PROGRAMS AND PLANS--Evaluation of Capsule Filters In Reply Refer To: January 21, 1993 Mail Stop 412 OFFICE OF WATER QUALITY TECHNICAL MEMORANDUM 93.05 Subject: PROGRAMS AND PLANS--Evaluation of Capsule Filters EVALUATION OF TRACE-ELEMENT AND NUTRIENT CONTAMINATION ASSOCIATED WITH CAPSULE FILTERS Synopsis An experiment was conducted to determine if capsule filters can be precleaned to be usable in a part-per-billion (ppb) protocol for dissolved trace elements. Based on the experimental results, the Gelman Supor 1/ capsule filters (Catalogue #12175) prerinsed/ conditioned with 1,000 milliliters (mL) of deionized water (DIW) are suitable for ppb-level determinations of trace elements, and also appear noncontaminating for nutrients at present reporting limits. At the ppb level for trace elements and present nutrient reporting limits, acid prerinsing the capsule filters does not appear to have any beneficial effect over the use of DIW. Background As indicated in Office of Water Quality (OWQ) Technical Memorandum 92.12, the OWQ has been conducting a series of studies designed to identify equipment, supplies, and cleaning procedures suitable for a ppb protocol for dissolved trace elements. Traditionally, the Water Resources Division (WRD) has processed whole-water samples (for the subsequent determination of dissolved constituents) through 0.45-um membrane plate filters. The continued use of such filters for the determination of dissolved trace elements at the ppb level is certainly feasible, but will entail much greater care than has been used in the past. The additional care will be required to limit potential sources of contamination associated with the filtration process. Some examples of contamination sources include: inadequate cleaning of the membrane filter and filtration system, excessive or improper handling of the filter membrane, atmospheric inputs during the filtration process, and carryover from one sample to another due to improper between- sample cleaning of the filter system. 1/ Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. government. Potential contamination during the filtration process can be minimized if cleaning and handling requirements are kept to a minimum, if the filtration system is used only once, and if the filtration system is a sealed unit until used. One way to address these requirements is to employ disposable (used only once) capsule filters. This would be especially true if the capsule filters were precleaned and prepackaged at a dedicated facility prior to taking them to the field. Because a number of capsule filters are commercially available, three questions must be addressed: 1. Which filter(s) should be used? 2. What concentration of trace-element and nutrient contaminants are associated with these filters? 3. If necessary, could the contaminants be removed using relatively simple cleaning procedures? Selection of Capsule Filters for Testing OWQ Technical Memorandum 80.22 recommended the use of Gelman Mini Capsule filters (Catalogue #12123) for processing whole-water samples containing high concentrations of suspended sediment. The reason for the recommendation was that these filters were substantially harder to clog than normal plate filters (142 mm diameter, 0.45 um pore size) because they had much higher surface areas (about 500 cm2 as opposed to 160 cm2). The Gelman Mini Capsule filters are 47-mm in diameter, with a 0.45-um pore size and contain a Versapor (acrylic copolymer on a nylon support) membrane on a polypropylene core, in a sealed polycarbonate housing. More recently, Windom and others (1991) used capsule filters to process whole-water samples to determine selected dissolved trace-element concentrations at the part-per-trillion (ppt) level in a number of east coast rivers. They used Gelman 47-mm, 0.45-um capsule filters (Catalogue #12175) that contain a Supor (polysulfone) membrane on a polypropylene core, in a sealed polycarbonate housing. These filters have an effective surface area of 600 cm2. Physically, both capsule filters are similar, albeit the Mini Capsule filter has a slightly smaller surface area. However, the filter membranes are chemically dissimilar and have different chemical compatibilities. Traditionally, trace-element chemists and geochemists working at low concentration levels use a weak acid rinse to preclean (decontaminate) filters prior to use. Some use a weak nitric acid (Shiller and Boyle, 1987) rinse; others use a weak hydrochloric acid (Windom and others, 1991) rinse. The WRD has traditionally used native water to clean and condition filters prior to use. However, it is very likely that any filtrations carried out with the intent of determining trace-element concentrations at the ppt level will require some form of acid washing. Currently, the OWQ is preparing a ppb-level trace-element protocol. Ultimately, a companion ppt protocol