Instrumentation for AES

The insnumentation used in AES is very similar to that used in XPS. The major difference is that the source used is a focused beam of electrons from an electron gun or a field emission source, not X-ray photons. A schematic diagram of an AES instrument is shown in Eigure 14.26. Many instrument manufacturers provide instfuments that permit both XPS and Auger spectra to be collected on one instrument. 

AES is primarily a surface elemental analysis technique. It is used to identify the elemental composition of solid surfaces and can be used to quantify surface components, although quantitative analysis is not straightforward. AES is a true surface analysis technique, because the low-energy Auger electrons can only escape from the first few (three to five) atomic layers or from depths of 0.2 to 2.0 nm. 

As can be done with XPS, another valuable application of AES has been developed by using an ion beam to progressively strip off the surface of a sample under controlled conditions from a sample. Spectra can be collected and the distribution of elements recorded as surface material is removed. The results show changes in distribution of different elonents with depth, called a depth profile. 

The Cl-containing region is only about 100 A wide. This extremely fine lateral resolution is one of the strengths of AES over other analytical techniques. 

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Atomic fluorescence spectrometry (AES) is an analytical method used to determine the concentration of elements in samples. The sample is converted to gaseous atoms, and the element of interest is excited to a higher electronic energy level by a light source. Following excitation, the atoms are deactivated by the emission of a photon. The measured fluorescence is this emission process. Instrumentation for AES... 

Principles and Characteristics Flame emission instruments are similar to flame absorption instruments, except that the flame is the excitation source. Many modem instruments are adaptable for either emission or absorption measurements. Graphite furnaces are in use as excitation sources for AES, giving rise to a technique called electrothermal atomisation atomic emission spectrometry (ETA AES) or graphite furnace atomic emission spectrometry (GFAES). In flame emission spectrometry, the same kind of interferences are encountered as in atomic absorption methods. As flame emission spectra are simple, interferences between overlapping lines occur only occasionally. 

Another procedure consists in finding the point on the curve where the first derivative, AE/Aml, or ApH/Aml, is maximal and its second derative, A2E/Aml2 or A2pH/Aml2, equals zero, which can be verified graphically or instrumentally for volume increments (ml) around the end-point sought. 

Quinby-Hunt MS, McLaughlin RD, QuintanihaA. 1986. Radiation monitoring In Greenberg AE, Morton GA, eds. Instrumentation for environmental monitoring. Vol. 2 Water. 2nd ed. New York, NY John Wiley and Sons, 696-742. 

The typical routine determination of a number of elements in a set of similar sample solutions will therefore no longer be the determination of element A in all the samples, followed by a change of radiation source, wavelength, flame conditions, burner height and so on, and determination of element B in the same samples, and so on, as it is common practice in LS FAAS. In HR-CS FAAS it will rather consist of a calibration of the instrument for all the elements of interest, followed by a determination of all elements in sample 1, all elements in sample 2, all elements in sample 3, and so on. It might be worth mentioning that, at least for a limited number of elements, the total analysis time required for HR-CS FAAS will be even shorter than that for a simultaneous ICP AES measurement, because of the much shorter equilibration time required for a typical AAS burner after changing the sample solution, compared to the spray chambers used in ICP AES. 

Due to its versatility, robustness, and relativel low cost, ICP-AES has started to be considered as routine instrumentation for elemental composition measurements in many laboratories involved in food analysis. Unquestionably, the acceptance of the technique has been further advanced by the commercial availability of rapid, simultaneous, and flexible, sequential instruments based on echelle grating crossed dispersion and solid state detectors. It is expected that novel nebulizer devices for liquid sample introduction will find increased applications in beverage analysis. 

For the in-situ FTIR analyses, mixed solutions were spread by the same hion onto a house-made minitrough that fitted into the sample chamber of the FTIR instrument. For every in-situ experiment, the close-packed monolayer was initially formed on DI water that was then replaced with ImM CdCl2 solution by careful circulation of the subphase using a peristaltic pump. The background spectrum was taken for a bare water surface when the sample was Ae monolayer on pure water, and was taken for a close-packed monolayer on pure water when the sample was the one on the cadmium-containing subphase. The latter technique enhanced the carboxylate peak relating to ion adsorption. All the experiments were done at room temperature. 

The published sensitivity factors are obtained with specific instruments and standard samples of pure elements. Sensitivity factors are subject to change with different samples and developments in instruments. For example, chemical contamination on a sample surface affects the accuracy of using Equation 7.3. For an AES spectrum, the matrix composition affects the efficiency of Auger emissions because the backscattered electrons in the matrix... 

