A pH Controller for HPLC Mobile Phases The pH of the mohile phase(s) used in reverse-phase chromatography hadhave a n important hearing on the effediveness of those ion-pairing reagents that might be present in solution. The more sophisticated chromatographic systems have the facilitv for continuouslv variable nH adiustment of the eluent bv incaboration of a ~ - & e , and 'attacged pump with assoeiat2 reagent reservoir, at the proximal end of the mlumn. This sllows the addition of basic or acidic reagent to the eluent, the pH of which can then he monitored using an in-line pH electrode. Addition of the pump, associated tubing, and the in-line pH electmde can increase the cost for the whale system by many hundreds of dollars. Afurther disadvantage is that the flow rate becomes eurnulatwe For Instance, a 2 0 mL mu-' eluent flowrate, added to w t h n 0 5-mL m u - ' reak-nt feed-stream, results in a 2 5-mL m m ' total flow mte. For dflmult u, aehleve srnarations, such increases in total flow rate can lead to a 10s; in resolution. Amore convenient, and far less expensive, approach that allows for continuously variable adjustment of the pH can he adopted by making use of easily available laboratory ware, as shown in the figure.It is a n approacb that is especially applicable to isoeratic systems and relies on the addition of suflicient mncentrated acid or base solution to the mobile phase reservoir in order to achieve the desired pH. The integrity of the mobile phase composition is maintained by sealing the storage battle with its own cap, drilled through with three or four holes. A larger hole, able to aecomodate a mred and sliced rubber stopner is a useful feature for enclosine the o H electrode cahle. bsmg this apparatus, a d p ~ t m e n t &the s mob~lephase pH are easy to make, and the result3 obtamed are comparable to those dcr~vedusing more expenswe components Michael N. Quigley Mellon Hall Duquesne University Pittsburgh, PA 15282
496
Journal of Chemical Education
