Microwave-induced plasma detectors

J. Brill, B. Narayanan, et al., Selective determination of organofluo-rine compounds by capillary column gas chromatography with an atmospheric pressure helium microwave-induced plasma detector, HRC CC J. High Res. Chromatogr. Chromatogr. Commun., 11 368-374(1988). 

The chromatograms were detected with the thermal conductivity and the microwave-induced plasma detectors in series. When the microwave-induced plasma detector was used as a detector, the emission signals were monitored at 253.7nm, and the solvent was vented through a four-way valve before reaching the microwave induced plasma detector. 

A method is presented to determine organomeicury compounds in aquatic sediments. The compounds are liberated from the sediment by addition of sulfuric acid and then immediately converted to the iodide form by iodoacetic acid in a closed vial. The liberated methylmercmy iodide is then Headspace-injected on a gaschromatograph and detected with a Microwave-induced-plasma detector. Analysis of 1 sample takes less than 15 minutes. Detection limit equals 0.014 p g methylmercury/g sediment. 

The first GC-microwave-induced plasma emission system was reported in 1965 [23]. During the past two decades GC-plasma emission systems have gained in popularity and have been used for the identification and quantification of mercury, lead, tin, selenium, and arsenic compounds [13]. The most frequently used plasma source is the microwave-induced plasma operated either at reduced pressure or at atmospheric pressure with helium or argon as the plasma gases at powers of 100 to 200 W The Beenakker cylindrical resonance cavity introduced in 1976 [24], and since then modified to achieve better detection limits, is most frequently used in the GC-microwave-induced plasma emission systems that are easily adaptable to capillary GC operation. These microwave-induced plasma detectors respond to non-metals (H, D, B, C, N, O, F, Si, F S, Cl, As, Se, Br, I) and metals, with absolute detection limits in... 

HPLC-QFAAS is also problematical. Most development of atomic plasma emission in HPLC detection has been with the ICP and to some extent the DCP, in contrast with the dominance of the microwave-induced plasmas as element-selective GC detectors. An integrated GC-MIP system has been introduced commercially. Significant polymer/additive analysis applications are not abundant for GC and SFC hyphenations. Wider adoption of plasma spectral chromatographic detection for trace analysis and elemental speciation will depend on the introduction of standardised commercial instrumentation to permit interlaboratory comparison of data and the development of standard methods of analysis which can be widely used. 

As SFC provides gaseous sample introduction to the plasma and thus near-100 % analyte transport efficiency, coupling SFC with plasma mass spectrometry offers the potential of a highly sensitive, element-selective chromatographic detector for many elements. Helium high-efficiency microwave-induced plasma has been proposed as an element-selective detector for both pSFC and cSFC [467,468] easy hyphenation of pSFC to AED has been reported [213]. 

While most preliminary SFC-plasma coupled techniques employed microwave-induced plasmas (MIPs), the use of ICP-MS is now increasing [469]. An advantage of microcolumn SFC-ICP hyphenation is the significantly reduced flow-rates of microcolumns compared with those of conventional columns. Both pSFC-ICP-AES [470,471] and cSFC-ICP-AES [472] were described. In the case of elemental detector selectivity (e.g. AES) complete chromatographic resolution is not required. The detector possesses linearity over several orders of concentrative magnitude. Minimum detectable quantities for nonmetals range from sub to low ng mL"1. 

Lobinski et al. [78] speciated organotin compounds in sediment samples by capillary gas chromatography using helium microwave induced plasma emission spectrometry as a detector. They used the procedure to determine mono-, di- and tri- and some tetraalkylated tin compounds in sediments. 

Microwave-induced plasma optical emission spectrometry (MIP-OES) is very sensitive for volatile species containing metals. Hence its use has been also proposed as a detector in the development of hyphenated techniques for speciation. GC MIP-OES has been successfully applied for the speciation of alkylmetal species of low molecular weight (Hg, Sn and Pb compounds) in many different environmental applications [23]. 

Kollotzek, D., Oechsle, D., Kaiser, G., Tschopel, R and Tolg, G. (1984) Application of a mixed gas microwave induced plasma to an on-line element-specific detector for high performance liquid chromatography. Fresenius Z. Anal. Chem., 318, 485-489. 

