PROGRAMS AND PLANS--Dissolved Trace-Element Data ARE DISSOLVED TRACE ELEMENTS LOST DURING FILTRATION AND STORAGE IN A PART-PER-TRILLION PROTOCOL?

In Reply Refer To:                               October 29, 1992
Mail Stop 412

OFFICE OF WATER QUALITY TECHNICAL MEMORANDUM 93.03

Subject:  PROGRAMS AND PLANS--Dissolved Trace-Element Data

     ARE DISSOLVED TRACE ELEMENTS LOST DURING FILTRATION 
         AND STORAGE IN A PART-PER-TRILLION PROTOCOL?

                          SYNOPSIS

The purpose of this memorandum is to summarize the findings of 
preliminary experiments conducted to assess whether analyte loss 
occurs during the filtration and sample storage steps of an "ultra 
clean" (part-per-trillion level) trace-element protocol.  The 
experiments were conducted under the supervision of Marty Shafer 
and Mark Walker, Water Chemistry Program, University of Wisconsin- 
Madison, Wisconsin.  Specific details and results of the 
experiments are given in two unpublished research reports dated 
June 1992.  Some of these details, included herein, are 
intentionally taken verbatim from the research reports to avoid 
misrepresentation. 

The major findings of the experiments are:

1.  In general, no statistically significant changes in 
concentrations occurred in cadmium (Cd), copper (Cu), lead 
(Pb), and zinc (Zn) during filtration; and aluminum (Al), 
cadmium (Cd), copper (Cu), lead (Pb), and zinc (Zn) during 
sample storage using ultra clean procedures.  This suggests 
that analyte loss, resulting from sorption of analyte onto acid 
cleaned surfaces is not a problem for part-per-trillion (ppt) 
trace element work.  

2.  For Cu, systematic low-level decreases were observed for both 
the filtration and sample storage experiments.  The decreases 
were not statistically significant except in one case.  It is 
probable that a portion of the difference could be attributed 
to error arising from a change in analytical sensitivity.

                           BACKGROUND

Office of Water Quality (OWQ) Technical Memorandum 91.10 
described our current understanding of observed discrepancies in 
the levels of dissolved trace-element results generated by the 
standard U.S. Geological Survey (USGS) protocol in comparison to 
"ultra clean" protocols used by academic researchers and Howard 
Taylor of the USGS National Research Program.  Median 
concentrations of arsenic, boron, beryllium, cadmium, chromium, 
copper, lead and zinc obtained by standard USGS methods were 
substantially higher than levels generated by ultra clean 
protocols.  In addition, a study in which both standard and ultra 
clean methods were applied to aliquots of the same samples showed 
that the discrepancy in trace element concentrations resulted 
primarily from sample collection and field processing, rather 
than from laboratory analysis.  Memo 91.10 listed three possible 
reasons for the observed discrepancies:  (a) contamination 
introduced by the standard USGS protocol; (b) variations in 
sample processing techniques (e.g., differences in particle 
removal procedures might cause differences in the amount of 
colloidal material incorporated and analyzed as "dissolved"); and 
(c) sorption loss of dissolved trace elements during the sample 
collection, sample handling, filtration, and sample storage steps 
of the ultra clean protocol.  Sorption loss could occur if: (a) 
exhaustive acid precleaning opens active sorption sites on 
sampling equipment, filters, and/or sample bottles, and (b) the 
sorption sites are incompletely equilibrated with excess sample 
during collection, processing, and storage of sample aliquots.  
It is expected that analyte loss would be minimal during sample 
storage because of the low pH resulting from sample preservation.

Contamination from selected surface-water samplers and from 
membrane filters has been described in OWQ Technical Memoranda 
92.12 and 92.13, respectively.  Also, the effects of filtration 
artifacts on iron and aluminum concentrations have been 
investigated by Art Horowitz (Horowitz, 1992).

To date, the USGS has not investigated analyte loss from sorption 
of dissolved trace elements onto equipment and materials during 
use of ultra clean methods.  Such studies have been delayed, 
pending the development of an "in-house" part-per-trillion (ppt) 
analytical capability in the National Water Quality Laboratory 
(NWQL).  Such a capability will not be available until October 1, 
1993, at the earliest.  However, the OWQ has obtained unpublished 
research reports from the University of Wisconsin that 
characterize the extent of analyte loss during use of ultra clean 
methods.  In these experiments, analyses were done by graphite 
furnace atomic absorption and at concentrations well above 
instrument detection levels.  This memorandum presents the results 
of those reports.

                 ANALYTE LOSS DURING FILTRATION

                        Design/Preparation

The water samples used in this experiment were taken from southern 
Lake Michigan and were filtered on board ship through an acid-
leached 0.4 5m track-etched filter.  Unacidified subsamples of the 
filtrates were transported to the lab on ice and refiltered to 
remove remaining particles and most colloids.  Particles/colloids 
were removed by filtering the samples twice through acid leached 
0.05 5m Nuclepore filters.  The 0.05 5m filtrates were split into 
two aliquots, one left untreated, the other spiked with an 
acidified mix of trace metals designed to approximately double the 
ambient concentration of targeted metals (Cd, Cu, Pb and Zn).  The 
spike additions lowered the pH of the sample from 7.6 to 7.4.

