Upon arrival at the National Atmospheric Deposition Program (NADP) central analytical lab (CAL) the National Trends Network (NTN) precipitation samples are assessed for volume and fall into one of three categories. The three categories are:

1. Wet Incomplete (WI) volume 1 -11 mL. Volume-permitting, samples are analyzed for as many chemical species as possible. Samples are not filtered or diluted (unless analytically necessary) and aliquots for pH and conductivity are not poured-off. Analysis priority starts with ammonium and continues in the priority order of NH₄⁺ first, then acid anions Cl^-, NO₃^-, SO₄^2- and then base cations Na⁺, K⁺, Ca²⁺, Mg²⁺.
2. Wet Dilute (WD) volume 12 to 30 mL. Aliquots are poured-off for pH and conductivity (this removes ~8 mL) and the remaining volume is filtered using PALL® 47 mm 0.45 um polyethersulfone filter and then diluted (by weight) to 25.0 mL to enable analyses for the full suite of analytes . 
3. Wet (W) volume > 30 mL. Aliquots are poured off for pH/conductivity, sample is filtered using a PALL® 47 mm 0.45 um polyethersulfone filter but not diluted (unless analytically necessary) and will be analyzed for the full suite of analytes.

There are several major potential disadvantages to the treatment of the WI samples. They are prepared differently than the W and WD samples due to lack of filtering and they often cannot be analyzed for all the analytes. The lack of filtering may be biasing the results for some analytes in comparison to a filtered samples. In addition, introducing samples with significant amounts of particulate matter directly into the instruments (Ion Chromatographs, Inductively Coupled Plasma and Flow Injection Analysis) can cause damage to instrument components and affect sample analysis.

Therefore, this study was undertaken to determine the comparability of PALL Acrodisc® 13mm (0.45 µM) Supor® syringe filters to the PALL® 47 mm (0.45 µM) filters currently used for the WD and W samples. The filter components have the same pore size and are made from the same kind of polyethersulfone material but the syringe components and process could have an effect on the sample. If the filtrates from both methods are comparable then a proposal to change the treatment of WI samples (and possibly WD samples) to incorporate syringe filtration and dilution will be made. The study utilized 12 NTN samples; most of which contained visible debris or pollen. Each sample had a variety of treatments analyzed for all NADP analytes (NH₄⁺, PO₄^3-, Cl^-, NO₃^-, SO₄^2-, Na⁺, K⁺, Ca²⁺, and Mg²⁺). The treatments included: unfiltered, filtered with 47mm PALL® “large” filters and analyzed at 100% as well as a variety of dilutions, filtered using PALL® 13mm Acrodisc® filters and analyzed at 100% as well as a variety of dilutions. In addition, 25 blanks were prepared including syringe blanks, Acrodisc® filter blanks, PALL® “large” filter blanks and “Type I” water blanks. The results of these comparisons and an outline of the proposed changes (if recommended) to the sample preparation regime for the three sample types (W, WD, WI) will be presented.