WATER QUALITY: Field filtering of water samples for chemical analysis April 5, 1978 QUALITY OF WATER BRANCH TECHNICAL MEMORANDUM NO. 78.06 Subject: WATER QUALITY: Field filtering of water samples for chemical analysis Technical reviews of District water-quality activities have revealed that a number of types of filtering devices and pressure sources are being used in the WRD to filter water samples. In most instances, correct procedures and equipment are being used; however, upon occasion incorrect procedures and equipment likely to result in sample contamination or possible injury to personnel have been encountered (see WRD Memo's Nos. 77.29 and 77.51). The purpose of this memorandum is to establish guidelines for filtering water samples for chemical analysis. Biological sample filtering guidelines already have been established and are presented in "Methods for Collection and Analysis of Aquatic Biological and Microbiological Samples" (Greeson, P. E., and others, 1977, TWRI, Book 5, Chapter A4). FILTER PORE SIZE By Federal interagency convention, the standard mean pore diameter size of filters used for separating water and sediment for chemical analysis is 0.45 micrometer (um). A prefilter with a larger pore diameter size may be used for samples containing large amounts of suspended materials; however, the final filter pore diameter size must be 0.45 um when the results of the chemical analysis are to be stated in terms of "dissolved" or "suspended" chemical constituents in reports or in the WATSTORE or STORET data systems. Thus, it is imperative that data resulting from analyses of samples passed through filters with pore diameter sizes other than 0.45 um be qualified separately in WRD publications to distinguish them from standard data. Such data should not be published in annual basic data reports unless they are clearly and adequately qualified, and the data cannot be stored in WATSTORE under the parameter codes now in use for the standardized data. These instructions should not be construed to discourage the use of finer filters where required for geochemical solubility studies. The Quality of Water Branch is currently investigating ways of storing data resulting from use of filters with pore sizes other than 0.45 um. Membrane filters made of mixed cellulose acetate and nitrate (Millipore HAWP, or equivalent) and polycarbonate film (Nuclepore, or equivalent) are recommended for processing water samples for inorganic analysis. The most common prefilters, when used, are the glass-fiber type (Gelman type E, or equivalent), having a mean pore size range from 1 to 5 um. It is recommended that the use of prefilters be kept to a minimum to reduce the possibility of contamination and sample alteration resulting from the possible presence of wetting agents and the ion exchange capacity of the filter. Most filters are known to leach contaminants in small quantities and some exhibit ion exchange properties for some of the substances in water; therefore, care must be taken to thoroughly equilibrate the filter with sample water when the filtrate is to be analyzed. Generally, 100-250 milliliters of sample passed through the filter is sufficient for equilibration. This rinse water should then be discarded and an additional quantity of sample filtered to provide the filtrate for chemical analysis. FILTERS AND FILTERING EQUIPMENT FOR INORGANIC CHEMICAL CONSTITUENTS Basically there are three types of filtering assemblies available for general purpose filtration work; plate-, funnel-, and pressure-type systems. Whichever type is used it should be constructed of nonmetallic materials such as plexiglass (lucite), polyvinylchloride, teflon, or polycarbonate. For some filtrating applications, for example when trace metal analyses are not required, the filter assemblies can be made of high grade stainless steel. Internal components of filtering assemblies, like the support screens, should be made of noncontaminating materials such as nylon, polyester, teflon, or high grade stainless steel. However, stainless steel should not be used where it will come in contact with water that is to be analyzed for trace metals. Gaskets and o-rings should be made of silicone, teflon, or Buna-N materials. Silicone and teflon are the preferred materials and are especially recommended for high precision/low concentration research-type work. Nylon or high grade stainless steel are recommended for filter clamps and bolts. The plate filter is the recommended filter assembly. It consists of two retainer plates with the filter, filter screen, and prefilter sandwiched between. When used the complete system is filled with liquid and no trapped compressed air is present. Plate filters are available in a variety of sizes and configurations. Normally a size from 102mm to 293mm diameter is used. Some smaller sizes such as the 47mm type can be purchased as complete disposable assemblies ready for immediate use. The larger types are designed for quick filter change capability without undue loss of sample. Some types feature a backflushing operation which enables repeated use of the same filter membrane in heavy sediment laden waters. For further information on backflushing filters, see open-file report 76-126 "Backflushing filters for field processing of water samples prior to trace-element analyses" by V. C. Kennedy and others. (See QW Branch