The mean concentration of trace elements in the unspiked and 
spiked filtrates, after completion of the described filtration 
steps, were:

                                 Nanogram/liter  (ng/L)	
                               Unspiked          Spiked
                   Cd              25.5             48 
                   Cu             780            1,460 
                   Pb              75              142 
                   Zn             725            1,415 

These spiked and unspiked filtrates became the "reference 
solutions" used to evaluate analyte loss associated with an "ultra 
clean" filtration apparatus and membrane filter.  The filtration 
apparatus was comprised of a segmented Teflon column to which an 
all Teflon 47 mm filter holder was attached.  Acid leached 0.4 5m 
Nuclepore filters were used and filtrates were obtained by 
pressurizing (10-20 psig filtered nitrogen) the column.  

The filter sorption experiment was run one day after the 
laboratory filtering and spiking.  The filtration apparatus was 
set up under a laminar flow hood, and the filter holder loaded 
with an acid leached filter.  Approximately 200 mL of reference 
solution was placed in the column and the filtration initiated.  
The first 25 mL of filtrate was discarded, and the next two 75 mL 
aliquots (filtrate "A" and filtrate "B") were collected in 
separate trace-metal cleaned polyethylene (LPE) bottles.  
Filtration rates were in the range of 0.5 to 1.0 mL min-1 cm-2.  
The complete experiment was run in duplicate for both the spiked 
and unspiked reference samples.  Two blank (Milli-Q water) 
filtrations were also performed.  After filtration, all samples 
were acidified with 150 5L of Ultrex HNO3, lowering the pH to 
about 1.6.

Pre-cleaning of the Nuclepore filters was done by placing filters 
individually into acid washed polystyrene petri dishes, and adding 
1M Ultrex HNO3.  Filters were soaked for a period of approximately 
24 hours, after which the acid was removed and filters thoroughly 
rinsed with Milli-Q water.  The filtration apparatus was cleaned 
by leaching in 20 percent HNO3 (ACS reagent) for 4 days at room 
temperature, followed by a leach in 2 percent HNO3 (Baker trace-
metal grade) for 4 days at room temperature.  The LPE sample 
bottles were cleaned as follows:  

1.  Fill with ACS reagent acetone, soak for approximately 2 
hours, remove acetone, rinse 3 times with Milli-Q water;

2.  Fill with 20 percent ACS reagent HCL, soak for 4 days at 
room temperature, remove HCL, rinse 3 times with Milli-Q 
water;

3.  Fill with 20 percent ACS reagent HNO3, soak for 4 days at 
room temperature, remove HNO3, rinse 3 times with Milli-Q 
water; and

4.  Fill with 0.5 percent Baker Ultrex HNO3, soak until needed 
(but at least 4 days), remove acid, rinse 3 times with 
Milli-Q water, dry under laminar floow hood, and bag.

                         Results

Table 1 summarizes the average of two replicated experiments.  
Changes in mean concentrations of trace elements through the 
filtration apparatus and membrane filter are shown, along with the 
mean percentage changes.  Positive values denote an increase in 
concentration; negative values a decrease.  The last column gives 
the calculated percentage changes in concentrations required to 
signal a statistically significant difference between the 
reference solutions and either filtrate A or filtrate B.  
Triplicate analyses and the t-test with a 95-percent confidence 
level were used to calculate the percentage changes shown.  Levels 
of Cd, Cu, Pb, and Zn in the filter blanks were undetectable; 
therefore, no blank corrections were made to the computed 
differences listed in Table 1.

Concentration changes during the filtration step were less than or 
equal to 12 ng/L for Cd and Pb (less than or equal to 16 percent 
difference), and less than or equal to 80 ng/L for Cu and Zn (less 
than or equal to 7.7 percent difference).  Only the -5.5 percent 
difference for Cu in the spiked sample for filtrate A was 
statistically significant.  It is noteworthy that the four 
observations for Cu show a negative change, suggesting that low 
levels of analyte loss may have occurred for this element.  

              ANALYTE LOSS DURING SAMPLE STORAGE

                            Design

Two separate studies were completed to evaluate the effect of 
refrigeration storage on trace-element concentrations (that is, 
analyte loss to sample bottles).  One study was conducted with 
southern Lake Michigan water; the second with samples from 
selected Wisconsin rivers.  Details and results of the former are 
described in this section, whereas, only those findings related to 
analyte loss are given for the latter.  Both studies involved: (a) 
analyzing samples several weeks or months after sample collection 
(first analysis), (b) storage of acidified samples at 4!C for 6 to 
8 months, and (c) then reanalysis of the samples (second 
analysis).  

