Analyte surface-active

API-electrospray ionization involves three stages. First, there is the formation of charged droplets. Once the droplets are formed, solvent evaporation and droplet fission occur. Droplet fission is due to an increase in charge repulsion at the surface of the droplet as the solvent evaporates. Once the droplets become small enough (<10 nm), it is believed that charge repulsion produces ion evaporation from the surface of the droplet. Thus, ions are transferred from the solution to the gas phase. Factors affecting the production of the desired ions include analyte concentration, flow rate, matrix content, and analyte surface activity. In... 

Electrospray ionization mass spectrometry is typically performed in polar solvents such as water, acetonitrile, methanol, or a combination of these. Thus, analytes with significant nonpolar regions should favor the air-solvent interface at the surface of electrospray droplets where these nonpolar regions can be desolvated. Such analytes are termed smface-active. A relationship between response in atmospheric pressure ionization mass spectrometry and analyte surface activity was postulated as early as 1983 by Mbame et al., the original authors of the ion evaporation theory. Such a relationship was also observed by Kebarle and co-workers," " who included a factor related to analyte surface activity in their models. 

Adsorptive stripping analysis involves pre-concentration of the analyte, or a derivative of it, by adsorption onto the working electrode, followed by voltanmietric iiieasurement of the surface species. Many species with surface-active properties are measurable at Hg electrodes down to nanoniolar levels and below, with detection limits comparable to those for trace metal detemiination with ASV. 

Selectivity of Membranes Membrane potentials result from a chemical interaction between the analyte and active sites on the membrane s surface. Because the signal depends on a chemical process, most membranes are not selective toward... 

The possible mechanism of ionization, fragmentation of studied compound as well as their desoi ption by laser radiation is discussed. It is shown that the formation of analyte ions is a result of a multi stage complex process included surface activation by laser irradiation, the adsoi ption of neutral analyte and proton donor molecules, the chemical reaction on the surface with proton or electron transfer, production of charged complexes bonded with the surface and finally laser desoi ption of such preformed molecules. 

Surface-active substances (SAS) are the most widespread contaminants of sewage and natural waters. They translate in small dispertion condition liquid and firm polluting substances - chlororganic, mineral oils, pesticides. Therefore, the SAS contents determination in water solutions is now one of actual tasks of analytical chemistry. 

The preparation and analytical results of semipolar organoboron compounds containing Cl or P were reported and the surface activity and fireproofing mechanism of these substances were described [283]. 

Vol. 9 Analytical Chemistry of Titanium Metals and Compounds. By Maurice Codell Vol. 10 The Chemical Analysis of Air Pollutants. By the late Morris B. Jacobs Vol. 11 X-Ray Spectrochemical Analysis. Second Edition. By L. S. Birks Vol. 12 Systematic Analysis of Surface-Active Agents. Second Edition. By Milton J. Rosen and Henry A. Goldsmith... 

Changes in the distribution of organic compounds in a seawater sample can be due to physical, chemical, or biological factors. As a physical factor, we might consider the absorption of surface-active materials on the walls of the sample container. While this effect cannot be eliminated it can be minimised by the use of the largest convenient sample bottle, and the avoidance of plastic (especially Teflon) containers. Another possible method of eliminating this source of error would be to draw the sample directly into the container in which the analytical reaction is to be run. 

There are two different classes of surface-active materials in seawater, those that are naturally present and those that have been added to the oceans by man s activities. Most of the analytical methods proposed for use in seawater actually measure the anthropogenic input, and attempt as much as possible to eliminate interferences from naturally occurring compounds. Yet sea foam was known to exist long before detergents. It is to be expected that both kinds of surfactants would be concentrated at the air-sea interface. 

The nature of the adsorbed species can be inferred from the usual chemical parameters, i.e. chemical shifts, linewidths and relaxation times. These latter allow the study of the mobility on the surfaces. As an analytical tool, C-NMR spectroscopy can also be used to determine the concentration of reactants or products as a function of time and hence kinetic constants can easily be determined. As a conclusion, a rather complete kinetic study can be carried out involving the nature of interaction between the admolecule and the surface and eventually the nature of the surface active centers. One can finally arrive at the proposition of a reaction mechanism. 

The surface-active properties of surfactants could give rise to sorption of analytes to glassware and equipment. However, the literature shows that when a comparison is made between the use of silanised glassware and careful rinsing of normal glassware with methanol during sample treatment, the latter (cheaper) solution is sufficient [5]. 