Memo No. 77.04). Funnel filter systems are of the types used for microbiological filtration work. For samples from which trace metals are to be analyzed, the funnel assembly and filter screen should be plastic such as polycarbonate, PVC, etc. The stainless steel microbiological assembly can be used to filter samples for inorganic constituents other than trace metals. It should be thoroughly cleansed of all sterilization liquids and gases before use. Funnel filter systems use vacuum as the power source. This is discussed in more detail in the next section. This method is best used for filtering small volumes of water that do not have high concentrations of filter clogging material. Pressure filtering systems use compressed nitrogen or air to force the water out of a reservoir through the filter. Most are similar in design to the recalled barrel filter unit which is no longer in use (see WRD Memo No. 77.147). Pressure filter units are usable for filtering moderate volumes of water without too much filter clogging material present. For a safety standpoint the pressure on these systems should be kept as low as possible and should not exceed 30 psi. Pressure filtration is useful for filtering samples for dissolved organic substances (see next section) and for certain biological applications such as ATP determinations where it is critical that a controlled low pressure be used to prevent rupture of algal cells. Pressure filter units are not recommended for general use unless the type and design is proven to be safe and reliable. Units that are used must have approved safety devices to prevent over pressurization and reliable regulation of the amount of pressure applied to the unit. Several types are available from the major filter manufactures. You should check with your Regional Safety Officer before purchasing any pressure units. The Ohio District has designed a PVC low pressure unit using a membrane safety valve and hand squeeze bulb as a pressure source. This unit has been approved for general field use. At present, though there is no commercial source from which the unit may be purchased. A description and plans of the unit can be obtained from the Ohio District. CLEANING FILTER EQUIPMENT Newly purchased filter assemblies should be thoroughly cleaned with a nonphosphate laboratory detergent to remove residual fabrication products such as oils, polishes, glues, and cleaners. Follow the cleaning with a tap-water rinse and an overnight soaking in 5 percent hydrochloric acid. Then rinse the filter assembly with tap water followed by distilled or deionized water. The filter assembly must receive the same thorough cleaning after each use in heavily polluted waters. Cleaning of the assembly after other types of usage consists of at least a thorough rinse in distilled/ deionized water or a detergent (nonphosphate) wash followed by a distilled/ deionized water rinse. wash porous plastic filter retainer pads and screens with distilled or deionized water after each use. Then, with the filter in place, the initial rinse water passing through the filter will serve to rinse and equilibrate both the filter itself and the filter support. Periodically clean all external components of the filter in order to avoid contamination of the filter assembly and the filter supports. Filter tubing should be of surgical silicone, Tygon, or other equivalent inert material. Discard thick-walled rubber tubing that often accompanies commercial filters and replace it with one of the above mentioned types. Inspect all tubing before each use for signs of possible contamination. Replace tubing that shows signs of residue accumulation, discoloration, weakness, or other deterioration. New tubing, regardless of quality, price, and manufacturing controls should be assumed to contain contaminants. Therefore, thoroughly clean before use all tubing used to transfer gas from a pressure source to the filter inlet, and all tubing used to transfer water for filtering from a sample container, stream, or well. Ideally, this consists of rinsing with the sample water to be filtered or, as an alternative, by soaking the tubing thoroughly in distilled or deionized water overnight before use. Do not use new uncleaned tubing for transferring water for filtration. FILTERS AND FILTERING EQUIPMENT FOR ORGANIC SOLUTES CHEMICAL CONSTITUENTS Filtering of samples for the determination of organic constituents requires the use of equipment constructed from lnorganic materials. Acceptable filtration equipment consists of a stainless steel assembly (Gelman No. 4280 filter assembly or equivalent) with a 0.45 um silver filter (Selas F.I47 or equivalent). Bear in mind that most stainless steel filter assemblies received new from the company are contaminated with grease. All parts must be cleaned sequentially with acetone, methylene chloride, and ethyl alcohol. After cleaning with these three organic solvents, wash the filter assembly carefully using a good quality laboratory detergent followed by a thorough rinsing with distilled water. with infrequent use, the stainless steel filter assembly may develop a few rust spots. These spots can be removed by rinsing the filter assembly in a dilute solution of sodium dithionite, Na2S204, (also called sodium hydrosulfite). After cleaning the filter assembly with the chemical, rinse it thoroughly with distilled water before