Trace Element Contamination:  Findings of Studies on the Cleaning of Membrane Filters and Filtration Systems

In Reply Refer To:                                  July 17, 1992
Mail Stop 412


Subject:  Trace Element Contamination:  Findings of Studies on
          the Cleaning of Membrane Filters and Filtration


In 1989 and 1990 Art Horowitz conducted a series of experiments 
with emphasis on the cleaning of filters and filtration systems.  
This memorandum describes the results and conclusions from those 
experiments.  Building on the information in Office of Water 
Quality (OWQ) Technical Memorandum 91.10, this memo provides 
background for some of the decisions that the OWQ is making to 
provide a supportable parts-per-billion (ppb) protocol for 
dissolved trace elements.

                     STUDY COMPONENTS

Figure 1 provides information on the various experimental designs 
and identifies the three filter brands tested--MFS, Millipore, and 
Nuclepore.  These brands were selected because Millipore and MFS 
represent approximately 80 percent of the Division's usage, and 
Nuclepore is the filter preferred by research chemists.  All 
experiments were made on 142-mm (millimeter) diameter filters.  
Four experiments were conducted:

1.  Cleaning of membrane filters for major and trace elements:
    A.  Concentration of elements in successive wash aliquots of 
        50, 50, 100, and 200 milliters (mL), which correspond to 
        cumulative wash volumes of 50, 100, 200, and 400 mL.
    B.  Comparison of deionized water (DIW) versus 2 percent HNO3.
    C.  Comparison of element concentrations in the 400-mL wash 
        volume to National Stream-Quality Accounting Network 
        (NASQAN) reporting limits (RLs).
    D.  Comparison of filter brands.

2.  Sample dilution effect of cleaning filters with DIW.

3.  Carryover contamination in equipment: Laboratory study.

4.  Carryover contamination in equipment: Field study.

Table 1 provides information on the National Water Quality 
Laboratory's (NWQL) RLs for each experiment and for NASQAN in 
1990.  Reporting levels changed from experiment to experiment 
depending on the analytical method used; namely, inductively 
coupled plasma (ICP) atomic emission spectroscopy, or graphite 
furnace atomic absorption spectroscopy (GFAAS).  


Five individual filters from a single batch of each of the three 
brands were tested.  This designation, together with the four wash 
volumes (50, 100, 200, and 400 mL) and the two wash solutions 
(DIW and 2 percent HNO3) gave a design of 120 data points 
(5 x 3 x 4 x 2).  Results were compared by computing mean 
constituent concentrations and the standard deviations for the 
five filters for each combination of brand, volume, and wash 
solution.  Ranks of constituent concentrations and standard 
deviations were also computed.  The rank results are not reported 
in this memo, but are cited in several places.

Analyses were made for 19 major and trace elements.  Of these, 
11 showed either: (a) no detectable concentrations throughout the 
entire experiment, or (b) concentrations below the study's 
reporting level before or in the aliquot corresponding to the 
200-mL cumulative wash volume.  The 200-mL volume is important 
because it has been the Division's "rule of thumb" in cleaning 
filters prior to collecting the filtrate for dissolved trace-
element analysis.  Elements in these two categories included 
silver (Ag), barium (Ba), beryllium (Be), cobalt (Co), lithium 
(Li), molybdenum (Mo), manganese (Mn), sodium (Na), strontium 
(Sr), vanadium (V), and zinc (Zn).  The remaining eight elements--
which are the focus of this memo--were cadmium (Cd), copper (Cu), 
lead (Pb), nickel (Ni), iron (Fe), silicon (Si), magnesium (Mg), 
and calcium (Ca).  For these elements, concentrations in the wash 
solutions were quite erratic, giving rise to high standard 
deviations (Tables 2, 3, and 4).  Thus, although differences based 
on means alone were observable for many tested comparisons, only a 
few differences were statistically significant.  The erratic 
results appear to have arisen from: (a) non-uniformity of 
contamination between individual filters in a batch, (b) 
differences in flow paths by which cleaning solutions passed 
through the 142-mm filters, and (c) in certain cases, 
contamination arising during the experiment or subsequent 
laboratory analyses.  

    Comparison of 400-ML versus 200-ML Wash Volumes

In this and following discussions, mean and standard deviations 
are described.  Five observations were available for each brand of 
membrane filter, wash volume and wash solution.  To compute means 
and standard deviations, "less than" values were assigned a value 
of one-half the reporting limit.  Table 2 shows the computed means 
and standard deviations for the eight elements for the DIW washes 
(by filter brand and wash volume), whereas Table 3 shows the 
comparative values for the 2 percent HNO3 washes.  From these 
tables, the following was observed:

1.  Elemental concentrations generally showed a decay function, 
with low concentrations reached by the 200-mL cumulative wash 

2.  In nearly two-thirds of 48 cases (8 elements x 3 filter brands 
x 2 wash solutions), the mean elemental concentrations were less 
in the 400-mL versus the 200-mL cumulative wash volume.  This 
trend was evident for both DIW and 2 percent HNO3 for the three 
filter brands, and for most of the elements.  Furthermore,the 
observation of lower concentrations in the 400- versus the 200-mL 
cumulative volume was confirmed by the mean of ranks.  Although 
the mean concentrations were typically less in the 400-mL wash 
volume, the difference between the 200-mL and 400-mL wash volumes 
tended to be small.  For example, in DIW washes, the difference 
for Cd, Pb, and Cu was