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


OFFICE OF WATER QUALITY TECHNICAL MEMORANDUM 92.13

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

                         PURPOSE

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).  

CLEANING OF MEMBRANE FILTERS FOR MAJOR AND TRACE ELEMENTS

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 
volume.  

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 </- 0.2 ug/L; and for Ca, Mg, and Si, </- 
0.04 mg/L.

3.  Mean constituent concentrations in the 400-mL cumulative wash 
volume were statistically different (at the +/- 1 standard 
deviation level) from the 200-mL volume in only 7 of the 48 cases 
(15 percent).  However, none of these 7 cases were confirmed by 
an evaluation of ranks at the +/- one standard deviation level.

The cited observations suggest there may be a slight, but 
statistically insignificant benefit for some constituents to 
cleaning filters with 400 mL of DIW or 2 percent HNO3, in 
comparison to 200 mL.  

       Comparison of DIW Versus Two Percent HNO3 

The results for the 400-mL DIW and 2 percent HNO3 washes are 
summarised in Table 4; the following was observed:

1.  For the 8 elements, the lowest mean concentrations were 
obtained as follows:
       *  Cd -  DIW, MFS; 0.06 +/- 0.02 ug/L 
       *  Cu -  DIW, MFS; 0.1 +/- 0.1 ug/L 
       *  Pb -  2 percent HNO3, Nuclepore; 0.05 +/- 0 ug/L, 
                followed closely by DIW, MFS 0.06 1 0.02 ug/L 
       *  Ni -  DIW, MFS; 0.2 +/- 0.2 ug/L 
       *  Fe -  2 percent HNO3, Nuclepore; 3.1 +/- 1.6 ug/L 
       *  Si -  2 percent HNO3, Nuclepore; 5 +/- 0 ug/L 
       *  Mg -  2 percent HNO3, Nuclepore; 5 +/- 0 ug/L 
       *  Ca -  2 percent HNO3, Nuclepore; 5 +/- 0 ug/L 

2.  For MFS and Millipore filters, the constituent concentrations 
were generally lower for DIW than for 2 percent HNO3.  The only 
exceptions were Fe with MFS, and Ca and Si for both brands 
(concentrations are identical for DIW and 2 percent HNO3).  For 
certain constituents--Cu and Ni--the concentrations in the DIW 
were much lower than in the 2 percent HNO3.

3.  For Nuclepore filters, the constituent concentrations were 
lower using 2 percent HNO3 (in comparison to DIW) for Pb, Ca, Mg, 
Fe, and Si, but higher for Ni, and identical for Cd and Cu.

These observations, based on means and standard deviations of 
concentrations, were confirmed in nearly all cases by the 
statistical analysis of ranks.

While comparisons of concentration means in the 400-mL cumulative 
wash volumes suggest that DIW and 2 percent HNO3 differ somewhat 
in their effectiveness for cleaning membrane filters, the 
differences were small for most elements.  Furthermore, in only 
three of the 24 cases--MFS-Pb (lower in DIW), Nuclepore - Pb 
(lower in 2 percent HNO3), and Nuclepore - Ni (lower in DIW), were 
the comparative concentrations (in DIW and 2 percent HNO3) 
statistically different (at the +/- one standard deviation level).

In summary, combining the cleaning results for the three filter 
brands, there was no advantage at the 400-mL cumulative wash 
volume to using 2 percent HNO3 as opposed to DIW for the eight 
elements.

 Comparison of Elemental Concentrations in the 400-mL DIW
        Wash Volumes to NASQAN Reporting Limits

Concentrations of the eight elements in the 400-mL DIW wash 
volumes were compared to the 1990 NASQAN RLs (compare results in 
Table 4 to RLs in Table 1).  Several observations were evident:

1.  For Cd and Pb, the individual, and hence the mean 
concentrations for the three brands of filters were at or below 
the NASQAN RLs.  For Cu and Ni, the individual concentrations 
exceeded the RL in 1 of 5 and 2 of 5 cases, respectively, for 
Millipore; the means for Cu and Ni were at or below the RLs for 
all three filter brands.

2.  For Ca, the individual concentrations were at or below the 
NASQAN RL for the three brands of filters, and the mean 
concentrations were all below the limit.

3.  For Mg, the individual concentrations exceeded the RL in 3 of 
5 cases for MFS, 1 of 5 for Millipore, and 0 of 4 for 
Nuclepore.  The mean concentrations were below the RL for 
Nuclepore, at the RL for Millipore, and slightly above for MFS.

