Atomic spectrometry systems

The major goals for the future development of analytical atomic spectrometry measurements are improved detection Hmits and the development of simple ways of couphng to other analytical techniques. The nebuHzer systems of the spectrometric instruments are the parts that need to be improved in order to achieve these goals. Typically, nebuHzer efficiencies are of the order of 1—2%, and, as a result, they are Hmiting factors for instruments which can cost between 100,000 and 150,000. 

Basic instrumentation systems used in analytical atomic spectrometry. 

On-line coupling between a gas chromatograph and an atomic spectrometry detector is fairly simple. Typically, the output of the CG capillary column is connected to the entrance of the atomisation-ionisation system simply via a heated transfer line. When separation is performed by liquid chromatography (EC), the basic interface is straightforward a piece of narrow-bore tubing connects the outlet of the EC column with the liquid flow inlet of the nebuliser. Typical EC flow rates of 0.5-2 ml min are within the range usually required for conventional pneumatic nebulisation. 

Figure A.3A.2 Selection of chromatography-mass spectrometry system for the analysis of a sample. Abbreviations APCI, atmospheric pressure chemical ionization CF, continuous flow Cl, chemical ionization El, electron impact FAB, fast atom bombardment GC/MS, gas chromatogra-phy/mass spectrometry LC/MS, liquid chromatography/mass spectrometry.

Every coupling application favors one part of the coupling system. A dominating chromatography part leads to the speciation analysis [5,6,26,27]. The elemental specific detection facilities of atomic spectrometry are strongly favored over the multielement capabilities. An inversion of this construction leads to multielement trace analysis in complex matrices with the use of chromatographic equipment as powerful preconcentration and matrix elimination tool [13k The ability of chromatography for a further time resolution between the separated traces is not really required because of the excellent elemental specific detection capabilities of atomic spectrometry. 

Trost et al. [11] reported another impressive example of bimetallic catalysts in which a Zn-Zn homobimetallic complex (17, Scheme 7) serves as an effective catalyst for direct aldol reactions [11-13]. The proposed structure of the catalyst was verified by mass spectrometry and the best ratio of Et2Zn and the ligand. The chemical yield was moderate in the reaction of methyl ketones (1) (Scheme 7, top) [11,12], but a highly atom-economic system was achieved when a-hydroxylated ketones (10) were used as a substrate (Scheme 7, bottom) [13]. Excellent diastereo- and enantioselectivity were obtained under mild conditions. In contrast to the case of Shibasaki s heteropolymetallic catalyst, syn-1,2-diols (syn-11) were obtained as the major diastereomers. 

The determination of chromium in most biological samples is difficult because of the matrix interference and the very low concentrations present in these samples. Prior to 1978, numerous erroneous results were reported for the chromium level in urine using electrothermal atomic absorption spectrometry (EAAS) because of the inability of conventional atomic absorption spectrometry systems to correct for... 

K. Julshamn, O. Ringdal, K. E. Slinning, O. R. Braekkan, Optimisation of determination of selenium in marine samples by atomic absorption spectrometry comparison of a flameless graphite furnace atomic absorption system with a hydride generation atomic absorption system, Spectrochim. Acta, 37B (1982), 473-482. 

A flame emission spectrometer therefore consists of an atom source, a monochromator and detector and is therefore simpler instrumentally than the corresponding atomic absorption system. Particular developments engendered by atomic absorption have restimulated interest in flame emission spectrometry after a dormant period. Chief of these is the use of the nitrous oxide—acetylene flame which is sufficiently hot to stimulate thermal atomic-emission from a wide range of metal elements. 

Gluodenis, T.J., Tyson, J.E. Elow injection systems for directly coupling on-line digestion with analytical atomic spectrometry. Part 1. Dissolution of cocoa under stopped-flow, high-pressure conditions. J. Anal. At. Spectrom. 7, 301-306 (1992)... 

LEI spectrometry using the total consumption burner, with greater sample throughput and a wider range of usable fuel-oxidant combinations, expands the possibilities for development of a more sensitive and versatile detection system for atomic spectrometry. In addition to furthering the analytical methodology, these results demonstrate that high-sensitivity LEI measurements are possible in adverse sample environments where traditional methods of optical spectrometry have proven inadequate. 

Atomic spectrometric methods of analysis essentially make use of equipment for spectral dispersion so as to isolate the signals of the elements to be determined and to make the full selectivity of the methodology available. In optical atomic spectrometry, this involves the use of dispersive as well as of non-dispersive spectrometers. The radiation from the spectrochemical radiation sources or the radiation which has passed through the atom reservoir is then imaged into an optical spectrometer. In the case of atomic spectrometry, when using a plasma as an ion source, mass spectrometric equipment is required so as to separate the ions of the different analytes according to their mass to charge ratio. In both cases suitable data acquisition and data treatment systems need to be provided with the instruments as well. 

