A microwave plasma cavity assembly for atomic emission spectrometry

Although inductively coupled plasmas (ICPs) are widely used for multielement analysis microwave induced plasma (MIP) offers a great potential for a variety of applications. Modifications to incorporate MIP into commercial ICP direct reading spectrometer systems have been developed. A direct reading échelle spectrometer is described which opens new possibilities for the successful construction of commercial MIP-AES systems with the potential to run all of the typical methods worked out for earlier ICP-AES applications. Use of flow injection techniques and automation to couple with in situ concentration will likely offer a further improvement in the analytical performance of this system. Due to the capabilities demonstrated by this spectrometer it appears that hybrid instruments will be increasingly important for future developments in optical spectrometry. This is particularly true for very demanding areas such as atomic emission spectrometry. The system could be readily commercialized.     

A microwave plasma cavity assembly for atomic emission spectrometry

Although inductively coupled plasmas (ICPs) are widely used for multielement analysis microwave induced plasma (MIP) offers a great potential for a variety of applications. Modifications to incorporate MIP into commercial ICP direct reading spectrometer systems have been developed. A direct reading échelle spectrometer is described which opens new possibilities for the successful construction of commercial MIP-AES systems with the potential to run all of the typical methods worked out for earlier ICP-AES applications. Use of flow injection techniques and automation to couple with in situ concentration will likely offer a further improvement in the analytical performance of this system. Due to the capabilities demonstrated by this spectrometer it appears that hybrid instruments will be increasingly important for future developments in optical spectrometry. This is particularly true for very demanding areas such as atomic emission spectrometry. The system could be readily commercialized.     

Spatially-resolved observation of glow discharge plasma for atomic emission spectrometry.

An imaging spectrograph equipped with a CCD detector was employed to measure two-dimensional emission images from a glow discharge plasma in atomic emission spectrometry. The emission images at Zn I 334.50 nm for a zinc sample and at Cu I 324.75 nm for a copper sample could be obtained. Their emission intensities were not uniform in the radial direction of the plasma region but became weaker at larger distance from the central zone. The two-dimensional distribution would result from a spatial variation in the excitation efficiency of the plasma and thus provide useful information for understanding the excitation processes occurring in the plasma.     

High-pressure microwave dissolution of ceramics prior to trace metal determinations by microwave induced plasma atomic emission spectrometry

A commercial laboratory microwave acid digestion system was evaluated for the acid dissolution of ceramic powders (Al₂O₃, AlN, BN and Si₃N₄) prior to the determination of their trace element content by microwave induced plasma atomic emission spectrometry. Newly designed vessels, capable of withstanding internal pressures of over 110 bar, provide rapid and satisfactory results for sample dissolution. Sample preparation time was approximately 30 min (including the subsequent cooling time and preparation of the final solution). Results from conventional stainless-steel acid digestion vessel (Teflon bomb) dissolution are compared with the microwave bomb results of microwave plasma atomic emission spectrometry.     

Multielement determination of major elements in polymer additives by microwave induced plasma atomic emission spectrometry after microwave digestion

A method for multielement determination of major elements in polymer additives by microwave induced plasma atomic emission spectrometry (MIP-AES) has been elaborated. Microwave digestion with nitric acid was selected for sample preparation because of its speed and versatility. Sodium nitrate was added to the digestion mixture in order to reduce phosphorus losses. The precision obtained varied between 2 and 4.5% depending on the element determined. The accuracy of the method was studied by analyzing the Spex 5-element oil standard. The method was applied to a variety of commercial and in-house prepared compositions.     

On-line preconcentration with activated carbon for microwave plasma torch atomic emission spectrometry

This paper presents a method whereby trace elements are adsorbed in NH(4)ClNH(3) medium on activated carbon and then determined by microwave plasma torch atomic emission spectrometry (MPT-AES). The working conditions (including microwave forward power, gas flow rate, NH(3)NH(4)Cl concentration in the sample solution, HCl concentration in the eluant, sample introduction rate and preconcentration time) were investigated in detail. The effects of concomitant ions were studied. The experimental results for such analytes as Pb, Mn, Cd, Cu and Fe indicate that the procedure can eliminate fundamentally the interferences caused by alkali and alkaline earth metal elements and the application of it to the determination of iron in industrial silicon and tap water samples is successful.     

