Ultratrace elements absorption

The most frequently applied analytical methods used for characterizing bulk and layered systems (wafers and layers for microelectronics see the example in the schematic on the right-hand side) are summarized in Figure 9.4. Besides mass spectrometric techniques there are a multitude of alternative powerful analytical techniques for characterizing such multi-layered systems. The analytical methods used for determining trace and ultratrace elements in, for example, high purity materials for microelectronic applications include AAS (atomic absorption spectrometry), XRF (X-ray fluorescence analysis), ICP-OES (optical emission spectroscopy with inductively coupled plasma), NAA (neutron activation analysis) and others. For the characterization of layered systems or for the determination of surface contamination, XPS (X-ray photon electron spectroscopy), SEM-EDX (secondary electron microscopy combined with energy disperse X-ray analysis) and... 

Graphite Furnace Atomic Absorption Spectrometry Graphite furnace atomic absorption spectrometry (GFAAS), the most popular form of ET-AAS, is today a common technique widely used in routine laboratories and has become a powerful tool for the analysis of trace and ultratrace elements in clinical and biological samples [61]. The main advantages of this technique are low cost, simplicity, excellent detection power, and the fact that it allows very low sample volumes to be used (5-20 p,L). In this sense, this technique allows LoDs for many elements in the order of 0.01 pgl-1 in solution or 1 pg g-1 in solid samples to be achieved [62]. However, the technique is prone to spectral and matrix interferences. 

Factors which are important for the lowering of human health risks due to mineral deficiency and intoxication are the normative requirements of the macro, trace and ultratrace elements, recommendations for their intake, their apparent absorption and excretion rates, their interactions, tolerance limits for chemicals in food and water, and occupational health standards. 

Fecal Excretion and Apparent Absorption of Ultratrace Elements... 

The majority of ultratrace elements are metals, and their fecal excretion rate is greater than 50% (Table 4.19). Fecal excretion of lead by people with a mixed diet is astonishingly low (50%), while the apparent absorption rate is very high (40% on average). The daily lead intake of test popula-... 

Tab. 4.19 Fecal excretion and apparent absorption rate of ultratrace elements by adults ...

Thus, the different absorption rates, the effects of other dietary components on the forms of vanadium in the stomach, and the rate of transformation into V appear markedly to affect the proportion of ingested vanadium that is absorbed (Patterson et al. 1986, Chasteen et al. 1986, Wiegman et al. 1982, Hansard etal. 1982b). Studies with humans and an intake of 11 to 39 pg V per day showed renal and lactational excretions of vanadium of approximately 5% and 16%, respectively. The apparent absorption rate varied between 10 and 25% (Table 27.5) (Anke et al. 1998d). Besides the form of vanadium, it is also the quantity of vanadium present that influences the absorption rate of this ultratrace element. 

Absorption of ultratrace elements from the intestinal lumen can occur in three ways. These are described below. 

Active transport - the accumulation within, or the extrusion from, a cell of an element in opposition to a concentration gradient. Active transport is saturable, is energy dependent and involves a carrier protein that usually is quite specific for an element. The mechanisms of absorption for the various ultratrace elements are given in Table 1 this table also lists the known transport and storage vehicles for these elements. 

Table 1 Absorption, transport, and storage characteristics of the ultratrace elements...

Advantages High analysis rate 3-4 elements per hour Applicable to many more metals than voltammetric methods Superior to voltammetry for mercury and arsenic particularly in ultratrace range Disadvantages Nonspecific absorption Spectral interferences Element losses by molecular distillation before atomisation Limited dynamic range Contamination sensitivity Element specific (or one element per run) Not suitable for speciation studies in seawater Prior separation of sea salts from metals required Suspended particulates need prior digestion About three times as expensive as voltammetric equipment Inferior to voltammetry for cobalt and nickel... 

For the determination of traces and ultratraces of Hg, As, Se, Te, As and Bi the formation of the volatile mercury vapor or of the volatile hydrides of the appropriate elements is often used, respectively. This allows a high sampling efficiency to be achieved and accordingly a high power of detection. The absorption measure-... 

Although originally FIA was conceived as a special technique for delivery of a sample segment into the instrument, the combination of flow injection as a sample pretreatment tool with atomic spectrometry has been shown to be of great potential for enhancing the selectivity and sensitivity of the measurements. Moreover, contamination problems are reduced due to the closed system used, making this interface suitable for ultratrace determination of metal species. Hyphenated techniques such as FIA/ SIA with flame atomic absorption spectrometry, inductively coupled plasma (ICP)-optical emission spectrometry, and ICP-mass spectrometry (MS) have been exploited extensively in recent years. The major attraction of FIA-ICP-MS is its exceptional multi-elemental sensitivity combined with high speed of analysis. In addition, the possibility of... 

For the determination of traces and ultratraces of Hg, As, Se, Te, As, and Bi, the formation of the volatile mercury vapor or of the volatile hydrides of the other respective elements is often used. This allows a high sampling efficiency to be achieved and accordingly a high power of detection. The absorption measurement is often performed in a quartz cuvette. Hg, for instance, can be reduced to the metal and then transported with a carrier gas into the cuvette. No heating is required for the absorption measurement. The other elements are reduced to the volatile hydrides, which are then transported with a carrier-gas flow (argon or nitro-... 

---