Atomic optical spectrometry detection limits

Highly sensitive iastmmental techniques, such as x-ray fluorescence, atomic absorption spectrometry, and iaductively coupled plasma optical emission spectrometry, have wide appHcation for the analysis of silver ia a multitude of materials. In order to minimize the effects of various matrices ia which silver may exist, samples are treated with perchloric or nitric acid. Direct-aspiration atomic absorption (25) and iaductively coupled plasma (26) have silver detection limits of 10 and 7 l-lg/L, respectively. The use of a graphic furnace ia an atomic absorption spectrograph lowers the silver detection limit to 0.2 l-ig/L. 

Plasma sources were developed for emission spectrometric analysis in the late-1960s. Commercial inductively coupled and d.c. plasma spectrometers were introduced in the mid-1970s. By comparison with AAS, atomic plasma emission spectroscopy (APES) can achieve simultaneous multi-element measurement, while maintaining a wide dynamic measurement range and high sensitivities and selectivities over background elements. As a result of the wide variety of radiation sources, optical atomic emission spectrometry is very suitable for multi-element trace determinations. With several techniques, absolute detection limits are below the ng level. 

Mass spectrometry is the only universal multielement method which allows the determination of all elements and their isotopes in both solids and liquids. Detection limits for virtually all elements are low. Mass spectrometry can be more easily applied than other spectroscopic techniques as an absolute method, because the analyte atoms produce the analytical signal themselves, and their amount is not deduced from emitted or absorbed radiation the spectra are simple compared to the line-rich spectra often found in optical emission spectrometry. The resolving power of conventional mass spectrometers is sufficient to separate all isotope signals, although expensive instruments and skill are required to eliminate interferences from molecules and polyatomic cluster ions. 

Table 8.76 shows the main characteristics of voltammetry. Trace-element analysis by electrochemical methods is attractive due to the low limits of detection that can be achieved at relatively low cost. The advantage of using standard addition as a means of calibration and quantification is that matrix effects in the sample are taken into consideration. Analytical responses in voltammetry sometimes lack the predictability of techniques such as optical spectrometry, mostly because interactions at electrode/solution interfaces can be extremely complex. The role of the electrolyte and additional solutions in voltammetry are crucial. Many determinations are pH dependent, and the electrolyte can increase both the conductivity and selectivity of the solution. Voltammetry offers some advantages over atomic absorption. It allows the determination of an element under different oxidation states (e.g. Fe2+/Fe3+). 

This section starts with a discussion of selectivity for the most extended analytical atomic techniques based on optical spectrometry. Then, aspects such as detection limits (DLs), linear ranges, precision, versatility and sample throughput will be presented. The section ends with a brief comparison of the... 

Each experiment was accortqjanied the determination of Pd in solution after hot filtration of the solid catalyst at the end of the reaction. Because simple Atomic Absorption Spectroscopy (AAS) was found to not be precise enough for the palladium analysis in this concentration range (detection limit too high.) ICP-OES and/or ICP-MS (Inductively Coupled Plasma - Optical Emission Spectroscopy or Inductively Coupled Plasma - Mass Spectrometry) were applied. To first approximation, the Pd leaching could not be correlated with the properties of the twelve different Pd/C catalysts described above ((1) Correlation of catalyst structure and activity.) There is, however, a strong correlation with the reaction parameters as described below. 

Laborious, yet very reliable, is the complete extraction of the dried, mortared and homogenized sediment sample using appropriate acids. Usually a microwave pressure extraction in a closed system composed of PTFE or a similarly inert and resistant material is applied for this purpose nowadays. With this method, all the sedimentary constituents are dissolved without exception by applying various mixtures of HF, HCl and HNO (HCLO can be avoided in most cases) at temperatures between 250 and 260 °C and a pressure ranging from 30 to 50 bars. In the ultimately received, slight nitric solution - hydrofluoric acid is left to evaporate in the course of the procedure -practically all elements (including the rare earth elements) can then be analyzed markedly above their limit of detection by atom absorption spectrometry (AAS), optical plasma emission spectrometry (ICP-OES) and/or plasma mass spectrometry (ICP-MS). 

Trace amounts of titanium can be determined by X-ray fluorescence spectrometry, neutron activation analysis (NAA), atomic absorption techniques (AAS) and inductively coupled plasma-optical emission spectrometry (ICP-OES). In case of AAS, a high-temperature flame (nitrous oxide, acetylene) is essential, and the optimum wavelengths are 364.3 and 365.4 nm the sensitivity is low. With the graphite furnace, a lower detection limit of approximately 0.5 xg L can be achieved. ICP-OES is especially sensitive, and is the recommended instrumental... 

Laser Fluorescence Noise Sources. Finally, let us examine a technique with very complex noise characteristics, laser excited flame atomic fluorescence spectrometry (LEAFS). In this technique, not only are we dealing with a radiation source as well as an atomic vapor cell, as In atomic absorption, but the source Is pulsed with pulse widths of nanoseconds to microseconds, so that we must deal with very large Incident source photon fluxes which may result in optical saturation, and very small average signals from the atomic vapor cell at the detection limit [22]. Detection schemes involve gated amplifiers, which are synchronized to the laser pulse incident on the flame and which average the analyte fluorescence pulses [23]. 

Various techniques can be used for quantitative analysis of chemical composition, including (i) optical atomic spectroscopy (atomic absorption, atomic emission, and atomic fluorescence), (ii) X-ray fluorescence spectroscopy, (iii) mass spectrometry, (iv) electrochemistry, and (v) nuclear and radioisotope analysis [41]. Among these, optical atomic spectroscopy, involving atomic absorption (AA) or atomic emission (AE), has been the most widely used for chemical analysis of ceramic powders. It can be used to determine the contents of both major and minor elements, as well as trace elements, because of its high precision and low detection limits. 

Another important difference between flame emission and atomic absorption is the difference in their signals. The analytical signal in flame emission is the sum of all energies emitted as excited atoms drop to the ground state. The signal comes entirely from the emitting atoms. In atomic absorption the signal is the difference between the intensity of the source in the absence of analyte atoms and the decreased intensity obtained when analyte atoms are present in the optical path. This basic difference in the two techniques also contributes to the fact that detection limits of many elements are quite similar in emission and absorption spectrometry. 

There has been renewed interest in the method, mainly due to the availability of improved Nd YAG laser systems. In addition, different types of detectors, such as microchannel plates coupled to photodiodes and CCDs, in combination with multi-channel analyzers make it possible for an analytical line and an internal standard line to be recorded simultaneously, by which the analytical precision can be considerably improved. By optimizing the ablation conditions and the spectral observation, detection limits obtained using the laser plume as a source for atomic emission spectrometry are in the 50-100 pg/g range and RSDs are in the region of 1% as shown by the determination of Si and Mg in low-alloyed steels [255, 259]. This necessitates the use of slightly reduced pressure, so that the atom vapor cloud is no longer optically very dense and the background emission intensities become lower. In the case of laser ablation of brass samples at normal pressure and direct... 

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