Consideration should be given to means of stabilizing p and to techniques allowing its precise measurement (5).The fact that higher precision requires lower counting rates places two parameters on a collision course. In the absurd extension, attainment of the highest precision would require the use of the lowest counting rate for an observation time approaching infinity. More practically, this opposition is expressed by Equations 16 and 25, which reveal the inverse cubic dependence of observation time on precision. Given present practical limitations (p 10~8 s,

Fllterlng a 200mL water sample through a palr of Chelex-100 Ion-exchange membranes, under a 2-3 bar pressure, at pH 7 to 8 In not less than 20 mln, leads to an efflclent collection of many trace metals In a form that Is Ideal for subsequent x-ray fluorescence analysls. Slnce enrlchmenl factors around 1250 are obtalned, the detectlon llmlts are at the 1-ppb level. The preclslon Is estlmated to be In the 10-15% range. A perfectly linear relatlonshlp between concentratlon and measured x-ray yleld Is obtalned up to the total membrane palr capaclty of 0.07 mequlv. The slgnlflcant afflntly of Chelex-100 membranes for the-usually abundant-alkall and alkallne earth Ions, together wlth thelr llmlted capaclty, restrlcts the appllcablllty of the descrlbed technlque to samples wlth a very low alkall and alkaline earth content.

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Energy-dispersive X-ray fluorescence might advantageously be used for the determination of trace metals in water. Suspended material can straightforwardly be collected on a 0.45 μm pore-size Nuclepore membrane by filtration. Dissolved ion analysis usually requires a physical or chemical enrichment step. Spotting a 1.5 ml water sample on a Whatman-41 filter within a 29 mm diameter hydrophobic ring and drying by a cold air stream from underneath yielded a 50 ppb sensitivity for many elements, and a 18% accuracy. Filtration of a 200 ml water sample at the natural pH through a pair of Chelest-100 chelating ion-exchange resin loaded filter papers allowed 1-5 ppb sensitivities but suffered from the combination of limited load capacity and significant uptake of usually abundant alkali earth metals.

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