Environ. Sci. Technol. 1996, 30, 954-963
Problems Associated with Using Filtration To Define Dissolved Trace Element Concentrations in Natural Water Samples ARTHUR J. HOROWITZ* U.S. Geological Survey, Peachtree Business Center, 3039 Amwiler Road, Atlanta, Georgia 30360
KEN R. LUM† Centre Saint-Laurent, Environment Canada, Montreal, Quebec H2Y 2E7, Canada
JOHN R. GARBARINO U.S. Geological Survey, Branch of Analytical Services, 5293 Ward Road, Arvada, Colorado 80002
GWENDY E. M. HALL Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada
CLAIRE LEMIEUX‡ Centre Saint-Laurent, Environment Canada, Montreal, Quebec H2Y 2E7, Canada
CHARLES R. DEMAS U.S. Geological Survey, 3535 South Sherwood Forest Boulevard, Suite 120, Baton Rouge, Louisiana 70816
Field and laboratory experiments indicate that a number of factors associated with filtration other than just pore size (e.g., diameter, manufacturer, volume of sample processed, amount of suspended sediment in the sample) can produce significant variations in the “dissolved” concentrations of such elements as Fe, Al, Cu, Zn, Pb, Co, and Ni. The bulk of these variations result from the inclusion/exclusion of colloidally associated trace elements in the filtrate, although dilution and sorption/desorption from filters also may be factors. Thus, dissolved trace element concentrations quantitated by analyzing filtrates generated by processing whole water through similar poresized filters may not be equal or comparable. As such, simple filtration of unspecified volumes of natural water through unspecified 0.45-µm membrane filters may no longer represent an acceptable operational definition for a number of dissolved chemical constituents.
† Present address: I.U.C.N.-World Conservation Union, 380 St. Antoine, W., Suite 3200, Montreal, Quebec H2Y 3X7, Canada. ‡ Present address: Multisources, 2875 Rue Holt, Montreal, Quebec H1Y 1P7, Canada.
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Introduction Some 25 years ago, marine chemists began to recognize that previously generated dissolved seawater trace element data were elevated due to contamination introduced by improper handling [e.g., sampling, processing, preservation, analyses (1-3)]. This led to the introduction of so-called “clean/ultraclean” sampling, processing, preservation, and analytical techniques to obtain reliable dissolved trace element data (4, 5). During the past 10 years, freshwater chemists have come to similar conclusions and have begun to employ similar techniques to those used by the oceanographic community (4-10). These changes have led to marked reductions in sample contamination and a concomitant decrease in the reported levels of ambient dissolved trace elements in marine and freshwater systems. During this same period, the importance of colloids to the concentration, transport, and redistribution of trace elements in aquatic systems has been demonstrated (1114). Colloids are capable of sorbing large concentrations of trace elements. They fall on the continuum between suspended sediment and dissolved constituents; as such, there is much controversy as to when a solid-phase material changes from being a suspended sediment to a colloid and when a colloid changes to a dissolved form (e.g., refs 15 and 16). However, colloidal material typically is considered to be finer than 1 µm. While improved sample handling has become more widespread and the impact of colloids on trace element distributions in natural waters has been recognized, there has not been a substantive change in how aquatic chemists define dissolved constituents. Examination of a wide variety of standard methods compendia dealing with water samples as well as regulatory requirements indicates that the current and almost universally accepted definition of a dissolved constituent is an operational onesonly substances which pass a 0.45-µm membrane filter are considered to be dissolved (17-20). The use of this definition continues, despite the fact that a “dissolved” trace element concentration obtained from a 0.45-µm filtered water sample may contain substantial amounts of colloidally associated trace elements (15, 21). Prior to the advent of clean/ultraclean techniques and major improvements in trace element analytical instrumentation (e.g., ICP-MS), the differences in dissolved concentrations caused by varying amounts of colloidally associated trace elements probably were masked by relatively high levels of contamination or relatively insensitive analytical techniques (e.g., ref 9). However, as the reported ambient levels of dissolved trace elements have declined from the tens of parts per billion (ppb, µg/L) into the single digit ppb range, and now well down into the parts per trillion (ng/L, ppt) range (e.g., refs 4-7, 9, and 10), the inclusion/exclusion of varying amounts of colloids and their affect on dissolved (filtered) trace element concentrations has become more significant. During the development of the U.S. Geological Survey’s (USGS) new “clean” protocol (8), it became apparent, based
0013-936X/96/0996-0954$12.00/0
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on Fe and Al concentrations, that the filtration step used to operationally define dissolved constituents significantly influenced the quantity of colloidally associated trace elements incorporated in the filtrate (21). This quantity was affected by the rate at which the membrane became clogged, thus reducing its nominal pore size (15, 21). Clogging could be affected by such diverse variables as (1) filter type, (2) filter diameter, (3) filtration method (vacuum or pressure), (4) suspended sediment concentration, (5) suspended sediment grain size distribution, (6) concentration of colloids, (7) concentration of organic matter, (8) volume of sample processed because of 4-6, and (9) method of sample collection (dip or depth- and width-integrated), again because of 4-6. The effects of these variables on dissolved Fe and Al concentrations were termed filtration artifacts (21). The pattern of decreasing trace element concentrations during the past 25 years due to contaminant reduction has led many water chemists to believe axiomatically that, within the context of the current definition of a dissolved constituent, the lowest concentration determined for a sample or a particular site was probably the most representative concentration (21). The results from this study indicate that this might not be true. The artifact study (21) implied that even the use of tightly controlled sampling and processing procedures might not eliminate filtration artifacts caused by randomly changing environmental factors (e.g., suspended sediment concentration, suspended sediment grain size distributions, concentration of organic matter). If these artifacts were widespread and affected trace elements other than just Fe and Al, they could affect a variety of environmental studies as well as water quality regulations. The underlying assumption behind many environmental studies attempting to identify long- and short-term trends as well as the common practice of combining dissolved trace element data from many sources to generate local, national, or worldwide average concentrations is that all samples processed through a 0.45-µm membrane filter are comparable because they all are based on the same definition, albeit an operational one. If this assumption proved to be invalid as a result of filtration artifacts, it could lead to major changes in how environmental scientists and regulatory agencies operate. Field and laboratory trials were undertaken by the USGS, the Geological Survey of Canada (GSC), and Environment Canada to evaluate the affects of filtration artifacts on the concentrations of many dissolved trace elements beside just Fe and Al. The studies were based on the premise that most of the observed artifacts resulted from the inclusion/ exclusion of colloidally associated trace elements in wholewater filtrates. These study results are reported herein.
