resolution limited to optimising column length, temperature, organic modaier content, ionic strength or pH adjustment of eluent in order to suppress both analyte dissociation and silanol ionisation. Retention underreversed-phase conditions is generally the result of several interaction mechanisms between polar solute, eluent, stationary phase and underlying silica with the consequenc that column selection and optimisation are rarely facile Recent HPLC methods for the determination of pantothenic acid in infant formulas have advocated reversed phase separation at both low pHand low organicmodier
concentration. It is recognised that both of these requirements, while essential for successful retention of
a polar and ionisable analyte, represent potential limitations to e€ective reversed-phase performance, especially with the older type A silica column chemistries.Thus, both conventional end-capping protection of
residual silanols and bonded-phase integrity are vulnerable to low pH, with variable selectivity commonly
reported under such conditions. Further, the predominantly aqueous eluents(>95%) required to elect retention of such polar analytes, can initiate collapse of the hydrophobic bonded phase, with potential variability, or loss of retention.
This problem has previously been described during pharmaceutical analysis (Woollard, 1986).
The six columns selected during column evaluation represent dierent solutions to the above considerations.
The Resolve C18 cartridge column is representative of arrst generation, non-endcapped type A silica chemistry,
Table 2
Pantothenic acid content in milk and infant formula powders (mg/100 g)a