4.  For Fe, the individual concentrations exceeded the RL in 4 of 
5 cases for MFS, 3 of 5 for Millipore, and 2 of 4 for 
Nuclepore.  The means exceeded the RL for all three brands.

5.  For Si, the individual concentrations exceeded the RL in 4 of 
5 cases for both MFS and Millipore, and in 1 of 4 for 
Nuclepore.  The mean concentration slightly exceeded the RL for 
the MFS and Millipore filters, and was at the RL for Nuclepore.

Thus, the mean concentrations of Mg and Si leached from certain 
brands, and for Fe from all three brands, were slightly above the 
current NASQAN RLs.  However, the higher concentrations for these 
elements are probably inconsequential when compared to the levels 
that occur in environmental samples from most NASQAN and WRD 
project sites.

Comparison of Filter Brands

Taking the lowest mean concentration for each element, whether for 
DIW or 2 percent HNO3, the results indicate: 

1.  The lowest mean concentrations of Cd, Cu, and Ni were found 
    with MFS filters.

2.  The lowest mean concentrations of Pb, Fe, Si, Ca, and Mg, were 
    found with Nuclepore filters.

However, for these cases, none of the lowest mean element 
concentrations were statistically different (at the +/- one 
standard deviation level) from concentrations measured for other 
filter brands.  The analysis of data by ranks generally confirmed 
the cited findings--for both the filter brand providing the lowest 
rank and the lack of statistically significant differences.

In summary, based on the results of this study and current NASQAN 
RLs, it appears that a 400-mL DIW wash of MFS, Millipore, or 
Nuclepore filters will sufficiently preclean the filters for: (a) 
trace elements at the ppb level, and (b) Fe, Si, Ca, and Mg at 
environmentally relevant concentrations.  In determining a 
preferred filter brand for the ppb trace element protocol, two 
additional factors are important:

1.  For the 142-mm diameter, Nuclepore filters are comparatively 
limp and awkward to use, and

2.  District projects have historically not used Nuclepore 
filters.

Thus, although the results from this study indicate no difference, 
statistically, between Nuclepore, MFS, and Millipore filters, 
there is no benefit and some disadvantages to the Division 
switching to Nuclepore for the ppb trace element protocol.

The OWQ will continue to evaluate this situation during the 
remainder of fiscal year (FY) 1992, and issue a directive in 
FY 1993 on membrane filters to be used in the new ppb protocol.  
The OWQ will also conduct a new set of filter cleaning studies as 
a basis for specifying procedures for a parts-per-trillion 
protocol for trace elements.


SAMPLE DILUTION EFFECT CAUSED BY CLEANING FILTERS WITH DIW

The second experiment was designed to determine the potential 
dilution effects on elemental concentrations of DIW remaining in 
the filter apparatus following the cleaning of membrane filters.  
The results are summarized in Table 5.

                            Design

Five native-water samples were collected and placed in a churn 
splitter.  Aliquots of each sample were processed in replicate in 
three different ways, using separate and clean filtration systems:

1.  A MFS filter was placed in the first filtration system and
    200 mL of DIW was passed through it and discarded; 1 liter
    of sample was processed and split into replicates labeled DIW 
    A and DIW B in Table 5.

2.  A MFS filter was placed in the second filtration system
    and 200 mL of native water was passed through it and 
    discarded; 1 liter of sample was processed and split into two 
    replicates labeled Native-A and Native-B in Table 5.

3.  A MFS filter was placed in the third filtration system and
    200 mL of DIW was passed through it and discarded; 1 liter
    of sample was processed, with the first 25 mL of filtrate 
    discarded; the remaining filtrate was split into two 
    replicates labeled DIW + Native-A and DIW + Native-B in Table 
5.

                           Results

For each of the five native water samples tested, the three 
processing systems produced similar results for all elements 
except Fe.  Filtrates from the native-water cleaning system 
(system 2) contained about 10 percent more Fe than filtrates from 
the DIW cleaned system (system 1).  This 10 percent difference was 
probably real because it was substantially higher than the 
analytical imprecision.  The difference probably results from 
dilution of the iron concentrations in the native water samples by 
some DIW retained in the filtration apparatus.  The dilution 
effect noted for Fe probably occurred for the other constituents, 
but could not be detected because the instrumental sensitivity was 
insufficient at the lower (in some cases, much lower) 
concentrations found for these elements.  However, the data 
indicated a slight tendency for the effect for Sr, Ba, and Ca.  

