Why run 100 mL through filter before sampling?

Why run 100 mL through filter before sampling?

The 100 mL "flush" (generally known as the "discard volume" or "preconditioning") serves to equilibrate the filter membrane with the groundwater chemistry. All filters, regardless of construction material or cleanliness, have some capacity to sorb target analytes from the water onto the filter membrane. If you sampled the first water through the filter, there is the potential that the sample results would be biased low due to this sorption. By passing groundwater through the filter before sampling, the target analyte concentration on the membrane will reach equilibrium with the concentration in the water, avoiding any sample bias due to sorption. Likewise, some filter membranes also have the potential to leach metals from the filter membrane, support material, or even the housing, and these may be present at greater initial concentrations of groundwater. Preconditioning with groundwater can reduce this leaching to avoid any measurable bias in the sample. An excellent paper, which covers this topic was published by Drs. Robert Puls and Robert Powell in Groundwater Monitoring and Remediation in 1992.

QED uses polyethersulfone (PES) for their filter membrane material. PES is considered to be a premium filter medium, far "cleaner" than any other materials that are commonly used for groundwater filters from other manufacturers. These other filters use filter media such as acrylic copolymer, polythylene and glass wool fiber materials. While they are priced comparably to QED's QuickFilter capsules, they typically require a much larger discard volume prior to collection, and some will leach metals such as zinc and barium into samples at measurable concentrations. There is one other company (Pall/Gelman) that offers a PES membrane filter under the brand name of "Supor" that is comparable in quality to our QuickFilter capsule, but it sells for about 35-50% more than our filters.