Methods Filtrate aliquots from the field study were analyzed by inductively coupled plasma-mass spectrometry (ICP-MS) and inductively coupled plasma-optical emission spectrometry (ICP-OES). Al, Sb, Ba, Be, Cd, Cr, Co, Cu, Pb, Mn, Mo, Ni, Ag, Sr, Tl, U, and Zn concentrations were measured by ICP-MS using VG Instruments PQ-1 (22). B, Ca, Fe, Li, Mg, Si (as SiO2), Na, and V were measured by ICP-OES using a Thermo Jarrell-Ash ICAP 61E nitrogen-purged instrument (23). System blanks were analyzed using both techniques; blank concentrations generally were less than the detection limit for all analytes (Table 1). Accuracy was monitored based on analyte results measured in USGS
TABLE 1
Analytical Limits for Field and Laboratory Studies U.S. Geological Survey
elementa
methodb
field and laboratory study method detection limit
Be Al Fe Cr Mn Co Ni Cu Zn Mo Ag Cd Sb Ba Tl Pb U Li V Sr Ca Mg Na SiO2
ICP-MS ICP-MS ICP-OES ICP-MS ICP-MS ICP-MS ICP-MS ICP-MS ICP-MS ICP-MS ICP-MS ICP-MS ICP-MS ICP-MS ICP-MS ICP-MS ICP-MS ICP-OES ICP-OES ICP-OES ICP-OES ICP-OES ICP-OES ICP-OES
0.6 1.0 3 0.3 0.3 0.2 0.8 0.6 1.0 0.6 0.2 0.2 0.2 0.1 0.2 0.2 0.1 1 3 0.1 0.002 0.001 0.03 0.02
Geological Survey of Canada
methodb
laboratory study reporting limit
ICP-MS ICP-MS ICP-MS ICP-MS ICP-MS ICP-MS ICP-MS ICP-MS ICP-MS
0.005 2 5 0.1‘ 0.1 0.02 0.1 0.1 0.5
ICP-MS ICP-MS ICP-MS ICP-MS ICP-MS ICP-MS ICP-MS ICP-MS ICP-MS ICP-MS AA AA AA
0.05 0.05 0.01 0.2 0.01 0.1 0.005 0.005 0.1 0.5 0.1 0.1 0.1
a All concentrations are in µg/L except for Ca, Mg, Na, and SiO , 2 which are in mg/L. b Analytical methods included inductively coupled plasma-mass spectrometry (ICP-MS), inductively coupled plasma optical emission spectrometry (ICP-OES), and flame atomic absorption spectrophotometry (AA).
standard reference water samples (measured reference concentrations had to be within (1.5σ of the certified value or the analytical results were discarded). Analyses for the laboratory study were performed by both the USGS in Denver, CO, and the GSC in Ottawa, Ontario (using a VG Plasmaquad 2+ ICP-MS and conventional nebulization, or flame atomic absorption spectrophotometry, see Table 1 for detection limits), on sample splits. The data generated by both facilities indicate that the trends shown in the various samples were the same for the USGS and the GSC data; however, there appears to be a bias in absolute concentrations between the sets for some elements. Considering the independent techniques/ methodologies used to produce the data sets, the agreement between both laboratories is quite good.
Results and Discussion The samples required for this study were collected from the Mississippi River at either St. Francisville or Baton Rouge, LA, and from the Tangipahoa River [a “blackwater” (organic-rich) stream] near Robert, LA. Sample collection and field processing (where required) were carried out following the new USGS sampling and processing protocols (8). Field Study. The purpose of this segment of the study was to evaluate the effect(s) of using different (e.g., manufacturer, diameter) 0.45/0.40-µm membrane filters to process whole-water samples for subsequent dissolved trace element quantitation. This was intended to evaluate how different filters affected the inclusion/exclusion of
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TABLE 2
Comparison of Selected Dissolved Trace Element Concentrationsa in Sample Filtrate Aliquots from the Mississippi River at St. Francisville, LAb [All Concentrations in µg/L Except Ca, Mg, Na, and Si (mg/L)] analytical methodc and constituent filterd and aliquot vol (mL) Nuclepore 0-100 MicroFiltration Systems 0-250 250-500 500-750 Gelman Capsule 0-250 250-500 500-750 750-1000 1125-1375 1375-1625 1625-1875 1875-2125 2125-2375 2375-2625 2625-2875 2875-3125 3125-3375
MS Al
OES Fe
MS Cr
MS MS Mn Co
MS Ni
MS Cu
8.2
7
0.5
14
0.4
