SORPTION OF AMMONIUM BY GLASS FRITS AND FILTERS - IMPLICATIONS FOR ANALYSES OF BRACKISH AND FRESHWATER

Glass support frits for 47-mm Millipore filter holders rapidly adsorb up to 600 nmol of ammonium from fresh or brackish waters (salinity < 5.permill.). Certain glass-fiber filters may also adsorb at least 200 nmol of ammonium. The adsorbed ammonium is released on contact with higher salinity or high K+ waters. Distilled deionized water is not an effective cleaning agent for the frits and filters, but either seawater or 2 mM K+ solution is efficient. Significant analytical errors can occur if one does not compensate for the effect by rinsing with copious amounts of sample before collection of water for analysis, by adding K+ to samples at levels of 2 mM or greater to reduce sorption, or by silanizing frits and filters to block sorption.