Galvanic detection

LEI as an analytical technique thus requires a tunable laser, an atom reservoir, a galvanic detection system and read-out electronics including a boxcar integrator. It was first realized experimentally with a flame as the atom reservoir using pneumatic nebulization for sample uptake [670]. 

LEI has been applied successfully to the trace determination of T1 [674] for certification purposes, and for combinations with laser evaporation and all other atomization techniques represents a powerful approach to detection. Laser photoionization and galvanic detection have been applied to hollow cathode dark space diagnostics [675]. Photoionization is produced to measure the dark space widths of linear field distributions directly. A theoretical model has been developed and its predictions verified with experimental findings for a uranium hollow cathode discharge operated in neon or xenon. Variations in the ground-state densities of sputtered neutrals have also been measured. 

Apart from the galvanic detection of the ion currents, direct mass spectrometric detection of the ions can also be applied, as is the case with resonance ionization mass spectrometry (RIMS) [676]. In addition, ionization can be performed by multiphoton absorption, which requires very intense primary sources. 

Babin F. J. and Gagne J. M. (1992) Hollow cathode discharge (HCD) dark space diagnostics with laser photoionisation and galvanic detection,... 

As we have seen, collisions are important for the signal generation in LEI. In low-pressure experiments photoionization instead is the principal origin of the signal. The term Resonance Ionization Spectroscopy (RIS) is then frequently used. Several examples of opto-galvanic detection schemes for different atoms are shown in Fig.9.11. If multi-photon excitation of the atoms to be studied is used the technique is referred to as REMPI (REsonance Multi-Photon Ionization) spectroscopy. The selectivity of RIS and REMPI can be further enhanced by using a mass spectrometer to ana-... 

As we have seen, collisions are important for the signal generation in LEI. In low-pressure experiments photoionization instead is the principal origin of the signal. The term Resonance Ionization SpectToscojjy (RIS) is then frequently used. Several examples of opto-galvanic detection schemes... 

Ma.rine. In the presence of an electrolyte, eg, seawater, aluminum and steel form a galvanic cell and corrosion takes place at the interface. Because the aluminum superstmcture is bolted to the steel bulkhead in a lap joint, crevice corrosion is masked and may remain uimoticed until replacement is required. By using transition-joint strips cut from explosion-welded clads, the corrosion problem can be eliminated. Because the transition is metaHurgicaHy bonded, there is no crevice in which the electrolyte can act and galvanic action caimot take place. Steel corrosion is confined to external surfaces where it can be detected easily and corrected by simple wire bmshing and painting. 

Cell formation can easily be detected by measuring potential if coated surfaces with no pores have a more positive potential than uncoated material. Usually this is the case with coated steel in solutions containing oxygen. More negative potentials can only arise with galvanized steel surfaces. Figure 5-4 shows examples of measured cell currents [9,10,16]. 

If the instrument indicates current surge in an air-free system, it generally implies hydrogen sulfide contamination, but the galvanic probe is usually best suited to detect corrosion influenced by oxygen contamination. 

In the galvanic detector, the electrochemical detector consists of a noble metal like silver (Ag) or platinum (Pt), and a base metal such as lead (Pb) or tin (Sn), which acts as anode. The well-defined galvanic detector is immersed in the electrolyte solution. Various electrolyte solutions can be used, but commonly they may be a buffered lead acetate, sodium acetate and acetic acid mixture. The chemical reaction in the cathode with electrons generated in the anode may generate a measurable electrical voltage, which is a detectable signal for measurements of DO. The lead is the anode in the electrolyte solution, which is oxidised. Therefore the probe life is dependent on the surface area of the anode. The series of chemical reactions occurring in the cathode and anode is ... 

It was mentioned that nitrite does not seem to accumulate in soils under most conditions, and the same generalization seems to be true for natural waters. Dressel reported N02" levels of up to 1 ppm in rivers (although considerably higher levels were detected in effluents of an experimental sewage plant and of a galvanizing plant). Keeney, found that... 

Galvan A, Smith Y, Wichmann T. 2003. Continuous monitoring of intracerebral glutamate levels in awake monkeys using microdialysis and enzyme fluorometric detection. J Neurosci Methods 126(2) 175-185. 

The interfacial chemistry of corrosion-induced failure on galvanized steel has been investigated (2) adhesion of a polyurethane coating was not found to involve chemical transformations detectable by XPS, but exposure to Kesternich aging caused zinc diffusion into the coating. Similar results were obtained with an alkyd coating. Adhesion loss was proposed to be due to formation of a weak boundary layer of zinc soaps or water-soluble zinc corrosion products at the paint metal Interface. 

Conductivity detection is a universal detection mode in which the conductivity between two inert electrodes comprising the detector cell is measured. The different arrangements employed for the construction of these detectors include apparatus with a galvanic contact of the solution with the sensing electrodes (contact conductivity detection) [51] and detection systems without galvanic contact of the solution with the sensing electrodes (contactless conductivity detection) [1]. 

When rebreathing systems are used for the delivery of xenon, its concentration within the system needs to be closely monitored. Infrared gas analysers cannot detect xenon, since it is a single atom, and as it is chemically inert its physical properties must be utilised. Mass spectrometry is the most accurate method but it is expensive and it is impractical for clinical use. A calibrated katharometer combined with a galvanic oxygen sensor is a satisfactory alternative which provides a reasonably accurate measure ( 1%). 

Sensor Cell Operating Mode. The simplest method of sensor operation is as a galvanic cell, whereby the sensor acts as a fuel cell and generates a current proportional to the gas concentration to be detected (1 ). However, when detecting certain species in air, it is difficult to obtain a counter-reference electrode in an acid system that will maintain the sensing electrode at a predetermined potential of approximately 1.0 V, to minimize interference. Counter-reference electrodes such as Pt/air (Op) or noble metal/ noble metal oxide structures have rest potentials in the 1.0 to... 

This type of detection has achieved much development in the last few years due to its simplicity. A specific revision on conductimetric (and potentiometric) detection in conventional and microchip capillary electrophoresis can be found in Ref. [57]. It is considered a universal detection method, because the conductivity of the sample plug is compared with that of the solution and no electroactivity of the analytes is required. Two electrodes are either kept in galvanic contact with the electrolyte (contact conductivity) or are external and coupled capaci-tively to the electrolyte (contactless mode). An alternating current potential is applied across the electrodes and the current due to the conductivity of the bulk solution is measured. As the signal depends on the difference in conductivity between solution and analyte zones, the choice of the electrolyte is crucial. It is necessary that it presents different conductivity without affecting sensitivity. 

With the arrangement shown above, the reaction proceeds spontaneously, in which electrons move from left to right and X ions from right to left so that the electroneutrality is maintained. This type of reactions which take place in an electrochemical manner is called electrochemical reaction. A device like the one shown above, which permits a spontaneous electrochemical reaction to produce a detectable electric current, is termed a galvanic cell. As shown in the above figure, oxidation occurs in one half-cell and reduction occurs in the other half-cell. The electrode at which oxidation occurs is referred to as the anode, while the electrode at which reduction occurs is termed cathode. 

Opto-galvanic spectroscopy detects the absorption spectra of atoms (6) and some molecules (7) in a flame by measuring current changes induced by optical irradiation at a wavelength corresponding to an electronic transition. Two steps are involved ... 

Uhlig was thermodynamically oriented and for him corrosion occurred because of the difference in the Galvanic potentials of the components making up a corroding surface. He believed that corrosion originated from the presence of inclusions, detected or not, in the corroding material. 

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