Acidic modifiers/buffers analyte retention

Particularly interesting is the mixture of an acid and a base, wherein we must then explicitly model the change in pfC of the base and mobile phase. Initially, we consider the aqueous systems that are shown in Fig. 4a and b. These are the pH/retention factors for an acid and a base with no acetonitrile. Based on this, the pH choices could be either less than 2.5, or more than 7.5. Most chromato-graphers would elect to choose a pH of 2.5 or so. However, if we are going to use an appreciable concentration of an organic modifier, the picture changes. If we assume that we are going to prepare an acid-based buffer, the elution profile for the acid remains the same however, for the base, the pH shift will be in opposite directions for buffer and analyte. The elution profile will shift. 

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. 

Two chlorophenoxyacetic acids, 2,4-D and dicamba, were extracted from soil and water and analyzed on a C g column (A = 236nm) using a 50/50 methanol/water (1% acetic acid) mobile phase [201,202], Detection limits of 0.1 pg/g in soil or l.Opg/mL in water were reported. In a similar fashion, eight chlorophenoxyacid residues (e.g., mecoprop, 2,4-D, dichlorprop, fenoprop) were extracted from water samples and analyzed on a C g column (A = 228nm) using a 60/40 methanol/water (30 mM phosphate buffer at pH 3.0) mobile phase [203]. The authors noted that 0.05% trifluoroacetic acid (TFA) used in place of the phosphate buffer as the mobile phase modifier was just as effective and produced significantly different retention times for the analytes. Molar absorptivities were tabulated for the analytes at 205 nm, 228 nm, and 280 nm. Detection limits of 0.5-2 pg/L were reported. 

---