Acidic modifiers/buffers modifier concentration, effect

Buffer—Mobile phase modifier used to control pH. Usually salts of weak acids or bases, most effective at their pKa, where concentrations of ionized and unionized form are equal. 

The FITC labeling method was also applied to chiral separations of amino acids on a microchip to determine the enantiomeric ratios of amino acids found on a meteorite [27], Since biotic amino acids are normally single enantiomers, chiral separations of amino acids are not truly clinical in nature, but illustrate the potential for chiral separations of small molecules of clinical interest. Ma-thies and co-workers used this technique to search for evidence of life in extraterrestrial environments. Enantiomeric forms of Val, Ala, Glu, and Asp could be discriminated by addition of a-, (3-, or y-cyclodextrin (CD) to the run buffer. Improved resolution with faster separations was found with respect to conventional CE. This method has been modified, by addition of SDS to the buffer, to perform cyclodextrin-modified micellar electrokinetic chromatography (CD-MEKC) [28]. Increasing the SDS concentration decreased the magnitude of elec-troosmotic flow (EOF), increasing the effective migration distance, and therefore the resolution on the microchips. 

The surface of the silica can be modified with the use of a buffer and this may be an effective alternative method for the separation of polar analytes. This may lead to the enhancement and change in selectivity of ionic samples while exhibiting no effect on the behavior of nonionic samples. The pH of the buffer, concentration, and the type of buffer used have a significant effect on the retention and peak shape of the analytes [4-6]. However, the most influential parameter is the pH of the buffer, where the pH of the buffer solution should be lower than the of the acidic analytes and be higher than the of the basic analytes. 

The most frequently used detector in FI systems with gas-diffusion separation is the spectrophotometer. Quite often the gas-diffusion process offers sufficient selectivity to allow relatively non-specific chemical reactions in the acceptor stream to detect the analyte. Thus, carbon dioxide, sulfur dioxide, hydrogen sulfide, ammonia may all be determined using suitable acid-base indicators in appropriate buffer solutions used as the acceptor streams. The concentration of the buffer solutions may be adjusted to suit a certain concentration range for the analyte. In order to further enhance the selectivity and/or sensitivity more specific reagents may be introduced in the acceptor streams. In the previously mentioned example on the determination of cyanide [20] a modified pyrazolone-isonicotinic acid reaction was used for such purposes. Interferences due to Schlieren effects seem not to have been reported in gas diffusion spectrophotometric systems. This is understandable, since the matrix composition of acceptor streams is usually quite uniform, and the refractive index is little affected after absorbing the gaseous analytes. 

Yang and Thyrion [1072] studied peak shape and retention on a wide range of basic amine-containing compounds procaine, adiphenine, drofenine, nafronyl, tetracaine, meclofenoxate, 4-aminobenzoic acid, and caffeine. The basic mobile phase was 65/35 acetonitrile/water (20 mM acetate buffer at pH 4.5) and the column a Ci8 (A =260 and 280 nm). The effect of the identity (i.e., di-n-butylamine and triethylamine) and concentration of mobile phase modifier from 0 to 0.3% was studied. Retention times from 2 to 20 min resulted and asymmetry factors in general were <2.5. 

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