Samples of southern Lake Michigan water were filtered on board 
ship through an acid leached 0.4 5m track-etched filter.  
Subsamples were pooled soon after filtration and placed into 
paired 125mL polyethylene (LPE) and Teflon (TEF) bottles.  (Both 
bottle types were pre-cleaned using the "ultra clean" procedure 
described previously for the filtration experiment).  Subsamples 
were then acidified at a rate of 2 mL of concentrated Ultrex HNO3 
per liter resulting in a pH of 1.4 to 1.5.  All samples were kept 
refrigerated for the entire cruise and transported to the lab on 
ice.  In the lab, samples were held at 4!C until first analyzed 2-
3 weeks after sample collection.  Considerable time elapsed before 
the first analysis was completed.  Therefore, analyte loss could 
have occurred to sample bottles between the time of sample 
collection and the first analysis.  After the first analysis, 
samples were stored for an additional 7.5 months at 4!C, and then 
re-analyzed for Cd, Cu, Pb, and Zn.  Two Milli-Q bottle blanks 
were also analyzed and, except for Zn (in LPE bottles only), the 
desorption of trace elements from sample bottles was not 
significant.  For Zn, the average increase in the blank for the 
LPE bottles over 7.5 months of refrigeration storage equated to 
approximately 10 percent of the sample's ambient concentration 
(about a 65-90 ng/L increase).

                              Results

A summary of the results from the study of Lake Michigan water is 
given in Table 2.  With the exception of Zn in the LPE bottles, 
none of the trace-element concentrations were statistically 
different (for both bottle types) between the two analyses.  The 
83 ng/L increase of Zn in the LPE sample bottles is similar in 
magnitude to the increase in the bottle blank and presumedly was a 
result of contamination, either during storage or during the 
second analysis.  In summary, there were no statistically 
significant losses of Cd, Cu, Pb, and Zn to either polyethylene or 
Teflon bottles during storage at pH 1.5 and 4!C for a period of 
7.5 months.  As noted above, the amount of analyte loss, if any, 
that occurred to samples between the time of collection and first 
analysis remains to be quantified.

Similar results were found for Al, Cd, Pb, and Zn in the storage 
loss experiment conducted on Wisconsin river waters.  However, 
although not statistically significant, a decrease in Cu 
concentration occurred in all four samples during storage in pre-
cleaned and refrigerated Teflon bottles (polyethylene bottles were 
not evaluated) for 5-8 months.  The decreases ranged from 35-131 
ng/L--5 to 15 percent of the sample's first analysis 
concentration.  The consistently lower copper concentrations 
during the second analysis might reflect some sorption loss to the 
Teflon sample bottles.  However, some of the difference was 
attributed by the University of Wisconsin researchers to an 
analytical "...downward sensitivity drift observed over the course 
of the storage check (period)".

                 SUMMARY AND FUTURE RESEARCH

The experiments conducted at the University of Wisconsin provide 
data for selected trace elements on the extent of potential 
analyte loss associated with ultra clean methods.  In general, 
changes in trace element concentrations from filtration and sample 
storage were not statistically significant.  Also, no systematic 
loss occurred: (a) for Cd, Pb, and Zn during filtration (that is, 
on the filter holder and membrane filter); nor (b) for Al, Cd, Pb, 
and Zn onto bottle walls during 7.5 months of storage at about 
pH 1.5.  However, systematic, low-level decreases in Cu 
concentrations were observed for the filtration experiment and 
for one of the sample storage experiments (Wisconsin River water), 
with decreases ranging from 30-131 ng/L.  

The University of Wisconsin preliminary test results suggest that 
analyte loss is not a problem for trace element work at the ppt 
level.  The University of Wisconsin is currently completing 
additional analyte loss experiments.  Once the NWQL establishes 
ppt analytical capability, the OWQ will collaborate with the 
University of Wisconsin to conduct analyte loss tests on sample 
collection equipment, filtration equipment, and storage bottles 
for those trace elements to be analyzed at the ppt level by the 
NWQL.

                            REFERENCES

Horowitz, A.J., Elrick, K.A., and Colberg, M.R., 1992, The effect 
of membrane filtration artifacts on dissolved trace-element 
concentrations: Water Research, v. 26, no. 6, p. 753-763.

Shafer, Martin, 1992, An evaluation of analyte loss resulting from 
the storage and filtration of Lake Michigan waters: Water 
Chemistry Program, Water Science and Engineering Laboratory, 
University of Wisconsin-Madison, 12 p.  [Unpublished report on 
file in the Office of Water Quality, Reston, Virginia]

Walker, Mark, 1992, An evaluation of trace element loss resulting 
from the storage of Wisconsin surface waters: Water Chemistry 
Program, Water Science and Engineering Laboratory, University 
of Wisconsin-Madison, 12 p.  [Unpublished report on file in the 
Office of Water Quality, Reston, Virginia]

						David A. Rickert
 						Chief, Office of Water Quality

Key words:  Trace elements, analyte loss, sorption to field 
            equipment

This memorandum refers to Office of Water Quality Technical 
Memorandums 91.10, 92.12, and 92.13.

Distribution:  A, B, S, FO, PO