Analytical atomic spectrometric techniques

Introduction Basis of Analytical Atomic Spectrometric Techniques... 

Generally physical, chemical, and ionization interferences are similar in terms of incidence and extent in all three flame analytical atomic spectrometric techniques, but they are not a severe problem provided the analyst is aware of their existence, and takes the necessary precautions. Spectral interferences are not regarded as a serious problem in flame AAS or flame AFS, but are potentially much more serious in FES. Unless the analyst is certain that a particular FES determination is spectral interference-free for the samples in question, scanning and careful scrutiny of emission spectra from samples and standards is advisable, together with reliability checks using certified reference materials and/or determination at more than one wavelength. 

TABLE 2.1. Analytical Characteristics of Solubilized Forms in Element Determination by Analytical Atomic Spectrometric Techniques... 

Elements Determined / Species Sample/Matrix Analytical Atomic Spectrometric Technique... 

Different analytical techniques such as ICP-OES (optical emission spectrometry with inductively coupled plasma source), XRF (X-ray fluorescence analysis), AAS (atomic absorption spectrometry) with graphite furnace and flame GF-AAS and FAAS, NAA (neutron activation analysis) and others, are employed for the trace analysis of environmental samples. The main features of selected atomic spectrometric techniques (ICP-MS, ICP-OES and AAS) are summarized in Table 9.20.1 The detection ranges and LODs of selected analytical techniques for trace analysis on environmental samples are summarized in Figure 9.15.1... 

Routine inorganic elemental analysis is carried out nowadays mainly by atomic spectrometric techniques based on the measurement of the energy of photons. The most frequently used photons for analytical atomic spectrometry extend from the ultraviolet (UV 190-390 nm) to the visible (Vis 390-750 nm) regions. Here the analyte must be in the form of atoms in the gas phase so that the photons interact easily with valence electrons. It is worth noting that techniques based on the measurement of X-rays emitted after excitation of the sample with X-rays i.e. X-ray fluorescence, XRF) or with energetic electrons (electron-probe X-ray micro-analysis, EPXMA) yield elemental information directly from solid samples, but they will not be explained here instead, they will be briefly treated in Section 1.5. 

For a given ICP-OES instrument, the intensity of an analyte line is a complex function of several factors. Some adjustable parameters that affect the ICP source are the radiofrequency power coupled into the plasma (usually about 1 kW), the gas flow rates, the observation height in the lateral-viewing mode and the solution uptake rate of the nebuliser. Many of these factors interact in a complex fashion and their combined effects are different for dissimilar spectral lines. The selection of an appropriate combination of these factors is of critical importance in ICP-OES. This issue will be addressed in Chapter 2, where experimental designs and optimisation procedures will be discussed. Many examples related to ICP and other atomic spectrometric techniques will be presented. 

The choice of the detector becomes crucial when the concentration of analyte species in the sample is very low and low limits of detection are required. For element-specific detection, the major atomic spectrometric techniques, flame AAS, OES, AFS and ICP-MS, are specially suited as chromatographic... 

A strong preference in speciation analysis is to use a separation step that can be combined with a detection step in an on-line system [45]. In such coupling, analytical selectivity relies on the application of different chromatographic or electrophoretic methods, while the use of atomic spectrometric techniques assures high sensitivity and f>t-for-purpose limits of detection (LoDs). However, hyphenated techniques with element-specif>c detection do not provide structural information on the species. If appropriate standards are available, the assignation of chromatographic peaks can be accomplished by spiking experiments. On the... 

Section I covers the more conventional equipment available for analytical scientists. I have used a unified means of illustrating the composition of instruments over the five chapters in this section. This system describes each piece of equipment in terms of five modules - source, sample, discriminator, detector and output device. I believe this system allows for easily comparing and contrasting of instruments across the various categories, as opposed to other texts where different instrument types are represented by different schematic styles. Chapter 2 in this section describes the spectroscopic techniques of visible and ultraviolet spectrophotometry, near infrared, mid-infrared and Raman spectrometry, fluorescence and phosphorescence, nuclear magnetic resonance, mass spectrometry and, finally, a section on atomic spectrometric techniques. I have used the aspirin molecule as an example all the way through this section so that the spectral data obtained from each... 

Equally impressively comprehensive reviews on atomic spectrometric techniques appear regularly in the Journal of Analytical Atomic Spectrometry these are referred to in appropriate determinative technique sections in this chapter. Other reviews in this journal target materials and emphasize various atomic spectrometric techniques for their analyses. Several recent materials-ori-ented reviews are by Taylor etal. (1997,... 

The review by Maenhaut (1990) (Recent advances in nuclear and atomic spectrometric techniques for trace element analysis - A new look at the position of PIXE) is an excellent review by a respected PIXE practitioner. It includes a comparison of DLs on a solid sample basis for seventeen elements for seven analytical techniques INAA, ED-XRE, PIXE, ICP-AES, ETA-AAS, LIE-ETA, ICP-MS, and also includes a good comparison of some characteristics (cost, spectral interferences, matrix effects, multielement capability) of these methods (and TXRE). 

The present book is designed to describe the basic theory of atomic spectroscopy, instrumentation, techniques, and the application of various analytical atomic spectrometric methods (AAS, plasma AES, AFS, and ICP-MS). 

The experimental arrangement involved in an AES measurement is shown in Fig. IE. The hot analyte environment, which is able to break down and excite atoms, is called an atom cell. The atom cell can be a flame, plasma, a heated graphite tube, or any other environment where the analyte is observed in a spatially confined arrangement. In Fig. I, the detector box is used to represent a detection system, which is able to identify the wavelength and measure the intensity of the emitted radiation. The experimental arrangement is the simplest of the three optical atomic spectrometric techniques. 

Measurement of U-series disequilibria in MORB presents a considerable analytical challenge. Typical concentrations of normal MORB (NMORB) are variable but are generally in the 50-150 ppb U range and 100-400 ppb Th range. Some depleted MORB have concentrations as low as 8-20 ppb U and Th. The concentrations of °Th, Pa, and Ra in secular equilibrium with these U contents are exceedingly low. For instance, the atomic ratio of U to Ra in secular equilibrium is 2.5 x 10 with a quick rule of thumb being that 50 ng of U corresponds to 20 fg of Ra and 15 fg of Pa. Thus, dissolution of a gram of MORB still requires measurement of fg quantities of these nuclides by any mass spectrometric techniques. 

The set of energy levels associated with a particular substance is a unique characteristic of that substance and determines the frequencies at which electromagnetic radiation can be absorbed or emitted. Qualitative information regarding the composition and structure of a sample is obtained through a study of the positions and relative intensities of spectral lines or bands. Quantitative analysis is possible because of the direct proportionality between the intensity of a particular line or band and the number of atoms or molecules undergoing the transition. The various spectrometric techniques commonly used for analytical purposes and the type of information they provide are given in Table 7.1. 

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. 

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