Ex-situ Method

1 Ex situ Methods These methods allow for the translation of the sputter time scale (that recorded during a SIMS measurement) into a depth scale using prior or retrospective methods. Ex-situ methods most conunonly applied in SIMS include 

Sputter rate or sputter yield equations/simulations 

Stylus profilometry represents by far the most heavily used of all the methods used to derive sputter rates, irrespective of the application field in SIMS. This is primarily due to its simplicity, cost effectiveness, and the speed in which accurate and precise data can be attained. 

The sputter rate (in units of A/nA.s or nm/nA.s) is then defined by dividing the crater depth (in units A or nanometer), by the sum of the sputter time (in units of seconds) and the primary ion current used in forming the respective crater (in units of nanoampere). This definition assumes a uniform sputter rate over the region (depth) sputtered, hence is only applicable to a specific matrix type. Such measurements should be carried out for each crater to ensure utmost in precision (averaging procedures are often employed to reduce statistical scatter). In the case of multilayered structures, surface profilometry measurements should be carried out once each subsequent layer is sputtered, such that sputter rates pertaining to each of the different layers can be de-convoluted (recall from Section 3.2.2 that sputter rates are dependent on many parameters including those defining the matrix). 

Interferometric methods operate by splitting a monochromatic coherent beam (all waves in phase) of hght into two parts. One part is directed at the surface from which it then reflects and the other part directed at some reference (optically smooth mirror) from which it reflects. When the two parts are recombined, their waves will either interfere in a constractive manner (waves in phase) or destructive manner (waves out of phase). As constructive interference enhances the amplitude of the combined beam, whereas destructive interference suppresses the amplitude, an interference pattern will be prodnced that is dependent on the crater depth. This interference pattern can also be nsed to derive the surface topography of the sputtered crater in both of the spatial dimensions, with the result being an AFM-like three-dimensional topographic image. 

The problem associated with the ex-situ technique is the fact that a finite time exists between radical generation at the electrode and its detection within the ESR cavity. The time for transfer of solution restricts the technique to long-lived radical species. This problem was alleviated, to some extent, without resorting to in-situ techniques by Albery et al. [29-33] who made use of the tubular electrode shown in Fig. 12. The electrode consisted of an annular ring which formed part of the wall of a tube through which solution flowed. The tube was situated centrally within the ESR cavity with the electrode immediately upstream outside the cavity. 

The flow of solution over the electrode was constrained to be laminar, within the range of flow rates used, enabling the distribution of radicals as a function of flow rate, electrode current, and cell geometry to be calculated. Theory was produced for stable radicals [30] and for radicals decaying by both first-order [31] and second-order kinetics [32], from which information of radical decay mechanisms was obtained by studying the ESR signal-current behaviour at constant flow rate. 

The laminar flow tube of Albery perhaps represents the best possible ex-situ generation technique, but nevertheless a finite transit time still exists between the electrode and the sensitive part of the ESR cavity. This limits the lifetimes of radicals that may be studied the upper limits for second- and first-order reactions have been estimated as 104dm3mol 1 s 1 and 102s 1, respectively, [34] but these are certainly very optimistic estimates. The range of lifetimes may be extended if the electrode is placed within the ESR cavity, that is by adopting in-situ techniques, as will be described next. 

In principle, any desired ex situ combination of VIM with other analytical methods can be applied, keeping in mind the necessary stability over a rather long time scale and the absolute amount one needs in order to gain significant signal-to-noise (S/N) ratios with such methods. Otherwise, in situ detection of products on a shorter time scale may be facilitated by suitable combination as exemplified further below. Of course, significant stability of the product of the VIM experiment is presumed in ex situ combinations. For example, one can conveniently probe the presence or absence of counterions after a VIM experiment by EDX [4, 22, 32-36] or the crystallographic properties by X-ray spectroscopy [6]. 

Ex situ specular reflectance IR spectroscopy was successfully applied to identify cis-[Cr(CO)2(dpe)2] which is stable on a long time scale only in the solid-state phase, but very rapidly isomerizes to the corresponding trans-isomer in solution [37]. 

Electrochemical analysis of dissolved solid products generated by VIM [34] was elegantly applied by complete electrolysis of the solid and, after having removed and dried the electrode, subsequent dissolution of the solid product in a drop of an organic solvent, and final detection of the now dissolved product with a microelectrode dipped into the drop, using the VIM electrode as counter electrode. 