Standards define a calibration curve that is used to convert the element intensity values into parts per million (ppm) over a specific measurement range. AE may utilize multiple sets of calibration curves for different analysis applications. Simultaneous instruments measure all elements in a single cycle and can provide data on 20-60 elements in less than a minute. The two most common excitation sources for AE oil analysers are the rotary disk electrode type, commonly referred to as RDE, Rotrode or Arc/Spark and the inductively coupled plasma (ICP) type. 

Until recently, analytical investigations of surfaces were handicapped by the lack of suitable methods and instrumentation capable of supplying reliable and relevant information. Electron diffraction is an excellent way to determine the geometric arrangement of the atoms on a surface, but it does not answer the question as to the chemical composition of the upper atomic layer. The use of the electron microprobe (EMP), a powerful instrument for chemical analyses, is unfortunately limited because of its extended information depth. The first real success in the analysis of a surface layer was achieved by Auger electron spectroscopy (AES) [16,17], followed a little later by other techniques such as electron spectroscopy for chemical analysis (ESCA) and secondary-ion mass spectrometry (SIMS), etc. [18-23]. All these techniques use some type of emission (photons, electrons, atoms, molecules, ions) caused by excitation of the surface state. Each of these techniques provides a substantial amount of information. To obtain the optimum Information it is, however, often beneficial to combine several techniques. 

Instruments for electron spectroscopy are offered by several instrument manufacturers. These products differ considerably in types of components, configurations, and costs.. Some are designed for a single type of application, such as XPS, and others can be adapted to AES and UPS by purchase of suitable accessories. All are expensive ( 300,(KK) to > 10 ),... 

A Fisher, SI Hill. Instrumentation for ICP-AES. In SI Hill, ed. Inductively Coupled Plasma Spectrometry and Its Applications. Sheffield Sheffield Academic Press, 1999, pp 71-97. 

Atomic emission spectroscopy (AES) and atomic absorption spectroscopy (AAS) are In a manner similar to our discussion of molecular spectroscopy, where we compared UV absorption with UV excitation and subsequent fluorescence, these two determinative approaches are the principal ways to identify and quantitate trace concentration levels of metal contamination in the environment. As the need developed to quantitate increasing numbers of chemical elements in the Periodic Table, so too came advances in instrumentation that enabled this to be achieved at lower and lower IDLs AES and AAS techniques are both complementary and competitive. Atomic fluorescence spectroscopy (AFS) is a third approach to trace metal analysis. However, instrumentation for this has not as yet become widespread in environmental testing labs and it is unlikely that one would see atomic or what has become useful x-ray atomic fluorescence spectroscopy. Outside of a brief mention of the configuration for AFS, we will not cover it here. 

Figure 4.68 Schematics of the three simplest instrument configurations for AE, AA, and AF.

The Chemistry and Life box in Section 6.7 described the techniques called NMR and MRI. (a) Instruments for obtaining MRI data are typically labeled with a frequency, such as 600 MHz. Why do you suppose this label is relevant to the experiment (b) What is the value of AE in Figure 6.27 that would correspond to the absorption of a photon of radiation... 

The elements are inevitably present in effluents produced by surface finishing industries. Determination of these elements is important to estimate the contamination level for monitoring purposes. We offer two types of instruments ICP-AES, and ICP-MS. The atomic absorption spectrometer requires a single lamp for each element, whereas in ICP-MS a multi-element determination is possible. The apparatus utilized for ICP-AES and ICP-MS, is to be cleaned with 5 % sodium hydroxide, 5 % nitric acid, and ultrapure water sequentially. Sampling of effluent requires the bottles to be cleaned three times. All reagents and water are purchased as ultrapure levels. Standard solutions are also purchased commercially as ICP-AES or ICP-MS solutions. Measurements by ICP-AES and ICP-MS are repeated at least three times, and a blank must be checked. A calibration line is calculated with three or four standard solutions and r should be ascertained almost 1. Concentration of the elements is calculated from the calibration line and errors are to be estimated. 

Bandpass filters have also been used extensively in AES instruments. For example, they are employed in the commercially available (TV- and HG-AFS instruments. For graphite furnace LEAFS, careful studies have been made that compare the use of monochromators and filters. Detection Hmits for lead and thallium were improved by two to three times by the use of very narrow (Inm) bandpass filters. 

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