Detectors plasma sources

Figure 6. Schematic outline of the first commercially available multiple collector ICPMS, the Plasma 54, after Halhday et al. (1995). This instrument uses Nier-Johnson double-focusing and is equipped with eight independently adjustable Faraday collectors. The axial collector can be wound down to provide access to a Daly detector equipped with ion counting capabilities and a second-stage energy filter for high abundance sensitivity measurements. The sample may be introduced to the plasma source by either solution aspiration or laser ablation.

This chapter also deals in particular with chromatographic detection by atomic plasma spectrometry and plasma mass spectrometry (AED, MIP, ICP). With the application of such detectors, metal-specific signals can be obtained - thus the information content of a separation increases significantly. The major objectives of interfaced chromatography-atomic plasma source emission spectrometry (C-APES) are ... 

GC-AAS has found late acceptance because of the relatively low sensitivity of the flame graphite furnaces have also been proposed as detectors. The quartz tube atomiser (QTA) [186], in particular the version heated with a hydrogen-oxygen flame (QF), is particularly effective [187] and is used nowadays almost exclusively for GC-AAS. The major problem associated with coupling of GC with AAS is the limited volume of measurement solution that can be injected on to the column (about 100 xL). Virtually no GC-AAS applications have been reported. As for GC-plasma source techniques for element-selective detection, GC-ICP-MS and GC-MIP-AES dominate for organometallic analysis and are complementary to PDA, FTIR and MS analysis for structural elucidation of unknowns. Only a few industrial laboratories are active in this field for the purpose of polymer/additive analysis. GC-AES is generally the most helpful for the identification of additives on the basis of elemental detection, but applications are limited mainly to tin compounds as PVC stabilisers. 

Plasmas compare favourably with both the chemical combustion flame and the electrothermal atomiser with respect to the efficiency of the excitation of elements. The higher temperatures obtained in the plasma result in increased sensitivity, and a large number of elements can be efficiently determined. Common plasma sources are essentially He MIP, Ar MIP and Ar ICP. Helium has a much higher ionisation potential than argon (24.5 eV vs. 15.8 eV), and thus is a more efficient ionisation source for many nonmetals, thereby resulting in improved sensitivity. Both ICPs and He MIPs are utilised as emission detectors for GC. Plasma-source mass spectrometry offers selective detection with excellent sensitivity. When coupled to chromatographic techniques such as GC, SFC or HPLC, it provides a method for elemental speciation. Plasma-source detection in GC is dominated by GC-MIP-AES... 

Besides liquid samples, gases and solids can be analysed after making the appropriate modifications to the sample introduction system. The application of plasma sources as detectors for gas chromatography of metal complexes have teen reviewed by Uden Literature dealing with the analysis of gas and liquid chromatographic effluents have been surveyed by Carnahan et al. 

SFC has received attention as an alternative separation technique to liquid and gas chromatography. The coupling of SFC to plasma detectors has been studied because plasma source spectrometry meets a number of requirements for suitable detection. There have been two main approaches in designing interfaces. The first is the use of a restrictor tube in a heated cross-flow nebuliser. This was designed for packed columns. For a capillary system, a restrictor was introduced into the central channel of the ICP torch. The restrictor was heated to overcome the eluent freezing upon decompression as it left the restrictor. The interface and transfer lines were also heated to maintain supercritical conditions. Several speciation applications have been reported in which SFC-ICP-MS was used. These include alkyl tin compounds (Oudsema and Poole, 1992), chromium (Carey et al., 1994), lead and mercury (Carey et al., 1992), and arsenic (Kumar et al., 1995). Detection limits for trimethylarsine, triphenylarsine and triphenyl arsenic oxide were in the range of 0.4-5 pg. 

An additional effusive beam source, operated with pure O2 as the precursor geis, is used for some studies. This source, shown schematically in Fig. 7, is a low pressure ( 100 mTorr) RF discharge tube, with a small hole (0.5 mm) in the end. The inductively coupled plasma is typically operated at a power of 50 W. The effluent (beam) from this source has been characterized by aligning the detector with the plasma source and placing... 