Bulska, E., Emteborg, H., Baxter, D.C., Freeh, W., Elligsen, D. and Thomassen, Y. (1992) Speciation of mercury in human whole blood by capillary gas chromatography with a microwave-induced plasma emission detector system following complexometric extraction and butylation. Analyst, 117, 657-663. 

Microwave-induced plasma (MIP), direct-current plasma (DCP), and inductively coupled plasma (ICP) have also been successfully utilized. The abundance of emission lines offer the possibility of multielement detection. The high source temperature results in strong emissions and therefore low levels of detection. Atomic absorption (AA) and atomic fluorescence (AF) offer potentially greater selectivity because specific line sources are utilized. On the other hand, the resonance time in the flame is short, and the limit of detectability in atomic absorption is not as good as emission techniques. The linearity of the detector is narrower with atomic absorption than emission and fluorescence techniques. 

Microwave induced plasma mass spectrometry has also been used as a detector for supercritical fluid chromatography (SFC) [113] for the separation of halogenated hydrocarbons. The design of an SFC-MIP interface must ensure that the frit restrictor temperature remains at a high temperature to prevent condensation of analytes. Stainless steel transfer lines may be used. The frit restrictor should be connected to a length of deactivated fused silica capillary, inserted through the transfer line, and positioned flush with the aluminum MIP torch inset (Fig. 10.21). 

As noted earlier, USNs have been employed for sample insertion into atomic spectrometers suoh as flame atomio absorption spectrometry (FAAS) [9,10], electrothermal atomic absorption speotrometry (ETAAS) [11], atomic fluorescence spectrometry (AFS) [12,13], induotively ooupled plasma-atomic emission spectrometry (ICP-AES) [14,15], inductively coupled plasma-mass spectrometry (ICP-MS) [16,17] and microwave induced plasma-atomic emission spectrometry (MIP-AES) [18,19]. Most of the applications of ultrasonic nebulization (USNn) involve plasma-based detectors, the high sensitivity, selectivity, precision, resolution and throughput have fostered their implementation in routine laboratories despite their high cost [4]. 

Because of its low specificity and sensitivity flame ionisation detection (FID) can only be used in the analysis of standard substances [37]. The same limited application is envisaged for the method with the microwave-induced plasma emission detector, which is not sensitive enough for environmental samples [2]. 

Beenakker C. I. M. (1977) Evaluation of a microwave-induced plasma in helium at atmospheric pressure as an element-spedfic detector for gas chromatography, Spectrochim Acta,... 

The gas chromatograph may be interfaced with atomic spectroscopic instruments for specific element detection. This powerful combination is useful for speci-ation of different forms of toxic elements in the environment. For example, a helium microwave induced plasma atomic emission detector (AED) has been used to detect volatile methyl and ethyl derivatives of mercury in fish, separated by GC. Also, gas chromatographs are interfaced to inductively coupled plasma-mass spectrometers (ICP-MS) in which atomic isotopic species from the plasma are introduced into a mass spectrometer (see Section 20.10 for a description of mass spectrometry), for very sensitive simultaneous detection of species of several elements. 

HPLC, coupled with element-specific detectors such as AAS, AFS, ICP-MS and microwave-induced plasma atomic emission spectrometry (MIP-AES). In general, methods for mercury speciation are classified according to the isolation/separation technique and the detection systems (Horvat 1996, Carro and Mejuto 2000). Most methods for isolation/separation are based on solvent extraction, differential reduction, difference calculation between total and ionic mercury, derivatization, or on paper- and thin-layer chromatography. 

In general, QE-AAS, AES, AES, microwave induced plasma (MIP) and ICP-MS are used as detectors rather than the less specific FID, FPD, and ECD. The overriding reason for this is the greater sensitivity and selectivity afforded by the element-specific detectors, without which it would not be possible to determine the chemical species of interest at the low concentrations generally present in biological and environmental samples. The other main detection method that has been used is mass spectrometry in its various configurations, but particularly electrospray ionization (ESI) and atmospheric chemical ionization (APCI), which are used with HPLC and CE separations, and... 

Such large amounts of data can only be sensibly and rapidly analysed and compared with reference spectra using microprocessors such as the fast 32 bit processors in PCs. The main systems in use today are discussed below, and in addition to the above mentioned techniques the microwave induced plasma (MIP) detector, a helium microwave plasma emission source coupled to a GC and an optical emission spectrometer are reviewed. 

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