The use of optical emission spectrometry with microwave induced plasma (MIP) discharges in a surfatron combined to different types of hydride generation for the determination of arsenic

The stability and analytical figures of merit of argon microwave induced plasma (MIP) discharges in a surfatron as sources for optical emission spectrometry (OES) are described. These MIPs have been used for the determination of arsenic after hydride generation. They could cope with the excess of hydrogen developed during the hydride generation step and thus not necessitated an isolation of the hydrides before releasing them into the MIP. Two methods for the generation of the volatile AsH₃ were applied. First a micro method was used with solid NaBH₄ on which 10 1 of the acidified sample solution is transferred. Its capabilities were compared to those of continuous hydride generation using a 5% (w/w) NaBH₄-solution and continuous liquid removal in a flow cell. Both methods were optimized for an argon MIP operated at a power of 120–160 W and gas flows of 20 l/h Ar. In the case of solid NaBH₄ the detection limit for As has been found to be 1.0 g/ml (10 ng) and with the flow cell hydride generation 50 ng/ml. The calibration curves are linear over three orders of magnitude. Interferences caused by Sb, Fe, Sn and NaCl were investigated. No interferences occurred for Sb up to an interferent concentration of 250 g/ml. The presence of Fe causes a significant depression of the As signal whereas an increase of the As signal was observed in the case of Sn. High NaCl concentrations did not influence the As signals when using continuous hydride generation, but had a great influence when using solid NaBH₄.     

Use of microwave plasma torch atomic emission spectrometry for the determination of silicon

 A method for the elimination of matrix effects was developed for the determination of trace amounts of silicon by microwave plasma torch atomic emission spectrometry (MPT-AES). The sample solution was introduced into the MPT with a pneumatic nebulizer (PN). When Ar was used as both carrier and support gas, a detection limit of 10.8 ng/ml was obtained. The precision was 4.2% (RSD). The characteristics of the emission spectrum of silicon in MPT was studied in detail. The interference of some concomitant cations with the silicon emission was eliminated by incorporation of a cation-exchange column into the flow injection system. The method has been applied to analyze some practical samples and the results obtained are satisfactory. Received: 23 October 1995/Revised: 16 April 1996/Accepted: 20 April 1996     

Thermospray nebulizer as sample introduction technique for microwave plasma torch atomic emission spectrometry

A thermospray nebulizer was used as a sample introduction device for microwave plasma torch (MPT) atomic emission spectrometry (AES). Experimental parameters, including the power supplied to the MPT, the flow rates of support and carrier gases, the observation height, the sample uptake rate, the thermospray working temperature, the temperature of the aerosol spray chamber and cooling water were optimized. Under optimum conditions, the relative standard deviation (RSD) of 10 measurements for 21 elements is in the range 0.3–2.0%. The detection limits were improved in comparison with the ultrasonic nebulizer as sample introduction technique for MPT–AES. The inclusion of 20% methanol into the MPT showed there is no effect on the stability of MPT discharge. The technique can thus be held to have the potential for interface to reverse-phase HPLC systems.     

Flow-injection on-line column preconcentration for low powered microwave plasma torch atomic emission spectrometry

This paper describes an improvement in detection capability of microwave plasma torch atomic emission spectrometry by using a flow-injection on-line column preconcentration system. The analytical performances of Cd, Cu, Mn and Zn were studied. The analytes were preconcentrated with a thiol resin. The preconcentration period, the pH of the sample solution and the HCl concentration in the eluant were examined in detail. Operating conditions were optimized as follows: sample uptake, 1.2 ml min(-1); preconcentration period, l min; pH of sample solution, 9; HCl concentration in the eluant, 1 mol 1(-1). The experimental results show that flow-injection on-line column preconcentration can not only eliminate the effect of some concomitant elements, such as Li, Na and K, on the determination of analyte, but also enhance the sensitivity.     

