Suppressing effect of weak acids

Table 1. Comparison of the suppressing effect of weak acids for the decomposition of wet aluminium powder and wet compositions which contain aluminium(100 grams of sajnples in a warm state)...

Table 14. Comparison of the suppressing effect of weak acids for...

Reverse-phase chromatography is used mainly for the separation of nonionic substances because ionic, and hence strongly polar, compounds show very little affinity for the non-polar stationary phase. However, ionization of weak acids (or weak bases) may be suppressed in solvents with low (or high) pH values. The effect of such a reduction in the ionization is to make the compound more soluble in the non-polar stationary phase but the pH of the solvent must not exceed the permitted range for bonded phases, i.e. pH 2-8. 

The ion suppression technique can be used to great effect for the analysis of weak acids or bases. For the analysis of acidic compounds the technique consists of the addition of a small amount of acetic or phosphoric acid to the mobile phase. By reducing the eluant pH dissociation of the sample molecules is suppressed. They thus have decreased affinity for the eluant and are retained to a greater extent by the ODS phase. The range of BPC is considerably extended using techniques such as ionic suppression and this mode of LC using ODS bonded phases finds wide application. 

PPI treatment leads to dose-dependent inhibition of H. pylori in the stomach [11, 12]. The explanation for this effect comes from in vitro observations. At high pH values, in the presence of urea and a weak buffer, urease activity produces very high periplasmatic pH values which are bactericidal [13-16]. The detailed explanation of this mechanism is given in the chapter dealing with interactions between PPIs and H. pylori in vitro. This inhibition is, in our view, the major explanation of the adjuvant effect of PPIs during anti-i/. pylori therapy. Other effects, such as a PPI-induced increase of antibiotic secretion by the gastric epithelium [17] and a diminished inactivation of antibiotics by lowered gastric acid [18], may play a less important role. The suppressive effect of PPIs on H. pylori is the basis for anti-Helicobacter therapies with a PPI plus one or, preferably, two antibiotics [19, 20]. 

For the Mo/H-Beta zeolites, the formation of the Al2(Mo04)3 phase and the decrease in the concentration of Brpnsted acid sites explains the low catalytic activity of Mo/H-Beta in metathesis of ethylene and 2-butylene to propylene [149]. A promoting effect of Mg was revealed in the Mo/H-Beta-Al203 catalyst for cross-metathesis of ethene and butene-2 to propene [150]. The stability is improved at the Mg content of l-2wt.% due to the elimination of weak acid sites and suppression of the side olefin oligomerization reaction. 

Table lo.ii, which deal with the narcotizing action of various substances on the worm Arenicola (Clowes and Keltch, 1931). It can be seen that the effect on non-electrolytes such as chloroform is independent of pH. These non-ionizing substances show that the changes of pH do not affect the worms. The weak bases, such as cocaine, behave differently. They become more effective as the pH is increased, i.e. in proportion as their ionization is suppressed. Similarly, the weak acids (four isomeric barbiturates) are more effective as the pH is decreased again this corresponds to the suppression of their ionization. By washing the worms with sea-water, the narcotic action is readily reversed in each case. Actually (although the authors did not do so), it is easily calculated that the ions make a small contribution to the toxic action in this series (see next section). 

Introduction of a suppression device between the column and the detector can be expected to cause some degree of peak broadening due to diffusional effects. The shape of the analyte band will also be influenced by hydrophobic adsorption effects, especially when the adsorption and desorption processes are slow. Examination of peak shapes and analyte losses can therefore provide important insight into the use of suppressors with organic analytes which are weakly acidic or weakly basic. It can be expected that peak area recovery rates after suppression are governed by a combination of hydrophobic interactions with the suppressor and permeation through the membranes with the balance between these mechanisms being determined by eluent composition, suppression conditions and analyte properties. 

These salt effects are schematically depicted in Scheme 8. As we will discuss later more in detail (Sections Vl.B.3 and VII.E.3), mechanistically, salts may act in two different ways. In polar solvents they will suppress the free ions and considerably reduce their lifetime. This often converts bimodal MWD to monomodal MWD and provides controlled polymers. However, in polymerization of vinyl ethers initiated by strong Lewis acids such as SnCl4, where only ion pairs are present after addition of a few percent of salts or in nonpolar toluene, control is still very poor (Fig. 17B). Controlled polymers can be obtained only after addition of a more than equimolar amount of tetra-n-butylammonium halides. This implies that the salts change the weakly nucleophilic counterion SnCIs-to the more nucleophilic SnCl62 , which faster converts growing carbo-cations to covalent species. Another effect of added salts is related to... 

Mobile-phase additives are used in HPLC to control the pH and ensure efficient and reliable separations. They also have to be compatible with ESI or APCI conditions. If the pH of the mobile phase needs to be reduced for better LC separations, the most suitable additives in LC/MS are acetic acid and formic acid with typical concentrations ranging from 0.1% to 1%. Note that addition of acids will suppress ionization in negative ion mode. Weakly acidic compounds may not form deprotonated ions under acidic conditions. If the pH of the mobile phase needs to be increased to enhance LC separations, ammonium hydroxide (0.1% to 1%) is suitable. Weakly acidic compounds can be ionized effectively in negative ion mode. Triethylamine is another additive that may be useful to enhance ionization of other compounds in negative ion mode because it is basic. It should be cautioned that the presence of triethylamine might suppress ionization of other compounds in the positive ion mode. A commonly used volatile salt in LC/MS to buffer mobile phases is ammonium acetate (<0.1 M). It is used to replace nonvolatile salts such as phosphates because these nonvolatile salts tend to crystalUze in the ion source and block the source, suppressing ionization of analytes. 

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