Change in Nitric Acid Preservative for Trace Element Samples
Date: Tue, 29 Sep 1998 14:29:30 -0400
From: Nana Frye
Reply-To: "Nana L Frye, Secretary (OA), Reston, VA "
Organization: U.S. Geological Survey
To: "A - Division Chief and Staff",
"B - Branch Chiefs and Offices",
"S - Special Distribution for Research",
"FO - State, District, Subdistrict and other Field Offices",
"PO - Project Offices",
CC: " WRD Archive File, "
Subject: OWQ Technical Memo 9806--Change in Nitric Acid Preservative for Trace Element Samples
In Reply Refer To: September 29, 1998
Mail Stop 412
OFFICE OF WATER QUALITY TECHNICAL MEMORANDUM 98.06
Subject: Change in Nitric Acid Preservative for Trace Element Samples
The purpose of this memorandum is to inform Division personnel of
a change in the concentration, volume, and containers for nitric
acid used for preservation of water samples for the determination
of trace elements.
For many years the accepted protocol for the preservation of
water samples for determination of trace elements has required
adjustment of sample pH to less than 2 pH units using nitric acid
(U.S. Environmental Protection Agency, 1979). The Division has
provided nitric acid in glass ampoules for this purpose since the
1970's (Wood, 1976). The purpose of adjusting sample pH to less
than 2 is to minimize metal cation precipitation and adsorption
onto the sample container wall (OWDC, 1982). However, it has
been known for some time that nitric acid will also solubilize
certain elements from glass ampoules, specifically aluminum,
barium, boron, silica, chromium, and zinc (Open-File Report (OFR)
As analytical detection limits for trace elements have become
more sensitive, the use of a higher purity ("ultra-pure" or
"Ultrex-grade") nitric acid (use of trade, product, or firm names
is for descriptive purposes only and does not imply endorsement
by the U.S. Geological Survey) was recommended to reduce possible
contamination of samples by the preservative (OFR 94-539). In
1994, concurrent with the promulgation of the new part-per-
billion sampling protocols, Ultrex-grade nitric acid in Teflon
vials were made available as an alternative to glass ampoules for
trace metals preservation. This change eliminated glass ampoules
as a source of contamination of field blanks and some
environmental samples. Teflon does not contribute detectable
inorganic contaminants, but the vials are costly and difficult to
handle in the field.
The Office of Water Quality (OWQ), National Water Quality
Laboratory (NWQL), and the Quality of Water Service Unit (QWSU)
have been evaluating polypropylene vials as an alternative to
both the glass ampoules and Teflon vials for Ultrex-grade nitric
acid preservative. This option would serve to improve data
quality at a reasonable cost. The polypropylene vials are less
fragile than glass, and the preservative is more easily dispensed
from them than from glass ampoules or Teflon vials.
Polypropylene is not recommended for storage of concentrated
nitric acid, but diluted nitric acid is reported to have only a
minor effect on the material, based on testing by Eagle-Picher
Industries, Inc. (Rod Neal, oral commun., 1998). The
manufacturer of Ultrex nitric acid has agreed to provide the acid
in a 50-percent concentration (7.5-7.7 Normal) for Division use.
This diluted form can be stored in polypropylene vials without
apparent degradation of the plastic for at least one year. The
acid will be provided in 2-milliliter quantities so that the
majority of natural water samples will have a pH below 2 after
addition of the contents of one vial. Field personnel should be
aware that unusual matrices may require additional acid to ensure
a sample pH of <2.0.
Effective October 1, 1998, glass ampoules containing Ultrex
nitric acid no longer will be available from QWSU. Districts may
continue to use Ultrex nitric acid in glass for major-ion
schedules, and in Teflon vials for trace-element schedules until
existing stocks are exhausted. Nitric acid in glass ampoules
should not be used to preserve samples for trace-element
In September 1998, Ultrex-grade nitric acid will be available
from the QWSU in polypropylene vials. Each vial will contain 2
milliliters (2 ml) of 50% (by volume) Ultrex-grade nitric acid
(7.5-7.7 Normal) which is sufficient to preserve the majority of
natural water samples (8-ounce, -250 ml) at a pH <2,0. The
addition of 2 milliliters of the preservative to a sample will
not introduce any significant dilution error. Some atypical
sample sources may require additional acid to ensure a sample pH
Each polypropylene vial will bear a label displaying the name of
the acid, concentration, volume, lot number, and date of
preparation. The preservative will be packaged and shipped in
boxes of 24 vials, with a Certificate of Analysis from the NWQL.
The packaging for these vials is approved by the U.S. Department
of Transportation (DOT), and the vials should be stored and
transported in the field in the original container. Empty PP
vials should be disposed in accordance with local regulations.
Available information indicates that the polypropylene vials are
suitable containers for 50 percent nitric acid. However, the OWQ,
the NWQL, and Ocala QWSU plan to test the vials and acid over
time to confirm stability. A shelf life cannot be provided at
this time. However, it is recommended that you buy no more than a
2-year supply. Until sufficient testing can be done, it is not
recommended that the vials be stored in field vehicles under high
heat conditions for extended periods.
The price of the polypropylene Ultrex nitric acid vials will be
somewhat more than the glass ampoules but much less than the
Ultrex acid in Teflon. The price (like other preservatives used
by the Division) will reflect the costs of the custom-prepared
acid, polypropylene vials, labels, packaging, and NWQL/QWSU
quality-assurance and handling. The vials will be listed in the
QWSU catalog as: 436FLD Vial (PP), HNO3, 7.5N-7.7N, 2ml, Ultrex.
Orders may be placed through One-Stop Shopping:
http://1stop.usgs.gov, from the QWSU webpage:
http://qwsu.er.usgs.gov, or Email to email@example.com. Ultrex
nitric acid will be available only in the polypropylene vials and
only in the 2 ml quantity and 7.5-7.7 Normal concentration.
Nitric acid is highly corrosive, even in this diluted form.
Suitable personal protective gear always should be used when
handling this chemical (gloves and eye protection). The Material
Safety Data Sheet for this chemical is available at:
U.S. Environmental Protection Agency, March, 1979, Methods of
Chemical Analysis of Water and Wastes.
Wood, W.W., 1976, Guidelines for collection and field analysis of
ground-water samples for selected unstable constituents:
Techniques of Water Resources Investigations of the U.S.
Geological Survey, Book 1, Chap. D2.
Office of Water Data Coordination, 1982, National Handbook of
Recommended Methods for Water-Data Acquisition, Chapter 5.
Horowitz, A.J., Demas, C.R., Fitzgerald, K.K., Miller, T.L., and
Rickert, D.A., 1994, U.S. Geological Survey Protocol for the
Collection and Processing of Surface-Water Samples for the
Subsequent Determination of Inorganic Constituents in Filtered
Water, Open-File Report 94-539.
Janice R. Ward
Acting Chief, Office of Water Quality
Distribution: A, B, S, FO, PO
District Water Quality Specialists
Regional Water Quality Specialists
This memorandum does not supersede any other Office of Water
Quality Technical Memorandum.
Key Words: Sample preservation, trace metals, Ultrex, nitric acid
OWQ Technical Memorandum 98.06