Determination of halides by microwave induced plasma and stabilized capacitive plasma atomic emission spectrometry after on-line continuous halogen generation

This paper describes a comparative study of the microwave induced plasma (MIP) and the stabilized capacitive plasma (SCP) for halide determinations. The MIP is generated in a Beenakker cavity TM(010) using a tangential flow torch and the SCP consists of a 27.12 MHz discharge sustained in a liquid-cooled, fused silica tube surrounded by two annular electrodes. Both discharges are operated in helium at atmospheric pressure and detection was carried out by Atomic Emission Spectrometry (AES). The halides (I(-), Br(-), Cl(-)) are converted to volatile halogens by continuous flow generation based on chemical oxidation and on-line separation from the aqueous phase, via a gas-liquid separator, to be finally introduced into the plasma. The different factors affecting the emission intensity of the volatile halogens generated are compared for both discharges and the analytical performance characteristics are also evaluated. Detection limits of 17 ng ml(-1), 24 ng ml(-1) and 55 ng ml(-1) are obtained for the determination of Cl(-), Br(-), and I(-), respectively, in the ultraviolet-visible (UV-VIS) region using the MIP-AES and 45 ng ml(-1), 135 ng ml(-1) and 400 ng ml(-1) for Cl(-), Br(-), and I(-) with the SCP-AES. Lines in the near infrared (NIR) region were also evaluated for the SCP-AES detection; improvements in detection limits higher than 30 times were observed in the NIR region as compared with the UV-VIS with detection limits in the NIR of 1.4 ng ml(-1) for Cl(-), 3 ng ml(-1) for Br(-) and 13 ng ml(-1) for I(-).     

Direct determination of trace amounts of sodium in water-soluble organic pharmaceuticals by microwave induced plasma atomic emission spectrometry

The direct determination of trace sodium by microwave induced plasma atomic emission spectrometry (MIP-AES) in water-soluble organic substances utilized in pharmaceutical preparations was developed. No decomposition of the organic constituents was required. Samples were dissolved with water and introduced to the plasma after ultrasonic nebulization without desolvation. A limit of detection (3sigma) of 0.91-3.0 ng ml(-1) was obtained under experimental conditions. The quantitative MIP-AES procedure involved the standard addition method. The sodium content determined in reference material NIST SRM 1568A Rice Flour agreed with the certified value (6.6+/-0.8 mug g(-1)). Physical and chemical interferences were investigated. It was found for the microwave plasma that it is possible to introduce organic substances solutions of concentration up to 5% without sensitivity losses. This direct technique is fast and sensitive and helps to reduce contamination connected with the sample preparation procedure.     

微波消解-电感耦合等离子体发射光谱法测定土壤中的硫

利用微波消解仪消解土壤样品,采用电感藕合等离子体发射光谱仪测定土壤中硫的含量。以浓王水为消解试剂,对土壤样品进行微波消解,优化了王水用量,考察了微波消解条件、谱线干扰情况和仪器参数对硫灵敏度的影响。硫的质量浓度在0~40 mg/L范围内与光谱强度具有良好的线性关系,线性相关系数为0.9997,硫的检出限为0.053 mg/L。土壤中硫测定结果的相对标准偏差为1.54%~7.84%(n=7)。对六种土壤国家一级标准物质进行了测定,测定结果与推荐值相符,无显著性差异。该方法简便、准确,分析效率高,能够满足日常批量样品分析的要求。     

Applicability of microwave induced plasma optical emission spectrometry (MIP-OES) for continuous monitoring of mercury in flue gases

