NSWC/WOL/TR 75-3 5 



(2) The rate of filtration should be high, and the filter 

 should not clog easily. 



(3) The suspended matter should not penetrate into the filter, 

 but should be retained on its surface, so that it can be readily- 

 recovered . 



(4) The filter should not absorb trace elements. 



(5) In order to prevent contamination of the water sample, it 

 should have a low ash content. This also enables the inorganic 

 components of the suspended matter to be recovered by ignition or 

 wet ashing with minimum contamination. 



(6) It should contain no loose fibers since these may 

 contaminate the water or suspended matter. 



The most satisfactory type of filter for the filtration of 

 water samples for analytical purpose is the molecular membrane 

 filters. These filters are composed of incompletely cross-linked 

 high polymers of partially substituted cellulose acetate and 

 cellulose nitrate, and are approximately 0.15 mm in thickness. 

 They are commercially available in pore sizes ranging from 10 my - 

 5. Op (Millipore Corporation Inc., Bedford, Massachusetts). The pore 

 sizes show a high degree of uniformity. There is little interconnec- 

 tion of adjacent pore channels, and the pore volume is about 80% of 

 the total volume of the filter, thus allowing a very high rate of 

 filtration. The ash weight of the filters is extremely small and 

 amounts to only 0.0001% of the original weight. Filtration is 

 carried out with suction. A low-cost, hand operated vacuum/pressure 

 pump (Cole-Parmer Instrument Co., Chicago, Illinois) is suitable for 

 field operation. The filter must be supported on a sinstered glass 

 plate or a fritted teflon support. A filtration apparatus of this 

 type is available from Wildco Wildlife Supply Co., Saginaw, Michigan, 



3.1.5 Storage of Water Samples 



One of the most important aspects of the sampling process is 

 the care and preservation of the sample prior to analysis. A water 

 analysis is of limited value if the sample has undergone physio- 

 chemical or biochemical changes during transportation or storage. 

 These changes are time dependent, but they usually proceed slowly. 

 In general, the shorter the time that elapses between collection of 

 a sample and its analysis, the more reliable will be the analytical 

 results . 



23 