Determination of metals in minerals or oxides in general is another example of VIM followed by an electrochemical measurement. The mineral is first reduced to generate metallic phases on the electrode. Subsequently, the metallic 

The instruments of surface analysis have become extremely important for electrochemistry. Investigations have been performed to study the double layer and corroborate results of electrochemical methods. 

One type of method is an energy analysis of electrons emitted from the surface with Auger-electron-spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS). A second way is to sputter the surface and analyze the emitted particles by mass spectroscopy. This method is called secondary ion mass spectroscopy (SIMS). A third method is based on the scattering of He ions from the surface and the analysis of the scattering parameters (Rutherford backscattering). 

Diffraction of electrons, either of high energy (RHEED) or of low energy (LEED), has been used for studying the structure of surfaces. 

FIGURE 27.39 Schematic diagram of a surface electrochemistry apparatus, showing UHV system, transfer manipulators, and interlocks. 

The surface sensitivity of most electron probe techniques is due to the fact that the penetration depth of electrons into metals falls to a minimum of 4 to 20 A when their kinetic energy is between 10 and 500 eV. It is also convenient that electrons at these energies have de Broglie wavelengths on the order of angstroms. With a monochromatic beam, it is possible to do LEED. 

LEED is a very specific tool for examination of the geometric pattern of atoms on a surface. In electrochemistry, it has been used to study surface reconstruction. 

LEED has also been used to study the adsorption of halide ions, cyanide and thiocyanate ions, and organic molecules on single-crystal metal surfaces. 

FIGURE 27.41 Electronic diagram of the Auger emission process. 

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Various in situ and ex situ methods have been used to determine the real surface area of solid electrodes. Each method10,15 32 67,73 74 218 is applicable to a limited number of electrochemical systems so that a universal method of surface area measurement is not available at present. On the other hand, a number of methods used in electrochemistry are not well founded from a physical point of view, and some of them are definitely questionable. In situ and ex situ methods used in electrochemistry have been recently reviewed by Trasatti and Petrii.73 A number of methods are listed in Table 3. 

Valette-Hamelin approach,67 and other similar methods 24,63,74,218,225 (2) mass transfer under diffusion control with an assumption of homogeneous current distribution73 226 (3) adsorption of radioactive organic compounds or of H, O, or metal monolayers73,142,227 231 (4) voltammetry232,233 and (5) microscopy [optical, electron, scanning tunneling microscopy (STM), and atomic force microscopy (AFM)]234"236 as well as a number of ex situ methods.237 246... 

Ex Situ Methods XPS and HREELS will continne to be very usefnl ex sitn methods for the stndy of electrode surfaces. Soft X-ray XAS in both the EY and FY modes shonld find wider application. 

Another classification of remediation technologies describes where the action is taking place. Ex situ methods are those applied to excavated soil and in situ processes are those applied to the soil in its original location. On-site techniques are those that take place on the contaminated site they can be either ex situ or in situ. Off-site processes treat the excavated soil in fixed industrial facilities, away from the contaminated site. 

The big difference in application from the in situ flushing method is that this ex situ method can apply to soils with lower permeability, because soil is excavated and can be sufficiently washed. The following presents two ex situ soil washing processes for organic contaminants the BioGenesis soil cleaning process and the BioTrol soil washing system. 

In principle, the analysis of molecules, ions and adsorbed intermediates is possible if they survive the emersion (no potential control) and UH V conditions (elimination of most of the solvent). The use of ex situ methods for the analysis of sub-monolayer quantities of oxygen-sensitive substances requires an extremely inert atmosphere when the electrode is emersed. In order to check whether a given adsorbate survives the experimental conditions, a control experiment must be carried out, as we describe here for adsorbed CO on Pt. 

In situ generation. In this technique, well-defined electrochemistry is sacrificed in favour of generating the radicals in situ within the cavity. The advantage is that more shorter-lived radicals can be observed than is possible with the ex situ methods. 

Methods that investigate the interface as such are called in situ methods. In ex situ methods the electrode is pulled out of the solution, transferred to a vacuum chamber, and studied with surface science techniques, in the hope that the structure under investigation, such as an adsorbate layer, has remained intact. Ex situ methods should only be trusted if there is independent evidence that the transfer into the vacuum has not changed the electrode surface. They belong to the realm of surface science, and will not be considered here. 

Definitions of in situ and ex situ preparation methods are frequently found in specialized articles, reviews or textbooks. In situ methods refer to the possibility to assemble the inorganic compounds directly on the pristine (or modified) CNTs and ex situ methods to binding such materials in a post-assembling step via some linking agent [96],... 