Over the past decade Inductively Coupled Plasma (ICP) sources, in particular coupled with Mass Spectrometry (MS) instruments, have shown an immense potential for multielement analysis in environmental samples (88). These capabilities have been obtained thanks to the combination of the great ionization energy of a plasma source with the high sensitivity and selectivity of the mass spectrometric detector. Since polar snow and ice are considered as the purest material on the earth surface, these environmental matrices constitute the ideal samples for ICP-MS since potential interferences formed in the plasma are kept at a minimum level. 

The introduction of an electro-thermal vaporisation (ETV) unit to an ICP-OES plasma source can be used for most solid and liquid samples with considerable ease. Drying and pyrolysis can remove the solvent and major components and the residual analytes are vaporised and transported by the argon gas flow to the ICP-OES plasma source where metals of interest are detected with a rapid CCD detector. The ETV sampling/analysis provides higher analytical transport efficiencies and can detect very low trace levels of metals (i.e. in the ppt range). 

At first glance the excellent detection limits obtained with the unintensified SPD, compared with those obtained with the high gain detectors may seem puzzling. An examination of the S/N characteristics of both the SPD and the plasma source can provide some insight into this pleasing performance. 

Figure 14.3 Plasmas obtained by inductively coupling or by microwaves, (a) Above left, (radial viewing to collect radial light). A radiofrequency current (between 27 and 50 MHz) that induces circulation of electrons in the inert gas drives the torch. The inner tube is used to inject the sample into the plasma. Right (for axial viewing), cooled device using fibre optic. The torch may consume up 10 to 15L/min of argon, which serves simultaneously as the ionization gas, nebulization gas and cooling gas (to avoid the torch melting ), (b) Below is a microwave plasma used at the outlet of a gas chromatograph for a specialized detector. These plasma sources must be stabilized for better reproducibility of the analyses.

The measurements were performed using a Thermo Elemental IRIS Inductively Coupled Plasma Atomic Emission Spectrometer (ICP-AES). A 2 kW crystal-controlled radio frequency (RF) generator operating at 27.12 MHz powers the plasma source. An Echelle optical system with a 381-mm focal length diffracts the light from the plasma source before it is focused onto the Charge Injected Device (CID) camera detector [4]. 

The AED employs a microwave-induced He plasma to dissociate eluted analyte molecules to their component atoms and excite them to emit at characteristic wavelengths. This is very similar to the mechanism in the argon plasma inductively coupled plasma source (cf. Section 7.3.1). A spectrometer with a diode array detector (Figure 7.26b and c) isolates and measures the intensity of sensitive emission lines unique to each element. Depending on the relative sensitivity and proportion of atoms in the molecules, separate element response channels may display peaks in several element-selective chromatograms. These data may be combined with retention... 

Wcssman reviewed a number of instruments used for uranium analyses and ranked their relative measurement sensitivities [32]. The methods include atomic absorption spectrophotometry, colorimetry, neutron bombardment, fission etched track detectors, fluorimetry, laser-induced fluorescence spectrometry, a-spectrometry, isotope dilution mass spectrometry, and spark source mass spectrometry. The majority of urinary bioassay measurements have been performed by fluorimetry, while environmental survey and baseline measurements have been performed by fluorimetry, a-spectrometry, and induced coupled plasma source mass spectrometry. 

Inductively coupled plasma-mass spectrometry is a very rapid technique for the determination of long-lived radionuclides. This technique is based on the ionization of elements in the plasma source. Typically, radiofrequency and argon are used to reach plasma excitation temperatures ranging from 4900 to 7000 K [18,19]. The ions produced are introduced through an interface into a vacuum chamber and are analyzed by a quadru-pole mass spectrometer. Other attempts are being made to use faster mass-spectrometer detectors, such as time-of-flight mass spectrometers, but methods are still not available. 

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