Ion desorption effect

At low pressures the measuring range is limited by two effects by the X-ray effect and by the ion desorption effect. These effects results in toss of the strict proportionality between the pressure and the ion current and produce a tow pressure threshold that apparently cannot be crossed (see Fig. 3.14). 

The Bayard-Alpert system with modulator (see Fig. 3.16 d), introduced by Redhead, offers pressure measurement in which errors due to X-ray and ion desorption effects can be quantitatively taken into account. In this arrangement there is a second thin wire, the modulator, near the anode in addition to the ion collector inside the anode. If this modulator is set at the anode potential, it does not influence the measurement. If, on the other hand, the same potential is applied to the modulator as that on the ion collector, part of the ion current formed flows to the modulator and the current that flows to the ion collector becomes smaller. The indicated pressure p, of the ionization gauge with modulator set to the anode potential consists of the portion due to the gas pressure pg and that due to the X-ray effect pg ... 

Ion desorption effects arise as a result of electron impact on a gas-covered surface. In an ionisation gauge, if gas is adsorbed on the anode, this can be partly desorbed, as ions, by the impacting electrons. Such ions reach the collector and lead to a pressure indication that increases initially with electron current. 

Bombardment of a liquid surface by a beam of fast atoms (or fast ions) causes continuous desorption of ions that are characteristic of the liquid. Where the liquid is a solution of a sample substance dissolved in a solvent of low volatility (often referred to as a matrix), both positive and negative ions characteristic of the solvent and the sample itself leave the surface. The choice of whether to examine the positive or the negative ions is effected simply by the sign of an electrical potential applied to an extraction plate held above the surface being bombarded. Usually, few fragment ions are observed, and a sample of mass M in a solvent of mass S will give mostly [M + H] (or [M - H] ) and [S -I- H]+ (or [S - H] ) ions. Therefore, the technique is particularly good for measurement of relative molecular mass. 

The reasons are analyzed in detail in Chapter 5. The equation is valid as long as the effective double layer is present at the metal/gas interfaces of the working and reference electrodes. Deviations are basically observed when ion backspillover is not faster than ion desorption or reaction (see also section 11.3). 

This was shown e.g. by investigating adsorption isotherms of Na dodecylbenzene-4-sulfonate and Na 4-hexadecyloxytolyl-2-sulfonate on various mineral surfaces differing from each other by the kind of PDFs86 . The potential value in relation to the surfactant concentration reached its maximum in the region of micelle formation and confirmed thus the shape of the adsorption isotherm. The presence of adsorption maxima is explained by a decrease in surfactant adsorption resulting from a desorption effect of micelles on the adsorption film, and by setting a three-component equilibrium (adsorption film - micelle - monomer) at concentrations CMC. This happens because of different ratios of the counter ions to the surfactant ions at the micelle and on the adsorption film. 

Particularly at low pressures, errors may arise with hot cathode ionisation gauges because of two effects the X-ray effect and gas-ion desorption. 

Laser-induced desorption of CO and CO+ from Pt(l 11) is observed by Fukutani et al. [12]. Upon laser irradiation on the CO-saturated Pt(l 1 1) surface at X = 193 nm with a laser fluence of 4mJ/cm2 for 10, 20, and 30 min, the on-top CO decreases gradually in RAIRS intensity, as shown in Fig. 12b-d, respectively. The effects of laser irradiation are clearly demonstrated in the difference spectra shown in Fig. 12. On-top CO decreases, while bridge CO remains unchanged. Neutral CO desorbed from the surface is clearly observed by the REMPI method. In addition to neutral species, CO+ ion desorption is also observed. Figure 19 displays the evolution of the desorption yield as a function of accumulated incident photon numbers at X = 193 nm with a laser fluence of 5.6 mJ/cm2 after a CO exposure of 2 L. Decay rates of the desorption intensity represented by an exponential decay are identical for neutral and ion species within the experimental error and the cross section s is 3 x 10 19 cm2. This result suggests that neutral CO molecules and CO+ ions are desorbed from the same adsorption species. 

When adsorbed molecules are bombarded with electrons, local heating effects occur that lead to thermal desorption. In addition, there is a small but finite probability that electrons in the chemical bonds that hold the adsorbate to the surface will be excited into a repulsive state, leading to the desorption of that molecule either as a neutral species or as a molecular ion. Desorption of neutral species under electron-beam bombardment is frequently observed in studies of electron-surface interactions. A fraction of the adsorbed molecules will be ionized. These can be detected as positive ions, and the spatial distribution of this ion flux can be imaged on a fluorescent screen. Electron-stimulated desorption ion-angular distribution (ESDIAD) [56, 61, 64, 79-84] is the name of the technique that is used to learn about the site symmetry and orientation of adsorbed molecular species, since the molecular ions are usually emitted in the directions of their chemical bonds with the surface and with an unchanged orientation with respect to the orientation of the molecule when it was adsorbed on the surface. 

Libong, D. Bruneau, S.P.C. Rogalewicz, F. Ricordel, I. Bouchonnet, S.J. Adsorption-desorption effects in ion trap mass spectrometry using in situ ionization. Chromatog. 2003, JOJO (issue 1), 123-128. 

After application of the matrix, the TLC plate is loaded directly into the MALDI device. The most obvious and simple approach is to fix the TLC plate with conductive adhesive tape onto a standard MALDI target (although a dedicated TLC adapter is now also available from Bruker Daltonics). To avoid charging effects in the MALDI source by residual charged particles on the surface and to enable successful ion desorption, an electrically conductive surface is needed. Therefore, the use of TLC glass plates is absolutely discouraged, whereas alumina TLC plates are perfect for this application. Such plates are commercially available with different stationary phases. 

Hayes and Altenau [34] were the first to report the use of MS to directly characterise antioxidants and processing oil additives in synthetic rubbers. Since then, various MS techniques have been applied to the analysis of rubber and polymer additives either as extracts or on the sample surface by laser techniques as reviewed by Lattimer and Harris [35]. Lattimer reviewed the present situation regarding MS in polymer analysis [36]. Analysis of polymer extracts by MS has proved challenging. Electron impact mass spectra (EI-MS) are often difficult to interpret due to the high concentration of processing oils and the additives in the extract, and excessive fragmentation of the molecular ions. Desorption/ionisation techniques such as field desorption (ED) and fast atom bombardment (FAB) have been found to be the most effective means for analysing polymer and rubber extracts [37, 38]. 

Unlike porous amorphous carbons, the high ratio of the external surface area to the total surface area of CNTs provides fast adsorption/desorption of electrolyte ions associated with the process of the formation of the electric double layer due to no ion-sieving effect occurring (Arulepp et al. 2006). Sorption of ions onto external surface area of CNTs makes the double-layer capacitance of CNT-based actuators less dependent on the ionic hquid species (ion dimensions) than the capacitance of amorphous carbon-based actuators, where the ion transport into the pores depends on the pore size and the size of electrolyte ions. The frequency dependence of generated strain has been attributed to the elecfrochemical kinetics different deflection amplitudes are the result of different ionic conductivities of EL species (Imaizumi et al. 2012). 

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