To understand the difference between in situ and ex situ methods. 

As mentioned by Mathias et al. [9], reliable methods to measure the thermal conductivity of diffusion layers as a function of compression pressures are very scarce in the open literature. Khandelwal and Mench [112] designed an ex situ method to measure accurately the thermal conductivities of different components used in a fuel cell. In their apparatus, the sample materials were placed between two cylindrical rods made out of aluminum bronze (see Figure 4.28). Three thermocouples were located equidistantly in each of the upper and lower cylinders to monitor the temperatures along these components. Two plates located at each end compressed both cylinders together. The temperatures of each plate were maintained by flowing coolant fluids at a high flow rate through channels located inside each of the plates. A load cell was located between two plates at one end so that the compression pressure could be measured. 

Some caution may be needed when evaluating in situ technologies for the remediation of sediments. In situ treatment of sediments may be less cost-effective than ex situ methods because the treatment level for in situ methods is not uniform and in some cases project goals cannot be met throughout the site (D20043R, p. 6). 

The ChemChar process is a patented, ex situ method for the treatment of hazardous and mixed wastes using reverse-burn gasification. Organic components of the treated waste are converted to a combustible gas and a dry, inert solid. The solid can be mixed with cement to prevent leaching of radioactive or heavy-metal constituents retained in the char residue after gasification, or the solid can be further reduced by forward-bum gasification. 

The RIMS library/database contains many technologies that can be operated in a pyrolysis mode. They include both in situ and ex situ methods, vitrification techniques, as well as technologies used for gasification. A list of these technologies can be found by searching under the technology category pyrolysis. ... 

Sevenson Environmental Services, Inc. (Sevenson), is the owner of the MAECTITE chemical treatment process for the precipitation and stabilization of toxic heavy metals. Chemical treatment by the MAECTITE process converts teachable lead, hexavalent chromium, or other heavy metals into insoluble minerals and mixed mineral forms within the material or waste matrix. The technology can be used as an in situ or an ex situ method and does not use pozzolanic or siliceous binders to stabilize the treated material. 

Exhibit high removal efficiency and are cost effective compared to traditional ex situ methods. 

The techniques used in studying interfaces can be classified in two categories in situ techniques and ex situ techniques. In situ methods are those where a surface is probed by one or several techniques while immersed in solution and under potential control. In contrast, in ex situ methods, an electrochemical experiment is first carried out. Then the electrode is removed from solution and examined by one or several spectroscopic techniques, which generally require ultrahigh vacuum (UHV) conditions. Figures 6.10 and 6.11 show some of the most common ex situ and in situ techniques applicable to the study of the metal/solution interface. 

Fig. 6.10. Some ex situ methods applied in the analysis of electrodes.

In situ measurements (i.e., those done on an electrode while it is in contact with the solution under a controlled potential) are described below (see also Section 6.2.4). However, there are plenty of reports in the electrochemical literature of the use of ex situ methods for looking at electrochemical situations. In these, the electrochemical reactions are duly carried out, sometimes using a thin-layer cell, and then the solution is rapidly removed from the thin-layer cell, e.g., by applying a vacuum. The electrode (one of the plates in the thin-layer cell) and whatever remains on it as a result of electrochemical activity while it was in contact with the solution, can then be examined at leisure, using a number of spectroscopic methods, including those that only function in vacuo. 

The big advantage of making ex situ measurements is that they allow the application of methods used in surface chemistry when no solution is present. Some of these ex situ methods (LEED or XPS) are described in Chapter 6. In electrochemical situations in which the critical questions concern, for example, passivation of metals involving oxides or sulfide films, there is no accompanying disadvantage in the use of these well-developed and accurate methods. 

The practice of physical electrochemistiy at the turn of the twenty-first century is to use both in situ and ex situ methods. In this section our description will be restricted... 

We begin with the most routine characterization methods—electrochemical methods. We then discuss various instrumental methods of analysis. Such instrumental methods can be divided into two groups ex situ methods and in situ methods. In situ means that the film on the electrode surface can be analyzed while the film is emersed in an electrolyte solution and while electrochemical reactions are occurring on/in the film. Ex situ means that the film-coated electrode must be removed from the electrolyte solution before the analysis. This is because most ex situ methods are ultra-high-vacuum techniques. Examples include x-ray photoelectron spectroscopy [37], secondary-ion mass spectrometry [38,39], and scanning or transmission electron microscopies [40]. Because ex situ methods are now part of the classical electrochemical literature, we review only in situ methods here. 

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