Spurious adsorption

Almost always, compounds of high polarity and low volatility tend to undergo adsorption on the chromatographic support or decomposition on contact with it. These phenomena usually result in peak tailing and the quantitative evaluation of the chromatograms is difficult or even impossible. A wellknown example is the GC analysis of cholesterol, which can be analysed as such or as the TMS derivative (Fig. 1.1). If the support is not modified, free cholesterol provides a wide, tailing peak which can be evaluated quantitatively only with difficulty, whereas the TMS ether provides a sharp, symmetric peak the retention time of which is, however, substantially shorter [1]. 

Tailing peaks may also originate if the amount of the solute in the chromatographic system is too great. If the linear range of the adsorption isotherm of the solute is exceeded, the chromatographic sorbent is overloaded, which results in asymmetry of the elution peak. By conversion into a derivative with other sorption properties, conditions may be attained that are suitable for operation in the linear range. 

Similarly, the calibration graph may be non-linear, particularly if peak heights are used as a quantitative parameter, and during manipulations with low concentrations of the solute when its adsorption on the surface of the support, column walls, etc., occurs to a significant extent. Fig. 1.2 illustrates the improvement that was obtained by the conversion of the sample compound. In the direct determination of morphine by GC, the dependence of the ratio of the peak height of morphine to that of squalene on the amount of compound injected is non-linear and therefore quantitative evaluation is difficult. An analogous calibration graph for the TMS derivative, in contrast, is linear. Hence, if a suitable derivative is used a drawback that could interfere with the GC analysis itself can be overcome [2]. 

In conclusion, it is necessary to add that a number of the problems mentioned, caused by adsorption in the chromatographic system, may be solved by new technological procedures for the preparation of columns and whole systems. Modern sorbents and procedures for deactivation have partly solved the initial difficulties in some instances, but the field is still wide enough for useful applications of derivatives. 

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According to general experience the Influence of sol concentration Is absent when the volume fraction remains of the order of a few percent. This may be concluded from older experiments and from recent electroacoustic studies, discussed in sec. 4.5d. Experiments involving mlcro-electrophoresls are not suited to stud3dng the volume fraction effect because the required extreme dilution may readily lead to spurious adsorption on the particles. Reed and Morrison studied theoretically the hydrodynamic interactions between pairs of different particles in electrophoresis they corrected the Helmholtz-Smoluchowski equation for various distances between the (spherical) particles and values of the electroklnetlc potentials of the particles, and... 

As Figure 21 shows, even closer are the two oxidations in triferrocenylphosphate, [O = P 0(C5H4)Fe(C5H5) 3]. The small spurious peak present at about + 0.2 V in the reverse scan of the cyclic voltammogram is likely due to electrode adsorption phenomena, given the ascertained stability of the corresponding trication.34... 

A fundamental improvement in the facilities for studying electrode processes of reactive intermediates was the purification technique of Parker and Hammerich [8, 9]. They used neutral, highly activated alumina suspended in the solvent-electrolyte system as a scavenger of spurious impurities. Thus, it was possible to generate a large number of dianions of aromatic hydrocarbons in common electrolytic solvents containing tetraalkylammonium ions. It was the first time that such dianions were stable in the timescale of slow-sweep voltammetry. As the presence of alumina in the solvent-electrolyte systems may produce adsorption effects at the electrode, or in some cases chemisorption and decomposition of the electroactive species, Kiesele constructed a new electrochemical cell with an integrated alumina column [29]. 

The answer is From harmful substances, i.e. elements or compounds which can interfere with the processes to be studied. One may encounter compounds which neutralise a catalyst or which generate spurious catalysts, compounds whose intense fluorescence in minute concentrations may frustrate a photochemical experiment, or others whose adsorption as a monolayer may alter the behaviour of an electrode or some other surface. 

Xe data would be even more useful in this sense, if there were any. The best data are probably those of Mazor et al. (1964) (Table 4.6). Normalized as for Kr previously, their data yield AXe = +7% in surface water and AXe = -21% in deep water. The surface water result is similar to more recent AKr (Table 4.7). Although the Mazor et al. data also include approximately 20% saturation anomalies in Ne and Ar that would be considered spurious today, it is tempting to speculate that their deep water Xe undersaturation is real, possibly also the scattered but very low values of Bieri, Koide, and Goldberg (1964), since of all the noble gases Xe is the best candidate for an adsorptive sink in ocean water (cf. Chapter 2). 