In optical atomic spectrometry the radiation emitted by the radiation source or the radiation which comes from the primary source and has passed through the atom reservoir has to be lead into a spectrometer. In order to make optimum use of the source, the radiation should be lead as complete as possible into the spectrometer. The amount of radiation passing through an optical system is expressed by its optical conductance. Its geometrical value is given by ... 

Thermal evaporation of the analyte elements from the sample has long been used in atomic spectrometry. For instance, it had been applied by PreuE in 1940 [170], who evaporated volatile elements from a geological sample in a tube furnace and transported the released vapors into an arc source. In addition, it was used in so-called double arc systems, where selective volatilization was also used in direct solids analysis. Electrothermal vaporization became particularly important with the work of L vov et al. [171] and Massmann in Dortmund [172], who introduced elec-trothermally heated sytems for the determination of trace elements in dry solution residues by atomic absorption spectrometry of the vapor cloud. Since then, the idea has regularly been taken up for several reasons. 

Fig. 57. G raphite atomizers used in atomic spectrometry. (A) Original graphite tube furnace according to MafSmann (a) graphite tube with sampling hide (reprinted with permission from Ref. [172]), (B) carbon-rod atomizer system according to West (a) support (b) clamps (cooled) (c) graphite rod or cup (reprinted with permission from Ref. [175]).

In a system for coherent forward scattering, the radiation of a primary source is led through the atom reservoir (a flame or a furnace), across which a magnetic field is applied. When the atom reservoir is placed between crossed polarizers scattered signals for the atomic species occur on a zero-background. When a line source such as a hollow cathode lamp or a laser is used, determinations of the respective elements can be performed. In the case of a continuous source, such as a xenon lamp, and a multichannel spectrometer simultaneous multielement determinations can also be performed. The method is known as coherent forward scattering atomic spectrometry [309, 310]. This approach has become particularly interesting since flexible multichannel diode array spectrometers have became available. 

Coherent forward scattering (CFS) atomic spectrometry is a multielement method. The instrumentation required is simple and consists of the same components as a Zeeman AAS system. As the spectra contain only some resonance lines, a spectrometer with just a low spectral resolution is required. The detection limits depend considerably on the primary source and on the atom reservoir used. When using a xenon lamp as the primary source, multielement determinations can be performed but the power of detection will be low as the spectral radiances are low as compared with those of a hollow cathode lamp. By using high-intensity laser sources the intensities of the signals and accordingly the power of detection can be considerably improved. Indeed, both Ip(k) and Iy(k) are proportional to Io(k). When furnaces are used as the atomizers typical detection limits in the case of a xenon arc are Cd 4, Pb 0.9, T11.5, Fe 2.5 and Zn 50 ng [309]. They are considerably higher than in furnace AAS. 

Glow discharges have also now been miniaturized. In the discharge in a micro-structured system, described by Eijkel et al. [607], molecular emission is obtained and the system can be used successfully to detect down to 10 14 g/s methane with a linear response over two decades. A barrier-layer discharge for use in diode laser atomic spectrometry has also been described recently (Fig. 122) [608],... 

You J., Dempster M. A. and Marcus R. K. (1997) Studies of analyte particle transport in a particle beam-hollow cathode atomic emission spectrometry system, J Anal At Spectrom 12 807-815. 

Sensitivities of atomic methods lie typically in the parts per million (mg dm ) to parts per billion (pg dm or pg kg ) range, although in some cases in the parts per trillion (ng dm ) range. (You may wish to think about the implications of this ). Additional virtues associated with these methods include speed, convenience, unusually high selectivity and moderate instrument costs (although not for an inductively coupled plasma-mass spectrometry system ). 

Perez-Jordan MY, Salvador A, de la Guardia M. 1998. Determination of Sr, K, Mg and Na in human teeth by atomic spectrometry using a microwave-assisted digestion in a closed flow system. Anal Lett 31(5) 867-877. 

B.F. Reis, M.A.Z. Arruda, E.A.G. Zagatto, J.R. Ferreira, An improved monosegmented continuous-flow system for sample introduction in flame atomic spectrometry, Anal. Chim. Acta 206 (1988) 253. 

M.F. Gine, A.P. Packer, T. Blanco, B.F. Reis, Flow system based on a binary sampling process for automatic dilutions prior to flame atomic spectrometry, Anal. Chim. Acta 323 (1996) 47. 

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