542F04006
Perchlorate in Ground Water: Update on the
Application of an Ion-Selective Electrode
Analytical Method
Through the Measurement and Monitoring Technologies for the 21 st Century (21M2) initiative,
EPA's Office of Solid Waste and Emergency Response (OSWER) is identifying and supporting
deployment of promising measurement and monitoring technologies by matching existing and
emerging technologies with OSWER program and client needs in the fields of waste management
and site cleanup. OSWER has identified a number of "needs areas" as the focus of 21M2. These
needs reflect evolving requirements across all waste programs. A recent list and description of
needs is available at the 21 M2 Internet site at htro://www.cluin.org/programs/21 m2/needs.cfm.
Background
EPA developed this fact sheet to summarize the
recent study of a field analytical method for
measuring perchlorate that was funded under the
21M2 program. This study resulted in a new
protocol for analyzing ground water samples for
perchlorate using ion-selective electrodes (ISE).
Project Objectives
EPA's goal was to develop an accurate and
reliable field method for measuring perchlorate in
ground water at concentrations as low as 15 ug/L.
Specific study objectives included:
• determination of the suitability of both
membrane and solid state electrodes;
• collection of information on the performance
of the method in terms of accuracy and
reliability;
• evaluation of the effect of common
environmental anions on the accuracy of
perchlorate data;
• development of protocols to handle
interferences; and
* development of a standard operating
procedure.
Project Results
Suitability of Ion Specific Electrode Designs.
The study evaluated an ISE that uses a plastic
membrane half-cell with a double junction
reference electrode and a solid state ISE with a
built-in reference element. The solid state ISE
does not require any filling solution or
maintenance and can be stored dry. The
membrane ISE uses a reference electrode that
requires maintenance of both the inner and outer
chamber filling solutions and must be stored in a
special solution or dismantled, cleaned, and stored
dry. It was found that the 18 (ig/L California
Action Limit concentration that had to be
achieved for the method to be of use in the field,
could not be met by the membrane ISE, but could
be met by the solid state one.
Method Detection Limit Study. A method
detection limit study showed that the solid state
ISE system could consistently detect perchlorate
concentrations at 3 ug/L when no interferents
were present. This allowed the project to set a
practical quantitation limit (PQL) or reporting
limit of 15 ng/L, which is 5 times the detection
limit and 20 percent lower than the target
concentration of 18 ug/L. Periodic tests of method
blanks and 5 |^g/L and 10 |ig/L perchlorate
standards indicated acceptable differentiation of
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Office of Superfund Remediation
and Technology Innovation
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millivolt readings at 5-10 |Jg/L for a distilled
water matrix. Although the MDLs were 3 u^g/L,
results for standards less than 5 ng/L could not
generally be considered differentiable from results
for blanks. The method was found to be linear
over a 5 to 100 ^ig/L range.
Results reported as non-detects (ND) at the PQL
or MDL indicate the absence of perchlorate at that
concentration. If results are to be reported as
detections between the PQL and MDL, they
should be considered quantiatively and
qualitatively uncertain due to possible matrix
effects. However, results below the PQL of 15
Hg/L should not be reported as detections unless
the matrix is demonstrated to be free of positive
interferences.
Interferences. A direct relationship between false
positive readings and conductivity was noted with
some samples causing significant loss of electrode
sensitivity as well. From background information
on the sampling locations, the most likely
significant interferents were determined to be
carbonate/bicarbonate, chloride, and nitrate.
Bromide, fluoride, iodide, phosphate, and other
anions such as thiocyanate were also identified as
interferents in the ISE manufacturer's
specifications. The project developed correction
factors for matrices containing concentrations in
excess of 50 mg/L chloride, 0.12 mg/L NO3-N, or
1.2 mg/L bromide. Interference due to carbonate
and bicarbonate is eliminated by the acidification
of all standards and samples to pH 4.0 (±0.1) with
sulfuric acid. The electrode must be reconditioned
after analyzing any sample containing
concentrations of nitrate greater than 0.2 mg/L
NO3-N. Reconditioning consists of placing the
ISE module in an acidified blank and an acidified
100-2,000 (jg/L perchlorate solution (depending
upon the concentration of nitrate) to maintain
adequate sensitivity to meet ±20% accuracy
criteria.
Standard Operating Procedure. A standard
operating procedure (SOP) was developed for
using ISE to analyze perchlorate in ground water.
The SOP can be found in Perchlorate Screening
Study: Low Concentration Method For the
Determination of Perchlorate in Aqueous Samples
Using Ion Selective Electrodes at http://clu-
in.org/programs/21 m2/letter_of_fmdings.pdf. It
was suggested that matrices with very high
concentrations of interfering anions be evaluated
before going into the field to determine if this
method will meet project objectives.
Recommendation for Further Studies. Further
studies are recommended to reconfirm the
accuracy of the correction factors and to study the
effects of other potential interferents, including
mixtures of interfering anions. Possible methods
to mitigate nitrate interference should be explored
to make the method more versatile. Further
studies to determine and maximize the
effectiveness of the method of standard additions
are required. The study also determined that
further improvements in cleanup technology, such
as ion exchange cartridges, needed to be
investigated to make the method more robust.
Project Information
For more information on this study, contact
Joe Eidelberg, U.S. EPA Region IX,
415-972-3809, Eidelberg.JoseDh@eoa.gov.
Additional Information
Also, more information can be found at the
following locations:
• Earthtech Inc., San Jose, CA, Chris Davis,
(408) 232-2829,
Chris.Davisrgiearthtech.com
21M2 program.
http://cluin.orff/profframs/21m2/OT
• Dan Powell, U.S.EPA,
703-603-7196,
powell.dan@,epa.gov
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