Mobile phase retention time, effect

Yu et al. (1986) achieved a detection limit of 3.8 pg for the TBA-MDA adduct using fluorometric detection (532 nm excitation 550 nm emission). The adduct was separated on a Zorbax Cl8, 3 //m column (0.63 x 15 cm) or an Altex ODS, 5 /rm analytical column (0.46x25 cm) the columns were eluted with 0.01-0.025 M phosphate buffer (pH 6.5)/methanol (60 40, v/v) at a flow rate of 1.0 ml/min. A presaturation column packed with Corasil silica (35-50 yum) was placed between the pump and the column. Reducing the pH of the mobile phase had little effect on retention time but symmetry and band broadening were worse at more acidic pH probably because the thionyl group of the TBA adduct is increasingly ionized at pH 6.5 and is less attracted to similarly charged residual silanols on the ODS columns. 

The addition of millimolar concentrations of most salts to the mobile phase has the effect of decreasing the retention time of a protein sample and improving peak shapes. This effect can be attributed to suppression of the interaction between polar groups on the solute and silanol groups. Other effects such as ion pairing can also operate and these will be described here. Higher salt concentrations may result in increased retention... 

The concentrations of benzoic acid, aspartame, caffeine, and saccharin in a variety of beverages are determined in this experiment. A Gig column and a mobile phase of 80% v/v acetic acid (pH = 4.2) and 20% v/v methanol are used to effect the separation. A UV detector set to 254 nm is used to measure the eluent s absorbance. The ability to adjust retention times by changing the mobile phase s pH is also explored. 

Concentrations of moderator at or above that which causes the surface of a stationary phase to be completely covered can only govern the interactions that take place in the mobile phase. It follows that retention can be modified by using different mixtures of solvents as the mobile phase, or in GC by using mixed stationary phases. The theory behind solute retention by mixed stationary phases was first examined by Purnell and, at the time, his discoveries were met with considerable criticism and disbelief. Purnell et al. [5], Laub and Purnell [6] and Laub [7], examined the effect of mixed phases on solute retention and concluded that, for a wide range of binary mixtures, the corrected retention volume of a solute was linearly related to the volume fraction of either one of the two phases. This was quite an unexpected relationship, as at that time it was tentatively (although not rationally) assumed that the retention volume would be some form of the exponent of the stationary phase composition. It was also found that certain mixtures did not obey this rule and these will be discussed later. In terms of an expression for solute retention, the results of Purnell and his co-workers can be given as follows,... 

As in SEC, the surface chemistry of the HdC gels should be similar to that of the mobile phase and the solute. Otherwise, the retention time may increase as with the nonsize exclusion effects. However, the tolerance of PCHdC for a poorer mobile phase is better than SEC. The polymer size under 0 conditions has been studied using PCHdC (19). 

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