Adsorption Isotherms with maxima are indicative of complications, such as formation of new phases, scavenging by micelles, competition of spurious contaminants (Including homologues of the adsorptive) or simply analytical artefacts. Such maxima should never be interpreted as a real decrease of F with increasing c because this would mean that a chemical potential would decrease with increasing amount of substance, which is thermodynamically impossible. For this reason, such isotherm types are not included in our scheme. 

Common problems with GC analysis include septum leaks and adsorption of components from the sample onto the septum durmg injection. In addition, because the septum is heated, decomposition products often form and bleed into the column. This results in spurious peaks, termed ghost peaks, appearing in the chi omatogram. Septum bleed is greater at higher injection-port temperatures. To minimize this problem, a Teflon-coated, low-bleed septum is used. The inner surface of the septum is purged continuously with the carrier gas that is vented before... 

The rates of hydrocarbon adsorption on all oxidation catalysts (VA, Cu20, etc.) are so high as to make kinetic studies impossible. Characteristic equilibrium isotherms for ethylene sorption on magnesium chromite are shown in Fig. 1. This is a spurious equilibrium,... 

First, the solvents should be relatively free of trace impurities while dissolving the compounds of interest effectively. The solvent trace impurities, while not detectable in the bulk, can produce spurious peaks after the concentration of the volume of extracted sample. Whenever an extensive solvent clean-up is either impossible or impractical, appropriate sample blanks should frequently be run. With today s capabilities of high-resolution GC and spectral identification, occasional solvent impurities are tolerated as convenient markers . A good sample solubility is required to minimize possible losses due to sample adsorption on the glassware or, simply, its unnoticed precipitation. 

Spurious frequency variations which may arise because of surface roughness, and complications because of contributions from adsorption pseudocapacitance make the interpretation of capacity data on solid electrodes even more qualitative. 

Note that the (mean) velocity Wf nt of the adsorption front does not depend on the length and shape of the MTZ, and corresponds to the velocity of the movement of the centroid of the adsorption front through the bed, which is constant for a constant pattern behavior. In addition, note that Ufront does not necessarily increase with increasing partial pressure of the adsorbent Pa, as we spuriously might expect on the basis of Eq. (3.3.82). We have to consider that the equilibrium loading Xads,A,sat probably also depends on Pa, for example, for a Langmuir-type adsorption we have ... 

Kaneko et al. (1992a) make the case that these considerations can be used to explain the high surface areas of activated carbon. A three-layered structure of molecule type B can possess a surface area >3000 m g assuming that adsorption occurs at the edges as well as on the surface and that the theoretical limitation of 2630 m g for a single layer of graphite need not be restrictive. (This argument is somewhat spurious.)... 

In this work we show that the pore size distributions of porous carbons, obtained from lignin pyrolysis and KOH activation, determined from molecular simulation and DFT are equivcdent. We suggest that the spurious minimum observed in the PSD of carbons obtained form inversion of the integral adsorption isotherms is due to artifacts of the model, such as the fact that, uniform size, infinite non-connected pores are considered. In such cases, for the pore width where transition Ifom two to three layers occurs, the difference in shape of the calculated adsorption isotherms results in an exclusion of some pore widths at the time of the inversion of the integral. Taking into account smooth changes between pore sizes, or pore connectivity may solve this problem. 

When solvent sublation is used to isolate cationics, an anionic surfactant is sometimes added in excess to minimize adsorption of the cationic. This requires that the ion pair be separated later in the analysis. One method to minimize loss of cationics by adsorption to the apparatus is to pretreat the apparatus with a solution of the cationic, then rinse with water and solvent. Obviously, blank determinations must be run to confirm that this treatment is not a source of spurious high results (151). Dioctyldimethylammonium bromide has been used as an internal standard to correct for incomplete recovery of cationics (82). 

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