Based on the Fe results, it appears that the use of the DIW pre-
wash can cause as much as a 10 percent dilution in the constituent 
concentrations of native-water samples.  Further, this dilution 
effect was only marginally reduced (range of 0-25 percent 
reduction for four native waters) by following the 200-mL DIW wash 
with a 25-mL native-water wash (system 3).  


    CARRYOVER CONTAMINATION IN EQUIPMENT: LABORATORY STUDY

                            Design 

Experiment 3 was conducted in the laboratory to determine whether 
carryover contamination would result if a sample having low 
concentrations of trace elements is processed after a sample 
having high concentrations.

The laboratory tests were designed to determine how much carryover 
would occur, and how much rinsing of the filtration system with 
DIW or 2 percent HNO3 would be required to eliminate it.  The 
procedure was:

1.  Place a clean filter in the pre-cleaned filtration system;

2.  Pass one liter of laboratory solution, containing a high 
    concentration (see concentrations below) of trace elements and 
    other constituents through the filtration system;

3.  Discard old membrane filter and replace with a new filter;

4.  Pass 200 mL of either DIW or 2 percent HNO3 through the 
    filtration system to simulate filter cleaning;

5.  Rinse the filter with four successive aliquots (50 mL, 50 mL, 
    100 mL, and 200 mL) of DIW or 2 percent HNO3,  representing 
    cumulative wash volumes of 250 mL, 300 mL, 400 mL, and 600 mL, 
    respectively; and

6.  Replicate the procedure five times for each type of membrane 
    filter, and for both DIW and 2 percent HNO3 washes.

The initial prepared solution contained the following:  Ba 
(5 mg/L), Fe (5 mg/L), Pb (5 mg/L), Sr (5 mg/L), Cu (5 mg/L), 
Zn (5 mg/L), Cd (5 mg/L), Ni (5 mg/L), Ag (5 mg/L), Ca (100 mg/L), 
Mg (100 mg/L), Si (100 mg/L), and Na (100 mg/L).  

                           Results

The concentration of elements in the wash solutions (see step 5) 
generally decreased with increasing wash volumes.  However, even 
after 600 mL of washing, the mean elemental concentrations coming 
from the filtration system were considerably higher than both the 
1990 NASQAN RLs and the NWQL RLs for this experiment.  This 
observation was evident for both DIW and 2 percent HNO3 washes, 
and 
for all three filter brands.

The mean concentration of constituents in the 600-mL wash aliquot 
(in this experiment) was also much higher than the mean 
concentration of constituents in the 400-mL cumulative wash 
aliquots from the filter-cleaning experiment (Experiment 1).

The laboratory carryover experiment was designed to mimic a worst 
case scenario of potential sample contamination.  The original 
trace element solution contained much higher concentrations than 
would be encountered in most natural waters, and the levels 
involved probably would never be encountered at either a NASQAN or 
a Benchmark sampling site.  

      CARRYOVER CONTAMINATION IN EQUIPMENT: FIELD STUDY

                              Design

This was a complex experiment incorporating a number of blanks and 
split samples to detect contamination and to define precision in 
field processing of sample aliquots and in laboratory analysis 
(Table 6).  However, as outlined in Figure 1, the main thrust of 
the experiment dealt with: 

1.  Processing sample aliquots from a contaminated site (site 1) 
    through two churns and filter systems, 

2.  Cleaning the churns and filter systems from site 1 
    differently, 

3.  Processing aliquots of a sample from an uncontaminated site 
    (site 2) through the differently cleaned churns and filter 
    systems (see figure 1 for details), and

4.  Processing equipment blanks at each site to determine if a 
    cleaning procedure was sufficient to eliminate detectable 
    carryover.

   Results to Determine if Carryover Occurs Between Sites

The following results compare the differences in concentrations 
for systems 1 and 2 at the uncontaminated site (site 2).

1.  Considerably higher mean concentrations of Cu, Zn, Fe, and Mn
    occurred in system 2, suggesting a large amount of carryover.  
    Mean concentrations of these elements at site 1 were 358, 
    5,350, 48,500, and 5,225 ug/L, respectively.  These 
    concentrations were, at a minimum, 2-3 orders of magnitude 
    higher than the levels in filter system 1 (the clean system) 
    at site 2.

2.  A small increase occurred in the mean concentration of Ca
    in System 2, indicating some carryover.  The mean 
    concentrations for Ca was  moderately high at site 1, and the 
    ratio for site 1/site 2 was about 22 fold.  

