PROGRAMS AND PLANS--Dissolved Trace-Element Data
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experiments are given in two unpublished research reports dated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