasma atomic spectrometry for trace element determinations and monitoring, their use as tandem sources and for particle sizing are discussed. Recent applications of the coupling of gas chromatography and MIP atomic spectrometry for the determination of organometallic compounds of heavy metals such as Pb, Hg, Se and Sn are reviewed and the possibilities of trapping for sensitivity enhancement, as required for many applications especially in environmental work, are showed at the hand of citations from the recent literature.     

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.     

Optimization of electrochemical hydride generation in a miniaturized electrolytic flow cell coupled to microwave-induced plasma atomic emission spectrometry for the determination of selenium

The optimization of a continuous flow system for electrochemical hydride generation coupled to microwave-induced plasma atomic emission spectrometry (MIP-AES) for the determination of Se is presented. A small electrolytic cell with a porous glassy carbon working electrode was used for hydride generation. When using an Ar MIP operated in a TE101 cavity a detection limit of 0.6 ng mL(-1) (3sigma) could be achieved. The calibration curve was linear up to 1 microgram mL(-1). A standard deviation of less than 2% (10 replicate analyses) could be achieved. It was shown that interferences of transition metals are of the same order of magnitude as with a larger electrolysis cell described earlier, and light elements hardly caused any signal depression as tested. It was possible to distinguish between Se(IV) and Se(VI) species and seleno-DL-methionine, because under optimized conditions of an electrolysis current of 10 mA, a microwave power of 210 W, an Ar flow rate of 15 L h(-1) and a sample flow rate of 2.5 mL min(-1) only Se(IV) was transformed to H2Se and transferred into the plasma. Finally, the possibility of an electrochemical pre-enrichment was shown to enable it to further decrease the detection limit.     

Design and modelling of a modified 2.45 GHz coaxial plasma torch for atomic spectrometry

The design and modelling of a stable and easy to build coaxial 2.45 GHz microwave plasma torch is presented. Its field modelling and operational principles based on transmission line theory are discussed. An experimental range of stable operation for usage in atomic spectrometry extends from 50 to 200 W microwave power and from an argon working gas flow of 50 to 1000 ml/min.     

The selective determination of halogens and sulphur in solution by atmospheric-pressure helium microwave-induced plasma emission spectrometry coupled to an electrothermal introduction system

Instrumentation is described for the direct determination of Cl, Br, I and S in dissolved samples. A tantalum furnace is coupled directly to a TM₀₁₀ cavity, in which a helium microwave plasma is generated at atmospheric pressure. Samples (20 μl) are dried, ashed and atomized; the free atoms are transported by helium to the cavity, where they are excited. Detection limits are in the sub-mg l^-1 region. The effect of counter cations is removed by the addition of 50 mg l^-1 potassium hydroxide, which also helps to suppress interferences by large amounts of matrix constituents, but the standard addition technique remains necessary to permit determinations that are reliable to within 5%.     

用于气相色谱的微波等离子体原子发射光谱检测器的发展

分别介绍和评价了用于气相色谱的微波诱导等离子体、电容耦合微波等离子体和微波等离子体炬等3种微波等离子体原子发射光谱检测器的发展、应用以及局限性。对用于气相色谱的微波等离子体原子发射光谱检测器的发展作了展望。     

Preliminary investigation of a tangential flow torch with coupling probe injector for helium microwave induced plasma mass spectrometry

A stable, low gas-flow torch has been developed for use with a helium microwave induced plasma (MIP). A toroidal plasma with central analyte introduction is obtained by the addition of a tantalum coupling probe injector tube. This injector penetrates through 100% of the total cavity depth and aids in the efficiency of power transfer to the cavity, in plasma initiation, and in circumventing the effects of a lack of homogeneity in the microwave field on analyte distribution in the plasma. The tangential helium flow was 41/min and the microwave power was 60 W.Presented in part at the 1989 European Winter Conference on Plasma Spectrochemistry, Reutte, Austria