using. Use extreme care when handling the stainless steel filter support screen in the bottom of the assembly. Burrs and projections on this screen may puncture the silver filter. Rinse the inside of the filter assembly thoroughly with the sample water before filtering. After use, rinse the assembly with distilled water and store in a plastic bag to avoid contamination. Handle the silver filters only with forceps. METHODS OF FILTERING Field filtration can be conducted using pump, compressed gas, or vacuum transfer methods. The preferred procedure is the peristaltic pump (pump transfer method) in conjunction with the plate filter assembly. The peristaltic pump operates on a squeeze and release principle using rollers and flexible tubing. In recent years many different designs and sizes of peristaltic pumps have become available on the market. They can be purchased as laboratory units running off AC power or as portable field pumps that are battery power. Many have reversible direction and variable speed motors which are desirable and necessary features for certain filtration methods such as backflushing filtration (see QW Branch Memo No. 77.04). Filtering with peristaltic pumps us done by placing the intake end of the tube directly into the sample-compositing container, such as the churn splitter, or directly into the stream, well, or lake. The discharge end is attached to the filtration unit and then the sample is pumped through the system using the peristaltic pump. The operating speed of the pump should be kept slow speed. As the filter becomes clogged, pressure will slowly build up to about 35 psi (depending on the type and size of tubing used) when the filter is completely clogged. Flow through a clogged filter can be reestablished by backflushing, if a backflushing plate filter unit is used, or by changing the filter membrane. Use of a compressed inert gas to force the liquid through the filter membrane is the primary method recommended for filtering organic carbon samples. It can also be used to filter other water samples when the proper approved equipment is used. The preferred gas for filtering is ultra-high purity, low moisture content nitrogen gas. This grade of nitrogen gas contains less than 0.5 ppm hydrocarbon, is water scrubbed, and is satisfactory for filtering all types of water samples. It is the same type of gas that is currently approved for use in manometer stage recording systems (bubble gages). Clean compressed air can be used if it is known that the presence of oxygen will not adversely effect the sample. The clean air can be pumped by either a peristaltic pump equipped with dry, clean tubing or a hand squeeze bulb such as used on blood-pressure measuring units. Bicycle pumps or air compressor sources should not be used because they can easily contaminate the water sample with their lubrication fluids. Air should not be used for filtering samples of water with low dissolved oxygen content, for example, waters highly polluted with oxygen-demanding substances, water at deeper depths in lakes, or ground waters. Cylinders of compressed gas must be equipped with an adjustable pressure regulator and the output line must have a safety pop-off valve set at 30 PSIG (see WRD Memo No. 77.113). Use the minimum pressure that will force water through the filter at an acceptable rate. Pressures above 15-20 PSIG have little effect on filtration rate when filters have become clogged. It should be remembered that some types of algal cells can rupture at pressures above 5 PSIG, releasing their cellular contents into the filtrate. However, many types of algal cells can withstand pressures greater than 5 PSIG, and in many waters the concentrations of algae ill not be great enough to influence the chemical quality of the water even if cells do rupture, except in cases where ATP is to be determined . Nevertheless, for samples from lakes, ponds, and estuaries, the presence of ruptured algal cells should be assumed and their influence on the chemical quality of the water considered. Vacuum or negative pressure also may be used to draw water through a filter. For very small volumes, the Millipore funnel and hand vacuum system may be used. For larger volumes, a vacuum pump or the engine manifold vacuum of an automobile may be needed; when using such equipment, a check valve should be installed in the vacuum line to prevent contamination by engine fumes. Vacuum filtering is most useful for the collecting of filterable ("suspended") material on a filter membrane for subsequent processing. If it is used to collect the filtrate, special care must be taken to avoid contamination. The filtrate vacuum flask must be thoroughly cleaned before use and rinsed with clean filtrate. The initial filtrate through a new filter membrane should be discarded, and all traces removed from the collection flask before the required filtrate can he collected. Remember that the purpose of filtering water is to remove suspended material larger than a known size without contaminating the sample. Care in both sample handling and filtering is required to insure against contamination. Common-sense practices and procedures will help insure that water samples collected and filtered by WRD personnel remain representative of the body of water that was sampled. R. J. Pickering Chief, Quality of Water Branch WRD Distribution: A, B, S, FO, PO