will be prepared when the National Water Quality Laboratory (NWQL) develops the appropriate analytical capability. Therefore, the ideal filter media chosen for divisionwide use should be amenable to either ppb- or ppt-level protocols. Conversations with Gelman technical representatives indicated that the Versapor filter is incompatible with acid of any sort, whereas the Supor filter can not resist nitric acid at any strength, but can resist hydrochloric acid (even at full strength). Thus, based on the expected potential requirement for acid precleaning in a ppt-level protocol, only the Supor filter was evaluated for possible sources of contamination. Testing Procedure The testing and analysis of capsule filters were carried out at the NWQL under tightly controlled conditions. The actual cleaning/conditioning tests were performed within a laminar-flow hood to limit atmospheric inputs. The test employed two cleaning/conditioning solutions: (1) DIW, and (2) 0.6 molar hydrochloric acid followed by DIW. Five separate capsule filters were used to test each cleaning/conditioning solution, as follows: 1. DIW cleaning/conditioning solutions: The filters were used as they came from the supplier; there was no precleaning procedure employed. Each filter was sequentially flushed with four 500-mL aliquots of DIW using a peristaltic pump and silicon tubing. 2. 0.6 molar HCl-DIW cleaning/conditioning solutions: Each filter was filled to capacity with 0.6 molar HCl. Pump tubing was attached to each end of the capsule filter and the tubing was inserted in a peristaltic pump. The acid solution was circulated for about 1 minute. At the end of that time, the filter was drained of all acid, and DIW was pumped through the filter until the pH of the DIW returned to normal (as determined by pH measurements). This was done to simulate the post acid washing with DIW that would be required with capsules being prepared for actual field use. The additional step is necessary because acid left on a filter during filtration of an actual sample could solubilize trace elements from the particulate matter retained on the filter. In most cases, about one liter of DIW was required to return the pH to normal. After this preparatory step, each filter was sequentially flushed with four 500-mL aliquots of DIW using a peristaltic pump and silicon tubing. In summary, each of the ten test filters was flushed with four sequential 500-mL aliquots of DIW. These aliquots were collected and stored in acid-rinsed polypropylene bottles and preserved with nitric acid prior to trace element analysis. The filter tests themselves accounted for a total of 40 samples (5 filters x 2 cleaning/conditioning solutions [DIW, HCl/DIW] x 4 [500-mL] aliquots) for trace-element analysis. In addition to the trace-element tests, the capsule filters were also evaluated for utility in processing samples for the subsequent determination of a variety of nutrient parameters. Separate aliquots from each of the 40 trace-element samples described above were collected, stored, and preserved in appropriate containers for nutrient analyses. During the course of the tests, quality control data were collected for both trace-element and nutrient analyses. Fourteen DIW blanks were collected; seven consisted of DIW poured directly into the appropriate sample containers from a reservoir filled prior to the initiation of testing, and seven consisted of DIW from the same reservoir which was run through a peristaltic pump and pump tubing, and collected in the appropriate sample containers (this DIW did not pass through the capsule filters). Two atmospheric blanks (open containers of DIW kept within the laminar flow hood) were collected during the course of the entire test. In addition, 10 samples from the capsule evaluation (from the second 500-mL rinse aliquot) were split to determine analytical precision. A total of 66 samples (40 capsule filter rinses + 14 DIW blanks + 2 atmospheric blanks + 10 splits) were analyzed for trace-elements and 66 samples for nutrients. Results and Discussion Table 1 presents data for the various blanks, and the sequential flushes for the capsule filters study. Trace-element analyses were carried out either by ICP-MS or ICP-AES techniques. The specific elements and their reporting limits for the ICP-MS determinations were: 0.5+/-1.0 5ug/L - Al and Zn 0.2+/-0.2 5ug/L - Sb, Ba, Be, Cd, Co, Cr, Cu, Mn, Mo, Ni, Pb, Ag, and Tl The specific elements and their reporting limits for the ICP-AES determinations were: Fe (3 ug/L), Sr (0.5 ug/L), B (2 ug/L), Ca (0.02 mg/L), Mg (0.01 mg/L), Na (0.2 mg/L), and Si (0.01 mg/L). Nutrient analyses were performed using standard techniques. Reporting limits for the various nutrient determinations were: total phosphorus (0.01 mg/L), total nitrogen (0.05 mg/L), ammonium ion (0.02 mg/L), orthophosphate (0.01 mg/L), nitrate plus nitrite (0.05 