3.  No discernable increase occurred in the mean concentrations 
    of Ba, Cd, Co, Li, Mg, Ni, Si, Na, and Sr in System 2.  This 
    indicates either that carryover did not occur or the extent 
    of carryover was very small and could not be detected at the 
    study's analytical sensitivity.  Mean concentrations of  
    these constituents at site 1 were moderately high, and the 
    site 1/site 2 ratio ranged from <1-41 fold.  

4.  No comparison is possible for Be, Cr, Mo, Pb, and V because 
    the reported concentrations were at or below the RLs at both 
    sites.

A possible hypothesis is that the higher mean concentrations of 
some constituents for System 2 at site 2 were due to sample-
handling and analytical imprecision.  However, because the higher 
concentrations for System 2 occurred for those constituents having 
very high concentrations at the first site (e.g., Cu, Zn, Fe, Mn), 
it seems more likely that the cause for the higher concentrations 
is carryover due to a lack of adequate cleaning of System 2's 
equipment.  

The churn splitter for System 2 was intentionally not cleaned 
after sampling the first site, whereas the filtration system was 
washed with 1 liter of DIW between samples.  The churn splitter 
may have been the source of contamination, although it is equally 
plausible that the DIW pre-washing of System 2's filtration 
apparatus was inadequate to eliminate carryover effects from this 
equipment.  Therefore, the specific equipment responsible for the 
carryover of Cu, Zn, Fe, and Mn can not be determined from this 
experiment.

                  Results of Acid Wash Cleaning

The experiment above indicates that contaminant carryover in field 
handling equipment can occur at the ppb level from one water 
sample to the next.  Therefore, a test was conducted to determine 
whether such contaminated equipment could be cleaned in the field.  
The filtration apparatus was disassembled, thoroughly cleaned 
(along with the churn splitter) with DIW, reassembled, and rinsed 
(together with the churn splitter) with 1-liter of 2 percent HNO3 
followed by 1-liter of DIW.  This procedure was done at both sites 
for system 1, after the native water sample was processed through 
the system, but prior to processing the respective field equipment 
blanks.  The analytical results for the two field equipment blanks 
shown in Table 6 confirm the efficacy of this cleaning procedure.

                          CONCLUSIONS

1.  Nuclepore filters--with intensive nitric acid cleaning--are 
the choice of scientists who conduct dissolved trace element work 
at the parts-per-trillion level.  However, with proper cleaning 
with DIW, MFS, and Millipore filters are acceptable for trace 
element work at the ppb level.  Nuclepore filters could be used, 
but accepting their relative difficulty of handling appears to be 
unncessary for ppb level work.  The results are only 
representative of the tested batches of filters and could change 
for different batches.  Thus, the OWQ will probably select a 
single brand of filter for Division use, identify a single 
supplier, purchase large batches, and quality assure each batch 
prior to distribution.  This implies a long-term commitment to 
quality control because each new batch of filters will have to be 
tested for contamination.

2.  For ppb level trace element work, a 400-mL DIW pre-wash is 
adequate for cleaning and pre-conditioning membrane filters.  
Existing procedures, which call for processing and discarding the 
first 200 mL of a native water sample to clean and condition 
filters, can lead to filter clogging and difficulties in obtaining 
sufficient additional filtrate for chemical analyses.  These 
difficulties could be eliminated by using a DIW wash on the order 
of 400-500 mL.  However, the results from this study indicate that 
this could produce about a 10 percent underestimation of Fe 
(because of its relatively high concentrations in examined 
environmental samples) due to dilution by DIW unavoidably retained 
in the filtration system.  The approximately 10 percent difference 
between DIW and native water pre-cleaning and conditioning can 
probably be reduced by processing and discarding limited volumes 
of native water prior to the actual sample collection.  However, 
this study did not determine the minimum volume of native water 
required to eliminate the difference.  On the other hand, the 10 
percent difference probably falls well within the errors 
associated with sample collection, field processing, and 
subsequent laboratory analysis.

3.  Constituent carryover represents a potential problem when 
multiple samples are obtained and processed with the same sample 
collection and processing equipment without proper cleaning 
between sites.  The cleaning procedure (complete DIW wash of the 
disassembled system and churn followed by a 1-liter, 2 percent 
HNO3 rinse and a 1-liter, DIW rinse through the complete system 
and churn) used during this study eliminated constituent 
carryover.




                                  David A. Rickert
                                  Chief, Office of Water Quality

This memorandum does not supersede any previous Office of Water
Quality Technical Memorandum.

Key Words:  NASQAN, trace elements, contamination, membrane 
            filters, equipment cleaning

Distribution:  A, B, S, FO, PO