THE ACTUAL PH VALUES OF THE STATIONARY-PHASE IN ION-PAIR THIN-LAYER CHROMATOGRAPHY

The pH values of different chromatographic layers, e.g. silica gel and physically and chemically bonded reversed-phase layers (C-18), were s tudied in the presence of Ion-pairing reagents with mobile phases cont aining methanol and water, Tetramethyl- and cetyltrimethylammonium bro mide (TMA and CTMA) were used as ion-pairing reagents, representing th e two major classes (symmetrical and asymmetrical) of this type of cat ionic compound, Six different methods were used to incorporate the buf fer into the chromatographic system, There were significant difference s between the pH values measured on chemically bonded reversed-phase l ayers previously treated with TMA or CTMA, especially when the buffer was added to the mobile phase only, Higher pH values were measured whe n both the stationary and mobile phase contained buffer, The presence of the ion-pairing reagent did not change the pH values, During the ch romatographic process a significant pH-gradient could not he detected. These results explain some of the contradictions observed in the effe ct of mobile phase pH on the retention of model compounds in ion-pair TLC.