mg/L), and nitrite (0.01 mg/L). All results, both in the blanks and in the sequential filter flushes, were below the respective reporting limits for: * ICP-MS limit of 0.2 ug/L for Sb, Ba, Be, Cd, Co, Cr, Pb, Mn, Mo, Ni, Ag, Tl; and 0.5 ug/L for Al and Zn; * ICP-AES limits of 0.5 ug/L for Sr, 0.01 mg/L for Mg, and 0.2 mg/L for Na; and * Nutrient limits of 0.01 mg/L for NO2, 0.02 mg/L for NH4, and 0.05 mg/L for NO3+NO2. Because all analytical results for each of the above are below reporting limits, these data are not included in table 1. In reviewing the results in table 1, the reader must note that the respective aliquots in the DIW-washed series (CF1-1 to CF5-4) represent dissimilar total volumes of fluid flushed through the filters from the respective aliquots in the acid washed series (ACF1-1 to ACF5-4). This stems from the extra volume represented by the acid wash step itself plus the volume represented by the subsequent DIW wash required to return pH to normal. Thus, where different concentrations of a given trace element occur for the same aliquot number in the two sets, the difference could result from the acid washing and/or the larger volume of fluid represented in the aliquots in the acid wash series. The two atmospheric blanks show no detectable concentrations of trace elements or nutrients except for a very small level of total phosphorus (Blank 2). Blanks from the DIW reservoir are similarly clean except for low levels of boron (DI-4) and total phosphorus (DI-1 and DI-4). The pump blanks have low levels of boron (PB-6), total phosphorus (PB-1), and total nitrogen (PB-1, PB-3, and PB- 6). Further, all of the pump blanks (as well as all capsule filter blanks) contain detectable but low levels of Si. The presence of Si can probably be ascribed to the use of silicon pump tubing, and occurred despite the use of new tubing that had been heavily soaked and rinsed with 0.6 molar hydrochloric acid prior to use. This conclusion is based on the facts that: (1) no detectable Si occurred in either the atmospheric or the reservoir blanks, and (2) the Si levels in the capsule filter blanks are consistent with the levels in the pump blanks. As cited above, the pump and/or reservoir blanks contain low levels of total nitrogen and total phosphorus. The concentrations are at or very near the reporting limits for these parameters. There is no detectable total nitrogen (>0.05 mg/L) in any of the capsule filtrates, whereas there is an occasional detectable total phosphorus concentration at the reporting limit (0.01 mg/L). We believe the sporadic low-level total nitrogen and total phosphorus concentrations result from analytical noise and do not represent contamination. At the detection limits used in this study, there is little or no contamination of trace elements and nutrients associated with the tested capsule filters. Further, there is little or no difference between the acid precleaned and non-acid precleaned filters. The only exceptions are Ca and Cu, both of which consistently show up in the initial 500-mL rinse for the non-acid precleaned filters (Cu shows up in the first 500-mL rinse for the first two acid pre- cleaned filters (ACF1-1 and ACF2-1) as well). Where detectable concentrations for both ele-ments occur in the first 500-mL rinse, they drop below their respective reporting limits in the second 500-mL rinse. Conclusions Based on these results, it appears that Gelman Supor capsule filters (Catalogue #12175) prerinsed/conditioned with 1,000 mL of DIW are suitable for ppb-level determinations of trace elements, and appear noncontaminating for nutrients at present reporting limits. At the ppb-level for trace elements and at present nutrient reporting limits, there appears to be no benefit to acid prerinsing. The use of silicon pump tubing, in conjunction with the tested capsule filters, appears to contribute minor amounts of Si contamination. However, the detected levels are probably insignificant relative to most environmental samples. If users desire to reduce/eliminate the Si contamination associated with the use of silicon pump tubing, Teflon tubing can be substituted, except for a very small length required for the peristaltic pump head. References Shiller, A.M., and Boyle, E.A., 1987, Variability of dissolved trace metals in the Mississippi River: Geochimica et Cosmochimica Acta, v. 51, p. 3273-3277. Windom, H.L., Byrd, J.T., Smith, R.G., Jr., and Huan, F., 1991, Inadequacy of NASQAN data for assessing metal trends in the nation's rivers: Environmental Science and Technology, v. 25, no. 6, p. 1137-1142. David A. Rickert Chief, Office of Water Quality Attachment: Table 1 This memorandum refers to Office of Water Quality Technical Memorandums 80.22, 92.12, and 92.13. Key words: Capsule filters, contamination, nutrients, trace elements Distribution: A, B, S, FO, PO [Table 1 available in hard copy only]