It is important to note that the ion series observed by the API-MS method may not be representative of all the products present, not the quantities thereof. Cleavage of the ethoxylate chain removes the capacity of silicone surfactants to be ionised and therefore detected by these methods. As such, for example, the cleaved silicone head group [M(D/CH2CH2CH2OH)M, 3] was never observed by API-MS. The nature of the API-MS process is such that competition between analytes for ionisation occurs, and as such compounds with higher surface activity and EO content can be expected to dominate in the resulting spectra. Suppression effects may thus preclude observation rather than confirm absence. As a consequence, the use of additional techniques such as FTICR-MS, GC-MS, HPLC and NMR to provide complementary data was also necessary. Furthermore, the high number of possible structures for each ion series observed, rendered it difficult to assign structures with confidence. Consequently simplified M2D-C3-0-(E0)n-R... 

Figure 1.35 On-demand protection of an electrochemical sensor. Trace analysis of metal (M) analyte in the presence of surface active compounds (S) using the active and passive states. (Reprinted with permission from Ref. [172]. 2006 American Chemical Society.)...

For the analysis of surface-active, electroactive organic compounds, the adsorptive stripping SWV was used. The method was applied to numerous analytes. Several of them are listed in Table 3.2. Some examples of metal complexes which were used for the quantitative analysis of metal ions by adsorptive... 

For systems, which cannot be marked easily, the displacement method is an alternative, in particular for low concentration ranges (36). This is a modification of isotachophoresis, because current flow is not constant with time and field strength is a function of the position along the capillary. Instead of analyte peaks, profiles are obtained (Fig. 6a) as injection and separation are carried out in one step. Although this method is not suitable for all micellar systems, one outstanding advantage is the higher UV sensitivity (which is important for most surface-active compounds). Because of the reliance of the method on displacement, the micellar phase is not diluted. 

One of the most important analytic solutions in the study of bubbles, drops, and particles was derived independently by Hadamard (HI) and Rybczynski (R5). A fluid sphere is considered, with its interface assumed to be completely free from surface-active contaminants, so that the interfacial tension is constant. It is assumed that both Re and Rep are small so that Eq. (1-36) can be applied to both fluids, i.e.,... 

Analytical Applications In addition to the above-mentioned analytical aspects of the processes at Hg electrodes, in this section, we briefly review the papers focused on the subject of the affinity of various compounds to the mercury electrode surface, which allowed one to elaborate stripping techniques for the analysis of inorganic ions. Complexes of some metal ions with surface-active ligands were adsorptively accumulated at the mercury surface. After accumulation, the ions were determined, usually applying cathodic stripping voltammetry (CSV). Representative examples of such an analytical approach are summarized as follows. 

If a dilute solution of a surface-active substance is brought in contact with a large adsorbing surface, then extensive adsorption will occur with an attendant reduction in the concentration of the solution. To meet the requirement of a large surface available for adsorption, the solid —which is called the adsorbent — must be finely subdivided. From the analytical data... 

However, tandem mass spectrometry, as a separation technique, does have limitations. It cannot easily differentiate between isomeric and isobaric species, and, in complex matrices, the presence of components with a high surface activity can suppress the ionization of components with a lower surface activity, leading to the nondetection of analytes (66). Therefore, the combination of MS-MS with a readily available chromatographic separation method such as TLC affords analysts real benefits. 

For aliphatic compounds with longer alkyl chains, such as surfactants, the NMR detector can contribute little to an increased selectivity of the LC-NMR coupling since, in the range of aliphatic protons, the spectra are often complex. Moreover, analyte signals around 2 ppm can be suppressed or influenced by the solvent suppression when acetonitrile is used as the organic component of the eluent. Since surfactants are present in many environmental samples, they pose problems for non-target analysis, not only because of their complex spectra but also because they can influence the separating properties of the analytical column by their surface activity [2]. 

Thermal desorption, on the other hand, makes use of the fact that the ability of a sorbent to retain compounds dramatically decreases at elevated temperatures. Therefore, heating of the sorbent under a continuous stream of an inert carrier gas can be used to transfer the adsorbed compounds into the GC system. In general the desorption temperature should be at least 20 °C above the boiling point of the adsorbed compounds, so the range of compounds which can be analyzed with thermal desorption is limited by the thermal stability of the sorbent. In order to avoid a loss of analyte due to decomposition on the sorbent s surface at elevated temperatures the surface activity needs to be carefully chosen. 

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