Buffer capacity acids, 183 reversible

Despite these considerations, the first approach in method development for ESl-MS is the formation of preformed ions in solution, i.e., protonation of basic analytes or deprotonation of acidic analytes. Thus, for basic analytes, mixtures of ammonium salts and volatile acids like formic and acetic acid are applied. Alternatively, formic or acetic acid may be added to the mobile phase, just to set a low pH for the generation of preformed ions in solution. The latter approach is successful if sufficient hydrophobic interaction between preformed aiialyte ions and the reversed-phase material remains. The concentration of buffer is kept as low as possible, i.e., at or below 10 nunol/1 in ESl-MS. The buffer concentration is obviously determined by the buffer capacity needed to achieve stable pH conditions upon repetitive injection of the samples. Constantopoulos et al. [99] derived an equilibrium partitioning model to predict the effect of the salt concentration on the analyte response in ESI. If the salt concentration is below 10 moFl, the analyte response is proportional to its concentration. The response is found to decrease with increasing salt concentration. 

Dividing both sides of the equation by d[B] gives the reverse of equation (1.3), defining the buffer capacity. Equations (1.2) and (1.3) have been defined for monoproteic acids, but are also applicable as an initial approximation to di-acids, such as tartaric and malic acids. 

A major part of the buffering capacity of blood is also due to the haemoglobin/oxyhaemoglobin equilibrium and its effect on carbon dioxide transport, but the quantitative treatment of acid-base equilibrium in blood is complicated by the rates of reversible hydration of carbon dioxide (catalysed by carbonic anhydrase) and by the chloride shift from erythrocytes. 

The peak capacity is not pertinent as the separation was developed by a solvent program. The expected efficiency of the column when operated at the optimum velocity would be about 5,500 theoretical plates. This is not a particularly high efficiency and so the separation depended heavily on the phases selected and the gradient employed. The separation was achieved by a complex mixture of ionic and dispersive interactions between the solutes and the stationary phase and ionic, polar and dispersive forces between the solutes and the mobile phase. The initial solvent was a 1% acetic acid and 1 mM tetrabutyl ammonium phosphate buffered to a pH of 2.8. Initially the tetrabutyl ammonium salt would be adsorbed strongly on the reverse phase and thus acted as an adsorbed ion exchanger. During the program, acetonitrile was added to the solvent and initially this increased the dispersive interactions between the solute and the mobile phase. 

For the reversed phase separation of acidic or basic compounds, it is strongly recommended to include a buffer to control the pH of the mobile phase. Capacity, UV absorbance, volatility, solubility, stability, and sample and stationary phase interaction are important to consider when selecting a buffer. For reversed phase separation, a buffer concentration in the range of 10-50mmoll is usually sufficient, when used within +1.0 pH units of the pK. Stationary phases recently introduced to the market easily withstand exceedingly higher pH, allowing... 

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