Electrostatic interactions - Organic mobile phases

Triazines belong to a group of widely used herbicides that have become major pollutants of soil and environmental waters [138,139]. Therefore, analytical methods are needed to allow them to be monitored at concentrations below the pg/h level. The triazines are available in large numbers with small structural differences and with different known basicity and hydrophobicity (Table 5.15). This class of compounds is therefore well suited as a model system in molecular imprinting [32]. Their conformers and their interactions with small ligands are known [101]. More recently they have also been imprinted and the resulting materials have been successfully incorporated in analytical methods based on competitive assays or selective sample pre-treatments (see chapter 15) [135,140,141]. 

A large number of polymers imprinted with A -protected amino acids have been prepared and studied for their chromatographic behaviour in organic mobile phases [17,18,47,72,131-133]. In most cases they have been prepared using a poorly hydrogen bonding diluent, sueh as chloroform [47,131] or acetonitrile [18,72], and 

MAA and/or VPY [132] or AAM [133] as functional monomers. They are then commonly evaluated in media consisting of the same solvent used as diluent together with a polar modifier. 

For templates interacting only weakly with the functional monomer, recognition is often only seen when using the same solvent used as diluent. Often, however, no selectivity is observed even then. The most common reasons for the lack of selectivity in these cases are sample overloading and/or slow mass transfer [59]. Decreasing the sample load leads to an increase in both retention and selectivity. Furthermore, 

CAPACITY FACTORS OF 9EA IMPRINTED POLYMERS PREPARED AND EVALUATED USING ACETONITRILE OR CHLOROFORM AS DILUENT AND MOBILE PHASE 

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HILIC uses a polar stationary phase to separate proteins and peptides. Samples are loaded onto a column using a low aqueous mobile phase and are eluted by increasing the water content of the mobile phase (152). HILIC is very suitable for separation of polar species such as sialic acid-containing glycopeptides, which have limited retention on most RP materials (153). A variant of HILIC is electrostatic repulsion-hydrophobic interactions chromatography and it utilizes IEX stationary phase and a high organic mobile phase (152,154). 

The nature of the analyte interactions with liophilic ions could be electrostatic attraction, ion association, or dispersive-type interactions. Most probably all mentioned types are present. Ion association is essentially the same as an ion-pairing used in a general form of time-dependent interionic formation with the average lifetime on the level of 10 sec in water-organic solution with dielectric constant between 30 and 40. With increase of the water content in the mobile phase, the dielectric constant increases and approaches 80 (water) this decrease the lifetime of ion-associated complexes to approximately 10 sec, which is still about four orders of magnitude longer than average molecular vibration time. 

Type V includes chiral stationary phases based on immobilized proteins as well as polysaccharide phases such as cellulose and amylose carbamate. They are used in conjunction with aqueous buffered mobile phases. The interaction between the stationary phase and the analytes is based on hydrophobic interaction as well as electrostatic interaction in the case of proteins. The retention of the analytes can be controlled by the addition of organic modifiers such methanol, ethanol, and 2-propanol. 

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