The applicability of microwave-induced plasma optical emission spectrometry (MIP-OES) for continuous monitoring of the environmentally hazardous element mercury in flue gases has been studied. Microwave induced plasmas have been sustained using both a TM₀₁₀ cavity (Beenakker resonator) and a so-called Surfatron. The analytical figures of merit for mercury in argon and helium discharges with both types of low-power micro-wave discharges have been examined. To determine mercury in artificial stack gases non-mixed argon/nitrogen discharges have been tested using a tangential flow torch design which allows to introduce a metal-loaded nitrogen gas flow as external gas and argon as internal gas. The addition of main flue gas components such as water vapour (concentration <6 g/m³), oxygen (<4% v/v) and carbon dioxide (<15% v/v) decrease the mercury line intensities to a considerable extent. Trace gases (CO, HCl, SO₂, NO) in concentrations typical to waste incineration processes have been found to have no effect on the mercury and the argon line intensities. The detection limit of mercury in nitrogen is 8 g/m³ using the TM₀₁₀ MIP and 10 g/m³ using the Surfatron. As such low detection limits are below the emission limit values of present-day environmental legislation MIP-OES is useful for on-line monitoring of mercury.Dedicated to Professor Dr. Dieter Klockow on the occasion of his 60th birthday     

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.     

Study on microwave induced plasma atomic absorption spectrometry (MIP-AAS) for the determination of calcium in axial viewing mode

A low-power surface-wave plasma device (surfatron) is employed as an atomizer for microwave induced plasma atomic absorption spectrometry (MIP-AAS) and a desolvation-condensation apparatus has been used to remove the solvent in the aerosol. An axial viewing absorption technique is adopted for the determination of calcium. The matrix effect on its determination has been investigated in detail. Some enhancing effects are observed on the calcium absorbance in the presence of organic additives. A possible mechanism for this enhancement is discussed.     

Embedding of small samples for spark discharge atomic emission spectrometry

One approach to the analysis of small samples by means of an ordinary point-to-plane spark spectrometer is by embedding them in an electrically conductive material. In order to do so for steel samples of any shape, embedding in a pure tin ingot was found to be appropriate. This was carried out by putting them into a graphite crucible along with fairly large tin pieces, followed by heating above the melting point of tin. Thus the samples became firmly attached to the tin ingots. Grinding was easy despite the different hardness of steel and tin. With this procedure, samples of various types of steel gave correct analytical results upon sparking except for tin, provided that their diameter was at least 6 mm. The method is fast, cheap and more versatile than alternative ones.     

Behaviour of a desolvation system based on microwave radiation heating for use in inductively coupled plasma atomic emission spectrometry

The present paper describes the preliminary results obtained with a desolvation system for inductively coupled plasma atomic emission spectrometry that incorporates a heating unit based on microwave (MW) radiation. This system has been called Microwave Desolvation System (MWDS). The results have proved that MW radiation can be considered as a good choice for aerosol heating in a sample introduction system. MW radiation seems to be a more uniform way of aerosol desolvation than conductive/convective heating (i.e. lower radial temperature gradients), the degree of vaporization of the droplets is less dependent on the liquid flow rate (Q_l), and also the background noise associated with the vaporization of droplets is reduced. As regards the results obtained with MWDS, in comparison with a conventional desolvation system (CDS), they are very dependent on Q_l. When heating is applied, the amount of analyte that leaves the heating step increases by 30–60% with the MWDS, irrespective of Q_l, whereas for the CDS this increase is very high (up to 300%) at low Q_l values (0.4 ml min⁻¹), but almost negligible at high Q_l values (2.4 ml min⁻¹). In agreement with this, the analytical figures of merit are favourable to the CDS at low flow rates, and to the MWDS at high liquid flows. Under all the conditions studied, the amount of solvent that leaves the condensation unit are lower for MWDS than for CDS.     

A helium inductively coupled plasma for atomic emission spectrometry

An annular helium inductively coupled plasma (He ICP) was generated at atmospheric pressure. No external cooling was used to stabilize the plasma. Aqueous solution was injected into the plasma without any difficulty. Preliminary results revealed that the annular He ICP was capable of exciting elements such as Cl and Br which possess high excitation energies. Atomic emission detection limits for Cl and Br were improved by factors of 63 and 34, respectively, as compared to the results obtained from the argon inductively coupled plasma. The excitation temperature of the annular He ICP (4180 K) was less than that of an Ar ICP (5570 K).