DETECTION OF ELEMENTS PRESENT

As one dilutes the amount of an element in an arc, the number of lines observable is reduced, and ultimately there remain only a few lines of the element which is diluted. These lines are referred to in Section 20.2 as the raies ultimes or persistent lines and tables of their wavelengths may be found in chemical handbooks. The identification of these lines will permit detection of elements present in low concentration, and all qualitative methods utilise the persistent lines. 

Figure 2.5 illustrates two spectra recorded from a sample of iron using (a) Al Ka radiation, and (b) Mg Ko, radiation. The binding energy of the peaks are characteristic of each element. There is a difference in hv between these sources of 233 eV, so, as expected from equation (2.1), the XPS peaks on spectrum (a) are displaced 233 eV relative to those in spectrum (b). The spectrum was taken over a wide energy range to detect all possible peaks of elements present in the surface. The 2p and 3p peaks from iron are identified, as well as the Is peak from carbon which was present as a contaminant. 

Computer software codes are available to deconvolute PIXE spectra and to calculate peak areas with accuracy, so that absolute amounts of elements present in the specimen may be derived. With a beam of 5 mm diameter incident on a thin organic specimen on a thin backing foil, trace elements can be detected at picogram levels. The x-ray production cross-sections, absorption coefficients and the various... 

Flame atomic absorption spectrometry (FAAS) can be used to detect most elements present at levels greater than about 100 pg 1 . For more sensitive determinations graphite furnace atomic absorption spectrometry (GFAAS) is the technique of choice. In addition, if the volume of the fraction is limited GFAAS is ideally suited for the determination because only a few microfitres (5-20 pi) of sample... 

The unique aspects of speciation procedures arise from the additional specification that the procedure adopted should not disturb existing equilibrium conditions. The choice of procedure is further restricted by the fact that the total concentration of element present in a sample (e.g. Cu, Pb, Cd, Zn in water samples) is often near the detection limits of many standard analytical techniques, and modified or refined techniques are required to handle the even lower levels present in isolated sub-categories. In biological matrices, the concentrations of inorganic and organo-metallic compounds present can range from 10 3 to 10 12 mol dm 3, and at the lower levels even the determination of total element content can be greatly in error, if suitable correction is not made for interference effects which can arise from the nature of the sample. 

Another more serious defect of the EDS technique concerns the inherent sensititivy of the measurements for light (low-Z) elements. Several factors prevent the efficient detection of elements within the first row of the Periodic Table and limit information from the second. Instrumentally, the efficiency of the Li-drifted Si detector, its resolution, and the transparency of the Be window commonly used between the former and the system vacuum all concern us here. Detection sensitivities may be as low as 10", thereby limiting MDM at a given S/N, whilst the resolution (typically 150— 200 eV) may make the analysis of, for example, magnesium (hco = 1253.6 eV) in the presence of aluminium (hcu = 1486.6 eV) difficult. The presence of the Be window presents a more immediate practical problem in that characteristic Z-rays softer than, say, Na Kct may be absorbed, and this certainly precludes the analysis of the majority of the first-row elements (Li to F). 

With proper safety procedures, radiation can be very useful in many scientific experiments. Neutron activation analysis is used to detect trace amounts of elements present in a sample. Computer chip manufacturers use this technique to analyze the composition of highly purified silicon wafers. In the process, the sample is bombarded with a beam of neutrons from a radioactive source, causing some of the atoms in the sample to become radioactive. The type of radiation emitted by the sample is used to determine the types and quantities of elements present. Neutron activation analysis is a very sensitive measurement technique capable of detecting quantities of less than 1 X 10 9 g. 

Analyses carried out using EDX on wear plate samples tested at 60°C are shown in Table 4. The results show that tests conducted in the FF oil using EN31 plates (with either pin material) show evidence of an adhered tribofilm as demonstrated by the detection of elements such as S, P, Zn, Mo and Ca. However, analysis of the Al-Si alloy plate systems shows no evidence of a tribofilm although there is evidence of material transfer from the cast iron pin (Fe peak). This suggests that tribofilms are either not formed at all on the surface of Al-Si or that this film is very much thinner since the EDX has a penetration depth of a few microns. Therefore a very thin layer of a few nm may mean that the elements are present beyond the detection limit. XPS analysis was therefore carried out to probe the near surface region of the wear scar. EDX analysis also shows more evidence of Zn on the wear scar... 

In these techniques, the excited atom source is not a flame but a plasma e.g. Ar plasma, a d.c. or a.c. arc or a spark. Since higher temperatures are achieved in plasmas, excited ions may be also formed as well as excited atoms. In any case, the lines are more numerous than in other techniques. The lines to be detected are selected by narrow band pass monochromator and are detected by sensitive photomultipliers. The functions of the instrument are controlled by a micro-processor and are displayed on a photographic plate, a cathode ray tube, a recorder or a printer. Qualitatively, the lines obtained from a sample are compared with tables compiled for various elements as atoms or ions. The latter are more intense when plasmas are used. The presence of 3 major lines of an element is taken as positive identification. Most elements can be identified by these methods. Solid samples can be embedded in an electrode of the source. Quantitative analysis is possible for instruments giving an intensity reading. By assigning a channel for each element, the measured intensity depends on the amount of element present. 

It is usually advisable to carry out the ignition test first. This will provide useful information as to the general properties of the compound and, in particular, the residue may be employed for the detection of any inorganic elements which may be present. 

Emission spectroscopy is the analysis, usually for elemental composition, of the spectmm emitted by a sample at high temperature, or that has been excited by an electric spark or laser. The direct detection and spectroscopic analysis of ambient thermal emission, usually ia the iafrared or microwave regioas, without active excitatioa, is oftea termed radiometry. la emission methods the sigaal iateasity is directiy proportioaal to the amouat of analyte present. 

It is well known that arsenic is one of the most dangerous elements in terms of its potential impacts to both to human and ecosystem health. Therefore the problem of As detection at ppb level remains very important from the point of environmental hazard investigation. The goal of the present work is the developing of very simple and inexpensive assay for arsenite and arsenate determination in environmental samples using whole-cell bacterial biosensors. 

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